Some scientists believe that dark clouds loom over the future of sea turtles as global warming continues to impact Earth. But new research from the Netherlands and the United Kingdom suggests that this may not be the case for some of these reptiles. The finding, presented in the journal Proceedings of the Royal Society B, shows that a number of sea turtles are naturally heat tolerant. The study was funded in part by the European Social Fund.
Led by the University of Exeter in the United Kingdom, the researchers investigated green turtles (Chelonia mydas) nesting on Ascension Island, a British overseas territory in the South Atlantic Ocean. These turtles lay eggs on a naturally hot beach, which enables them to tolerate high temperatures better than eggs from turtles nesting on a cooler beach just a few kilometres away.
For the purposes of their study, the researchers placed several eggs laid on each beach into incubators of either 32.5 °C or 29 °C, and kept an eye on their progress. The eggs from the warmer beach succeeded in thriving in the hot incubator while those from the colder beach did not.
'We believe this is the first time that adaptation to local environmental conditions has been demonstrated in sea turtles,' says senior author Dr Jonathan Blount. '[This] is all the more remarkable because the beaches in question are just six kilometres apart.'
According to the researchers, the warmer beach has dark sand while the colder beach has white sand. Green turtles make their way from the coast of South America to the small island to nest. The majority of these turtles nest on the beaches where they were hatched as well; the team says turtle populations have the capacity to adapt to specific nesting locations.
'Migratory species often present particular problems for conservationists when defining relevant spatial units for protection,' the authors write in the paper. 'Given that local adaptation is opposed by gene flow and disrupted in temporally variable environments, species that migrate widely across a variety of different habitats might be expected to exhibit relatively low levels of adaptive differentiation. However, in some cases, the tendency for migrants to return to their own natal sites to reproduce has driven local adaptation at surprisingly fine spatial scales (e.g. in fishes). Marine turtles are renowned for migrating long distances to breed at their natal nesting beaches, and have become flagship species for conservation initiatives.'
But they note that recent assessments of the structure of the nesting population as well as the designation of management units in these species have for the most part been based on molecular genetics approaches. Information about the extent of adaptive phenotypic variation in thermal tolerance remains a mystery. 'This is surprising, given that temperature has a profound effect on hatching success, embryonic development and sex in marine turtles, leading to growing concerns regarding the impacts of climate warming on their reproductive success,' they write.
The researchers say their finding indicates that the thermal tolerances of green turtle embryos at Ascension Island have diverged in response to the different incubation temperatures of the dark and white sand nesting beaches located only a few kilometres apart.
Experts from the Centre for Behaviour and Neuroscience at the University of Groningen in the Netherlands and the Ascension Island Turtle Group in Ascension Island contributed to this study.