Optimal sex ratio as a function of EGG incubation temperature in the crocodilians

Abstract

Recently, there has been a growing consensus as to the adaptive significance of temperature-dependent sex determination in the Crocodilia. The observationally and experimentally motivated hypotheses are that male fitness depends more strongly on quality of incubation environment than female fitness, and that there is a strong correlation between a female's egg incubation temperature choice and her own egg incubation temperature. A population genetics model based on these hypotheses is derived. A method for finding the optimal sex ratio as a function of temperature, which is an evolutionary stable strategy (ESS), is stated and applied under various assumptions. This extends ESS theory to the functional case. Cases where there is no ESS and the population sex ratio oscillates in evolutionary time are discovered. Numerical computation is needed to solve the full problem and the resulting optimal sex ratio is compared to laboratory sex ratio data. The general pattern of TSD in crocodilians (female-male-female with female biased overall sex ratio) agrees well with the theory, but details of the pattern are problematic.