NCBI Bookshelf. While the sex of most snakes and most lizards is determined by sex chromosomes at the time of fertilization, the sex of most turtles and all species of crocodilians is determined by the environment after fertilization. In these reptiles, the temperature of the eggs during a certain period of development is the deciding factor in determining sex, and small changes in temperature can cause dramatic changes in the sex ratio Bull There is only a small range of temperatures that permits both males and females to hatch from the same brood of eggs. Figure At temperatures in between, the broods will give rise to individuals of both sexes. Variations on this theme also exist.
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By Emma Young. Australian central bearded dragon, Pogona vitticeps. High temperatures can make an Australian lizard that is genetically male develop into a female. The finding throws new light on how sex is determined in reptiles. For most reptiles, a gene on a sex chromosome triggers an embryo to develop as either a male or a female. In some species, males have an X and a Y chromosome, while females are XX, as in mammals. In other species of lizards, males are ZZ while females are ZW, as in birds. But for a third group of reptiles, which includes all crocodiles, alligators and marine turtles, temperature, rather than a gene on a sex chromosome, triggers either male or female differentiation. Extreme low or high temperatures generally lead to more females.
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Alex Quinn, a Ph. Sex-determining mechanisms in reptiles are broadly divided into two main categories: genotypic sex determination GSD and temperature-dependent sex determination TSD. Species in the genotypic group, like mammals and birds, have sex chromosomes, which in reptiles come in two major types. Many species—such as several species of turtle and lizards, like the green iguana—have X and Y sex chromosomes again, like mammals , with females being "homogametic," that is, having two identical X chromosomes. Males, on the other hand, are "heterogametic," with one X chromosome and one Y chromosome. Other reptiles governed by GSD have a system, similar to one found in birds, with Z and W sex chromosomes.
Duck mating can be surprisingly brutal, but females have a way to avoid giving birth to offspring from aggressive males. Richard O. Ducks are different from most birds in the fact that male ducks have a penis, analogous with the mammalian or human penis. And the fact that ducks still have a penis allows them to force copulation in ways that are unavailable to other birds. Unpaired males will attempt to force copulation during the egg-laying season. There are even socially organized groups of males pursuing females to force copulation.