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Green Turtle's Mating Habits Protect Against the Effects of Global Warming

Wright, L.I., Stokes, K.L., Fuller, W.J., Godley, B.J., McGowan, A., Snape, R., Tregenza, T. and Broderick, A.C. 2012. Turtle mating patterns buffer against disruptive effects of climate change. Proceedings of the Royal Society B: Biological Sciences Published online before print January 25, 2012 doi: 10.1098/rspb.2011.2285.
Sea turtles, like many reptiles, have what is called "temperature-dependent sex determination" (TSD), which means that the temperature experienced by each developing embryo (within its individual eggshell casing, laid within a nest built by the mother on a terrestrial beach) determines its ultimate sex (Janzen, 1994). In turtles, males require somewhat cooler embryonic conditions than females: a temperature of 29°C generates a sex ratio among hatchlings of about 50:50, while at higher temperatures, more females are produced (Hawkes et al., 2009). (Strangely enough, in the tuatara-another TSD reptile-the opposite is true: more males are produced at higher temperatures.) As a result, strongly-skewed offspring sex ratios are common in TSD reptiles.

The amount of global warming predicted for the next century by some climate scientists is considered a threat to TSD-species because it is feared that such extreme sex ratio biases will be generated (e.g. 100% females) that extinction will be inevitable (e.g. Fuentes et al., 2010; Janzen, 1994; Witt et al., 2010). As a highly female-skewed hatchling sex ratio already occurs in marine turtles (Hawkes et al. 2009), "predicted increases in global temperatures are expected to exacerbate this trend." (Wright et al., 2012).

While it is clear that living sea turtle species have survived substantial temperature fluctuations in the recent past (Janzen and Paukstis, 1988), will they be able to adapt to future warming?

In the present study, Wright et al. (2012) examined the mating behavior of green sea turtles (Chelonia mydas) to see if it was compensating for the extreme female-skewed sex ratios of hatchlings currently produced in the Mediterranean (86-96% female). In doing so, they undertook genetic typing of tissue samples taken in 2008 from 20 nesting females and 809 of their offspring (from 37 clutches of eggs), from a single rookery beach in northern Cyprus. Paternity analysis of the hatchlings allowed the researchers to determine how many males had mated with the females they sampled. They also put a satellite transmitter on a single male to map his movements during the breeding season.

The authors point out that sea surface temperatures at this site have risen by, on average, less than 1°C over the past 50 years and that the female bias that exists today in adult green sea turtles is probably the result of a similar extent of female bias among hatchlings produced approximately 30 years ago (due to the late age at sexual maturity in this species) -- that is, they assumed in their study that many more breeding-aged adult turtles in the population were female than were male.

The authors found that "despite an offspring sex ratio of 95 per cent females, there were at least 1.4 reproductive males to every breeding female" -- 28 males breeding with 20 females tested. This suggested to the authors that males may breed more often than females (females breed every 2-4 years). In addition, the fact that the male they tracked visited a number of rookeries in the region suggested that males may mate with females on a number of different beaches. Together, the authors suggest, these mating behaviors partially compensate for the lower abundance of males within the population such that "male mating patterns have the potential to buffer the disruptive effects of climate change on marine turtle populations, many of which are already seriously threatened." Furthermore, Wright et al. conclude that "current mating patterns will help to preserve genetic variation that may be critical if marine turtles are to adapt behaviorally or physiologically to a warming climate and have, no doubt, contributed to their persistence through historical climatic upheaval."

Additional References
Hawkes, L.A., Broderick, A.C., Godfrey, M.H. and Godley, B.J. 2009. Climate change and marine turtles. Endangered Species Research 7: 137-154.

Fuentes, M.M.P.B., Hamann, M. and Limpus, C.J. 2010. Past, current and future thermal profiles of green turtle nesting grounds:implications from climate change. Journal of Experimental Marine Biology and Ecology 383: 56-64.

Janzen, F.J. 1994. Climate change and temperature-dependent sex determination in reptiles. Proceedings of the National Academy of Science USA 91: 7487-7490.

Janzen, F.J. and Paukstis, G.L. 1988. Environmental sex determination in reptiles. Nature 332: 790.

Witt, M.J., Hawkes, L.A., Godfrey, M.H., Godley, B.J. and Broderick, A.C. 2010. Predicting the impacts of climate change on a globally distributed species: the case of the loggerhead turtle. Journal of Experimental Biology 213: 901-911.

Archived 7 February 2012