Global warming and temperature-dependent sex determination in Reptillia
Written by: Madelaine Huygens
Many reptile (Reptillia) species have the sex of their offspring determined by the environment through the mechanism of temperature-dependent sex determination (TSD), instead of genetically like Homo sapiens. TSD is where the sex of offspring is determined by the temperature embryos are exposed to during their development. This mechanism is seen in most reptile species, including all crocodile (Crocodillia) species, tuatara (Rhynchocephalia), and certain turtle (Testudines) and lizard (Squamata) species (L. Schwanz and F. Janzen, 2008).
The issue reptile populations with temperature-dependent sex determination have is the current increasing global temperatures due to global warming and climate change, putting them at risk of endangerment. As of 2019, Earth’s surface temperature has risen approximately 0.95°C in the past 10 years due to increasing greenhouse gas emissions, making current global temperatures the highest recorded (Kai Zheng, et al., 2019). This could be detrimental to certain reptile populations because the temperature range that determines if male or female offspring hatch is very narrow. Generally, lower incubation temperatures, around 22–27°C, will produce one sex and temperatures of around 30°C and higher produce the other (Gilbert SF., 2000). This means any small changes in temperature during development can change the ratio of sexes in TSD clutches, with only a small temperature range allowing both sexes to hatch from the same clutch (Gilbert SF., 2000). Global warming could lead to sex ratio bias, most likely skewed toward the sex produced by the upper temperature limit. This would have detrimental effects on TSD populations, causing a disproportion in sexes resulting in a reduction in availability of mates (Standora, E., & Spotila, J., 1985).
Species where higher temperatures produce male offspring are more at risk because a reduction in females within a population can result in reduced growth rates for the population (Mitchell, Nicola & Janzen, F.J., 2010). Overproduction of one sex also reduces the genetic diversity of the population, causing heterozygosity to be lost at a greater rate than a population that does not have a sex-ratio bias (Mitchell, Nicola & Janzen, F.J., 2010). This means global warming could result in a reduction in the stability and adaptive potential of TSD populations due to sex ratio bias and diversity loss, which could lead to the endangerment or even extinction of certain reptilian species.
Crocodiles (Crocodillia) and tuatara (Rhynchocephalia) would be at most risk to the effects of increasing global temperatures as all species rely on temperature to determine offspring sex, both lacking sex chromosomes. Turtles (Testudines) and snakes and lizards (Squamata) only have some species that are true TSD species, putting them at lower risk of impact. However, the species that have TSD could be largely affected by only a small increase in global temperatures (Mitchell, Nicola & Janzen, F.J., 2010).
With global temperatures predicted to be rising approximately 0.1°C per decade, human intervention in nesting areas and controlled incubation may need to occur to prevent the overproduction of one sex in reptilian populations (Kai Zheng, et al., 2019). Taking clutches from nesting sites to then incubate in a temperature-controlled environment or providing shade in nesting areas could help neutralise sex ratio biases and prevent the negative impacts global warming has on TSD reptile populations (Mitchell, Nicola & Janzen, F.J., 2010).
Lisa E. Schwanz and Fredric J. Janzen, “Climate Change and Temperature‐Dependent Sex Determination: Can Individual Plasticity in Nesting Phenology Prevent Extreme Sex Ratios?” Physiological and Biochemical Zoology 81, no. 6 (November/December 2008): 826-834.
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