Planet Earth Online

New insight into Hawksbill Turtles' mating patterns

4 February 2013, by Harriet Jarlett

Dr David S Richardson and Karl Phillips from the University of East Anglia looked at turtle nests on Cousine Island in the Seychelles, an eco-tourism holiday destination favoured by the rich and famous.

Working with the islands' conservation staff they used a non-harmful procedure to collect DNA from the hard skin of the flipper, or around the shell.

The researchers looked at 43 mothers and 1600 young to reconstruct a genetic model of each nestling's father, without having to catch the males out at sea. 'It's important to do this so that we can understand their mating system and see how much genetic variation there is within the population' says Richardson.

This is the first comprehensive study into Hawksbill Turtle mating patterns and the findings may help conservationists refocus efforts to where they're needed.

Each season female Hawksbills can lay up to 1000 eggs in four to five nests, with a few weeks' break in between. Until now scientists assumed that during this break female's return to sea to mate again.

The scientists were surprised then to find all of the eggs from an individual female are normally fathered by the same male. This means the mothers mate only once and then store the sperm for the entire mating season, fertilising all their eggs with it.

Monogamy in the animal kingdom is the exception rather than the rule. Many females are forced to mate multiple times by aggressive males, or prefer to mate multiply to ensure they have the best quality sperm.

'If they mate with one male and then met another one that was in some way better, bigger or stronger, they might re-mate and have a mix of eggs fertilised by both,' Richardson explains. 'However, it seems female Hawksbills would rather mate just once, far away from their nesting site, then store the sperm, perhaps to avoid unwanted male attention.'

The study, published in Molecular Ecology, showed each female was mating with a different male, proving that there are lots of males out at sea. This demonstrates that conservation efforts so far have been working, and Hawksbill numbers are growing.

With so many males contributing to the next generation it also means there is lots of different DNA circulating amongst nestlings, so if new diseases or predators appear some are better adapted to cope, and the population isn't in immediate danger of extinction.

Hawksbill Turtles had been hunted for their shell for over 100 years, until it was banned in 1996 when they became critically endangered. 'Since hunting was banned, this population has started to recover, but we never knew how damaged they've been by the hunting,' says Richardson. 'We didn't know how many individuals were out there, nor the genetic diversity of the population.'

Conservation on Cousine Island's beaches had mostly involved monitoring the females by tagging them as they came onshore to build nests. But there had been little information about the males, who were hidden far out in the ocean, until now.

'When you only see females and never see males you don't know the mating system. You don't know if there is just one male monopolising female turtles, one male with a harem, like you see in elephant seals or red deer.'

Now the scientists know that not only are there lots of males fathering the nestlings, but there are many male turtles scattered far across the Indian Ocean. Conservationists can use this information to widen their efforts, from the beaches where the females lay their nests to the entire Indian Ocean area, in order to protect males too.

Keywords: Amphibians, Biodiversity, Biology, Conservation, Extinction, Genetics, Indian Ocean, Marine life,

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