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Geographic variation in the pattern of temperature-dependent sex determination in the American snapping turtle (Chelydra serpentina)

Published online by Cambridge University Press:  03 December 2004

Michael A. Ewert
Affiliation:
Department of Biology, Indiana University, 1001 East 3rd Street, Bloomington, IN 47405-3700 U.S.A.
Jeffrey W. Lang
Affiliation:
Department of Biology, University of North Dakota, Grand Forks, ND 58202 U.S.A.
Craig E. Nelson
Affiliation:
Department of Biology, Indiana University, 1001 East 3rd Street, Bloomington, IN 47405-3700 U.S.A.
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Abstract

Sex ratios of the American snapping turtle Chelydra serpentina are female-biased at cool temperatures, male-biased at moderate temperatures, and only females are produced at warm temperatures. The laboratory incubation of eggs at several constant temperatures yields patterns of skewed sex ratios. In this study, incubated eggs represented regional samples of C. serpentina from six different latitudes. The sample from the highest latitude yielded male-biased sex ratios across the broadest range of cool temperatures. This male-biased zone and those of the other five samples collectively contributed to a nested arrangement that is approximately symmetrical around a temperature of 24.5°C. Two indices of temperature-dependent sex determination (TSD) patterns (MF pivotal temperature, width of the male-biased zone) were significantly associated with latitude and with published data on several climatic variables (e.g. sunshine and rainfall). At high latitudes, females nest in exposed locations, locally the warmest places. At low latitudes females place their nests in shaded locations. Such nest site selection renders high latitude nests warmer than low latitude nests, even if only intermittently. Given that constant conditions show faster embryogenesis in high vs low latitude eggs, exposed nest sites seem to be an additional adaptation to hasten embryogenesis in high latitude nests. At low latitudes, shading prevents nests from overheating. We propose that a trade-off between evolution, (1) to balance sex ratio and (2) to maintain male-biased sex ratios in a temperature range that favours male fitness, leads to the observed arrangement of concentric TSD patterns centred around male production at 24.5°C.

Type
Research Article
Copyright
© 2005 The Zoological Society of London

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