Ethologists tend to wave off attempts to classify animal behavior within the realm of human feelings. But it is hard to avoid talking about the concept of friendship after reading a study of the small blacktip reef shark published in the latest edition of the journal Animal Behaviour.

The paper, which examines the behavior of the shark species also known as carcharhinus melanopterus, was conducted by Johann Mourier and his colleagues at the Island Research Centre and Environmental Observatory (CNRS, Ecole Pratique des Hautes Etudes). The team has discovered nothing less than the enigmatic existence of friendship links being forged in this marine species, which was thought to be devoid of social structure.

If the research is correct, it would mean that these sharks are not only lonely hunters who prowl the reefs randomly: affinities between individual sharks seem to form communities within larger populations that share a territory.

How did the research team come to this counter-intuitive conclusion? "Before conducting this study, I noticed while I was diving in Moorea – an Island of French Polynesia – that I would often see the same sharks swimming together," says Mourier. "I asked myself if it was a coincidence, an effect of my memory, or if this was real."

To confirm or refute Mourier's impression, the authors of the paper conducted nearly 200 dives in seven sites along ten kilometers of the Moorean coastline. During each dive, they photographed sharks staying together -- each identified by the shape of the pattern adorning his black fin -- and carefully noted the associations between individuals.

"We used the research techniques that were originally used to study social networks like Facebook," Mourier says. "The principle idea is that by analyzing the connections between people, we can identify common interest groups, or communities. We used these tools to see if there were actually sharks in these communities of individuals who tend to cluster preferentially."

Hunting buddies

The researchers found that the population of blacktip sharks in this region is fragmented into four observable communities; the largest of these groups is itself divided into two subgroups. The sharks do not wander near the reefs at random, guided only by their search for prey. They associate more favorably with some peers while avoiding others.

So how are these social links forged? In order to find out, the authors looked for correlations between the sharks' affinities and their sex or size (that is to say, age). Still, these parameters were not enough to explain the social phenomenon uncovered in this case: the groups are all mixed.

"However," Mourier says, "within each group, the strength of the link between individuals appears to be related to these similarities." The team is currently conducting additional research based on DNA sampling in order to determine whether the groups are determined by family relationships. Depending on their findings, it may in fact be more appropriate to refer to the structures as tribalism rather than friendship.

The usefulness of these social groups among the blackfin shark population is not obvious. The authors suggest that the social ties may exist to reduce aggression between individuals, to protect the group against larger predators, or to allow certain forms of cooperation in the hunt.

Work on the sociability of sharks is rare, and this work could provide valuable insight into the communities of these animals. However, research conducted in aquariums has already shown that female sharks tend to form more cohesive groups when males are introduced into the same basin. Also juveniles from the same litter of lemon sharks (Negaprion brevirostris) tend to stay together during the first years of their lives after they are abandoned by their mothers.

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