Most male loggerhead turtles go back to the nesting beaches to breed -a common behaviour among female turtles-, according to a study in which the researchers Marta Pascual, Àlex Aguilar, Carles Carreras, Lluís Cardona and Marcel Clusa, from the Faculty of Biology and the Biodiversity Research Institute of the University of Barcelona (IRBio) took part.

Also, experts from the Cyprus Wildlife Society (Cyprus), University of Tripoli (Libya) and the University Adnan Menderes (Turkey), among other institutions, collaborated in the study.

The study, published in the journal Marine Ecology Progress Series, outbreaks the classical view on the breeding behaviour of these marine turtles, and explains how the species could also breed in feeding areas or during their journey towards nesting beaches.

New paradigm: male turtles return to the nesting beach to breed

The loggerhead turtle (Caretta caretta) is a marine species that travels long journeys to tropical and temperate areas around the world. In the eastern Mediterranean, in particular, it nests in the coasts of Greece, Turkey, Cyprus, Libya, Lebanon and Israel, although there have been some sporadic nesting episodes in the western Mediterranean.

It was believed that only female turtles went back to the nesting areas to lay the eggs -philopatric behaviour- after reproducing with male turtles from different areas. Philopatry is a studied phenomenon among female C. caretta turtles. The process of detection, marking, and the chelonian genetic study (for instance, with the mitochondrial DNA, transmitted by maternal inheritance), are easily conducted if females are the ones that go back to the beach of birth to lay the eggs.

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However, markers in males are not abundant and results have never been conclusive. Previous studies with few genetic nuclear markers ─microsatellite loci, the biparental inheritance─ suggested male turtles did not show philopatric behaviour and mated with females from different areas.

"Our study reveals the breeding behaviour of the C. caretta marine turtle can be more complex. In most populations, female turtles are not the only ones with philatropic behaviour: males also mate near nesting beaches," says the lecturer Marta Pascual, member of the Department of Genetics, Microbiology and Statistics of the UB and IRBio.

Breeding far from nesting beaches

In the study, the UB-IRBio team increased the number of microsatellite markers to analyse the gene flow among loggerhead turtle populations in the Mediterranean. The results show a higher gene differentiation in the nesting beaches in the Mediterranean and suggest the possibility that turtles breed in feeding areas or during their journey towards nesting beaches.

"Therefore, the accepted belief that males do not display philopatry could be due -in some cases- the low number of molecular markers that were used so far," states Marta Pascual. "Also, if we compare mitochondrial and nuclear markers, we can compare the spreading behaviour of male and female turtles in different areas, which shows complex and particular breeding behaviours in each area."

Higher temperatures, more female turtles in the marine habitat

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In most cases, philopatry happens in male and female turtles. However, there are cases of opportunistic breeding patterns between males and females in different areas other than their place of birth. According to the experts, the obtained results could be explained with some hypotheses that have to be tested in future studies.

"The breeding behaviour can change depending on the population; it can even be affected by the amount of male turtles that are born in a specific area," says Pascual. The sex of marine turtles is determined by the temperature of incubation. If the temperature is high, there will only be female turtles: "with global warming, high temperatures would cause a feminization of the populations, a phenomenon that could be balanced through opportunist breeding with males from other areas," concludes the expert.

Protecting an emblematic species in the Mediterranean

Although the Mediterranean can be understood as a regional unit to manage globally, when it comes to the loggerhead turtle there are genetically differentiated units that should be protected. In some cases, these are big populations -according to the annual number of nests in their beaches- but there are examples that show a lower balance. In a planet affected by global change, a more comprehensive study of different areas is necessary to identify bottlenecks -which reduce the number of population individuals- and to study the impact of the increase of consanguinity over the viability of the different units.

There are still many unknown issues on the breeding biology of the species C. caretta. Migratory routes that have been observed with telemetry on females in Cyprus show that they feed in Libya and travel near this area's nesting beaches. New studies with genomic scale markers are necessary to get deeper in the biology and ecology of the most abundant marine turtle in the Mediterranean (sporadic nesting, non-philopatric breeding, ect.).