Age at Maturity Influences Adult Sex Ratio in the Turtle Malaclemys terrapin

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Abstract
Numerous explanations have been offered for biased sex ratios including sampling bias, skewed primary sex ratios, differential mortality, differential immigration and emigration, and differential maturity of the sexes. We observed a strongly male-biased population of the turtle Malaclemys terrapin in South Carolina, USA. The overall adult male:female sex ratio was 1.78:1. Although males predominated numerically, the probability of recapturing individuals of either sex was the same (0.44 for males, 0.38 for females). Sex ratios were consistently male biased when based on monthly and yearly samples. This bias was not altered by the use of different collecting techniques or restriction of the sample to major habitats within the study area. Previously reported adult sex ratios for other populations range from strongly male biased to strongly female biased. Reports of female-biased samples appear to be a result of sampling bias or the artificial conditions under which terrapins have been cultivated in the past. The excess of males in our population appears to be a result of differences in the timing of maturity between the sexes. Male Malaclemys mature after their third year and females after their sixth year. Assuming a minimal effect from the other potential factors and regular juvenile recruitment, we expect that male Malaclemys will maintain numerical superiority in most populations. We propose that adult sex ratio variation in turtles, and other organisms with sexual differences in the timing of maturity, can be explained with a simple model: the earlier maturing sex will predominate numerically.