DIVING LEATHERBACKS REGULATE BUOYANCY

Leatherback turtles are remarkably versatile divers. Routinely diving to depths of several hundred metres, leatherbacks are occasionally known to plunge as deep as 1250 m. The animals probably plumb the depths to avoid predators, search for prey and avoid heat in the tropics. However it wasn't clear how these mammoth reptiles regulate their buoyancy as they plunge down. Sabrina Fossette from Swansea University explains that no one knew how the turtles descended so far: do they swim down or become negatively buoyant and plummet like a stone? Curious to find out how nesting leatherbacks dive, Rory Wilson and his long time collaborator, Molly Lutcavage, decided to deploy data loggers containing triaxial accelerometers on leatherback females as they nested on beaches on St Croix in the US Virgin Islands (p. 4074).

‘When you first see a leatherback turtle coming out of the water, it's like a dinosaur, it's really impressive,’ says Fossette, having just returned from collecting data in the Indian Ocean. Three members of the team, Andy Myers, Nikolai Liebsch and Steve Garner, attached accelerometers to five females as they laid their eggs, and then waited 8–12 days for the reptiles to return to the beach to lay more eggs. Retrieving the accelerometers, the team found that only two of the five had collected usable data, but the data loggers that functioned showed 81 dives that the team could analyse, ranging from 64 m down to 462 m.

Back in Swansea, Fossette, Wilson and their colleagues Adrian Gleiss and Graeme Hays analysed the temperature, pressure and acceleration data collected by the loggers. Describing the accelerometer data, Fossette says, ‘You can almost see the animal swimming. It's the first time we could see the locomotor activity during those deep dives.’

Extracting the acceleration data that showed the leatherbacks' movements, the team could see that the turtles dived deeply at an average angle of 41 deg as they began their descent. Initially the turtles swam with each flipper stroke lasting 3 s, but as they descended further they swam less hard until they stopped swimming altogether, became negatively buoyant and began gliding down. At the bottom of the dive, the turtles began swimming as they headed to the surface and continued swimming until they regained buoyancy near the surface and began gliding again.

Fossette explains that many diving animals exhale before they leave the surface to minimise the risk of decompression sickness; however, leatherbacks do not. They dive carrying a lungful of air. Curious to find whether leatherbacks vary the amount of air that they inhale to regulate their buoyancy, Fossette and Gleiss compared the depths at which the turtles became negatively buoyant with the maximum depth that they reached. The team found that the deepest divers remained buoyant the longest and started gliding at deeper depths. So the turtles probably regulate their buoyancy before diving by varying the amount of air they inhale. Fossette also says, ‘The nesting turtles may glide for 80% of the dive's descent to optimise their energetic reserves, which is crucial for the production of eggs.’

The team is now keen to look at the diving patterns of leatherbacks in their foraging grounds in the North Atlantic. Fossette explains that nesting turtles lose weight while foraging turtles are gaining weight and this could affect their buoyancy and diving behaviour. However, tagging a 400 kg turtle in the ocean is a much bigger problem than tagging them on a beach.