When one of the largest animals in the ocean takes a nap, it can look pretty weird.
Popular photos have surfaced online showing sperm whales gathered together, seemingly motionless and arranged vertically in the water. The whales, which are roughly the size of school buses, almost always appear to be "standing" and clustered in pods of five or six.
French photographer and filmmaker Stephane Granzotto captured this behaviour while diving in the Mediterranean, where he was documenting sperm whales for his photo book on the creatures titled Cachalots. In the photo above, submitted to National Geographic's YourShot photography community, he noted that the whales had been napping for an hour.
A study published in 2008 in the journal Current Biology was the first to conclusively document the whale's vertical sleeping position.
Sleep had previously been observed in some captive cetaceans by monitoring eye movements, but how whales in the wild rested was significantly less understood. Using data-collecting tags suction cupped to 59 sperm whales, researchers from the University of St. Andrews and the University of Tokyo measured the animals' periods of inactivity.
The whales were found to spend seven percent of their day in these vertical sleeping positions near the surface of the water, where they napped from 10 to 15 minutes. Researchers suggested at the time that they might be one of the world's least sleep-dependent animals.
Whales in captivity have been found to use only half their brain while sleeping, a behaviour scientists think could help them avoid predators, maintain social contact, control breathing, or continue swimming.
The study also noted observations from a video shot in northern Chile that showed whales did not wake from their surface naps until a ship approaching with its engines off unintentionally bumped into them. This suggests whales in the wild might enter a full sleep, unlike their captive counterparts.
Header Image: In the Indian Ocean, a group of more than 30 adult sperm whales are sleeping at a depth of 15 meters. They stand like this without moving for minutes or hours. All are females, and all the calfs are at the surface while parents are sleeping. PHOTOGRAPH BY STEPHANE GRANZOTTO