How Do Killer Whales Sleep?

Conscious Breathing

Killer whale in Prince William Sound

Like dolphins, killer whales are voluntary breathers, meaning their breathing is not automatic like it is for humans. Instead, they must consciously rise to the surface and breathe. This makes sleeping for them much different than it is for other animals. Studies on dolphins and beluga whales have shown that they sleep by shutting down only one hemisphere of the brain at a time, thereby allowing them to both rest and have control over their breathing. It is thought that killer whales, being closely related to dolphins, do the same thing.

Resting

Resting behaviors differ depending on whether the killer whale is by itself or part of a pod. If it is alone, it will rest by staying near the surface, with dorsal fin, blowhole and part of the head out of the water. Usually this only lasts for a few minutes at a time, but there have been reports of killer whales resting this way for up to eight hours.
Killer whales in pods sleep differently. The pod will form a tight circle, and their breathing and movements will synchronize. They take several breaths at the surface, and then submerge for a period of time before surfacing again. The time spent submerged increases as the "sleep" becomes deeper, and they may take up to ten breaths near the surface before submerging for as long as ten minutes. Pods resting like this will avoid boats and contact with other animals during this process. It is thought that this kind of group resting may strengthen the animals' bond and cohesiveness as a unit.

Mothers and Calves

Mother and calf

Studies indicate that during a killer whale's first few months of life, neither the calf nor the mother sleeps at all. Unlike other mammals, which sleep more in infancy and gradually become more active, both calf and mother are active non-stop. Scientists believe that this may be due to the fact the calves are not very strong swimmers, and the mothers are protecting them by keeping them moving in the "slipstream" under the mother's body, created by her motion in the water.

Resources

• Killer whales info book from SeaWorld
• Calves' sleeping habits and unihemispheric sleep patterns