Most so-called ‘cold-blooded’ animals (ectotherms) are anything but. Deriving heat from the surroundings and their own muscles, ectothermic animals are usually quite warm to the touch. However, retaining heat in water can be more of a challenge. ‘Since fish live in water with high thermal conductivity, it has been assumed that they lose most of their metabolic heat through their gills’, says Itsumi Nakamura from Nagasaki University, Japan. And body size can also affect how quickly fish lose warmth to their surroundings, with tiny species (<10 g) losing heat much more rapidly than medium sized creatures (0.1–5 kg). But Nakamura and Katsufumi Sato from The University of Tokyo, Japan, were curious to find out how much heat the largest fish of all, whale sharks (Rhincodon typus), lose while plumbing the depths. ‘They can dive to depths exceeding 1000 m’, says Nakamura, where the temperature can be 20°C lower than at the surface (∼27°C). So, when Nakamura and Sato heard from Rui Matsumoto of Okinawa Churaumi Aquarium, Japan, that the facility planned to release two giant whale sharks that had been in captivity for almost 20 years, their chance had come.
However, before the huge animals (∼1600 kg and ∼1500 kg) could be released, the scientists needed to attach a data logging tag that would record the animals’ movements, seawater temperature and the temperature of the muscle behind the head. Manoeuvring each shark into a massive container filled with seawater, the team anaesthetised the animals before gently securing the tags; ‘it took about 5–10 minutes to attach the device’, says Nakamura. Then the team carefully towed the container to a release site in the East China Sea. ‘I prayed that they would return safely to the wild’, says Nakamura, who admits that he was relieved to see that the first shark had been swimming vigorously when its tag bobbed safely to the surface 7 days later.
Back in the lab after retrieving two additional tags, one from a smaller wild whale shark caught in a net off Okinawa and the other from the second captive shark, the team began to unpick how the massive animals’ body temperatures fluctuated as they swept through the water. Despite descending regularly to depths of ∼100 m, where the water was 20°C, the whale shark body temperatures barely varied from 27°C. However, the first shark had plunged to 390 m immediately after release, remaining submerged for 12 h. Yet, even at this depth – where the water was 14°C – the animal's muscle temperature only declined slowly, eventually falling to 19°C. On other occasions, the sharks plunged even further, with the first shark setting a record descent to 1427 m at a water temperature of 3.4°C.
But how did the whale shark's heat loss rates compare with those of other fish? It turns out that whale sharks don't top up their temperature with warmth generated by their muscles and they lose heat far more slowly than any other species. Plotting the heat losses of fish ranging in size from 0.3 g bluefin tuna youngsters up to the colossal whale sharks, the team found that the whale sharks lost heat at ∼0.0022°C min−1 in contrast to much smaller species that lost heat rapidly at ∼1°C min−1. Nakamura says, ‘The large body size of whale sharks aids in preventing a decrease in body temperature during deep excursions’, explaining why the massive animals can dive safely to depths that smaller species never dare reach.