By Victoria Gill
Science and nature reporter, BBC News
Leatherbacks use their lungs as buoyancy aids
Leatherback turtles, the ocean's deepest-diving reptiles, control their buoyancy simply by breathing in, scientists have found.
Researchers describe in the Journal of Experimental Biology how the turtles regulate their dives through the volume of air in their lungs.
This allows them to glide and forage for food at a variety of depths.
Scientists believed that the animals would exhale before diving, to avoid gas bubbles forming in their bodies.
US and UK scientists monitored the turtles' diving by attaching small data loggers to the animals' backs.
Sabrina Fossette from the University of Swansea in Wales led the study.
She and her colleagues attached the data loggers to five female leatherbacks in a wildlife refuge in the Caribbean.
"In addition to the depth, we could measure their acceleration," Dr Fossette told BBC News. "So we were able to model each dive in 3-D.
The researchers saw that the leatherbacks started their dives by actively swimming as they descended.
"Then, at some point during the dive they started gliding," said Dr Fossette.
"The turtles started gliding at deeper depths during deeper dives, suggesting they regulate the amount of air they inhale before diving."
This enables them to use their lungs as buoyancy aids to precisely counteract their weight.
The ability to glide at a variety of depths allows leatherbacks to conserve energy; it also means they are more flexible in terms of where in the ocean they can feed.
"Leatherbacks forage on gelatinous plankton," said Dr Fossette, "which can be found either at the surface or really deep in the ocean."
Dr Fossette and her team attached the loggers to five female turtles
Many other diving animals, including hard-shelled turtles and penguins also inhale before they dive, but the researchers we were surprised to see the same behaviour in leatherbacks because the creatures are such deep divers.
Scientists have recorded the animals reaching depths of up to 1,000m - the deepest leatherback dive ever recorded was more than 1,200m.
"Stick-on" data loggers allowed the scientists to monitor the turtles' dives
So the researchers expected that the animals would exhale before a dive, in order to avoid decompression sickness.
Otherwise known as the bends, this can occur when dissolved gases in an animal's (or human's) blood form bubbles inside their bodies.
"Many deep divers - notably deep-diving mammals - exhale before diving to minimise the effects of decompression," said Dr Fossette.
"Leatherback turtles share many physiological and physical features with deep-diving mammals and therefore we would expect them to exhale."
It seems that the turtles' body temperature increases the solubility of the gases and therefore decreases the risks of bubbles forming.
This tagging experiment was designed by Rory Wilson, also from the University of Swansea, and Molly Lutcavage from the University of New Hampshire, US.