Maintaining a warm body temperature gives you a head start in life. It allows you to function effectively in cold environments that are unavailable to warm-adapted, cold-blooded (ectothermic) animals and enables you to hunt and forage uninterrupted by inconvenient temperature fluctuations. This is exactly why leatherback turtles are able to take full advantage of the rich foraging grounds off the coast of eastern Canada. James Casey, from the University of North Carolina Wilmington, USA, explains that leatherbacks are able to maintain body temperatures that can be as much as 10°C higher than their surroundings, and so can survive water temperatures that could cold-stun, or even kill, less resilient and smaller turtle species. However, it was not entirely clear how the colossal creatures manage to maintain their elevated body temperatures. Casey says that, in addition to generating heat by muscle activity and other metabolic processes, the turtles may also regulate their body temperature by swimming in warm surface water and basking, but it wasn't clear exactly how much each factor contributed (p. 2331).
Teaming up with Michael James, from Fisheries and Oceans Canada, and fishermen and scientists from the Canadian Sea Turtle Network, Casey headed out into the waters off Nova Scotia in late summer in search of foraging leatherbacks, before the animals began their migration south. Carefully capturing each turtle in a gigantic net before manoeuvring it onto a hydraulic platform, the team then gently lifted the animal out of the water to measure its mass and size. ‘Working safely with such large and powerful animals required extensive teamwork and near-perfect weather conditions’, recalls Casey, who adds that this was probably the first time that any of the males had been out of the water since they took to the sea as hatchlings. Then, Casey gently inserted a tiny temperature logger pill into the throat of each turtle to record the animal's body temperature, and attached a platform transmitter to the turtle's shell to relay the temperature information to a satellite when the animal surfaced.
After months of patiently waiting for the data to be collected by the satellite, Casey, James and their colleague Amanda Williard plotted the animals' body temperature (which ranged from 25.4 to 27.3°C) and behaviour patterns and were impressed to see that the turtles were able to maintain temperatures that were 10–12°C above the sea temperature. They also noticed that the animals' body temperature dropped during the day – as they consumed cold prey while foraging – and rose at night. But when the team analysed the animals' dive pattern, they found this could not account for the nocturnal body temperature increase. They realised, instead, that the animals must be generating heat at night while digesting dinner and that this must account for the increase in nocturnal body temperature. They also suspect that processing and digesting a meal could account for 30–50% of the animal's metabolic rate and that the animals could expend as much as 90% of that energy associated with digestion on simply warming the cold food in their bellies.
And when the team investigated the amount of time that the turtles spent in warm water at the surface of the sea, they noticed that there was a strong correlation with the turtles' body temperature. They suggest that altering the dive pattern to spend more time in warm waters reduces the amount of heat that the turtles lose to their surroundings, helping them to maintain a high body temperature even in relatively cool conditions.