How Can Marine Turtles Always Return to the Same Sex Beach?

Scientists have been puzzled for long by the ability of the marine turtles to return their entire life to the same beach to depose their eggs.

These egg-laying sites can be often located far from the feeding areas and the individuals (both females and males, as they mate in the same area where the females lay their eggs) swim several hundred kilometers of ocean with no visual landmarks.

A new research led by Simon Benhamou of the Center for Functional and Evolutionary Ecology at Montpellier1, France, points out that turtles' exceptional navigational ability relies on detecting the earth's magnetic field, against the deflection induced by the ocean currents.

Each 4 years, Indian Ocean green turtles (Chelonia mydas) make a journey of hundreds of kilometers to specific breeding shores, where they will lay 4 to 6 successive clutches.

The research team made two experiments.

The turtle populations investigated were from Islands of Europa and Mayotte, off the Madagascar coast.

Female turtles were captured at the beginning of their egg-laying cycle (thus strongly impelled to return to this area to complete their cycle), tagged with Argos transmitters to satellite track them and released in open sea, several hundred kilometers from the egg-laying site.

The researchers wanted to see if turtles' movements were controlled by the currents or if they used them to ease their journey.

The marine turtles' navigation system permitted them to keep their direction towards their natal beaches wherever they find themselves, like a compass pointing towards them.

They were able to fix any deflection they were subjected to, like boat transportation or ocean currents.

The animals could not stand up against ocean drift plotting their track, thus the movements made by the animals were a mix of deliberate actions by the turtles and the slide induced by the oceanic currents, that makes them wander for long periods during adverse ocean weather.

One turtle released 250 km away from its egg-laying beach on Europe swam over 3 500 km (2 200 mi) in two months before returning there!

In the other investigation, the team checked how much the earth's magnetic field counts on the turtles' navigation system.

For the first time was proven what was before just a supposition: marine turtles employ the magnetic field of the earth to navigate.

Impairing its detection using a powerful magnet attached on the turtles' heads decreased their navigational sense.

They were still able to return to their original egg-laying beach, thus the turtles do not rely only in the geomagnetic field.

They probably use olfaction, like some sea birds (petrels and albatrosses) or homing pigeons do, but this is just a supposition by now.