December 12, 2011
Every year in False Bay, South Africa, great white sharks congregate as Cape fur seal pups are weaned. Seals feed offshore, swimming out together in groups of five to 20. They spend a few days foraging, depending on each other to protect against shark attacks. The sharks, though, have many advantages, such as big bodies and sharp teeth. And they can use the power of physics–specifically, water optics–to aid in their attacks, say scientists in a new study in Marine Biology Research.
Seal vision is best adapted to seeing through the shallow coastal waters where the animals spend most of their time. When Cape fur seals watch out for sharks, they do so by lying at the surface, looking down into the depths. This is good enough to find bright objects below them, but great white sharks, despite the name, have dark grey backs that camouflage well against the dark reef floor near the island where the seals live. In low light conditions, a seal won’t be able to see a shark until it’s about 2.6 meters away.
Shark vision is different from seal vision. No one has determined directly how well a great white sees, but studies of its eyes have shown that the shark has a high rod-to-cone ratio in its retina, which should give it good vision in low light conditions, like those in early morning when they most often hunt. And when they look up to the surface where the seals swim, they see an easy-to-spot dark flippered body silhouetted by the sun.
The researchers observed sharks in False Bay as they leaped out of the water in their attacks on seals and calculated the maximum speed reached, about 35 kilometers per hour (22 mph). To reach those speeds, the scientists further calculated, the shark would have to start its attack from at least 7 meters away, and the seal would have only a tenth of a second to react. “Stealth and ambush are key elements in the white shark’s predatory strategy,” said study co-author Neil Hammerschlag of the University of Miami.
Real-world observations seem to match up with these calculations. Most shark attacks occur over a water depth range of 7 to 31 meters. And great whites are more successful in their attacks during low-light conditions; 55 percent of their attacks are successful during those times versus less than 40 percent in bright light.
But if a great white doesn’t make a kill in that first strike, its chance of success decreases with longer it tries to catch his dinner. Young Cape fur seals can reverse direction in a much smaller distance than their shark attacker needs, which lets them take evasive action, leaping away from the shark’s jaws before it can take a second bite.
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