Sporting a deep-diver's physiology when you spend most of your time paddling near the surface seems a little extravagant. But leatherback turtles are equipped with myoglobin-rich blood, which provides the oxygen storage capacity required to allow them to dive to impressive depths, sometimes exceeding 1 km. Near their breeding sites in the Caribbean the turtles perform deep dives very occasionally, which has puzzled researchers for decades; why bother going to such great depths when all of the turtles' needs can be met nearer the surface? So when satellite relay data loggers became available to record dives, Jonathan Houghton, Graeme Hays and colleagues at the University of Swansea, UK, decided to study the turtles' diving behaviour during the long voyages from their breeding grounds to discover the reason for these rare extreme dives (p. 2566).
The team equipped 13 turtles with the data loggers before letting the animals roam free. `We tie them on after the turtles finish laying eggs, when they flick sand around to cover the nest. They stay completely still during handling,' Houghton says. The device, which records location, temperature,dive depth and duration, collects data as soon as it is submerged in saltwater and transmits the information to satellites when the turtle returns to the surface. Using this system, Houghton recorded over 26,000 dives, spanning the entire North Atlantic Ocean. He discovered that extraordinarily deep dives were always rare. Only 95 of the dives, 0.4%, went to depths beyond 300 m.
Why might turtles dive deep? According to one idea, they do it to escape predators. But the data loggers revealed that the reptile's diving speed remained normal during deep dives, suggesting that they were in no hurry to escape. Moreover, they spent several hours at the surface both before a deep dive – probably to slow their metabolism for increased oxygen efficiency– and afterwards, presumably to repay the oxygen debt created by anaerobic conditions during the dive. `Hanging out at the surface would be a daft strategy for avoiding predators, because that's where they can spot your silhouette,' says Houghton.
A second hypothesis speculates that deep dives help turtles cool down. But water temperatures only decrease marginally beyond 350 m, which fails to explain why turtles would bother diving deeper.
This left Houghton with a third hypothesis; that turtles dive deep searching for food. Leatherbacks like to eat surface-dwelling jellyfish, which are common only in northern waters. However, during the months spent travelling from their tropical breeding grounds the turtles rely on other jellyfish-like animals that form long colonies and spend their days at depths around 600 m, only coming to the surface at night. Houghton's data show that most deep dives occurred around midday during this transit period, often just before a turtle settled for a few days or weeks in the same area. From this he suspects that the turtles dive deep to locate prey during daylight hours,harvesting the animals later when they come to the surface at night. If the turtles have identified a particularly rich site, they may stay a while after a deep dive to replenish their energy reserves before moving on.
So leatherback turtles probably use deep dives to find their next feeding station when travelling where jellyfish swarms are sparse.