BioEd Online

Bottlenose dolphins are clever, sociable beasts that feed in packs. But a study carried out off the coast of Florida has revealed another layer of complexity in their hunting: group members have specialized jobs that they stick to time and time again.

Cooperative hunting is fairly widespread among animals and is found, for example, in chimpanzees, colobus monkeys and Harris' hawks. But the phenomenon of specific jobs for individuals, like the different positions in a football team, is much rarer, say Stefanie Gazda and her colleagues, who studied the dolphins off Florida's remote Cedar Key.

Gazda and her colleagues watched two groups of bottlenose dolphins (Tursiops truncatus), one always consisting of three individuals and the other ranging from two to six members. During group hunts, one dolphin always took the role of 'driver', harrying shoals of small fish towards a waiting cordon of 'barrier' dolphins, then herding them up to the surface.

The researchers identified individual dolphins by examining their fin markings, and observed at least 60 group hunts for each pack. Both groups had a particular individual who took the driver role in every single group hunt, the researchers report in Proceedings of the Royal Society of London B¹.

Special strategy

Such 'role specialization' has been seen only once before, in African lionesses, the researchers add. In that case, lionesses try to outflank prey and herd them towards a lioness waiting in the centre of a hunting ground.

It's not yet clear whether specialized hunting is common to all bottlenose dolphins, or whether it is particular to certain groups or areas, says Gazda. "It's exciting because it's never been seen before [in marine mammals]," she says, adding that few researchers have visited Cedar Key because of its isolated location and marshy surroundings.

But witnessing the hunts is worth the effort, enthuses Gazda, who carried out the work while at the University of Massachusetts Dartmouth, and now teaches at the New England Aquarium in Boston. "It's really cool, the dolphins with their heads out of the water, splashing, and the fish flying through the air," she says.

Barrier method

The question remains of what's in it for the 'barrier' dolphins, because in the group of three individuals, the driver consistently got more food than the others. During solo hunts, the researchers saw non-driver dolphins using driver-like attacking manoeuvres. "Maybe they all possess the ability but one is the leader or is better at it," Gazda speculates.

Another possibility is that groups of dolphins are related, making them more likely to cooperate, Gazda suggests. "There are lots more questions than answers," she says. "When they're not feeding do they still socialize together? And what happens if the driver dolphin dies?"

Gazda hopes that the sea will throw up more examples of animals that take special roles in their group hunts. This seems likely, given the relatively wide range of intelligent marine mammals, and the possibilities offered by the open sea for rounding up prey.

And bottlenose dolphins may surprise us further with their inventiveness. They have even been known to help human fishermen while hunting: they can use fishing nets in a similar way to their own 'barriers' to improve their haul.