Potential Project Description for 2008-09


Title: Natal homing as a factor in sex ratios in freshwater turtle populations
Faculty: Steve Freedberg, Biology

In many animals, sex is determined by the temperature of the nest during incubation, a phenomenon known as environmental sex determination (ESD). Studying the factors affecting maternal nest-site choice under ESD can be critically important in understanding both the evolutionary dynamics of the population sex ratio and the conservation implications of human-induced changes in the environment. Previous field work has revealed that turtles may “transmit” their nest-site location across generations through natal homing, whereby females return to nest at the site from which they were hatched. If ESD species exhibit natal homing, it may cause entire families to inherit similar nesting locations and thus similar offspring sex ratios. This interaction is predicted to impact population sex ratios and the evolutionary dynamics of these species.

This study seeks to examine the role of natal homing on the sex ratios of local populations of freshwater turtles. If females are homing to their natal sites, there is an empirical expectation of high genetic relatedness among females nesting in close geographic proximity. Furthermore, because of the influence of local thermal environment on sex ratios, females nesting in close proximity may have positively correlated nest sex ratios. In an ongoing field study, students are using three sets of tools to collect data from nesting snapping turtles in Southern Minnesota; data loggers to track thermal variation throughout incubation, GPS to record nest location, and genetic analysis to determine relatedness among females. These data sets will allow us to construct distance matrices of temperature, geographic location, and genetic distance for the nests throughout our population. While the Mantel test is traditionally used for testing for correlations among two distance matrices, this study presents the interesting mathematical challenge of testing for correlations among three such matrices. While designing an appropriate method to study these relationships may present some interesting challenges, such an analysis will be a crucial first step in addressing these important questions.