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When did birds and mammals diverge?

When did birds and mammals diverge?


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Which was the most recent common ancestor between mammals and birds?

Form a rapid google research I could not find good answers. This site seems to imply it was at least 250 milions of years ago. This scientific article mentions 600 milion years but the sentence is a little unclear.


You can check it easily in the timetree website: http://www.timetree.org/

Put a mammalian species as taxon 1 (Homo sapiens) and a bird as taxon2 (Gallus gallus) in the first pair of boxes and hit run.

You will see a compilation of studies, with their references, with different calculations about the time when the last common ancestor of both species lived. A median and a mean are also provided.

In this case the values are Median Time: 320 MYA Estimated Time: 312 MYA CI: (297 - 326 MYA)


The Most Recent Common Ancestor (MRCA) of birds and mammals was the first amniotes.

Fossil record

The latest known fossil of amniotes are 312 million years old (Bentou and Donoghue, 2007).

Genetic data

From EOL, using data from The Paleobiology Database, the first amniote is in between 314.6 and 323.2 million years old. Those results are a little bit older than the latest known fossil which very much makes sense.

The (apparent) contradiction in the sources you found

This site seems to imply it was at least 250 milions of years ago.

They actually say 250 to 320 millions years. This broad estimates match the above studies and data.

This scientific article mentions 600 milion years but the sentence is a little unclear.

It is actually a news feature published by a science writer (former neuroscientist according to her twitter account), not a researcher. She indeed made the claim (Even so, 600 million years of evolution and radically different brain architectures separate birds from humans.). There is no reference for the claim and based on above evidences, it appears at first to be a mistake. It is not impossible though that she considered the evolutionary time in both branches in her statement but that would be very unusual If two lineages diverged $t$ years ago, then they both evolved for $t$ years and it could arguably be said that they are separate by $2 t$ years of evolution. If this is what she meant, then the claim is correct but quite misleading.


When did birds and mammals diverge? - Biology

Thanks to amazing new fossil discoveries in China, the evolutionary history of birds has become clearer, even though bird bones do not fossilize as well as those of other vertebrates. As we’ve seen earlier, birds are highly modified diapsids, but rather than having two fenestrations or openings in their skulls behind the eye, the skulls of modern birds are so specialized that it is difficult to see any trace of the original diapsid condition.

Birds belong to a group of diapsids called the archosaurs , which includes three other groups: living crocodilians, pterosaurs, and dinosaurs. Overwhelming evidence shows that birds evolved within the clade Dinosauria, which is further subdivided into two groups, the Saurischia (“lizard hips”) and the Ornithischia (“bird hips”). Despite the names of these groups, it was not the bird-hipped dinosaurs that gave rise to modern birds. Rather, Saurischia diverged into two groups: One included the long-necked herbivorous dinosaurs, such as Apatosaurus. The second group, bipedal predators called theropods, gave rise to birds. This course of evolution is highlighted by numerous similarities between late (maniraptoran) theropod fossils and birds, specifically in the structure of the hip and wrist bones, as well as the presence of the wishbone, formed by the fusion of the clavicles.

The clade Neornithes includes the avian crown group, which comprises all living birds and the descendants from their most recent common maniraptoran ancestor. One well-known and important fossil of an animal that appears “intermediate” between dinosaurs and birds is Archaeopteryx (Figure 1), which is from the Jurassic period (200 to 145 MYA). Archaeopteryx has characteristics of both maniraptoran dinosaurs and modern birds. Some scientists propose classifying it as a bird, but others prefer to classify it as a dinosaur. Traits in skeletons of Archaeopteryx like those of a dinosaur included a jaw with teeth and a long bony tail. Like birds, it had feathers modified for flight, both on the forelimbs and on the tail, a trait associated only with birds among modern animals. Fossils of older feathered dinosaurs exist, but the feathers may not have had the characteristics of modern flight feathers.

Figure 1. (a) Archaeopteryx lived in the late Jurassic period around 150 million years ago. It had cuplike thecodont teeth like a dinosaur, but had (b) flight feathers like modern birds, which can be seen in this fossil. Note the claws on the wings, which are still found in a number of birds, such as the newborn chicks of the South American Hoatzin.

The Evolution of Flight in Birds

There are two basic hypotheses that explain how flight may have evolved in birds: the arboreal (“tree”) hypothesis and the terrestrial (“land”) hypothesis. The arboreal hypothesis posits that tree-dwelling precursors to modern birds jumped from branch to branch using their feathers for gliding before becoming fully capable of flapping flight. In contrast to this, the terrestrial hypothesis holds that running (perhaps pursuing active prey such as small cursorial animals) was the stimulus for flight. In this scenario, wings could be used to capture prey and were preadapted for balance and flapping flight. Ostriches, which are large flightless birds, hold their wings out when they run, possibly for balance. However, this condition may represent a behavioral relict of the clade of flying birds that were their ancestors. It seems more likely that small feathered arboreal dinosaurs, were capable of gliding (and flapping) from tree to tree and branch to branch, improving the chances of escaping enemies, finding mates, and obtaining prey such as flying insects. This early flight behavior would have also greatly increased the opportunity for species dispersal.

Although we have a good understanding of how feathers and flight may have evolved, the question of how endothermy evolved in birds (and other lineages) remains unanswered. Feathers provide insulation, but this is only beneficial for thermoregulatory purposes if body heat is being produced internally. Similarly, internal heat production is only viable for the evolution of endothermy if insulation is present to retain that infrared energy. It has been suggested that one or the other—feathers or endothermy—evolved first in response to some other selective pressure (e.g., the ability to be active at night, provide camouflage, repel water, or serve as signals for mate selection). It seems probable that feathers and endothermy coevolved together, the improvement and evolutionary advancement of feathers reinforcing the evolutionary advancement of endothermy, and so on.

Figure 2. Shanweiniao cooperorum was a species of Enantiornithes that did not survive past the Cretaceous period. (credit: Nobu Tamura)

During the Cretaceous period (145 to 66 MYA), a group known as the Enantiornithes was the dominant bird type (Figure 2). Enantiornithes means “opposite birds,” which refers to the fact that certain bones of the shoulder are joined differently than the way the bones are joined in modern birds. Like Archaeopteryx, these birds retained teeth in their jaws, but did have a shortened tail, and at least some fossils have preserved “fans” of tail feathers. These birds formed an evolutionary lineage separate from that of modern birds, and they did not survive past the Cretaceous. Along with the Enantiornithes, however, another group of birds—the Ornithurae (“bird tails”), with a short, fused tail or pygostyle —emerged from the evolutionary line that includes modern birds. This clade was also present in the Cretaceous.

After the extinction of Enantiornithes, the Ornithurae became the dominant birds, with a large and rapid radiation occurring after the extinction of the dinosaurs during the Cenozoic era (66 MYA to the present). Molecular analysis based on very large data sets has produced our current understanding of the relationships among living birds. There are three major clades: the Paleognathae, the Galloanserae, and the Neoaves. The Paleognathae (“old jaw”) or ratites (polyphyletic) are a group of flightless birds including ostriches, emus, rheas, and kiwis. The Galloanserae include pheasants, ducks, geese and swans. The Neoaves (“new birds”) includes all other birds. The Neoaves themselves have been distributed among five clades: [1] Strisores (nightjars, swifts, and hummingbirds), Columbaves (turacos, bustards, cuckoos, pigeons, and doves), Gruiformes (cranes), Aequorlitornithes (diving birds, wading birds, and shorebirds), and Inopinaves (a very large clade of land birds including hawks, owls, woodpeckers, parrots, falcons, crows, and songbirds). Despite the current classification scheme, it is important to understand that phylogenetic revisions, even for the extant birds, are still taking place.


Contents

Ornamentation and coloration Edit

Common and easily identified types of dimorphism consist of ornamentation and coloration, though not always apparent. A difference in coloration of sexes within a given species is called sexual dichromatism, which is commonly seen in many species of birds and reptiles. [4] Sexual selection leads to the exaggerated dimorphic traits that are used predominantly in competition over mates. The increased fitness resulting from ornamentation offsets its cost to produce or maintain suggesting complex evolutionary implications, but the costs and evolutionary implications vary from species to species. [5] [6] The costs and implications differ depending on the nature of the ornamentation (such as the colour mechanism involved).

The peafowl constitute conspicuous illustrations of the principle. The ornate plumage of peacocks, as used in the courting display, attracts peahens. At first sight one might mistake peacocks and peahens for completely different species because of the vibrant colours and the sheer size of the male's plumage the peahen being of a subdued brown coloration. [7] The plumage of the peacock increases its vulnerability to predators because it is a hindrance in flight, and it renders the bird conspicuous in general. [7] Similar examples are manifold, such as in birds of paradise and argus pheasants.

Another example of sexual dichromatism is that of the nestling blue tits. Males are chromatically more yellow than females. It is believed that this is obtained by the ingestion of green Lepidopteran larvae, which contain large amounts of the carotenoids lutein and zeaxanthin. [8] This diet also affects the sexually dimorphic colours in the human-invisible ultraviolet spectrum. [9] [10] Hence, the male birds, although appearing yellow to humans actually have a violet-tinted plumage that is seen by females. This plumage is thought to be an indicator of male parental abilities. [11] Perhaps this is a good indicator for females because it shows that they are good at obtaining a food supply from which the carotenoid is obtained. There is a positive correlation between the chromas of the tail and breast feathers and body condition. [12] Carotenoids play an important role in immune function for many animals, so carotenoid dependent signals might indicate health. [13]

Frogs constitute another conspicuous illustration of the principle. There are two types of dichromatism for frog species: ontogenetic and dynamic. Ontogenetic frogs are more common and have permanent color changes in males or females. Ranoidea lesueuri is an example of a dynamic frog that has temporary color changes in males during breeding season. [14] Hyperolius ocellatus is an ontogenetic frog with dramatic differences in both color and pattern between the sexes. At sexual maturity, the males display a bright green with white dorsolateral lines. [15] In contrast, the females are rusty red to silver with small spots. The bright coloration in the male population serves to attract females and as an aposematic sign to potential predators.

Females often show a preference for exaggerated male secondary sexual characteristics in mate selection. [16] The sexy son hypothesis explains that females prefer more elaborate males and select against males that are dull in color, independent of the species' vision. [17]

Similar sexual dimorphism and mating choice are also observed in many fish species. For example, male guppies have colorful spots and ornamentations while females are generally grey in color. Female guppies prefer brightly colored males to duller males. [18]

In redlip blennies, only the male fish develops an organ at the anal-urogenital region that produces antimicrobial substances. During parental care, males rub their anal-urogenital regions over their nests' internal surfaces, thereby protecting their eggs from microbial infections, one of the most common causes for mortality in young fish. [19]

Most flowering plants are hermaphroditic but approximately 6% of species have separate males and females (dioecy). [20] Males and females in insect-pollinated species generally look similar to one another because plants provide rewards (e.g. nectar) that encourage pollinators to visit another similar flower, completing pollination. Catasetum orchids are one interesting exception to this rule. Male Catasetum orchids violently attach pollinia to euglossine bee pollinators. The bees will then avoid other male flowers but may visit the female, which look different from the males. [21]

Various other dioecious exceptions, such as Loxostylis alata have visibly different genders, with the effect of eliciting the most efficient behaviour from pollinators, who then use the most efficient strategy in visiting each gender of flower instead of searching say, for pollen in a nectar-bearing female flower.

Some plants, such as some species of Geranium have what amounts to serial sexual dimorphism. The flowers of such species might for example present their anthers on opening, then shed the exhausted anthers after a day or two and perhaps change their colours as well while the pistil matures specialist pollinators are very much inclined to concentrate on the exact appearance of the flowers they serve, which saves their time and effort and serves the interests of the plant accordingly. Some such plants go even further and change their appearance again once they have been fertilised, thereby discouraging further visits from pollinators. This is advantageous to both parties because it avoids damage to the developing fruit and avoids wasting the pollinator's effort on unrewarding visits. In effect the strategy ensures that the pollinators can expect a reward every time they visit an appropriately advertising flower.

Females of the aquatic plant Vallisneria americana have floating flowers attached by a long flower stalk that are fertilized if they contact one of the thousands of free floating flowers released by a male. [22] Sexual dimorphism is most often associated with wind-pollination in plants due to selection for efficient pollen dispersal in males vs pollen capture in females, e.g. Leucadendron rubrum. [23]

Sexual dimorphism in plants can also be dependent on reproductive development. This can be seen in Cannabis sativa, a type of hemp, which have higher photosynthesis rates in males while growing but higher rates in females once the plants become sexually mature. [24]

Every sexually reproducing extant species of vascular plant actually has an alternation of generations the plants we see about us generally are diploid sporophytes, but their offspring really are not the seeds that people commonly recognise as the new generation. The seed actually is the offspring of the haploid generation of microgametophytes (pollen) and megagametophytes (the embryo sacs in the ovules). Each pollen grain accordingly may be seen as a male plant in its own right it produces a sperm cell and is dramatically different from the female plant, the megagametophyte that produces the female gamete.

Insects display a wide variety of sexual dimorphism between taxa including size, ornamentation and coloration. [25] The female-biased sexual size dimorphism observed in many taxa evolved despite intense male–male competition for mates. [26] In Osmia rufa, for example, the female is larger/broader than males, with males being 8–10 mm in size and females being 10–12 mm in size. [27] In the hackberry emperor females are similarly larger than males. [28] The reason for the sexual dimorphism is due to provision size mass, in which females consume more pollen than males. [29]

In some species, there is evidence of male dimorphism, but it appears to be for the purpose of distinctions of roles. This is seen in the bee species Macrotera portalis in which there is a small-headed morph, capable of flight, and large-headed morph, incapable of flight, for males. [30] Anthidium manicatum also displays male-biased sexual dimorphism. The selection for larger size in males rather than females in this species may have resulted due to their aggressive territorial behavior and subsequent differential mating success. [31] Another example is Lasioglossum hemichalceum, which is a species of sweat bee that shows drastic physical dimorphisms between male offspring. [32] Not all dimorphism has to have a drastic difference between the sexes. Andrena agilissima is a mining bee where the females only have a slightly larger head than the males. [33]

Weaponry leads to increased fitness by increasing success in male-male competition in many insect species. [34] The beetle horns in Onthophagus taurus are enlarged growths of the head or thorax expressed only in the males. Copris ochus also has distinct sexual and male dimorphism in head horns. [35] These structures are impressive because of the exaggerated sizes. [36] There is a direct correlation between male horn lengths and body size and higher access to mates and fitness. [36] In other beetle species, both males and females may have ornamentation such as horns. [35] Generally, insect sexual size dimorphism (SSD) within species increases with body size. [37]

Sexual dimorphism within insects is also displayed by dichromatism. In butterfly genera Bicyclus and Junonia, dimorphic wing patterns evolved due to sex-limited expression, which mediates the intralocus sexual conflict and leads to increased fitness in males. [38] The sexual dichromatic nature of Bicyclus anynana is reflected by female selection on the basis of dorsal UV-reflective eyespot pupils. [39] The common brimstone also displays sexual dichromatism males have yellow and iridescent wings, while female wings are white and non-iridescent. [40] Naturally selected deviation in protective female coloration is displayed in mimetic butterflies. [41]

Many arachnid groups exhibit sexual dimorphism, [42] but it is most widely studied in the spiders. In the orb-weaving spider Zygiella x-notata, for example, adult females have a larger body size than adult males. [43] Size dimorphism shows a correlation with sexual cannibalism, [44] which is prominent in spiders (it is also found in insects such as praying mantises). In the size dimorphic wolf spider Tigrosa helluo, food-limited females cannibalize more frequently. [45] Therefore, there is a high risk of low fitness for males due to pre-copulatory cannibalism, which led to male selection of larger females for two reasons: higher fecundity and lower rates of cannibalism. [45] In addition, female fecundity is positively correlated with female body size and large female body size is selected for, which is seen in the family Araneidae. All Argiope species, including Argiope bruennichi, use this method. Some males evolved ornamentation [ vague ] including binding the female with silk, having proportionally longer legs, modifying the female's web, mating while the female is feeding, or providing a nuptial gift in response to sexual cannibalism. [45] Male body size is not under selection due to cannibalism in all spider species such as Nephila pilipes, but is more prominently selected for in less dimorphic species of spiders, which often selects for larger male size. [46] In the species Maratus volans, the males are known for their characteristic colorful fan which attracts the females during mating. [47]

Ray finned fish are an ancient and diverse class, with the widest degree of sexual dimorphism of any animal class. Fairbairn notes that "females are generally larger than males but males are often larger in species with male-male combat or male paternal care . [sizes range] from dwarf males to males more than 12 times heavier than females." [48]

There are cases where males are substantially larger than females. An example is Lamprologus callipterus, a type of cichlid fish. In this fish, the males are characterized as being up to 60 times larger than the females. The male's increased size is believed to be advantageous because males collect and defend empty snail shells in each of which a female breeds. [49] Males must be larger and more powerful in order to collect the largest shells. The female's body size must remain small because in order for her to breed, she must lay her eggs inside the empty shells. If she grows too large, she will not fit in the shells and will be unable to breed. The female's small body size is also likely beneficial to her chances of finding an unoccupied shell. Larger shells, although preferred by females, are often limited in availability. [50] Hence, the female is limited to the growth of the size of the shell and may actually change her growth rate according to shell size availability. [51] In other words, the male's ability to collect large shells depends on his size. The larger the male, the larger the shells he is able to collect. This then allows for females to be larger in his brooding nest which makes the difference between the sizes of the sexes less substantial. Male-male competition in this fish species also selects for large size in males. There is aggressive competition by males over territory and access to larger shells. Large males win fights and steal shells from competitors. Another example is the dragonet, in which males are considerably larger than females and possess longer fins.

Sexual dimorphism also occurs in hermaphroditic fish. These species are known as sequential hermaphrodites. In fish, reproductive histories often include the sex-change from female to male where there is a strong connection between growth, the sex of an individual, and the mating system it operates within. [52] In protogynous mating systems where males dominate mating with many females, size plays a significant role in male reproductive success. [53] Males have a propensity to be larger than females of a comparable age but it is unclear whether the size increase is due to a growth spurt at the time of the sexual transition or due to the history of faster growth in sex changing individuals. [54] Larger males are able to stifle the growth of females and control environmental resources.

Social organization plays a large role in the changing of sex by the fish. It is often seen that a fish will change its sex when there is a lack of dominant male within the social hierarchy. The females that change sex are often those who attain and preserve an initial size advantage early in life. In either case, females which change sex to males are larger and often prove to be a good example of dimorphism.

In other cases with fish, males will go through noticeable changes in body size, and females will go through morphological changes that can only be seen inside of the body. For example, in sockeye salmon, males develop larger body size at maturity, including an increase in body depth, hump height, and snout length. Females experience minor changes in snout length, but the most noticeable difference is the huge increase in gonad size, which accounts for about 25% of body mass. [55]

Sexual selection was observed for female ornamentation in Gobiusculus flavescens, known as two-spotted gobies. [56] Traditional hypotheses suggest that male-male competition drives selection. However, selection for ornamentation within this species suggests that showy female traits can be selected through either female-female competition or male mate choice. [56] Since carotenoid-based ornamentation suggests mate quality, female two-spotted guppies that develop colorful orange bellies during the breeding season are considered favorable to males. [57] The males invest heavily in offspring during the incubation, which leads to the sexual preference in colorful females due to higher egg quality. [57]

In amphibians and reptiles, the degree of sexual dimorphism varies widely among taxonomic groups. The sexual dimorphism in amphibians and reptiles may be reflected in any of the following: anatomy relative length of tail relative size of head overall size as in many species of vipers and lizards coloration as in many amphibians, snakes, and lizards, as well as in some turtles an ornament as in many newts and lizards the presence of specific sex-related behaviour is common to many lizards and vocal qualities which are frequently observed in frogs.

Anole lizards show prominent size dimorphism with males typically being significantly larger than females. For instance, the average male Anolis sagrei was 53.4 mm vs. 40 mm in females. [58] Different sizes of the heads in anoles have been explained by differences in the estrogen pathway. [59] The sexual dimorphism in lizards is generally attributed to the effects of sexual selection, but other mechanisms including ecological divergence and fecundity selection provide alternative explanations. [60] The development of color dimorphism in lizards is induced by hormonal changes at the onset of sexual maturity, as seen in Psamodromus algirus, Sceloporus gadoviae, and S. undulates erythrocheilus. [60]

Male painted dragon lizards, Ctenophorus pictus. are brightly conspicuous in their breeding coloration, but male colour declines with aging. Male coloration appears to reflect innate anti-oxidation capacity that protects against oxidative DNA damage. [61] Male breeding coloration is likely an indicator to females of the underlying level of oxidative DNA damage (a significant component of aging) in potential mates. [61]

Sexual dimorphism in birds can be manifested in size or plumage differences between the sexes. Sexual size dimorphism varies among taxa with males typically being larger, though this is not always the case, e.g. birds of prey, hummingbirds, and some species of flightless birds. [62] [63] Plumage dimorphism, in the form of ornamentation or coloration, also varies, though males are typically the more ornamented or brightly colored sex. [64] Such differences have been attributed to the unequal reproductive contributions of the sexes. [65] This difference produces a stronger female choice since they have more risk in producing offspring. In some species, the male's contribution to reproduction ends at copulation, while in other species the male becomes the main caregiver. Plumage polymorphisms have evolved to reflect these differences and other measures of reproductive fitness, such as body condition [66] or survival. [67] The male phenotype sends signals to females who then choose the 'fittest' available male.

Sexual dimorphism is a product of both genetics and environmental factors. An example of sexual polymorphism determined by environmental conditions exists in the red-backed fairywren. Red-backed fairywren males can be classified into three categories during breeding season: black breeders, brown breeders, and brown auxiliaries. [66] These differences arise in response to the bird's body condition: if they are healthy they will produce more androgens thus becoming black breeders, while less healthy birds produce less androgens and become brown auxiliaries. [66] The reproductive success of the male is thus determined by his success during each year's non-breeding season, causing reproductive success to vary with each year's environmental conditions.

Migratory patterns and behaviors also influence sexual dimorphisms. This aspect also stems back to the size dimorphism in species. It has been shown that the larger males are better at coping with the difficulties of migration and thus are more successful in reproducing when reaching the breeding destination. [68] When viewing this in an evolutionary standpoint many theories and explanations come to consideration. If these are the result for every migration and breeding season the expected results should be a shift towards a larger male population through sexual selection. Sexual selection is strong when the factor of environmental selection is also introduced. The environmental selection may support a smaller chick size if those chicks were born in an area that allowed them to grow to a larger size, even though under normal conditions they would not be able to reach this optimal size for migration. When the environment gives advantages and disadvantages of this sort, the strength of selection is weakened and the environmental forces are given greater morphological weight. The sexual dimorphism could also produce a change in timing of migration leading to differences in mating success within the bird population. [69] When the dimorphism produces that large of a variation between the sexes and between the members of the sexes multiple evolutionary effects can take place. This timing could even lead to a speciation phenomenon if the variation becomes strongly drastic and favorable towards two different outcomes.

Sexual dimorphism is maintained by the counteracting pressures of natural selection and sexual selection. For example, sexual dimorphism in coloration increases the vulnerability of bird species to predation by European sparrowhawks in Denmark. [70] Presumably, increased sexual dimorphism means males are brighter and more conspicuous, leading to increased predation. [70] Moreover, the production of more exaggerated ornaments in males may come at the cost of suppressed immune function. [66] So long as the reproductive benefits of the trait due to sexual selection are greater than the costs imposed by natural selection, then the trait will propagate throughout the population. Reproductive benefits arise in the form of a larger number of offspring, while natural selection imposes costs in the form of reduced survival. This means that even if the trait causes males to die earlier, the trait is still beneficial so long as males with the trait produce more offspring than males lacking the trait. This balance keeps the dimorphism alive in these species and ensures that the next generation of successful males will also display these traits that are attractive to the females.

Such differences in form and reproductive roles often cause differences in behavior. As previously stated, males and females often have different roles in reproduction. The courtship and mating behavior of males and females are regulated largely by hormones throughout a bird's lifetime. [71] Activational hormones occur during puberty and adulthood and serve to 'activate' certain behaviors when appropriate, such as territoriality during breeding season. [71] Organizational hormones occur only during a critical period early in development, either just before or just after hatching in most birds, and determine patterns of behavior for the rest of the bird's life. [71] Such behavioral differences can cause disproportionate sensitivities to anthropogenic pressures. [72] Females of the whinchat in Switzerland breed in intensely managed grasslands. [72] Earlier harvesting of the grasses during the breeding season lead to more female deaths. [72] Populations of many birds are often male-skewed and when sexual differences in behavior increase this ratio, populations decline at a more rapid rate. [72] Also not all male dimorphic traits are due to hormones like testosterone, instead they are a naturally occurring part of development, for example plumage. [73] In addition, the strong hormonal influence on phenotypic differences suggest that the genetic mechanism and genetic basis of these sexually dimorphic traits may involve transcription factors or cofactors rather than regulatory sequences. [74]

Sexual dimorphism may also influence differences in parental investment during times of food scarcity. For example, in the blue-footed booby, the female chicks grow faster than the males, resulting in booby parents producing the smaller sex, the males, during times of food shortage. This then results in the maximization of parental lifetime reproductive success. [75] In Black-tailed Godwits Limosa limosa limosa females are also the larger sex, and the growth rates of female chicks are more susceptible to limited environmental conditions. [76]

Sexual dimorphism may also only appear during mating season, some species of birds only show dimorphic traits in seasonal variation. The males of these species will molt into a less bright or less exaggerated color during the off breeding season. [74] This occurs because the species is more focused on survival than reproduction, causing a shift into a less ornate state. [ dubious – discuss ]

Consequently, sexual dimorphism has important ramifications for conservation. However, sexual dimorphism is not only found in birds and is thus important to the conservation of many animals. Such differences in form and behavior can lead to sexual segregation, defined as sex differences in space and resource use. [77] Most sexual segregation research has been done on ungulates, [77] but such research extends to bats, [78] kangaroos, [79] and birds. [80] Sex-specific conservation plans have even been suggested for species with pronounced sexual segregation. [78]

The term sesquimorphism (the Latin numeral prefix sesqui- means one-and-one-half, so halfway between mono- (one) and di- (two)) has been proposed for bird species in which "both sexes have basically the same plumage pattern, though the female is clearly distinguishable by reason of her paler or washed-out colour". [81] : 14 Examples include Cape sparrow (Passer melanurus), [81] : 67 rufous sparrow (subspecies P. motinensis motinensis), [81] : 80 and saxaul sparrow (P. ammodendri). [81] : 245

In a large proportion of mammal species, males are larger than females. [82] Both genes and hormones affect the formation of many animal brains before "birth" (or hatching), and also behaviour of adult individuals. Hormones significantly affect human brain formation, and also brain development at puberty. A 2004 review in Nature Reviews Neuroscience observed that "because it is easier to manipulate hormone levels than the expression of sex chromosome genes, the effects of hormones have been studied much more extensively, and are much better understood, than the direct actions in the brain of sex chromosome genes." It concluded that while "the differentiating effects of gonadal secretions seem to be dominant," the existing body of research "support the idea that sex differences in neural expression of X and Y genes significantly contribute to sex differences in brain functions and disease." [83]

Pinnipeds Edit

Marine mammals show some of the greatest sexual size differences of mammals, because of sexual selection and environmental factors like breeding location. [84] [85] The mating system of pinnipeds varies from polygamy to serial monogamy. Pinnipeds are known for early differential growth and maternal investment since the only nutrients for newborn pups is the milk provided by the mother. [86] For example, the males are significantly larger (about 10% heavier and 2% longer) than the females at birth in sea lion pups. [87] The pattern of differential investment can be varied principally prenatally and post-natally. [88] Mirounga leonina, the southern elephant seal, is one of the most dimorphic mammals. [89]

Sexual dimorphism in elephant seals is associated with the ability of a male to defend territories and control large groups of females, which correlates with polygynic behavior. [90] The large sexual size dimorphism is partially due to sexual selection, but also because females reach reproductive age much earlier than males. In addition the males do not provide parental care for the young and allocate more energy to growth. [91] This is supported by the secondary growth spurt in males during adolescent years. [91]

Primates Edit

Humans Edit

Top: Stylised illustration of humans on the Pioneer plaque, showing both male (left) and female (right).
Bottom: Comparison between male (left) and female (right) pelvises.

Sexual dimorphism among humans includes differentiation among gonads, internal genitals, external genitals, breasts, muscle mass, height, the endocrine (hormonal) systems and their physiological and behavioral effects. Human sexual differentiation is effected primarily at the gene level, by the presence or absence of a Y-chromosome, which encodes biochemical modifiers for sexual development in males. [92] According to Clark Spencer Larsen, modern day Homo sapiens show a range of sexual dimorphism, with average body mass difference between the sexes being roughly equal to 15%. [93]

The average basal metabolic rate is about 6 percent higher in adolescent males than females and increases to about 10 percent higher after puberty. Females tend to convert more food into fat, while males convert more into muscle and expendable circulating energy reserves. Aggregated data of absolute strength indicates that females have, on average, 40–60% the upper body strength of males, and 70–75% the lower body strength. [94] The difference in strength relative to body mass is less pronounced in trained individuals. In Olympic weightlifting, male records vary from 5.5× body mass in the lowest weight category to 4.2× in the highest weight category, while female records vary from 4.4× to 3.8×, a weight adjusted difference of only 10–20%, and an absolute difference of about 30% (i.e. 472 kg vs 333 kg for unlimited weight classes)(see Olympic weightlifting records). A study, carried about by analyzing annual world rankings from 1980 to 1996, found that males' running times were, on average, 11% faster than females'. [95]

Females are taller, on average, than males in early adolescence, but males, on average, surpass them in height in later adolescence and adulthood. In the United States, adult males are, on average, 9% taller [96] and 16.5% heavier [97] than adult females. There is no comparative evidence of differing levels of sexual selection having produced sexual size dimorphism between human populations. [98]

Males typically have larger tracheae and branching bronchi, with about 30 percent greater lung volume per body mass. On average, males have larger hearts, 10 percent higher red blood cell count, higher hemoglobin, hence greater oxygen-carrying capacity. They also have higher circulating clotting factors (vitamin K, prothrombin and platelets). These differences lead to faster healing of wounds and higher peripheral pain tolerance. [99]

Females typically have more white blood cells (stored and circulating), more granulocytes and B and T lymphocytes. Additionally, they produce more antibodies at a faster rate than males. Hence they develop fewer infectious diseases and succumb for shorter periods. [99] Ethologists argue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as a selective advantage. [100] [101] [102] [103] [104]

Considerable discussion in academic literature concerns potential evolutionary advantages associated with sexual competition (both intrasexual and intersexual) and short- and long-term sexual strategies. [105] According to Daly and Wilson, "The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals." [106]

In the human brain, a difference between sexes was observed in the transcription of the PCDH11X/Y gene pair unique to Homo sapiens. [107] Sexual differentiation in the human brain from the undifferentiated state is triggered by testosterone from the fetal testis. Testosterone is converted to estrogen in the brain through the action of the enzyme aromatase. Testosterone acts on many brain areas, including the SDN-POA, to create the masculinized brain pattern. [108] Brains of pregnant females carrying male fetuses may be shielded from the masculinizing effects of androgen through the action of sex hormone-binding globulin. [109]

The relationship between sex differences in the brain and human behavior is a subject of controversy in psychology and society at large. [110] [111] Many females tend to have a higher ratio of gray matter in the left hemisphere of the brain in comparison to males. [112] [113] Males on average have larger brains than females however, when adjusted for total brain volume the gray matter differences between sexes is almost nonexistent. Thus, the percentage of gray matter appears to be more related to brain size than it is to sex. [114] [115] Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Haier et al. found in a 2004 study that "men and women apparently achieve similar IQ results with different brain regions, suggesting that there is no singular underlying neuroanatomical structure to general intelligence and that different types of brain designs may manifest equivalent intellectual performance". [116] (See the sex and intelligence article for more on this subject.) Strict graph-theoretical analysis of the human brain connections revealed [117] that in numerous graph-theoretical parameters (e.g., minimum bipartition width, edge number, the expander graph property, minimum vertex cover), the structural connectome of women are significantly "better" connected than the connectome of men. It was shown [118] that the graph-theoretical differences are due to the sex and not to the differences in the cerebral volume, by analyzing the data of 36 females and 36 males, where the brain volume of each man in the group was smaller than the brain volume of each woman in the group.

Sexual dimorphism was also described in the gene level and shown to extend from the sex chromosomes. Overall, about 6500 genes have been found to have sex-differential expression in at least one tissue. Many of these genes are not directly associated with reproduction, but rather linked to more general biological features. In addition, it has been shown that genes with sex specific expression undergo reduced selection efficiency, which lead to higher population frequencies of deleterious mutations and contributing to the prevalence of several human diseases. [119] [120]

Sexual dimorphism in immune function is a common pattern in vertebrates and also in a number of invertebrates. Most often, females are more ‘immunocompetent’ than males. The underlying causes are explained by either the role of immunosuppressive substances, such as testosterone, or by fundamental differences in male and female life histories. It has been shown that female mammals tend to have higher white blood cell counts (WBC), with further associations between cell counts and longevity in females. There is also a positive covariance between sexual dimorphism in immunity, as measured by a subset of WBC, and dimorphism in the duration of effective breeding. This is consistent with the application of ‘Bateman’s principle’ to immunity, with females maximizing fitness by lengthening lifespan through greater investment in immune defences. [121]

Phenotypic differences between sexes are evident even in cultured cells from tissues. [122] For example, female muscle-derived stem cells have a better muscle regeneration efficiency than male ones. [123] There are reports of several metabolic differences between male and female cells [124] and they also respond to stress differently. [125]

In theory, larger females are favored by competition for mates, especially in polygamous species. Larger females offer an advantage in fertility, since the physiological demands of reproduction are limiting in females. Hence there is a theoretical expectation that females tend to be larger in species that are monogamous. Females are larger in many species of insects, many spiders, many fish, many reptiles, owls, birds of prey and certain mammals such as the spotted hyena, and baleen whales such as blue whale. As an example, in some species, females are sedentary, and so males must search for them. Fritz Vollrath and Geoff Parker argue that this difference in behaviour leads to radically different selection pressures on the two sexes, evidently favouring smaller males. [126] Cases where the male is larger than the female have been studied as well, [126] and require alternative explanations.

One example of this type of sexual size dimorphism is the bat Myotis nigricans, (black myotis bat) where females are substantially larger than males in terms of body weight, skull measurement, and forearm length. [127] The interaction between the sexes and the energy needed to produce viable offspring make it favorable for females to be larger in this species. Females bear the energetic cost of producing eggs, which is much greater than the cost of making sperm by the males. The fecundity advantage hypothesis states that a larger female is able to produce more offspring and give them more favorable conditions to ensure their survival this is true for most ectotherms. A larger female can provide parental care for a longer time while the offspring matures. The gestation and lactation periods are fairly long in M. nigricans, the females suckling their offspring until they reach nearly adult size. [128] They would not be able to fly and catch prey if they did not compensate for the additional mass of the offspring during this time. Smaller male size may be an adaptation to increase maneuverability and agility, allowing males to compete better with females for food and other resources.

Some species of anglerfish also display extreme sexual dimorphism. Females are more typical in appearance to other fish, whereas the males are tiny rudimentary creatures with stunted digestive systems. A male must find a female and fuse with her: he then lives parasitically, becoming little more than a sperm-producing body in what amounts to an effectively hermaphrodite composite organism. A similar situation is found in the Zeus water bug Phoreticovelia disparata where the female has a glandular area on her back that can serve to feed a male, which clings to her (note that although males can survive away from females, they generally are not free-living). [129] This is taken to the logical extreme in the Rhizocephala crustaceans, like the Sacculina, where the male injects itself into the female's body and becomes nothing more than sperm producing cells, to the point that the superorder used to be mistaken for hermaphroditic. [130]

Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss Dicranum [131] and the liverwort Sphaerocarpos. [132] There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome, [132] [133] or to chemical signalling from females. [134]

Another complicated example of sexual dimorphism is in Vespula squamosa, the southern yellowjacket. In this wasp species, the female workers are the smallest, the male workers are slightly larger, and the female queens are significantly larger than her female worker and male counterparts. [ citation needed ]

Sexual dimorphism by size is evident in some extinct species such as the velociraptor. In the case of velociraptors the sexual size dimorphism may have been caused by two factors: male competition for hunting ground to attract mates, and/or female competition for nesting locations and mates, males being a scarce breeding resource. [136]

In 1871, Charles Darwin advanced the theory of sexual selection, which related sexual dimorphism with sexual selection.

It has been proposed that the earliest sexual dimorphism is the size differentiation of sperm and eggs (anisogamy), but the evolutionary significance of sexual dimorphism is more complex than that would suggest. [137] Anisogamy and the usually large number of small male gametes relative to the larger female gametes usually lies in the development of strong sperm competition, [138] [139] because small sperm enable organisms to produce a large number of sperm, and make males (or male function of hermaphrodites [140] ) more redundant. This intensifies male competition for mates and promotes the evolution of other sexual dimorphism in many species, especially in vertebrates including mammals. However, in some species, the females can be larger than males, irrespective of gametes, and in some species females (usually of species in which males invest a lot in rearing offspring and thus no longer considered as so redundant) compete for mates in ways more usually associated with males.

In many non-monogamous species, the benefit to a male's reproductive fitness of mating with multiple females is large, whereas the benefit to a female's reproductive fitness of mating with multiple males is small or nonexistent. [141] In these species, there is a selection pressure for whatever traits enable a male to have more matings. The male may therefore come to have different traits from the female.

These traits could be ones that allow him to fight off other males for control of territory or a harem, such as large size or weapons [142] or they could be traits that females, for whatever reason, prefer in mates. [143] Male-male competition poses no deep theoretical questions [144] but mate choice does.

Females may choose males that appear strong and healthy, thus likely to possess "good alleles" and give rise to healthy offspring. [145] In some species, however, females seem to choose males with traits that do not improve offspring survival rates, and even traits that reduce it (potentially leading to traits like the peacock's tail). [144] Two hypotheses for explaining this fact are the sexy son hypothesis and the handicap principle.

The sexy son hypothesis states that females may initially choose a trait because it improves the survival of their young, but once this preference has become widespread, females must continue to choose the trait, even if it becomes harmful. Those that do not will have sons that are unattractive to most females (since the preference is widespread) and so receive few matings. [146]

The handicap principle states that a male who survives despite possessing some sort of handicap thus proves that the rest of his genes are "good alleles". If males with "bad alleles" could not survive the handicap, females may evolve to choose males with this sort of handicap the trait is acting as a hard-to-fake signal of fitness. [147]


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Materials and Methods

Genomic DNA was isolated with the DNeasy tissue kit following the manufacturer's protocol (Qiagen, Valencia, CA). Tissues from zebra finch (Taenopygia guttata), house sparrow (Passer domesticus), eastern phoebe (Sayornis phoebe), ruby-throated hummingbird (Archilochus colubris), Anna's hummingbird (Calypte anna), budgerigar (Melopsittacus undulatus), and American alligator (Alligator mississippiensis) came from the University of Michigan Museum of Zoology. DNA for pygmy hippopotamus (Hexaprotodon liberiensis) came from the Zoological Society of San Diego, and that for bottle-nosed dolphin (Tursiops truncatus) was a gift from Dr. A. P. Rooney (U.S. Department of Agriculture). Primers for polymerase chain reaction (PCR) amplification of exon 7 of FoxP2 are 5′-GAAGACAATGGCATTAAACATGGAGG-3′ and 5′-GAATAAAGCTCATGAGATTTACCTGTC-3′. Primers for amplification of cytochrome b came from Parson et al. (2000). PCR was conducted with MasterTaq under the manufacturer's recommended conditions (Eppendorf, Hamburg, Germany) and products were sequenced from both directions with the dideoxy chain termination method on an automated sequencer. GenBank accession numbers for the new sequences are AY726626–AY726635 and AY724762–AY724767. After removal of the primer sequences, a total of 124 nucleotides per sequence were compared. Synonymous nucleotide substitution rates were computed by the method of Zhang et al. (1998). Tajima's (1993) test of the molecular clock was computed with the MEGA3 program ( Kumar et al. 2004).


Biologists use fossils to pinpoint when mammal and dinosaur ancestors became athletes

Photomicrograph showing a blood smear with the large red blood cells of the African clawed frog and the small red blood cells of a rodent side-by-side (400x magnification). Credit: Photomicrograph by Adam Huttenlocker, wood mouse by Anne Burges, African clawed frog by Tim Vickers.

Many mammals and birds are remarkable athletes mice work hard to dig burrows for protection and sparrows fight gravity with each flap of their wings. In order to have the energy to sustain vigorous exercise, the body's tissues need a steady supply of oxygen, and red blood cells (RBCs) are the center of the oxygen delivery system. Size matters, too athletic mammals and birds have much smaller RBCs than other vertebrates with lesser capacities for exercise. Biologists have long been puzzled over the evolutionary origins of RBC size. Were predecessors of mammals and birds—including dinosaurs—athletes and did they have tiny red blood cells? How do you measure the blood of extinct animals?

Now, biologists at the University of Utah and the Natural History Museum of Utah have established a 'fossilizable' indicator of athleticism in the bones of extinct vertebrates.

The study, which published online in Current Biology on Dec. 22, is the first to draw a link between RBC size and the microscopic traces of blood vessels and bone cells inside the bone. The researchers measured the bony channels that deliver oxygen to bone tissue to pinpoint when our mammal ancestors, bird and dinosaur predecessors evolved small RBCs. They found that extinct mammal relatives, or cynodonts, and extinct bird relatives had smaller RBCs and were likely better athletes than earlier terrestrial vertebrates. The timing of RBC-size reduction coincided with the greatest mass extinction event on Earth 252 million years ago, an event that paved the way for the age of the dinosaurs.

"Some people look at fossils, and they see rocks—but these were living and breathing organisms. To be able to find proxies that tell us something like this, it gets us to think about living organisms in their environments," says lead author Adam Huttenlocker of the Keck School of Medicine at the University of Southern California, who completed the research as a postdoctoral fellow at the U and THE MUSEUM. "It allows us to think about the overall implications for mass extinction. What were some of the physiological innovations that allowed them to be successful? That's really exciting."

Red blood cells: oxygen delivery system

It may seem counterintuitive that tiny RBCs deliver oxygen more effeciently than larger cells, says Huttenlocker, but the smaller size corresponds to densely packed vascular networks that are far more efficient than big blood cells that are sparsely distributed.

"Think about it like this it's much easier to unload people from two small sedans quickly than it is to unload a 15-passenger van at the same rate," says Huttenlocker. "That's basically what red blood cells are—vehicles for oxygen. When you have lots more very small ones, you can pick up and drop off oxygen very quickly."

Huttenlocker knew from previous research that the diameters of capillaries that deliver oxygen to muscles tend to have similar widths as the RBCs of the organism. This makes it easier for the oxygen molecules to leave the blood cells, pass through the capillary wall and push into various tissues. Athletic mammals' and birds' RBCs are much smaller than the blood cells of less active animals.

For a paleontologist hoping to find clues in the blood of extinct animals, this presents a problem capillaries don't fossilize. So Huttenlocker measured the diameter of the microscopic canals imbedded in fossilized bones that allow capillaries to deliver oxygen to bone tissue. He also measured tiny cavities that hold bone cells, called lacunae.

"This aspect—the diameter of the canals—has been overlooked by paleontologists," says Colleen Farmer, senior author of the paper and biologist at the U and the museum. The size of the canals is a proxy for the size of the red blood cells, which indicates whether or not an organism had the ability to sustain vigorous exercise, she continues. "Aerobic capacity coevolved with many key life-history traits—mode of locomotion, ability of an animal to migrate, whether the animal experienced intense intraspecific competition—it's not trivial, aerobic capacity is a central and key aspect of life history."

Cynodonts and other extinct mammal predecessors exhibited RBC sizes similar to modern day mammals by the Permian-Triassic transition. Credit: Adam Huttenlocker

A look into a long history

Huttenlocker and Farmer looked at three major lineages of terrestrial animals the mammals and their extinct relatives, non-avian reptiles and birds and their extinct relatives, and amphibians. The researchers chose these three groups, called the tetrapods, because of their evolutionary history they all shared the same, ancient four-legged ancestor before branching off on their own evolutionary courses by 320 million years ago.

The amphibians, including water-loving frogs and salamanders, diverged from the common tetrapod ancestor first in the early Carboniferous period, more than 320 million years ago. The mammal and reptile lineages branched off next. The extinct mammal-like cynodonts first appeared in the late Permian period, about 260 million years ago. They looked more like stout, possibly furred lizards than the mammals we see today. Around the same time, the reptiles diverged into two groups—the forebears to dinosaurs and birds, called the archosaurs, and the other non-avian reptiles.

At the beginning of the Triassic period 252 million years ago, 90 percent of life died off. The so-called Permian-Triassic mass extinction was the largest extinction event in Earth's history, and left room for the survivors to diversify and fill in the newly vacant niches.

"We're dealing with a long timescale. Right before the extinction in the early Permian and Carboniferous are these generalized-looking four-legged animals. By the time you get to the Triassic, they look more like what we see today, but still alien in some ways," says Huttenlocker.

First, Huttenlocker analyzed the bones of 14 species of living tetrapods: six mammals, two birds, four non-avian reptiles and two amphibians. He made paper-thin sections of each animal's forelimb bone and digitally visualized the image to see the microscopic canals and bone cell lacunae under a microscope. Then he measured the diameters of the smallest canals and lacunae and came up with equations that could predict the size of the RBCs. He tested his predictions against blood smears to measure the actual blood cell size. He found he was able to predict the size based on the microstructures in the bone.

The researchers found that mammals and birds had smaller canal and lacunae sizes than the non-avian reptiles and amphibians. Additionally, smaller blood cells corresponded to a higher density of canals, which allows oxygen to diffuse into tissues more rapidly. Huttenlocker and Farmer were confident that bone microstructure would be a proxy for RBC size in the extinct animals. They found that all of the Triassic mammal ancestors (the cynodonts) and some earlier mammal predecessors, had similar RBC sizes as modern mammals. The archosauromorphs had a wide range of cell sizes, however the smallest RBCs were about the same size as their contemporary Triassic cynodonts.

Under selective pressure

The fossilized proxy for RBC size gives clues to the extinct animals' environment. The cynodonts existed 70 million years before true mammals appeared, yet they lived similar lifestyles. The small cynodonts nested in underground burrows, which often have lower oxygen levels than at the surface. The cynodonts may have evolved smaller RBCs to dig tunnels, move dirt around, and maintain active lifestyles in low-oxygen environments.

A more controversial hypothesis has to do with a long-term drop in atmospheric oxygen at the beginning of the Triassic. Some people have suggested that adaptations for more efficient oxygen distribution could have been caused by low oxygen levels. Whatever the cause, Huttenlocker says that there must have been some selective factors present during the Triassic period that promoted these traits.

"Similar environmental pressures can result in similar solutions to problems in these very different groups of animals," says Huttenlocker. "In this paper, we're just focusing on one little nugget of that. But the forerunners of mammals and birds were able to exercise and be athletes in the Permian-Triassic world."

Farmer says that they need to analyze lots more living animals to strengthen the case that bone microstructures can be a proxy for athleticism. The tool will enable paleontologists to assess the athletic ability of many types of extinct animals.

"Many fossils have been analyzed without this insight, and the data are just sitting there, and can readily be viewed through this lens," says Farmer. "This is transformative in providing a new avenue to peer into the past to see what these animals were like."


Acknowledgements

We thank A. Phillimore, N. Kane, P. Nosil, B. Langerhans, D. Kissling, D. Rabosky and CD Cadena for comments on earlier versions of this manuscript L.H. Liow and K. Lintulaakso for assistance with mammalian hibernation data and R. Stepp for GIS layers with slope values for elevation. The National Evolutionary Synthesis Center, NESCent, fostered this collaboration through partial funding and logistic support to CB, RD and RS (#EF-0905606 and short-term visiting scholar fellowship to RD). This work was also supported by the US National Science Foundation (Safran: IOS #0717421 and DEB CAREER #1149942 McCain: DEB 0949601), Postdoctoral Fellowships from NESCent (CAB) and North Carolina State University (CAB) and by Grant/Cooperative Agreement # G10AC00624 from the United States Geological Survey (CAB).

Data S1 Spatial sensitivity analysis.

Data S2 Sample code for Bayesian Phylogenetic Mixed Models (BPMM) in R.

Table S1 Principal components analyses of continuous bio-ecological variables in the spatial sensitivity analysis.

Table S2 Summary of results for the Bayesian Phylogenetic Mixed Models of subspecies richness in the spatial sensitivity analysis.

Table S3 Summary of results for the Bayesian Phylogenetic Mixed Models of subspecies richness in north temperate mammals and birds.

Fig. S1 Elevation gradients of the world and three representative examples of species whose breeding ranges are dissected by mountains.

Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.


Researchers Compare Chicken, Human Genomes: Analysis Of First Avian Genome Uncovers Differences Between Birds And Mammals

BETHESDA, Md., Wed., Dec. 8, 2004 &ndash An international research consortium has found that chickens and humans share more than half of their genes, but that their DNA sequences diverge in ways that may explain some of the important differences between birds and mammals. The consortium's analysis is published in the Dec. 9 issue of the journal Nature.

The International Chicken Genome Sequencing Consortium analyzed the sequence of the Red Jungle Fowl (Gallus gallus), which is the progenitor of domestic chickens. The National Human Genome Research Institute (NHGRI), part of the National Institutes of Health, provided about $13 million in funding for the project, which involved researchers from China, Denmark, France, Germany, Japan, Poland, Singapore, Spain, Sweden, Switzerland, the United Kingdom and the United States.

The chicken is the first bird, as well as the first agricultural animal, to have its genome sequenced and analyzed. The first draft of the chicken genome, which was based on 6.6-fold coverage, was deposited into free public databases for use by researchers around the globe in March 2004. Over the past nine months, the consortium carefully analyzed the genome and compared it with the genomes of organisms that have already been sequenced, including the human, the mouse, the rat and the puffer fish.

"The chicken genome fills a crucial gap in our scientific knowledge. Located between mammals and fish on the tree of life, the chicken is well positioned to provide us with new insights into genome evolution and human biology," said NHGRI Director Francis S. Collins, M.D., Ph.D. "By comparing the genomes of a wide range of animals, we can better understand the structure and function of human genes and, ultimately, develop new strategies to improve human health."

In their paper published in Nature, members of the International Chicken Genome Sequencing Consortium report that the chicken genome contains significantly less DNA than the human genome, but approximately the same number of genes. Researchers estimate that the chicken has about 20,000-23,000 genes in its 1 billion DNA base pairs, compared with the human count of 20,000-25,000 genes in 2.8 billion DNA base pairs. The difference in total amount of DNA reflects a substantial reduction in DNA repeats and duplications, as well as fewer pseudogenes, in the chicken genome.

About 60 percent of chicken genes correspond to a similar human gene. However, researchers uncovered more small sequence differences between corresponding pairs of chicken and human genes, which are 75 percent identical on average, than between rodent and human gene pairs, which are 88 percent identical on average. Differences between human and chicken genes were not uniform across the board, however. Chicken genes involved in the cell's basic structure and function showed more sequence similarity with human genes than did those implicated in reproduction, immune response and adaptation to the environment.

The analysis also showed that genes conserved between human and chicken often are also conserved in fish. For example, 72 percent of the corresponding pairs of chicken and human genes also possess a counterpart in the genome of the puffer fish (Takifugu rubripes). According to the researchers, these genes are likely to be present in most vertebrates.

"Genomes of the chicken and other species distant from ourselves have provided us with a powerful tool to resolve key biological processes that have been conserved over millennia," said Richard Wilson, Ph.D., of Washington University School of Medicine in St. Louis, the consortium's leader and senior author of the Nature article. "Along with the many similarities between the chicken and human genomes, we discovered some fascinating differences that are shedding new light on what distinguishes birds from mammals."

Like all birds, chickens are thought to have descended from dinosaurs in the middle of the Mesozoic period and have evolved separately from mammals for approximately 310 million years. Chickens were first domesticated in Asia, perhaps as early as 8000 B.C.

As might be expected, genomic researchers determined that chickens have an expanded gene family coding for a type of keratin protein used to produce scales, claws and feathers, while mammalian genomes possess more genes coding for another type of keratin involved in hair formation. Likewise, chickens are missing the genes involved in the production of milk proteins, tooth enamel and the detection of hormonal substances called pheromones, which researchers say may mirror the evolution of the mammary glands and the nose in mammals and the loss of teeth in birds. But other results of the analysis caught even the researchers by surprise.

The analysis showed that a group of genes that code for odor receptor proteins is dramatically expanded in the chicken genome &ndash a finding that appears to contradict the traditional view that birds have a poor sense of smell. And, as it turns out, birds might not have such a great sense of taste. When compared with mammals, chickens have a much smaller family of genes coding for taste receptors, particularly those involved in detecting bitter sensations.

Other intriguing findings from the Nature paper include:

* Alignment of chicken and human genes indicate that approximately 2,000 human genes may actually start at different sites than scientists thought. The discovery of these "true" start sites, which appear to lie inside the previously hypothesized boundaries of the genes, may have implications for the understanding of human disease and the design of new therapies.

* Chicken genes that code for eggshell-specific proteins, such as ovocleidin-116, have mammalian counterparts that play a role in bone calcification. Previously, such genes were not known outside of birds. However, the analysis also showed that, in contrast to chickens, mammals are missing key genes coding for proteins involved in egg production, such as egg whites and yolk storage.

* Chickens have a gene that codes for interleukin-26 (IL-26), a protein involved in immune response. Previously, this immune-related gene was known only in humans. The discovery means that the chicken may now serve as a model organism in which researchers can investigate the function of IL-26.

* Chickens possess genes coding for certain light-dependent enzymes, while mammals have lost those genes. It is thought losses reflect a period in early mammalian history in which mammals were active mainly at night.

* The avian genome contains a gene that codes for an enzyme involved in generating blue color pigments, while mammals are lacking that gene.

Besides providing insights into gene content and evolution of genes, the consortium's analysis offers new perspectives on the evolution of portions of the genome that do not code for proteins. Less than 11 percent of the chicken genome consists of interspersed segments of short, repetitive DNA sequences, compared with 40 to 50 percent of mammalian genomes. With genes comprising another 4 percent of the chicken genome, researchers say that leaves them with no explanation for the function of more than 85 percent of the chicken genome. They hypothesize this genetic "dark matter" may contain previously unrecognized regulatory elements, but also may include ancient DNA repetitive elements that have mutated beyond recognition. Furthermore, researchers said it appears that the 571 non-coding RNA "genes" that they identified in the chicken genome may use different duplication and/or translocation mechanisms than do regular protein-coding genes, opening the door to a whole new realm of scientific inquiry.

In addition to its tremendous value as a resource for comparative genomics, the chicken is widely used in biomedical research. It serves as an important model for vaccine production and the study of embryology and development, as well as for research into the connection between viruses and some types of cancer.

Recent outbreaks of avian flu have accelerated agricultural researchers' interest in learning more about the chicken genome and how genetic variation may play a role in susceptibility of different strains to the disease. The chicken genome sequence will also serve as a resource for researchers seeking to enhance the nutritional value of poultry and egg products. Furthermore, as the first of 9,600 species of birds to have its genome fully sequenced and analyzed, the chicken genome will help to further understanding of avian genomics and biology in general.


Evolution of Reptiles

The earliest amniotes evolved about 350 million years ago. They resembled small lizards, but they were not yet reptiles. Their amniotic eggs allowed them to move away from bodies of water and become larger. They soon became the most important land vertebrates.

Synapsids and Sauropsids

By about 320 million years ago, early amniotes had diverged into two groups, called synapsids and sauropsids. Synapsids were amniotes that eventually gave rise to mammals.Sauropsids were amniotes that evolved into reptiles, dinosaurs, and birds. The two groups of amniotes differed in their skulls. The earliest known reptile, pictured in Figure below, dates back about 315 million years.

Earliest Reptile: Hylonomus. The earliest known reptile is given the genus name Hylonomus. It was about 20 to 30 centimeters (8 to 12 inches) long, lived in swamps, and ate insects and other small invertebrates.

At first, synapsids were more successful than sauropsids. They became the most common vertebrates on land. However, during the Permian mass extinction 245 million years ago, most synapsids went extinct. Their niches were taken over by sauropsids, which had been relatively unimportant until then. This is called the Triassic takeover.

Rise and Fall of the Dinosaurs

By the middle of the Triassic about 225 million years ago, sauropsids had evolved into dinosaurs. Dinosaurs became increasingly important throughout the rest of the Mesozoic Era, as they radiated to fill most terrestrial niches. This is why the Mesozoic Era is called the Age of the Dinosaurs. During the next mass extinction, which occurred at the end of the Mesozoic Era, all of the dinosaurs went extinct. Many other reptiles survived, however, and they eventually gave rise to modern reptiles.

Evolution of Modern Reptile Orders

Figure below shows a traditional phylogenetic tree of living reptiles. Based on this tree, some of the earliest reptiles to diverge were ancestors of turtles. The first turtle-like reptiles are thought to have evolved about 250 million years ago. Ancestral crocodilians evolved at least 220 million years ago. Tuataras may have diverged from squamates (snakes and lizards) not long after that. Finally, lizards and snakes went their separate ways about 150 million years ago.

Traditional Reptile Phylogenetic Tree. This phylogenetic tree is based on physical traits of living and fossil reptiles. Trees based on DNA comparisons may differ from the traditional tree and from each other, depending on the DNA sequences used. Reptile evolution is currently an area of intense research and constant revision.


Biologists follow 'fossilizable' clues to pinpoint when mammal, bird and dinosaur ancestors became athletes

Many mammals and birds are remarkable athletes mice work hard to dig burrows for protection and sparrows fight gravity with each flap of their wings. In order to have the energy to sustain vigorous exercise, the body's tissues need a steady supply of oxygen, and red blood cells (RBCs) are the center of the oxygen delivery system. Size matters, too athletic mammals and birds have much smaller RBCs than other vertebrates with lesser capacities for exercise. Biologists have long been puzzled over the evolutionary origins of RBC size. Were predecessors of mammals and birds -- including dinosaurs -- athletes and did they have tiny red blood cells? How do you measure the blood of extinct animals?

Now, biologists at the University of Utah and the Natural History Museum of Utah have established a 'fossilizable' indicator of athleticism in the bones of extinct vertebrates.

The study, which published online in Current Biology on Dec. 22, is the first to draw a link between RBC size and the microscopic traces of blood vessels and bone cells inside the bone. The researchers measured the bony channels that deliver oxygen to bone tissue to pinpoint when our mammal ancestors, bird and dinosaur predecessors evolved small RBCs. They found that extinct mammal relatives, or cynodonts, and extinct bird relatives had smaller RBCs and were likely better athletes than earlier terrestrial vertebrates. The timing of RBC-size reduction coincided with the greatest mass extinction event on Earth 252 million years ago, an event that paved the way for the age of the dinosaurs.

"Some people look at fossils, and they see rocks -- but these were living and breathing organisms. To be able to find proxies that tell us something like this, it gets us to think about living organisms in their environments," says lead author Adam Huttenlocker of the Keck School of Medicine at the University of Southern California, who completed the research as a postdoctoral fellow at the U and THE MUSEUM. "It allows us to think about the overall implications for mass extinction. What were some of the physiological innovations that allowed them to be successful? That's really exciting."

Red blood cells: oxygen delivery system

It may seem counterintuitive that tiny RBCs deliver oxygen more effeciently than larger cells, says Huttenlocker, but the smaller size corresponds to densely packed vascular networks that are far more efficient than big blood cells that are sparsely distributed.

"Think about it like this it's much easier to unload people from two small sedans quickly than it is to unload a 15-passenger van at the same rate," says Huttenlocker. "That's basically what red blood cells are -- vehicles for oxygen. When you have lots more very small ones, you can pick up and drop off oxygen very quickly."

Huttenlocker knew from previous research that the diameters of capillaries that deliver oxygen to muscles tend to have similar widths as the RBCs of the organism. This makes it easier for the oxygen molecules to leave the blood cells, pass through the capillary wall and push into various tissues. Athletic mammals' and birds' RBCs are much smaller than the blood cells of less active animals.

For a paleontologist hoping to find clues in the blood of extinct animals, this presents a problem capillaries don't fossilize. So Huttenlocker measured the diameter of the microscopic canals imbedded in fossilized bones that allow capillaries to deliver oxygen to bone tissue. He also measured tiny cavities that hold bone cells, called lacunae.

"This aspect -- the diameter of the canals -- has been overlooked by paleontologists," says Colleen Farmer, senior author of the paper and biologist at the U and the museum. The size of the canals is a proxy for the size of the red blood cells, which indicates whether or not an organism had the ability to sustain vigorous exercise, she continues. "Aerobic capacity coevolved with many key life-history traits -- mode of locomotion, ability of an animal to migrate, whether the animal experienced intense intraspecific competition -- it's not trivial, aerobic capacity is a central and key aspect of life history."

A look into a long history

Huttenlocker and Farmer looked at three major lineages of terrestrial animals the mammals and their extinct relatives, non-avian reptiles and birds and their extinct relatives, and amphibians. The researchers chose these three groups, called the tetrapods, because of their evolutionary history they all shared the same, ancient four-legged ancestor before branching off on their own evolutionary courses by 320 million years ago.

The amphibians, including water-loving frogs and salamanders, diverged from the common tetrapod ancestor first in the early Carboniferous period, more than 320 million years ago. The mammal and reptile lineages branched off next. The extinct mammal-like cynodonts first appeared in the late Permian period, about 260 million years ago. They looked more like stout, possibly furred lizards than the mammals we see today. Around the same time, the reptiles diverged into two groups -- the forebears to dinosaurs and birds, called the archosaurs, and the other non-avian reptiles.

At the beginning of the Triassic period 252 million years ago, 90 percent of life died off. The so-called Permian-Triassic mass extinction was the largest extinction event in Earth's history, and left room for the survivors to diversify and fill in the newly vacant niches.

"We're dealing with a long timescale. Right before the extinction in the early Permian and Carboniferous are these generalized-looking four-legged animals. By the time you get to the Triassic, they look more like what we see today, but still alien in some ways," says Huttenlocker.

First, Huttenlocker analyzed the bones of 14 species of living tetrapods: six mammals, two birds, four non-avian reptiles and two amphibians. He made paper-thin sections of each animal's forelimb bone and digitally visualized the image to see the microscopic canals and bone cell lacunae under a microscope. Then he measured the diameters of the smallest canals and lacunae and came up with equations that could predict the size of the RBCs. He tested his predictions against blood smears to measure the actual blood cell size. He found he was able to predict the size based on the microstructures in the bone.

The researchers found that mammals and birds had smaller canal and lacunae sizes than the non-avian reptiles and amphibians. Additionally, smaller blood cells corresponded to a higher density of canals, which allows oxygen to diffuse into tissues more rapidly. Huttenlocker and Farmer were confident that bone microstructure would be a proxy for RBC size in the extinct animals. They found that all of the Triassic mammal ancestors (the cynodonts) and some earlier mammal predecessors, had similar RBC sizes as modern mammals. The archosauromorphs had a wide range of cell sizes, however the smallest RBCs were about the same size as their contemporary Triassic cynodonts.

Under selective pressure

The fossilized proxy for RBC size gives clues to the extinct animals' environment. The cynodonts existed 70 million years before true mammals appeared, yet they lived similar lifestyles. The small cynodonts nested in underground burrows, which often have lower oxygen levels than at the surface. The cynodonts may have evolved smaller RBCs to dig tunnels, move dirt around, and maintain active lifestyles in low-oxygen environments.

A more controversial hypothesis has to do with a long-term drop in atmospheric oxygen at the beginning of the Triassic. Some people have suggested that adaptations for more efficient oxygen distribution could have been caused by low oxygen levels. Whatever the cause, Huttenlocker says that there must have been some selective factors present during the Triassic period that promoted these traits.

"Similar environmental pressures can result in similar solutions to problems in these very different groups of animals," says Huttenlocker. "In this paper, we're just focusing on one little nugget of that. But the forerunners of mammals and birds were able to exercise and be athletes in the Permian-Triassic world."

Farmer says that they need to analyze lots more living animals to strengthen the case that bone microstructures can be a proxy for athleticism. The tool will enable paleontologists to assess the athletic ability of many types of extinct animals. "Many fossils have been analyzed without this insight, and the data are just sitting there, and can readily be viewed through this lens," says Farmer. "This is transformative in providing a new avenue to peer into the past to see what these animals were like."


Watch the video: Parental Care in Birds u0026 Mammals (June 2022).