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Thursday, July 12, 2001

NATURAL SELECTIONS

Jaws was born a rambling shark


A dark dorsal fin breaks the surface of a gleaming seascape. A ghost-faced killer glides silently through the water . . . the theme tune to "Jaws" automatically plays in the brain.

Peter Benchley has a lot to answer for, and he knows it. Attacks on surfers don't do much for the public image of the great white shark, but as the author of "Jaws" -- made into one of the highest-grossing films of all time -- Benchley is partly responsible for creating the image of the shark as a ruthless, lone, blood-crazed human killer.

It was never meant to be like that. Benchley wrote the book because of a fascination for the ocean and awe for its creatures. "Even in 'Jaws' I never advocated wiping out sharks," he told Sports Afield magazine. "The voice of me in 'Jaws' is Hooper, the biologist, who is saying 'This is not an evil animal; it's just doing what it does.' "

Despite the voice of the valiant Hooper, hunting in the 1970s and '80s drove the great white to the brink of extinction.

It is now a protected species in many countries, and real scientists are finally learning more about its behavior. In a paper in today's issue of Nature, an international team of scientists report that the notorious shark's ecology has more in common with whales and dolphins than fish. The study will have implications for conservation, as well as being long overdue PR for one of the most misunderstood animals of the sea.

Amanda Pardini at the Department of Zoology, University of Aberdeen, Scotland, and colleagues studied great whites in South Africa, Australia and New Zealand. They analyzed mitochondrial DNA (which is only passed down to the next generation through the female line) and found that sharks from Australia and New Zealand were different from those off South Africa. When they looked at "regular" nuclear DNA, however, which is passed on through both the male and female line, they found no such differences.

This means that females prefer to stick to a home ground -- hence there is no gene flow between females in the Australasian and South African populations. Males, on the other hand, like to roam the oceans, resulting in gene flow between populations.

The scientists inferred this from DNA sampling alone, because there is little direct evidence to show that males and females differ in their range of habitats. But the DNA evidence, and the occasional report of migratory behavior, suggest that like some birds and mammals, male great whites are the dispersers while females are the stay-at-homers. Conservation strategies for the shark, which is listed as "vulnerable" on the World Conservation Union's red list, should encompass this new work.

"Exploitation of a population in which . . . females are philopatric [stay-at-homers] could lead to a rapid decline in stock size and future sustainability," write the authors. "The global dispersion of males may indicate that the demography of widely separated populations is linked, underscoring the need for international regulations to govern exploitation of great white sharks."

As the need to conserve this magnificent animal seeps into fishermen's consciousness, behavioral observations of the shark in the wild are gradually dispelling its "Jaws" image. Great whites inhabit temperate waters throughout the world -- a 3-meter specimen was caught off Wakayama Prefecture in July last year -- and are well-known for sometimes attacking, but not eating, surfers. It was once thought that they mistook the surfboard for a seal (cold comfort for a surfer with a severed leg), but it is now believed that they can distinguish seals from surfboards. They attack humans for different reasons.

Far from being the coldblooded killers of "Jaws," great whites are partly warmblooded and have well-developed social behavior. There is a pecking order when feeding at carcasses, and extremely mild aggressive behavior is used to settle territorial disputes. Gentle biting is also thought to take place during mating.

The puncture wounds usually inflicted on humans are the same low-intensity attacks inflicted on other sharks during territorial incursions. Attacks on real prey (sea lions, elephant seals and even small-toothed whales) are completely different: a swift, surprise attack from below, inflicting a devastating bite. Prey usually die from blood loss or massive trauma.

Faced with a 6-meter-long great white shark underwater, a diving human can be forgiven for misinterpreting the shark's body language. According to some scientists, the terrifying, iconic gape of the shark may be the equivalent of a dog snarling and baring its teeth: It means "keep out of my personal space." The problem is that even a low-intensity attack from a great white shark can be quite damaging: This is one dog with a bite worse than its bark. Divers might like to stay in their cages.

See www.greatwhites.com for more information, including a scientist's response to the question: "In a fight between a killer whale and a great white shark, which would win?"E-mail Rowan Hooper at rowan@ japantimes.co.jp


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