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Many of us think of sharks as lone hunters. We imagine them spending most of their lives swimming alone, briefly seeking out the company of another shark for the purposes of reproduction, then going back to their solo ways. We also tend to think of sharks as dumb machines, capable of little more than hunting. We certainly do not think "social network" when we think of sharks. But shark scientists have known for some time that a wide variety of shark species, ranging from plankton eaters to reef dwellers to large predators, often aggregate into groups. Some scientists have suggested that sharks get together for reproduction, for cooperative hunting, or for protection from sexual harassment, and there is data supporting each of these hypotheses.
Until recently, we did not know much about the groups of sharks that formed, beyond the fact that they formed in the first place. This seemed like an important question to French shark scientists Johann Mourier, Julie Vercelloni, and Serge Planes. They realized that nobody before them had determined whether the aggregations of sharks that formed could be explained by social factors, or only by the environmental context in which they occurred. They decided to begin studying this question with the blacktip reef shark, Carcharhinus melanopterus, which is a species common among Indo-Pacific coral reefs. Blacktip reef sharks are not particularly solitary, but they don't school either. The researchers conducted their observations of these sharks at seven sites along 10km of the north shore of Moorea Island, which is part of French Polynesia.
If sharks tend to have overlapping home ranges, and food tends to occur in a particular place, the same sharks could routinely encounter each other by accident. Or are shark aggregations actually social, formed by social preferences? If sharks are more (or less) likely to be found together than would be expected by things like territory boundaries and food locations, then perhaps social factors are at work.
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They found that the sharks of northern Moorea Island - 133 individuals in all - organized themselves into four communities, with one really being two overlapping subcommunities. The co-occurrence of individual sharks was found to be non-random, and tended to persist over time. That is, the same sharks were sighted with the same companions several times, often engaging in actual interaction. In addition, the communities were fairly stable - individuals tended to stick to their communities.
And the communities were social in nature. That is, they weren't just occupying the same space at the same time. For example, the researchers found that some pairs of sharks were seen together more often than you would expect based solely on the overlap of their home regions. They also found that some pairs of sharks actually avoided each other, despite a high degree of territory overlap. On the other hand, the communities themselves were mostly explained by non-social factors such as sex, age, and territory. Taken together, the data suggest that communities are created by environmental factors, but within-community interaction is more genuinely social.
The types of social interaction occurring among the blacktip reef sharks in this study were actually similar to those previously observed in social fish species like guppies, and in marine mammals like dolphins or sea lions. The researchers write, "the grouping patterns displayed by this shark species indicate that the structure of this population does not reflect passive aggregations at specific resources but rather developed from an active choice of individuals similar in some ways to some other social animals." In other words, sharks have friends.
Related:
What Can Dolphins Teach Us About The Evolution Of Friendship?
On Ya Like Dags?: The Social Shark Network
Mourier, J., Vercelloni, J., & Planes, S. (2011). Evidence of social communities in a spatially structured network of a free-ranging shark species Animal Behaviour DOI: 10.1016/j.anbehav.2011.11.008
