Many studies are now focused on how climate change is going to impact organisms with temperature-dependent sex determination (TSD). TSD is where during embryonic development ambient temperature determines the sex of the offspring, in most reptiles high temperatures induce a high female percentage. Climate change is an extreme threat for these organisms as global warming produces an extreme sex ratio that could possibly lead to extinction. Marine turtles are one of those such species, increases in temperature cause a skewed ratio of more females than males. Wright et al. (2012) examined the effect of climate change on marine turtles by testing to see if skewed offspring sex ratios persist into adulthood and whether variation in male mating success impacts the population’s genetic variance. They tested their hypothesis by doing genetic tests in conjunction with a tracker turtle to observe the migration route. The author’s found that despite a 95% female sex ratio there were at least 1.4 reproductive males per breeding female, suggesting that the interval between male reproducing periods is quite short. The scientists suggest this shows that male mating patterns have the potential to buffer the disruptive effects of climate change but that the growing temperatures still might increase sex ratios to the point that genetic diversity becomes so limited that populations could become endangered. –Connor O’Boyle
Wright, L., Stokes, K., Fuller, W., Godley, B., McGowan, A., Snape, R., Tregenzal, T., Broderick, A., 2012. Turtle mating patterns buffer against disruptive effects of climate change. Proc. R. Soc. B., doi:10.1098/rspb.2011.2285.
Understanding how climate change might impact organisms with TSD is a crucial part in conservation of those organisms. Usually TSD is experienced during embryonic development; even small temperature changes can have a profound effect on offspring sex ratios. Because most organisms with TSD already have skewed offspring ratios, studying the mechanisms and implications of temperature-dependent sex determination was mostly academic. Ever since scientists recognized steady increases in global temperatures, studies on TSD and the impact of rising temperatures on these organisms has become more conservation based. Most species of marine turtles have TSD, with females being reproduced at a much higher rate at high temperatures. The 50% sex ratio temperature for green turtles is 29°C; however this species lives in areas above that temperature so a female-skewed ratio is common. This is true for green turtles, which are the species the authors looked at. Due to the lack of information regarding adult sex ratios in marine turtles and mating behavior in male turtles the authors set out to find out more and figure out what could possibly protect the turtles from climate change effects.
Wright et al. conducted their study in a wild population of green turtles at Alagadi beach, northern Cyprus, during the breading season of 2008. They obtained tissue samples from twenty nesting females with known identity and twenty-three offspring from one or more clutches (A clutch is basically a dog litter but for turtles) per female. The dataset was made up of 809 offspring from 37 clutches. The sex of the offspring was estimated based on incubation time. The scientists genotyped the individuals whose tissue samples they obtained, then used these samples to determine population allele frequency to see if they deviated from Hardy-Weinberg equilibrium (HWE). Also using the genotyped samples, they ran a paternity analysis using parentage inference software. The authors placed a transmitter to a male turtle from the same study site and the male was tracked via satellite for 81 days.
The authors found that the offspring tested were 95% female and a minimum of twenty-eight unique males sired offspring from twenty nesting females, showing that there was a higher sex ratio of breeders than expected at 1.4 males to each female. The runs identified twenty family clusters with a single mother and all her offspring with one or multiple fathers. The results also indicated that no male produced offspring using multiple mothers. The loci the authors tested were within HWE and the probability of multiple paternities, assuming two fathers had skewed paternal contributions, was 0.876. The turtle with the transmitter travelled close to multiple nesting beaches in Cyprus and Turkey then traveled to North Africa. The route is consistent with a mate searching behavior as the breeding sites within 20 km of the route account for 58% of green turtle nesting areas in the Mediterranean.
The findings that more males contributed to reproduction than females were surprising due to the female dominant hatchling ratio seen at the studied rookery. The author’s results suggest that males have more frequent breeding periods than females, resulting in sex ratios of adults on breeding grounds that are much less female dominant. This more frequent breeding period for males also helps explain why the skewed ratio for turtles is able to persist. The frequent breeding period for males however will not keep the turtles safe from a low population variance. Wright et al. propose an alternative explanation for the breeding behavior, males breeding at the study site might have originated from other locations that produce a more even offspring ratio, sometimes region courtship allows a male to migrate and have multiple breeding females along the migration path. This is supported by the results from the tracker. The turtle’s journey suggests the male stops to mate at multiple breed grounds. Wright et al. highlight the role of mating behavior in maintaining relatively equal operational sex ratios with their study. The breeding behavior in green turtles warrants more analysis as it might be a crucial part of the population variance. Understanding mating patterns might help researchers preserve genetic variance that might be a crucial part of marine turtle’s ability to cope with climate change.