In a new study, Professor Bruce Mate from the Hatfield Marine Science Center in Oregon working with Jorge Urban of the Autonomous University of Baja California Sur, placed data-recording tags on sperm whales feeding on Humboldt squid in the Gulf of California.

The tags use GPS and depth sensors to track the whales' movements in three dimensions for up to 28 days, then detach and float to the surface.

Mate says the data show that three whales they tracked seemed to move around the gulf together and dive together. But one of the three almost always dove deeper. On successive dives, a different whale assumed the deep-diving role. As the huge mammals made their dives - hunting squid at depths reaching more than 1,000m - their behaviour varied with each dive.

Whale groups 'may corral deep squid'
The researchers think the whales may be rounding up squid to be more easily picked off by the hunting party - with the deep diver cutting off any escape route. "we're speculating that the animals are herding a ball of squid."

Group hunting is well known in humpback whales, which enclose prey in a virtual net of bubbles. But individual humpbacks appear to always play the some position in a hunting team, unlike the rotation of tasks apparent in sperm whales.

Since diving to extreme depths to chase squid from below is the most physically demanding task, Mate said it makes sense for a hunting party to rotate that burdensome job.

"It may be that each individual takes it in turns to do the most physiologically demanding task - the deep dive,"