If there is one thing more terrifying than a white shark (popularly known as a great white) it would have to be a collection of these apex predators. Fortunately for ocean swimmers, white sharks are usually the ultimate loners, roaming the seas in splendid isolation. Yet when they do come together, Carcharodon carcharias spend more time with certain individuals than others, something which in mammals we might regard as indicating friendship networks, although the biologists who observed this pattern doubt that is what is going on here.
Every year South Australia's Neptune Islands abound with seals and white sharks gather to the feast, hosting possibly the greatest concentration of the species in the world. Even when seal numbers drop, there are still enough of them to maintain a substantial shark presence.
Dr Charlie Huveneers of Flinders University helped photograph 282 of these sharks, noting their proximity to each other, over a period of 4.5 years. In Behavioral Ecology and Sociobiology, Huveneers and co-authors report certain sharks were often seen at the same location on the same day as each other, even years apart.
“Rather than just being around randomly, the sharks formed four distinct communities, which showed that some sharks were more likely to use the site simultaneously than expected by chance,” said senior author Dr Stephan Leu of Macquarie University.
As true apex predators, large white sharks have nothing to fear but humans and each other, but this week's photographs of the unfortunate “Vimy” demonstrate there are good reasons for them to avoid larger members of their own species.
Huveneers ruled out some obvious explanations to IFLScience. “Sharks in general and white sharks in particular are not known to play or form bonds in the standard meaning of the term,” he said. Unlike orcas or dolphins, there is no evidence of them herding prey together.
The sample Huveneers investigated was almost two-thirds male, which he told IFLScience reflects the population in these waters for most of the year, but in late fall or early winter, females can outnumber the males.
Lest anyone is thinking of the shark support group from Finding Nemo, Huveneers makes clear the associations are very loose, and may not involve a preference for their fellows at all. Instead, if particular environmental conditions attract certain individuals, while others are drawn elsewhere, apparent associations will emerge. However, we don’t know what these preferences might be.
Although the findings were particularly surprising for white sharks, Huveneers told IFLScience: “We are increasingly seeing such non-random associations in a number of species including lemon sharks and small-spotted catsharks.” Earlier this year persistent social relationships were identified in manta rays, who like sharks are cartilaginous fish.