As global temperatures continue to climb due to climate change, animals around the world will face new threats. Among the most animals most vulnerable to increasing temperatures are the turtle and other reptile species whose sex is determined by environmental conditions such as temperature. Unlike in mammals, whose sex is determined before development by their genes, many reptile species’ sex is determined by environmental conditions during development (Moyes & Schulte, 2005). Temperature, for example, determines sex in many turtle, lizard, and crocodile species. This mechanism of sex determination is called temperature-dependent sex determination or TSD (Bull, 1980). Climate change is expected to have drastic impacts on sex ratios in species with TSD and could affect the ability of these species to successfully reproduce and survive (Janzen, 1994a).
How does temperature determine sex in species with TSD?
There are many different patterns of TSD that may alter the effect climate change has on sex ratios. TSD species have a temperature at which an equal number of males and females develop, called their pivotal temperature. In some species, temperatures above the pivotal temperature result in eggs developing into females, as shown in the figure below. In other species, temperatures above the pivotal temperature cause eggs to develop into males. Some species have two pivotal temperatures. When temperatures during development are between the two pivotal temperatures, most eggs develop into males. Temperatures more extreme than the pivotal temperatures in either direction cause most eggs to develop into females (Ewert et al., 1994). In all of these cases, extremely warm temperatures could result in eggs developing into individuals of a single sex, skewing sex ratios and making reproduction impossible.
Temperatures during development affect the embryo’s sex by changing the expression of sex hormones. Hormones are molecules that travel through the body to particular organs or tissues and cause that target organ or tissue to do something particular (Randall et al., 2008). Androgens, including testosterone, are an example of sex hormones that influence sexual maturity and stimulate the production of sperm. Estrogens, which are produced from androgens, are another sex hormone, and they play a vital role in egg development in females. Androgens and estrogens are present and play important roles in both male and female development, but the concentration of these hormones affects sex. Aromatase is the enzyme responsible for converting androgens to estrogens (Moyles & Schulte, 2005). In animals with TSD, the external temperature affects how much aromatase is produced. In red-eared slider turtles (Trachemys scripta), higher temperatures result in a higher expression of aromatase. High aromatase concentrations cause more androgens to be converted to estrogens, causing the egg to develop as female. At temperatures below the pivotal temperature, red-eared sliders have very low aromatase concentrations, which results in high androgen concentrations and male offspring (Ramsey et al., 2007).
What will happen to sex ratios if temperatures continue to rise?
Based on current predictions for future climate change scenarios, models suggest that many turtle species will face problems within the next decade. Loggerhead sea turtles (Caretta caretta) in the southeastern United States are expected to produce almost exclusively female offspring if temperatures rise as little as 1º C (Hawkes et al., 2007). Painted turtles (Chrysemys picta) are expected to have highly skewed sex ratios with an increase of less than 2º C and offspring will be exclusively female if temperatures increase 4º C (Janzen, 1994a). The good news is that not all nesting areas are affected by temperature changes equally. The amount of vegetation shading a nest and the depth to which the nest is dug can significantly affect nest temperatures, and therefore sex ratios (Thompson, 1988; Janzen 1994b; Booth & Astill, 2001). Nests that are artificially shaded with screens have lower temperatures and more even sex ratios (Hawkes et al., 2007). Protecting nesting sites that have naturally lower temperatures, like heavily shaded sites, and artificially protecting nests from increasing temperatures may help mitigate the effects of increasing temperatures on turtle sex ratios in the short-term. If turtles and other TSD species are to survive long-term, however, we need broader solutions to climate change that will reduce the expected global temperature increases.
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