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ABSTRACT.-We studied the sex ratio of a population of common musk turtles (Sternotherus odoratus) from Dewart Lake in north-central Indiana. From 1979 to 2000 we trapped S. odoratus nearly annually using wire traps or fyke nets. Sex ratios were consistently male biased (average = 64.9% males) and did not vary between trap types or time of day. Available evidence points to differential mortality of the sexes as a possible cause for the biased sex ratios. However, additional work is needed.
INTRODUCTION
Observed sex ratios can be indicators of ecological processes occurring in a population. Because selection is likely to lead to the evolution of a 1:1 sex ratio (Fisher, 1930), deviations from a 1:1 sex ratio in a population demand an explanation. Biased sex ratios (male or female) could be caused by a number of possible mechanisms, each of which might be important to understanding a population's dynamics and characteristics. For example, sex ratios in turtle populations diverging from 1:1 may be the result of differential mortality, differential activity, differential habitat use, temperature-dependent sex determination or simply trapping bias.
In natural populations of turtles, sex ratios often vary significantly from the expected 1:1 ratio (see review in Gibbons, 1990). In the common musk turtle, Sternotherus odoratus, observed sex ratios have been observed to vary from strongly female biased (74% female; Dodd, 1989) to strongly male biased (72.1 % male; Edmonds and Brooks, 1996). We studied the sex ratio of a single population of S. odoratus in a lake in northern Indiana for 21 y (1979-2000).
We attempted to answer the following questions. What is the sex ratio of this population of Sternotherus odoratus? Have sex ratios varied over the 21 y of this study? Do sex ratios vary during the day? Do they differ between trap types? The answers to these questions can lead us to a better understanding of the structure of this and other populations of turtles and indicate areas for future investigation.
MATERIALS AND METHODS
We studied a population of Sternotherus odoratus in the marsh at the SE end of Station Bay in the SE corner of Dewart Lake (area of our study area = 4.5 ha) near Syracuse, Kosciusko Co., Indiana (41 deg...