Storing sperm helps female sea turtles remain monogamous

By Joshua Rapp Learn

Researchers took genetic samples from baby sea turtles to determine paternity. ©Florida Atlantic University

Many female loggerhead sea turtles stay monogamous during mating seasons, likely by storing sperm, research reveals.

Female loggerhead sea turtles (Caretta caretta) often nest only once every couple of years, giving time for their bodies to recover after resource-depleting pregnancies. When they do lay eggs, they often return to the same beach two or three times in a season — sometimes every few weeks — to lay new clutches of eggs. While researchers previously believed the females were likely mating again between every clutch, new research shows this often isn’t the case. In fact, they may be mating just once and storing the sperm throughout their egg-laying seasons.

“The turtles that we sampled were vastly monogamous,” said Jacob Lasala, a postdoctoral research fellow at Mote Marine Laboratory and Aquarium in Sarasota, Florida, and the lead author of a study published recently in Ecology and Evolution.

He and his colleagues studied female loggerheads on several keys near Sarasota, Fla. The population is part of ongoing research and monitoring dating back 40 years. They samples 36 egg clutches from 16 females in the 2015 population and analyzed the animals’ genetics to determine paternity.

Baby loggerhead sea turtles make their way along a Florida beach. ©Florida Atlantic University

They found that only four of the 16 turtles laid clutches that had been fertilized by different fathers. The rest were monogamous. And the turtles that laid more eggs tended to be more monogamous.

“The longer-term nesters were much more monogamous than we were anticipating,” Lasala said. It’s unlikely the females are encountering the same male again in between laying clutches since the males typically don’t hang around that long in the area, Lasala said. So this discovery likely means that that the females are capable of storing sperm for subsequent fertilizations.

There is some precedent for this.

A previous Japanese study published in 2011 showed that a female loggerhead in captivity laid egg clutches two years in a row without any contact with another male. Other research has shown that gopher tortoises (Gopherus Polyphemus) can also store sperm.

Further research will be necessarily to determine how long wild loggerheads can store sperm, and Lasala said that he’s working on more research that looks at other populations to determine whether they show similar trends in terms of monogamy.

But he said this discovery is good news, because it shows that females have an ability to reproduce even if males are hard to find.

In earlier research in 2018, he and his colleagues looked at the paternity and breeding sex ratios of loggerhead turtles in a population that nested on Sanibel Island in southwestern Florida. They found that due to temperature-dependent sex determination found in earlier research, the warmer water was causing loggerheads to skew increasingly female.

If climate change causes water temperatures to continue to warm, loggerhead females may not have to work as hard as previously believed to find mates.

Joshua LearnJoshua Rapp Learn is a science writer at The Wildlife Society. Contact him at jlearn@wildlife.org with any questions or comments about his article.

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