Studying any animal in the wild is hard enough, let alone one that spends 30 years at a time out at sea. Because of its ocean-faring lifestyle, scientists know next to nothing about the life style of the critically endangered hawksbill sea turtle, save for the fleeting observations they make when female turtles come to beaches to lay their eggs.
Genetics, however, is giving researchers a glimpse into the sex lives of these elusive creatures ? and insight into their preservation.
?People have looked at turtle mating systems before, but not at any depth,? said David S. Richardson, a molecular ecologist at the University of East Anglia in Britain. ?The trouble is that it?s very hard because most of what happens happens far out at sea.?
Hawksbill turtles live in warm waters throughout the world, starting their lives as hatchlings on tropical shores. Female turtles lay up to five clutches of around 200 eggs every three years. Once the baby turtles break free of their sandy cradles, only the females will ever return to dry land. Sometimes, that first return visit takes three decades or more.
As for what happens in between, researchers could only guess at whether females mate with single or multiple males ? whether a handful opportunistic alpha males surround beaches to monopolize mating, or males compete for females that then chose the most robust candidate. ?All we see is the females coming and laying nests,? Dr. Richardson said.
Dr. Richardson, working with Karl Phillips, a doctoral candidate, and other colleagues, decided to pull back the curtain on the turtles? mating patterns by reconstructing paternal genotypes. Comparing DNA between mother and offspring can provide roundabout insights into a father?s genetic identify. To fill in the missing male piece, the researchers took genetic samples from 53 females and 1,600 hatchlings from 85 nests from the Seychelles, which lie in the Indian Ocean off East Africa.
Genetic material in hand, they used a powerful microsatellite array ? an analysis of unique repeating sequences of DNA ? to identify the number of males that sired the females? young.
Surprisingly, the researchers found that more than 90 percent of females mated with a single male, and the remaining four females mated with just two males each, they report in the journal Molecular Ecology.
Over all, the researchers determined that 47 males fathered the baby turtles, and none of them mated with more than one female that visited the beach. Comparing the males? genetics revealed no patterns indicating that females choose the most genetically impressive or compatible mates, suggesting that females may just mate with the first male that turns up when the time is right.
?We were very surprised at this level of monogamy,? Dr. Richardson said. ?To put it mildly, monogamy is the exception, not the rule, in the animal world.?
The females achieved this reproductive feat with sperm storage, a trick that many animal species rely upon in which females store sperm after mating for future fertilization. In the case of the hawksbill turtles, females store sperm for up to 75 days, an impressive length of time, to make it last throughout the duration of their egg-laying period.
By storing enough sperm to get through a full breeding season, females can avoid the hassle of dealing with other males after that first encounter. ?Females separate having to mate with having to fertilize eggs in space and time,? Dr. Richardson said. ?Basically, they can get on with it and lay their eggs in peace.?
Beyond shedding light on fundamental turtle biology, the findings also present good news for conservation. In follow-up work in which they examined mating patterns across multiple years, the researchers found evidence suggesting that females mate with new males each reproductive season.
Because every female mates with a different male and switches males between years, the researchers can safely assume that male populations are plentiful, at least in the Seychelles.
They also found a significant amount of genetic diversity within their samples, which gives animal populations the evolutionary flexibility to respond to new environmental challenges, such as disease.
Like most sea turtle species, hawksbill turtles are hunted around the world for their shells and meat, and to poach their eggs. Sea turtles become entangled in pollution or snared by fishing lines, and coastal development destroys their nesting habitat.
As a result, hawksbills have declined from historic numbers by an estimated 80 percent. But in the Seychelles, at least, populations seem to be on the rise.
?Combined with our estimate that there are lots of genetically viable males out there, hawksbill turtles may actually be in a better place than we thought,? Dr Richardson said.
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