Sleeping like a leaf

Elephant seals are the largest living sea predators that can weigh up to 3000 kg (males). Elephant seals can take very deep dives in search of food, and the animals can also stay under water for a very long time without breathing.
But how does an elephant seal sleep? The elephant seals swim for two to eight months in a row, without coming to land. Elephant seals have to keep swimming, as they do not float automatically.
Dolphin let half of their brains sleep in turns. Elephant seals have found a different solution, Japanese and American biologists discovered.

Six young northern elephant seals (Mirounga angustirostris) were tagged with depth meters, thermometers, movement sensors and transmitters. The elephant seals swam near the Californian coast. The researchers discovered that the elephant seal sleeps while diving. During a ‘sleep dive’ the sea elephant sinks down, wobbling like a falling leaf.

A sleep dive starts like a regular dive. The animal swims down fast. At a depth of at least 135 metres the seal turns on its back and stops swimming. Every ten seconds its whole body is moved up and down, left and right. This slows down the descent and avoids diving too deep.

The biologists think that the elephant seals sleep. When the sea is not too deep, the elephant seals end up on the sea bottom and stay there lying still for a few minutes. The researchers believe they already fall asleep at 135 metres and then turn automatically because most of the fat is on their bellies. The depth at which the sleep dives start (more than 135 metres) probably has to do with safety: there are hardly any orcas and sharks at this depth.

The results of this study were published in the scientific journal Biology Letters.