Swimming in deep cold waters hammerhead sharks they have an unusual way of keeping warm: they close their gills and hold their breath, a behavior first recorded in fish.

“It was a complete surprise,” says Mark Roger, a research fellow in the shark department at the Hawaiian Institute of Marine Biology. “It was absolutely amazing that the sharks held their breath and hunted like diving mammals. This is incredible behavior from an incredible animal,” he adds.

Hammers are ectothermic or cold-blooded creatures, meaning that their body temperature is determined by their environment.

Their gills, which allow them to breathe by collecting oxygen dissolved in water, are important points of loss of body heat, explains Roger for Nature News. If their body temperature drops too low, they become lethargic and cannot swim. And if they stop moving, water won’t be able to circulate through their gills, which can lead to suffocation.

Despite this, hammerheads manage to dive hundreds of meters into the oceans in search of prey, however, without using already known methods increase in body temperature of other fish.

For example, notes Nature News, tuna, swordfish and mako shark have a temperature switching system in the gills – in bluefin tuna, with the help of this system, the body temperature rises to 3.5 degrees Celsius.

To determine their strategy, Roger and his colleagues attached special sensors to the backs of fish in the shallow bay of Oahu. These sensors recorded each shark’s muscle temperature and orientation, the temperature of the surrounding water, and the depth at which it swam.

Hammerhead sharks swam quickly and repeatedly from shallows (where the water temperature is about 26 degrees Celsius) to 800 meters and in cold water of 5 degrees. They stayed at this depth to eat for only four minutes before returning to shallow water – and on a near-vertical return path. However, their body temperature remained stable for approximately 17 minutes after their return, after which they slowed down their movements to allow the heat to subside.

“We could see that these sharks were doing something very interesting and very different from any other fish. It’s obvious how they are not based on simple thermal inertia to maintain body temperature at these depths. There is something much more complex,” says Roger.

Computer simulations have shown that they must close their gills to prevent cold water from passing through them, which would lower their body temperature, the researchers said in a statement.

As Julia Spath, an environmental scientist at the University of Cambridge, explains: although it’s quite possible these sharks save energy by sealing their gills, more evidence needed using cameras or other methods to confirm such behavior.

Source: Smithsonian Journal.