Effects of Incubation Temperature on Sex Ratios in Pine Snakes: Differential Vulnerability of Males and Females

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Abstract
Sex ratios in vertebrates are the result of primary sex ratios and subsequent mortality rates. Mortality rates are generally higher in males, leading to differential longevity and biased sex ratios. Sex determination by heteromorphic sex chromosomes is common in mammals and birds and less so in other vertebrates. In many reptiles, sex is determined by incubation temperature, although in snakes, sex is determined genetically. Snakes are usually believed to have a male:female primary sex ratio of unity, although a male-biased sex ratio has recently been reported for hatchlings and adults. Here, sex ratios from 134 adult pine snakes observed over an 11-yr period in nature indicate a 0.39 sex ratio for all snakes and 0.66 for all non-gravid snakes. By size class (generally indicative of age), ratios varied (from smallest to largest snakes) from 0.78, 0.23, 0.54, to 0.40. For 37 clutches incubated at room temperature (cycling temperatures), the sex ratio varied yearly from 0.50 to 1.78. Under controlled laboratory conditions, sex ratios of hatchlings varied from 0.11 at an incubation temperature of 21$^\circ$C to 1.40 at an incubation temperature of 32$^\circ$C. However, primary sex ratios of all embryos (hatched and unhatched) did not differ significantly from unity at different incubation temperatures. These data indicate differential mortality of males and females during embryonic development as a function of incubation temperatures in pine snakes, potentially resulting in different hatching sex ratios in nature depending on environmental temperatures. These results suggest that biases in secondary or tertiary sex ratios could result from differential effects of an environmental gradient (temperature) on the sexes during development in poikilotherms.