Worries about climate change threatening sea turtles may have been misdirected.
Warming that could lead to far more female hatchlings than males isn’t the most immediate danger from climate shifts. Lethally overheated beach nests are more important, researchers argue February 8 in Proceedings of the Royal Society B.
Climate change can meddle with sex ratios of the seven species of sea turtles because their embryos start life without a genetically fixed sex. Nest temperatures greater than roughly 29° Celsius tip the ratio toward more female hatchlings, explains marine ecologist Graeme Hays of Deakin University in Warrnambool, Australia.
Warm the nesting beaches enough, and sea turtle populations with few to no male mates might get feminized to extinction, biologists have warned. Yet records from 75 sea turtle nesting sites around the world suggest that many still-abundant populations are skewed to extreme female bias. “That’s not really the No. 1 concern,” Hays says. “A few male turtles already go a long way.” Instead, youngsters dying in overheated nests appears to be a more serious problem and needs a worldwide effort at data collection, Hays and colleagues say.
One reason heavily female turtle populations haven’t crashed yet is the difference between male and female breeding frequency, the researchers say. A male shows up to mate in the waters off the nesting beach roughly twice as frequently as a female does, perhaps every two years instead of every four, Hays says.
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Disproportionately higher death rates among female hatchlings also could temper the female bias generated by upward creeping nest temperatures. Warmer spots in nesting beaches that are more likely to make embryos turn female are also more likely to cook them.
At extreme temperatures, however, everybody loses. Should a nest reach 35° C, a group of 100 eggs would yield on average only five living female hatchlings plus a ghost of a fraction of a male, mathematical simulations predict.
Vincent Saba has seen hatchling losses firsthand at the leatherback turtle nesting site he studied in Costa Rica. There, computer simulations suggest that as the beach overheats the dwindling survival of young turtles could nearly wipe out the population within a century, says Saba, a biological oceanographer with the National Oceanic and Atmospheric Administration’s Northeast Fisheries Science Center at Princeton University.
Just how much a beach heats up depends largely on rainfall, which in turn depends on how the climate changes, says Saba. Sorting out the effects on sea turtle populations is going to take science done “beach by beach.”