Around the world, many conservationists are celebrating increases in sea turtle population growth. Cape Verde in West Africa now has 100 times as many loggerhead turtle nests each year as there were in 2008. But scientists warn that this apparent success could be hiding an impending population collapse.
Using drone surveys and 15 years of nesting data, scientists at Queen Mary University of London report that booming nest counts could be misleading. As global warming causes temperatures to rise, more sea turtle eggs are developing into females. Without enough males, even a seemingly thriving population can collapse. Their findings, which are yet to be peer-reviewed, were posted January 20 to bioRxiv.org.
โWe think that thereโs a bit of a mirage,โ says Christopheย Eizaguirre, a conservation geneticist at Queen Mary University. Conservation efforts, such as marine protected areas, fishing regulations and protecting egg clutches, are helping populations, but may not provide the full picture.
As in some other reptiles, such as crocodiles, a sea turtleโs sex is dependent on the eggโs incubation temperature. Eggs that develop in warmer sand become female, while cooler conditions create males. โThe way we describe it in my lab is โhot chicks and cool dudesโ,โ says Jeanette Wyneken, a biologist at Florida Atlantic University in Boca Raton, who wasnโt involved in the study.
Using drones, the scientists photographed the breeding population and found a 9โ1 ratio of females to males. โWe can differentiate between females and males by the tails,โ says biologist Fitra Aryaย Dwi Nugraha, also at Queen Mary University. โMales have longer and thicker tails.โ
Itโs believed that a temperature of about 29 degrees Celsius produces a roughly even split between male and female hatchlings. โWe donโt know what a perfect population should look like,โ Eizaguirre says.
The authors hypothesize that the extreme skew toward female loggerheads (Caretta caretta) that go on to lay eggs inflates the nest count, making the population seem healthier than it is. Without enough males to sustain breeding, population growth could quickly vanish, though itโs hard to predict when this tipping point could arrive.
Wyneken has concerns about the studyโs methods, particularly in determining the ratio. Although adult males can be identified by drone, itโs harder to tell with subadult individuals, which can be comparable in size to females. โItโs possible that they are counting some immature males as females,โ she says. โThe 9โ1 may be more skewed than normal if itโs got that error.โ Itโs more accurate, she says, to confirm hatchling sex through a laparoscopy surgery.
Turtles have evolved strategies to protect against a female bias: Males can mate more frequently than females, females store sperm to maximize how many clutches of eggs they can fertilize and both sexes mate with several partners. Some conservation initiatives relocate eggs to hatcheries to give them the best chance of survival by protecting them from predators, poachers and environmental threats. If something goes wrong, โyouโre putting all your eggs in one basket,โ Wyneken says.
Having too many females across global populations would be a concern, the scientists agree. โYou expect more females,โ Wyneken says, but โseasons where we get 100 percent female, again and again and again, or 98 percent female? Thatโs not sustainable.โ These dramatically skewed populations are already popping up in some warmer regions, such as beaches in the northern Great Barrier Reef.
The authors hope that their work will prevent people from winding down conservation initiatives, thinking that theyโve worked. โThatโs probably not the case,โ Eizaguirre says. Ongoing and adapting conservation efforts are vital in giving turtle populations time to respond to changes in their environment. โWhat we really donโt want is the effort to stop.โ