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Featured Story October Sociable Killers New studies of the white shark aka great white show that its social life and hunting strategies are surprisingly complex. Aidan Martin and Anne Martin. Photo by Neil Hammerschlag. Our research vessel drifts off Seal Island, South Africa. A lone Cape fur seal pup porpoises through the gently rolling swells toward the island. Suddenly, a ton of white shark launches from the water like a Polaris missile, the little seal clamped between its teeth.
Framed against purple clouds washed with the orange light of breaking dawn, the shark clears the surface by an astonishing six feet. It hangs, silhouetted in the chill air for what seems an impossibly long time before it falls back into the sea, splashing thunderous spray beneath a gathering mob of seabirds. We and our crew of five student volunteers watch breathlessly as the drama unfolds. Now mortally wounded and lying on its side at the surface, the seal raises its head and weakly wags its left foreflipper.
The shark, an eleven-and-a-half-foot male we call Sneaky, circles back unhurriedly and seizes the hapless pup again. He carries it underwater, shaking his head violently from side to side, an action that maximizes the cutting efficiency of his saw-edged teeth. An ominous blush stains the water and the oily, coppery smell of the wounded seal prickles our nostrils. The seal carcass floats to the surface while gulls and other seabirds compete vigorously for its entrails, squawking avian obscenities at one another.
The sharks rush their quarry from directly below; the power of the attack often hurtles both predator and prey out of the water. Photo by Neil Hammerschlag For white sharks also known as great whites , socializing trumps dining. Sneaky turns his attention to Couz. Is he friend or foe? Of higher or lower rank? For half a minute, Sneaky and Couz swim side by side, warily sizing each other up as white sharks do when they meet.
All of a sudden, Sneaky hunches his back and lowers his pectoral fins in response to the threat posed by the larger shark, whereupon he and Couz veer apart. Then calm returns to the sea. Just six minutes have passed since the seal pup was innocently making its way to shore. Its migration routes and favorite hunting grounds, aside from the waters around Seal Island and several other places, remain largely unknown. And even less is known about its social behavior.
But after observing the white sharks at Seal Island for eight seasons, and documenting more than 2, predatory attacks, we have arrived at quite a different opinion. Our research demonstrates that white sharks are intelligent, curious, oddly skittish creatures, whose social interactions and foraging behavior are more complex and sophisticated than anyone had imagined. Nowhere is that social and foraging behavior on more vivid display than at Seal Island, a rocky, five-acre islet in False Bay, twenty-two miles south of Cape Town.
The island is home to some 64, Cape fur seals, plus thousands of cormorants, gulls, penguins, and other seabirds. Mother fur seals give birth here in the spring, around the end of December.
By early May the pups are joining their older siblings on fishing trips into False Bay and beyond. Both behaviors can be observed elsewhere, but far less frequently than at Seal Island. Since then we have returned each southern winter to continue studying the remarkable behavior of the white shark. In the popular imagination, Carcharodon carcharias is the quintessential predator. The largest of all predatory sharks, it reaches a length of more than twenty feet and a weight of 4, pounds.
The white shark possesses acute color vision, the largest scent-detecting organs of any shark, and sensitive electroreceptors that give it access to environmental cues beyond human experience. As for habitat, the white shark prefers cool and temperate seas worldwide.
Its brain, swimming muscles, and gut maintain a temperature as much as twenty-five Fahrenheit degrees warmer than the water. That enables white sharks to exploit cold, prey-rich waters, but it also exacts a price: Pound for pound, fat has more than twice as many calories as protein. By one estimate, a fifteen-foot white shark that consumes sixty-five pounds of whale blubber can go a month and a half without feeding again. A decoy draws an early morning attack.
Photo by Neil Hammerschlag In fact, a white shark can store as much as 10 percent of its body mass in a lobe of its stomach, enabling it to gorge when the opportunity arises such as when it encounters a whale carcass and live off its hoard for extended periods. Usually, though, white sharks eat more moderately. How does a white shark decide what to eat? A model known as optimal foraging theory offers a mathematical explanation of how predators weigh the calorie content of food against the energetic cost of searching for it and handling it.
According to the theory, predators employ one of two basic strategies: Energy maximizers selectively eat only high-calorie prey.
Their search costs are high, but so is the energy payoff per meal. Numbers maximizers, by contrast, eat whatever kind of prey is most abundant, regardless of its energy content, thereby keeping per-meal search costs low. Based on optimal foraging theory, A.
Peter Klimley, a marine biologist at the University of California, Davis, has proposed an intriguing theory about the feeding behavior of the white shark. That neatly explains why they often feed on seals and sea lions but rarely on penguins and sea otters, which are notably less fatty. As we mentioned earlier, however, white sharks eat many other kinds of prey. Although those prey may be low-cal, compared with sea mammals, they may also be easier to find and catch, and thus sometimes energetically more attractive.
It seems likely that white sharks follow both strategies, depending on which is the more profitable in a given circumstance. Of all marine mammals, newly weaned seals and sea lions may offer the best energy bargain for white sharks. Another shark falls for the decoy. After one or two strikes, however, most get wise to the ruse. Each winter, white sharks drop by Seal Island for between a few hours and a few weeks, to feast on young-of-the-year Cape fur seals.
White sharks that visit either Seal Island or the Farallon Islands come back year after year, making those islands the marine equivalent of truck stops. Far from being the indiscriminate killers the movies have portrayed, white sharks are quite selective in targeting their prey.
But on what basis does a shark select one individual from a group of superficially similar animals? No one knows for sure. Many investigators think predators that rely on single-species prey groups, such as schools of fish or pods of dolphins, have developed a keen sense for subtle individual differences that indicate vulnerability.
Such cues may be at work when a white shark picks a young, vulnerable Cape fur seal out of the larger seal population at Seal Island. The location and timing of predatory attacks are also far from indiscriminate. At high tide on the Farallon Islands, for instance, there is heavy competition for space where northern elephant seals can haul themselves onto the rocks, and the competition forces many low-ranking juve-nile seals into the water.
Similarly, at Seal Island, Cape fur seals leave for their foraging expeditions from a small rocky outcrop nicknamed the Launch Pad. Coordinated groups of between five and fifteen seals usually leave together, but they scatter while at sea and return alone or in small groups of two or three.
The incoming seal pups have fewer compatriots with which to share predator-spotting duties than they do in the larger outgoing groups.
The white shark relies on stealth and ambush when hunting seals. It stalks its prey from the obscurity of the depths, then attacks in a rush from below. Most attacks at Seal Island take place within two hours of sunrise, when the light is low. The shark thus maximizes its visual advantage over its prey. The numbers confirm it: As the sun rises higher in the sky, light penetrates farther down into the water, and by late morning their success rate falls to about 40 percent.
After that the sharks cease hunting actively, though some of them return to the hunt near sunset. Photo by Neil Hammerschlag But Cape fur seals are hardly helpless victims. They are big, powerful predators in their own right, and take defensive advantage of their large canine teeth and strong claws. They also exhibit a remarkable range of antipredator tactics. Swimming quickly in small groups to or from the Launch Pad minimizes their time in that high-risk zone, and they remain in the relative safety of the open sea for extended periods.
When they detect a white shark, seals often do a headstand, vigilantly scanning underwater with their rear flippers in the air. They also watch one another closely for signs of alarm. Alone, in pairs, or in threes, Cape fur seals occasionally even follow a white shark, swirling around it as if to let the would-be predator know its cover has been blown.
If an attacking shark does not kill or incapacitate a seal in the initial strike, superior agility now favors the seal. Cape fur seals never give up without a fight. One has to admire their pluck against such a formidable predator. 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 [see illustrations below]. 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.