Why Do Loggerhead Females Return To Their Place Of Birth To Mate
May 31, 2013

Passing On Immunity Drives Female Loggerhead Sea Turtles To Go Home To Mate

Enid Burns for redOrbit.com — Your Universe Online

New research is shedding light on why female loggerhead sea turtles return to their place of birth to mate, while males jump to different locations.

Scientists at the GEOMar Helmholtz Centre for Ocean Research Kiel in Germany used molecular tools to research turtle migratory patterns around the Cape Verde islands to find out why female turtles return home.

The reason that female loggerhead sea turtles return to their place of birth to mate is that females from different islands have different immune genes. By returning home these, females give their offspring better parasite resistance since hatching on a different island could expose baby turtles to disease and death.

Male loggerhead sea turtles are not as bound to their place of birth. Male turtles "appear less selective," and mate at multiple locations. By mating in multiple locations, the male sea turtles might be sharing their immune genes, and giving their offspring resistance to parasites and other germs in multiple locations.

Scientists have wondered for years why female loggerhead sea turtles behave this way. These turtles return to their birthplace about 25 years after they are born. Many travel great distances - up to several thousands of kilometers - to return home.

The migratory study is important. Loggerhead sea turtles have an endangered status on the Red List of Threatened Species. This is due to turtles being slaughtered for their meat, marine pollution, coastal development in nesting areas and fisheries by-catch.

The archipelago of Cape Verde, which includes numerous islands, serves as an optimal location for the GEOMAR scientists to conduct their study. GEOMAR collected tiny skin samples from turtles on four different islands of the archipelago.

"Using multiple genetic tools, the scientists found that Cape Verdean female loggerheads not only return to Cape Verde to breed, but also that they show a remarkably accurate philopatric (returning to reproduce at the place of birth) behavior of a couple tens of kilometers," the report said. "It was fascinating to demonstrate that most female turtles actually return to the exact island where they were born," said lead author Victor Stiebens.

Turtles returning to their original birthplace have advantages and disadvantages for the species. The study identifies the so-called major histocompatibility complex, a genome identified in the turtles responsible for fighting parasites.

"Indeed, the study shows that turtles nesting at the most distant islands of the archipelago have different sets of these genes, providing the right genetic make-up to pass to the offspring to fight off the local parasite fauna present in that specific place," said senior author Dr. Christophe Eizaguirre.

Returning to their place of birth also gives female turtles and their offspring a disadvantage. Always returning to the same island may be detrimental for a species with small population sizes. This may lead to mating with relatives and inbreeding. This may be why males mate in different locations, which counteracts the risks.

"Males seem to look for females over large regions of the archipelago, whereas females are more faithful to their place of birth to mate," said Stiebens.

"These gender-specific behaviors assure genetic transfer between the nesting islands but also the existence of genes needed in these local environments," said Dr. Eizaguirre.

The study is published in the journal Proceedings of the Royal Society B: Biological Sciences.