Dr Holly Shiels at the university's Faculty of Life Sciences says: "When tunas dive down to cold depths their body temperature stays warm but their heart temperature can fall by 15°C within minutes. The heart is chilled because it receives blood directly from the gills which mirrors water temperature. This clearly imposes stress upon the heart but it keeps beating, despite the temperature change. In most other animals the heart would stop."

The mis-match between oxygen demands of the tunas' warm swimming muscles and the cardiac system that operates at water temperature is a puzzle the team has long been trying to solve.

"Tunas are at a unique place in bony fish evolution" says Professor Barbara Block at Stanford. "Their bodies are almost like ours - endothermic, but their heart is running as all fish at ambient temperatures. How the heart keeps pumping as the fish moves into the colder water is the key to their expanded global range."