SAN FRANCISCO, CALIFORNIA—Sea turtles, like other reptiles, don’t have sex chromosomes; instead, their sex is determined by the temperature of the nest as it incubates. Because warmer temperatures produce females, scientists have been warning that the species could face extinction, as higher global temperatures mean fewer males. But a new study suggests that reports of the male sea turtle’s death could be greatly exaggerated. Since 2013, biologists have collected blood samples from hundreds of turtle hatchlings (Caretta caretta) on Sanibel Island in Florida, and identified the paternity of each. As they looked at hundreds of nests year after year, they found something surprising: DNA from male turtles never appeared in the nests of more than one female. Females can mate multiple times before the nesting season, then store sperm and lay eggs in several nests, each with a different repertoire of fathers. After calculating the number of breeding males from the DNA tests, the researchers determined that for every sea turtle mother, there are nearly three fathers, they reported here last week at the Society for Integrative and Comparative Biology’s annual meeting. That means this population has far more breeding males than anyone realized. To find out why—one theory is that more males survive to adulthood—the researchers plan to conduct paternity testing on more Florida beaches, with two of the six other sea turtle species.