By Matt Walker
Editor, Earth News
Right or leftie?
Leatherback turtles tend to be the reptilian equivalent of "right-handed".
Across a population studied by scientists, more turtles preferred to use their right rear flipper rather than their left when laying eggs.
The result, published in the journal Behavioural Brain Research, is the first time a species of turtle has found to prefer one limb over another.
The discovery adds to growing evidence that even lower vertebrates prefer to use one side of the body more often.
Such preference is known by scientists as a "lateralised functional behaviour", and it usually indicates that an animal's brain function is also lateralised, with one side of the brain dominating control of certain tasks.
Studies on relatively small numbers of reptiles have shown that some species display lateralised behaviour.
For example, cottonmouth snakes (Agkistrodon piscivorus) tend to prefer to coil one way more than the other, while upturned Mediterranean tortoises (Testudo hermanni) prefer to right their bodies to one side.
But a team of US-based researchers led by Annette Sieg of Drexel University in Philadelphia, Pennsylvania investigated whether such a similar preference occurs across a large wild population of leatherback turtles (Dermochelys coriacea).
When female leatherbacks come ashore to lay their eggs, they clear out an chamber for the eggs using repeated movements of their rear flippers.
Then as they lay their eggs, they move one rear flipper back over the opening from which the eggs emerge, called the cloaca, obscuring it, perhaps to prevent the eggs being spotted by predators.
The turtles do this spontaneously, and it is the only time when leatherbacks use a single flipper to perform a particular task.
Dr Sieg and her colleagues observed flipper use among 361 females laying at the Playa Grande rookery on the northern Pacific Coast of Costa Rica.
Over five years, they watched as these females laid eggs on 1889 occasions.
Overall, the turtles preferred to use their right hind flippers 54% of the time.
Although the preference is subtle, it is statistically significant, revealing a bias in flipper use at the population level.
The turtles can grow up to 2.7m (8.8ft)
Due to their size, adults have few natural predators
The species is endangered
They are accidentally trapped in tuna and swordfish nets
They can weigh up to a tonne
It also compares with the proportion of people or apes that are right or left-handed.
For example, 54% of people across different cultures who have yet to learn how to write spontaneously prefer to use their right hands.
In chimpanzees, 53.8% prefer to use one limb over the other.
"We call this asymmetry in the use of the leatherback's flipper "flipperedness" because "handedness" is used in primates, "footedness" in birds and "pawedness" in rodents, other mammals and several amphibian species," the researchers write in the journal.
The study is the first to show a limb preference among Testudinata, the group that comprises turtles and tortoises, and is the largest multi-year study of any spontaneous behaviour in a lower vertebrate.
Why the turtles prefer to use one flipper more than the other remains a mystery.
While researchers have recently found differences in brain structure between left- and right-handed primates, with the left and right brain hemispheres having slight structural differences, there is no evidence as yet that the left and right sides of a turtle's brain are different.