Sandra Hochscheid is fascinated by loggerhead turtles. ‘I was always interested in what they do under the surface,’ says Hochscheid from the Stazione Zoologica Anton Dohrn, Italy. But when Hochscheid began hearing reports that the champion divers had been spotted on the surface on calm days, she was intrigued. Having spent most of the last decade tracking loggerhead diving behaviour with satellite technology, Hochscheid had never noticed the turtles taking extended breaks at the surface. ‘They only need a few minutes to recharge their oxygen supplies,’ explains Hochscheid, so why were the reptiles taking such long surface breaks? Curious to find out whether the anecdotes were true and, if so, why, Hochscheid and her Italian colleague Flegra Bentivegna, Abdulmaula Hamza from the Libyan Environmental General Authority and Graeme Hays from Swansea University, UK, decided to track loggerhead's diving behaviour to see if they could find any evidence of the surfacing behaviour (p. ).
First the team had to find turtles that could be tagged with a sophisticated data logger that would send information back when the animals surfaced. Closely involved with a rehabilitation programme that returns injured loggerheads to the Mediterranean, the team had access to eight juvenile turtles from the Stazione Zoologica's Rescue Centre, as well as two turtles that had been caught in a net off the coast of Libya. Attaching the tracking devices to the tops of the animals' shells, the team released them near the sites where they had been rescued, and waited for the data to start coming in.
Over a period of more than a year, the dataloggers contacted their satellites, which calculated the turtles' locations on the surface, and then transmitted: how deep and long they had dived; the temperature of the water surrounding them; and the length of time they spent at the surface. ‘It was a really huge data set,’ remembers Hochscheid, but as she was analysing the data she realised that the anecdotes were true. The turtles did occasionally spend extended periods at the surface, ranging from tens of minutes up to a marathon 17 h on one occasion, with more than 80% of the visits occurring during daylight.
Looking closer to see if the behaviour was related to the turtle's diving activity, Hochscheid noticed that the majority of stays at the surface occurred around noon, when the sun was at its highest, and that some of the turtles were diving much deeper than had been expected. ‘A few of the larger turtles were diving off the continental shelf and were going into very deep waters experiencing a 10°C temperature difference between surface and depth,’ says Hochscheid, who suspects that these animals remain at the surface to warm up after their cold deep dive and also to top up their vitamin D levels. But this couldn't explain the night time surfacing events.
Scrutinising the duration of the dives that preceded the long nocturnal surface breaks, Hochscheid realised that some of the turtles were remaining submerged well after their oxygen stores must have run out. Knowing that turtles can extend the length of a dive by switching to anaerobic metabolism after exhausting their oxygen supply, Hochscheid suspects that the turtles may be clearing lactic acid – which accumulates during anaerobic metabolism – from their muscles during their long stopovers.
So loggerhead turtles seem to extend surface breaks for at least two reasons: to soak up the sun and get warm by day and to recover from anaerobic exercise at night.
