Effects of incubation temperature on sex ratios in pine snakes: differential vulnerability of males and females

Sex ratios from 134 adult Pituophis melanoleucus 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 0.50-1.78. Sex ratios of hatchlings varied from 0.11 at an incubation temperature of 21°C to 1.40 at an incubation temperature of 32.C. However, primary sex ratios of all embryos (hatched and unhatched) did not differ significantly from unity at different incubation temperatures. 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. Biases in secondary or tertiary sex ratios could result from differential effects of an environmental gradient (temperature) on the sexes during development in poikilotherms. -from Authors