Threats to reptiles and amphibians include a litany of familiar factors including habitat destruction, pollution, disease, and over-exploitation by an increasingly hungry world. In the face of this onslaught, turtles face a more insidious threat: global increases in temperature caused by greenhouse gases (Lovich, 1997). This is an especially large threat to turtles because the sex of many turtle species is determined by the incubation temperature of the egg (Bull, 1980). This phenomenon is known as temperature-dependent or environment-dependent sex determination. Temperature-dependant sex determination is found in a number of species of turtles in the Great Lakes region. .
The temperature threshold which determines the sex of the embryo varies slightly between species (Bull, 1980). However, the threshold for the red-eared slider will suffice as a reference temperature scale for our purposes. The threshold temperature where a 1:1 ratio between male and female young will occur is 29.2aC (www.ourstolenfuture.com). Any embryos under that temperature will be male and any embryos above that temperature will be female. It is thought that this effect is seen because higher temperautres speed the conversion of testosterone to estradiol. Embryos incubated at a warmer temperature are thus exposed to more estradiol during the critical period for sex determination, and therefore become female (www.ourstolenfuture.com). .
Temperature-dependent sex determination might be expected to cause a wide variation in sex ratios of turtles and it is true that hatchling sex ratios can range from all male in some years to all females in others. However, over long periods of time, the hatchling sex ratios of most well-studied turtle populations averages 1:1 (Lovich, 1997).
Temperature-dependant sex determination is unique because a variety of factors interact to determine the sex ratio. These factors include maternal behavior in choosing a nest site, the zygote's response to temperature in becoming male or female, and environmental effects on the temperature of the nesting area.
