The Mechanism of Temperature Dependent Sex Determination in Crocodilians: A Hypothesis

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Abstract
Incubation of alligator eggs at 30°C produces 100% females, at 33°C 100% males; temperatures in between produce varying sex ratios. Wild nests of Alligator mississippiensis show similar effects and the populations are biased towards females. The incidence and patterns of temperature dependent sex determination (TSD) in other crocodilians are reviewed. Temperature also affects hatchling size and pigmentation patterns, post-hatching growth rates and thermoregulation by juvenile crocodilians. The significance of temperature sensitive periods defined by temperature shift experiments is questioned in relation to a hypothesis to explain the mechanism of TSD in crocodilians. It is postulated that there is an initial sex differentiation mechanism which involves a quantum period of time and a threshold for a dose of a male determining factor. The conditions for induction of males are precise but exhibit variation between individuals within the population. Females develop by default. The hypothalamus may have an important role in a later sex differentiation mechanism. The hypothesis is used to explain the late temperature sensitive periods defined by high to low temperature shift experiments, why cooler temperatures are more effective at determining sex, how intermediate temperatures can produce both sexes, the differences in the pattern between turtles and crocodilians and geographical similarities in the pattern of TSD within crocodilians despite differing climates. The phylogenetic advantages of TSD in crocodilians are concerned with the overall reproductive strategy of the animals. Those crocodilians which are incubated and grow to maturity under optimal environmental conditions will be both large and male. Larger males are more likely to produce more offspring. A review of the effects of the environment on sex determination in amphibians and fish suggests that there is a general relationship between size and sex in vertebrates

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