> To understand how sharks were coping with the temperature changes, Royer and his colleagues developed a device consisting of instruments that measured depth,

> To understand how sharks were coping with the temperature changes, Royer and his colleagues developed a device consisting of instruments that measured depth, water temperature, location and movement, as well as a probe embedded into muscles near the dorsal fin that recorded the shark’s core temperature. > In a paper published in Science1, the team reported that the sharks would dive several times — six in an evening, for one shark — into deep water at temperatures of 5–11 °C, around 20 °C colder than at the surface, and remain there for 5–7 minutes at a time before surfacing. > Body temperature remained constant for most of the dive until the final stage of their ascent back to warmer waters, when it would decline rapidly. > Royer suggests that the sharks are keeping their core temperature stable by simply not opening their gills or mouth during the dive; effectively ‘holding their breath’. “If you don’t have water going over your gills, then you won’t be dumping your body heat into the environment,” he says. So they only measured the shark's temperature, the water temperature, and location (in 3 dimensions). Hammerhead sharks holding their breath is a hypothesis that aligns with the rapid heat loss as they ascend and reenter medium-temperature waters (and start breathing again presumably). It seems a bit early to say this is the exact mechanism, but I am no marine biologist. edit: ah, they have more evidence, including videos of sharks closing their gills https://www.science.org/doi/10.1126/science.add4445#sec-4 https://www.science.org/doi/10.1126/science.add4445#sec-4