Are we heading to a world of all female sea turtles?
Sea turtles just can’t seem to catch a break. There are seven species of the marine reptiles, all belonging to the superfamily Chelonioidea. The Kemp’s ridley, hawksbill, and leatherback sea turtles are all classified by the IUCN as critically endangered. The loggerhead and green species are endangered, while the olive ridley is considered vulnerable. The flatback is data deficient, which only means that scientists don’t really know what its conservation status even is, not that it has somehow escape the same existential threat as its green-flippered relatives.
Sea turtles have plenty of natural threats. They get gobbled up by sharks, and their eggs, silently incubating on the world’s sandy beaches, are an easy snack for raccoons, foxes, and shorebirds. But those are all threats to which they’ve adapted. Females lay a hundred eggs at a time knowing that only a few hardy individuals – maybe just a single hatchling! – will survive to adulthood.
But we humans have made things quite a bit worse for them. They get caught up in fishing lines as bycatch. Some are poached illegally, because their shells are prized both as decorative items and for traditional medicine. Others, having evolved to navigate to the ocean by moonlight after they hatch, are instead distracted by the bright artificial lights of beachside development, luring them instead away from the surf. Now, research published this week in Nature Climate Change finds that the warming world in which the turtles now find themselves could spell even more trouble for the charismatic critters.
Many reptiles, including all sea turtles, have “temperature-dependent sex determination.” That means that the ambient temperature surrounding the eggs determines whether the animal that hatches will be a male or a female. For sea turtles, warmer sand – higher than 29 Celsius or 84.2 Fahrenheit – means that the hatchlings are more likely to be female. Could our warming world lead sea turtles to an overwhelming, unsustainable boom in females and a diminishing number of males? Despite what Jurassic Park may have taught you, a single-sex population of animals would have a hard time reproducing. Swansea University researcher Jacques-Olivier Laloë and colleagues used historical records and climate projections to try to predict the future of sea turtle population dynamics.
The researchers compared the cooler light-colored beaches with the warmer dark-colored beaches of Sal, an island in the Atlantic Ocean’s Cape Verde archipelago, just off the western coast of Senegal. Lighter beaches reflect more light, keeping the sand, and the hidden eggs, cooler. The beaches of Sal are an important rookery for loggerheads.
Surprisingly, they find that the outlook for sea turtles isn’t so bad, at least when it comes to climate change. While future warming will indeed likely lead to a higher proportion of female hatchlings, that might not be a death sentence for the turtles. By the middle of the 22nd century, Laloë predicts, the Cape Verde rookery will be between 4 and 17 percent male. However, while females always return to lay eggs on the beaches they hatched from, males that hatched on light colored beaches likely still fertilize females from dark colored beaches. In addition, a few males can breed with multiple females, and females can store sperm to fertilize multiple clutches of eggs. A few males could go a long way to sustaining a population, in other words.
Most interestingly, the researchers predict “that the increasing skew towards more female hatchlings will actually lead to increased recruitment of females to the adult population and to an increase in nesting numbers.” In that way, warmer temperatures may ironically confer a conservation benefit on sea turtles, with more females laying more eggs, increasing the growth rate of the population.
There aren’t many optimistic stories on wildlife conservation, and while this story may seem like a rare beacon of hope, the story isn’t yet over. That’s because this study only considered the impact of global warming on incubation temperatures within sandy beaches. But global warming is also associated with sea level rise. Rising waters will likely mean that some rookeries will have to find new beaches to lay their eggs as their old beaches are washed away. Indeed, some local populations have already been observed migrating to new nesting sites. But rising sea levels coupled with increasing beachside development could quickly eliminate whatever suitable alternative nesting sites there are.
This research highlights the importance of light-colored beaches for sea turtle conservation. In a world in which some conservation efforts must be chosen over others, lighter colored beaches may therefore be a priority. – Jason G. Goldman | 21 May 2014
Source: Laloë J.O., Cozens J., Renom B., Taxonera A. & Hays G.C. (2014). Effects of rising temperature on the viability of an important sea turtle rookery, Nature Climate Change, DOI: 10.1038/nclimate2236
Header image: shutterstock.com
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