In most animals the sex of an individual is determined at the moment of fertilization; when the egg and the sperm fuse together it is fixed if that animal will be male or female. Yet in many reptilian groups sex determination is established later during incubation, and the determinant external factor is the incubation temperature of the eggs. In reptiles, this means that the environment plays a crucial role in determining the sex ratio emerging from an egg clutch, and that these animals are very susceptible to alterations in temperature caused, for example, by climate change. In the majority of animal species, sexual differentiation the development of ovaries or testes is determined genetically GSD.
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How temperature determines sex in alligators -- ScienceDaily
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.
How do temperature and global warming affect the sex of reptiles?
To our knowledge, there is, so far, no evidence that incubation temperature can affect sex ratios in birds, although this is common in reptiles. Here, we show that incubation temperature does affect sex ratios in megapodes, which are exceptional among birds because they use environmental heat sources for incubation. Chicks from lower temperatures weigh less, which probably affects offspring survival, but are not smaller. Megapodes possess heteromorphic sex chromosomes like other birds, which eliminates temperature-dependent sex determination, as described for reptiles, as the mechanism behind the skewed sex ratios at high and low temperatures. Temperature-dependent sex determination TSD during incubation is a well-known phenomenon in reptiles, whereas birds have genotypic sex determination GSD in which sex is determined at fertilization long before the incubation of eggs begins Hardy
Some reptiles such as crocodilians and some turtles are known to display temperature-dependent sex determination TSD , where the ambient temperature of the developing eggs determines the individual's sex. The research has been published in Scientific Reports. Guillette Jr.