Thursday, 23 April 2015 08:00

Theory behind a Great White's hunting strategy...

Ah, a beautiful romantic sunset over the ocean can inspire one to burst out into a loving serenade.  However, after reading this, you may never look at the sun setting over water in the same way again. . .

Sitting comfortably at the top of the marine food chain, Great White Sharks have no natural predators of their own and are equipped with an impressive set of features that collectively make them the skilled hunters that they are. Their streamlined, torpedo-shaped bodies and powerful tails allow them to propel through the water at speeds of up to 35 miles (50 km) per hour, which is fast enough for them to burst out of the water entirely.

But great whites are far more than just straight-up brawn: their brains have to coordinate all of their highly-developed senses. Great white sharks exploit the angle of the sun to hunt down their prey, perhaps concealing themselves in the reflected glare.  It's common knowledge that sharks like to hunt at dawn and dusk.

But researchers from Flinders University in Australia wanted to test if the sun's low position on the horizon may play a role in why they hunt at those times. So they sailed 30 kilometres offshore from South Australia and lured great white sharks to their boat using trails of chum, fish oil and minced fish. Then they threw 6-kilogram chunks of tuna into the water and watched how the sharks approached it when attacking.

An astounding 44 different great white sharks were recorded, making 1000 approaches, 37 of which were actual attacks.  From this data, they quickly saw a pattern emerge: Sharks tend to beeline along the direction of the sun, positioning it directly behind them. At dawn, they usually approached from the east, but at dusk they came in from the west. Interestingly, they also found that when the sun’s rays were obscured by clouds, they had uniformly distributed approach directions, which further suggests that they are exploiting the fluctuating sun during predation.

Photo by: Chris Fallows

There are numerous reasons why the sharks could benefit from approaching with the sun behind them. It could be that the prey is better lit from that angle or perhaps the view of the shark is obscured by the glaring sun. "With the sun in the eyes of the prey, their pupils contract," says Rob Harcourt from Macquarie University in Sydney, Australia. Harcourt thinks it's probably a combination of all those things that dictate the sharks' strategy. "In almost all biology, there are often multiple contributing factors," he says. Also, an approach from the direction of the sun would conceal the shark in the glare reflecting off the water when prey have their heads above the surface – like birds. 

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