Dolphins swim in circles as they snooze.© GettyDolphins, when dozing, swim in lazy circles. A new paper uncovers the odd fact that dolphins in the Northern Hemisphere swim in anticlockwise circles, whereas dolphins in the Southern Hemisphere swim in clockwise circles.
The marine mammals only sleep with one half of their brains at a time, and continuously swim as they snooze. Wild dolphins, not just captive ones, have been seen to swim in circles when they sleep, and dolphins of various species had previously been shown to move preferentially anticlockwise.
That prompted speculation as to why they choose this direction, with explanations relating mainly to the animals' anatomy, perhaps an asymmetry in their brain.
But when Paul Manger, a dolphin neuroethologist from Sweden, moved to the University of Witwatersrand in South Africa, he realized that all reports of dolphins swimming anticlockwise round their pools came from the Northern Hemisphere. On a hunch, he went down to the local dolphinarium with a video camera and a Thermos of hot coffee. Four nights of watching the animals produced the surprising result that these Southern dolphins spent 86% of their time swimming clockwise.
Now that it seems clear that the chosen direction depends on hemisphere, explanations relying on anatomy must be thrown out, says Manger. The behaviour must be due to "global forces", he believes. He reports the results with colleague Guinevere Stafne in a forthcoming issue of Physiology & Behavior1.
Circular arguments
The finding is compelling and quirky, but some specialists have scoffed at it. Andrew Read, who studies dolphins at Duke University in Beaufort, North Carolina, says he will remain unconvinced until dolphins are observed after being moved from one hemisphere to another.
Dawn Goley, an expert on dolphin sleep from Humboldt State University in Arcata, California, is also sceptical. "This is going to be related to the size of the group, the natural history of the individual species and the shape of the tank. I just don't think you can make a blanket statement like that," she says.
If the dolphins are in fact circling in different directions, what kind of "global forces" could cause this? One possibility is the Coriolis force - an effect of Earth's rotation that produces large-scale currents in the ocean and atmosphere. This is the force that is said to be responsible for water spinning down a drain in different directions in the Northern and Southern Hemispheres - although physicists disagree over whether seeing the Coriolis force on this tiny scale is possible, even in a hypothetically flawless sink. Do the dolphins somehow sense this force and choose to swim against the prevailing currents? Manger thinks that the scale of a dolphin's slumberous circuit is also too small to be related to the ocean-sized currents.
But David McIntyre, a Coriolis expert at Oregon State University in Corvallis, likes the idea. "If on a large scale, ocean currents are doing a certain thing, I imagine the dolphins may be able to pick up on that," he says.
Another possibility is that the animals all swim in the same direction to stay together during the relatively vulnerable hours when they are half asleep. "When dolphins are awake they use their signature whistle to keep together," says Manger. "When they are sleeping, they don't want to be vocalizing, because they don't want to attract attention."
If they have all learned - or been genetically programmed - to swim in the same direction, they could stay together silently. But Manger doesn't know why the choice of direction would vary by hemisphere. "That's beyond me," he says.
</p><p>Manger's observations also show that dolphins seem to start and stop swimming, or change direction, about once every 40 seconds, which is the same duration as their breathing rate. He hypothesizes that this less-than-a-minute stretch between breaths may be the "attention span" of a half-asleep dolphin.
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