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Grey reef sharks hang out with the same 鈥渇riends鈥� in the same spot for years, a four-year study at the remote Palmyra Atoll in the Pacific Ocean has revealed.

鈥淲e don鈥檛 think of sharks as social animals, but they do have social groups,鈥� says Yannis Papastamatiou at Florida International University in Miami.

Grey reef sharks (Carcharhinus amblyrhynchos) are most active at night. During the day, they return to a particular spot on the reef, forming groups of 20 or so. They do catch prey during the day, but feed less than at night.

To study the shark鈥檚 social behaviour, Papastamatiou and his colleagues tagged around 40 individuals with acoustic transmitters that each emit a unique high-frequency sound. The batteries on the transmitters last for four years. A network of 65 receivers recorded the identity of any tagged shark that came within 300 metres or so of any one receiver.

The recordings reveal that the social groups of grey reef sharks are remarkably stable, with the same individuals associating together year after year and movements between groups being rare.

鈥淭hey purposely associate with the same individuals,鈥� says Papastamatiou. That suggests they can recognise other sharks individually, though how they do this isn鈥檛 clear.

It is also unclear whether the same individuals hunt together when they leave the home area at night. While grey reef sharks were thought to hunt mostly on reefs, the researchers found that those at Palmyra catch most of their prey in open waters at night, too far from the reef to be detected by the receiver network.

Papastamatiou describes the individuals that any one sharks hangs out with as associates. 鈥淭hey are not friends in the sense of having any emotional bond with each other,鈥� he says.

The team hasn鈥檛 seen any sign of one shark deliberately assisting others, but the sharks may increase the odds of catching prey by hunting together. The shark that first spotted the prey might lose out initially, but the benefits should even out over the group with time.

Grey reef sharks are unusual among sharks in being social animals. Other species, such as the great hammerhead, spend most of their time alone.