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This applies to most if not all reptiles, but I'll write about leopard geckos since that's what I know.

Incubation temperature of the eggs makes a big difference in the development of the gecko. Obviously excessively high or low temperatures will kill the embryo, but inside the safe temperature range there are some interesting effects. The most well-known is Temperature-Dependant Sex Determination (TDSD).

Studies have been done and it is quite well-documented that:

  • Incubating eggs at the mid-point of the safe range results in predominantly male offspring.
  • Incubating eggs at the high or low end of the range both result in predominantly female offspring.

According to my gecko book, there are also some interesting effects regarding males that hatch from female-biased temperatures being more sexually active but less aggressive to females than the "macho" males produced at male-biased temperatures, females at female-biased temperatures being the most fertile and high-temperature females being more aggressive than lower-temperature females.

(Incubation temperature also affects colour - specifically melanin production levels. AFAIK it's not known whether this is a direct effect of the temperature or an indirect effect from the changes in sex hormones at the different temperatures. It's probably a different enough issue to warrant a separate question, though.)

Going back to TDSD, my question is: what theories have put forward to explain this sex determination? Evolutionarily speaking, how did this come about? Are males less likely to survive so they are only produced at safe temperatures? Does the optimal male:female ratio for propagating the species change in adverse conditions?

I'm not looking for random guesses. I'm interested in knowing if there have been any theories put forward from knowledgeable and reputable sources.

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  • $\begingroup$ How is this possible? per Wikipedia Y chromosome defines gender in mammals but it is different in Non-mammals i.e for reptiles. $\endgroup$ – James Jenkins May 7 '14 at 12:57
  • $\begingroup$ @JamesJenkins Not really sure if you're attempting to answer the question or just providing extra info. In case it's not clear in the question, I'm not asking how it's possible, I'm asking why reptiles have evolved that way; what advantage it gives. Though I did find that link interesting as I was unaware of the differences. $\endgroup$ – starsplusplus May 7 '14 at 13:00
  • $\begingroup$ Not answering, I was only aware of the mammal process and thought it applied to all species. $\endgroup$ – James Jenkins May 7 '14 at 13:18
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    $\begingroup$ I had the chance to read part of this book before its releasing. I'm sure you'll like it. $\endgroup$ – Remi.b May 7 '14 at 19:50
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Welcome to Biology StackExchange!

Environmental sex determination is tremendously interesting. It looks like the most common explanation is that temperature-dependent sex determination (TSD) is the original form of sex determination, and genetic sex determination has evolved multiple times in different animal groups since then. Reptiles are a mish-mash of different mechanisms.

If TSD came first, then the real question is not why did it evolve, but why is it still around. And the answer is likely because it works well enough. I couldn't find a lot of evidence that females make varying choices for next temperature, so in "real life" it might mostly be that plenty of males and females are always born. There is also recent evidence that genes affect when the temperature has an effect, so again, temperature alone is unlikely to drive sex ratios.

As far as possible advantages of TSD there is no definitive explanation that has held up to testing. It is possible that ideal female development has slightly different requirements than male development, so nest location choice by the parent may act to produce more females if the current environment supports them more than males. For instance, it has been shown that having more female offspring when temperatures are higher results in larger females.

Sorry to not have a simple answer for you. The whole idea is fascinating, along with other environmental triggers to development, like worker ant larvae becoming a winged queen if given the right nutrition, or butterflies that add "eyespot" pigmentation to their wings in wet years with increased predators.

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