The effect of sex-specific terrestrial movements and roads on the sex ratio of freshwater turtles

Differential road mortality may affect the demography of vertebrate populations when movements associated with foraging, reproduction, or dispersal cause a greater proportion of one sex or stage to frequently contact roads. In the case of turtles, roadsides create artificial disturbed and open habitats that may be attractive to nesting females but may cause significant road mortality. I tested this possibility by comparing the sex ratios of turtle populations in a lake that is bisected by a high-traffic, four-lane highway to those in nearby ponds located away from major roads in northwestern Florida (USA). I evaluated the potential role of road mortality as the cause of skewed sex ratios by determining whether adult female freshwater turtles move overland more frequently than that expected from actual sex ratios and the proportions of turtles of four species that attempt to nest annually along the roadside. Population sex ratios were dramatically biased toward males in the Florida cooter (Pseudemys floridana) (80% males), yellow-bellied slider (Trachemys scripta) (73% males), and common musk turtle (Sternotherus odoratus) (65% males) in the lake adjacent to the highway compared to those in ponds not affected by road mortality, where the proportion of males ranged from 39% to 60%. Despite mate-biased population sex ratios, under normal (non-drought) conditions a significantly greater proportion of adult females (57-72%) than males were found on land along the highway and, thus, have a greater annual probability of being killed by vehicles. An estimated 6-29% of all adult females in four turtle species nested annually along the highway shoulder. If this phenomenon is general for freshwater turtle populations located near roadways, male-biased sex ratios and low numbers of adult female turtles elsewhere may be the result of cumulative differential mortality of nesting females struck by vehicles as they enter the highway during annual nesting forays. (C) 2004 Elsevier Ltd. All rights reserved.