According to study leader Dr Yuuki Watanabe of the National Institute of Polar Research, Japan, "The physiological mechanism of keeping heat in the body is well understood. But, more a fundamental question is, why this unique evolution occurred in the first place. In other words, what kind of advantages does the fish gain from being warm-bodied?"

The team collected and analyzed data from previous studies as well as accumulating their own data. Speed sensors were attached to sharks in Alaska, the Bahamas and the central Pacific, to compare swim speeds of warm and cold-bodied fishes.

Warm or cold

"Fishes are generally considered cold-bodied in that their body temperatures are very similar to that of the water they reside in," said Dr Yannis Papastamatiou of the University of the St Andrews Scottish Oceans Institute, one of the researchers who made the discovery.

"However, amazingly a small number of tunas and sharks, including white sharks, have evolved the ability to maintain their body temperatures higher than the surrounding water - sometimes up to 20°C warmer. We found that on average, warm-bodied fishes can swim almost 2.7 times as fast as cold-blooded species, likely because the warmer muscle temperatures enhance power output.”

Evidence was also discovered that increased swim speeds allow some warm-bodied fish to migrate large distances in a relatively short period of time. "As such these animals can swim to distant locations and back again which may allow them to take advantage of seasonal pulses in food or other resources," added Papastamatiou. The results of the study were published in the Proceedings of the National Academy of Sciences.

What kind of advantages do the fish gain from being warm bodied?