eggs - Have there been any theories put forward as to the reasons behind Temperature-Dependant Sex Determination in reptiles?

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.