Picks from the Past: October 2006

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After the morning flush of predatory activity at Seal Island, white sharks turn to socializing. We have discovered, by observing both from the surface and with underwater cameras, that the social behavior of these sharks is astonishingly complex. During the past five years, we have cataloged twenty distinct social behaviors in white sharks at Seal Island, half of which are new to science. We are just beginning to understand their significance, but many are related to establishing social rank.

Rank appears to be based mainly on size, though squatter’s rights and sex also play a role. Large sharks dominate over smaller ones, established residents over newer arrivals, and females over males. Why such a focus on rank? The main reason is to avoid combat. As many as twenty-eight white sharks gather at Seal Island each day during the winter seal-hunting season, and competition among them for hunting sites and prey is intense. But since white sharks are such powerful, heavily armed predators, physical combat is a risky prospect. Indeed, unrestrained combat is extremely rare. Instead, the white sharks at Seal Island reduce competition by spacing themselves while hunting, and they resolve or avert conflicts through ritual and display.

At Seal Island, white sharks arrive and depart year after year in stable “clans” of two to six individuals. Whether clan members are related is unknown, but they get along peacefully enough. In fact, the social structure of a clan is probably most aptly compared to that of a wolf pack: each member has a clearly established rank, and each clan has an alpha leader. When members of different clans meet, they establish social rank nonviolently through any of a fascinating variety of interactions.

For example, as was the case with Sneaky and Couz, two white sharks often swim side by side, possibly to compare their relative sizes; they may also parade past each other in opposite directions or follow each other in a circle. One shark may direct splashes at another by thrashing its tail, or it may leap out of the water in the other’s presence and crash to the surface. Once rank is established, the subordinate shark acts submissively toward the dominant shark—giving way if they meet, or avoiding a meeting altogether. And rank has its perks, which can include rights to a lower-ranking shark’s kill.

Another form of nonviolent, tension-diffusing behavior often takes place after a shark repeatedly fails to catch bait (typically a tuna head) or a rubber seal decoy: the shark holds its head above the surface while rhythmically opening and closing its jaws. In 1996 Wesley R. Strong, a shark investigator then affiliated with the Cousteau Society in Hampton, Virginia, suggested the behavior might be a socially nonprovocative way to vent frustration—the equivalent of a person punching a wall.