If you’re going to have a tail that stretches as long as your body, you might as well whack things with it.
For years it’s been suspected that a small-mouthed, slender family of sharks called thresher sharks would actively use their disproportionately long tails for hunting, but until recently, no one had managed to film it. Then three years ago, a team of scientists in the US, led by marine biologist Dr Chugey Sepulveda from the Pfleger Institute of Environmental Research, captured the first ever footage of 14 wild thresher sharks attacking baited lines with their tails in the waters of southern California. While this discovery answered the question of what exactly the thresher shark did with that incredible tail shaped like a waning moon, no one knew quite how they would react when faced with live and very active prey.
Having studied thresher sharks for the past eight years, Dr Simon Oliver from the University of Liverpool begun filming pelagic thresher sharks – the smallest of the three species in the thresher shark family – near Pescador Island in the Philippines. Over five months in 2010, he captured footage of the sharks interacting with huge, pulsating schools of sardines, and watched as they picked off the unlucky ones with a powerful flick of the tail.
“Initially I was surprised because this is the holy grail of thresher shark research,” ays Oliver, also the founder of the Thresher Shark Research and Conservation Project. ”To observe this behaviour in the wild was unprecedented and I felt extremely privileged to be there. Once I got over the sheer joy of being there, tail-slaps occurred quite regularly when a shark was in the vicinity of the bait ball [a large aggregation of prey fish].”
Of the 25 instances of tail-slapping filmed, 22 were initiated by the thresher shark lowering its snout and charging forward, then flexing the base of its tail to make it flick vigorously over the back of its head – just like a whip – to collide with as many sardines as possible. If the tail caught the sardines with enough force, it could stun or even kill them, and the shark would then spin around to collect its meal. The larger, stronger sharks with the longest tails were able to achieve the fastest, most powerful tail-slaps. On the other three occasions, the sharks opted for a ‘sideways tail-slap’, charging their bodies forward to postion themselves alongside some sardines before whacking them with a horizontal, sideways slap.
While the sharks didn’t manage to stun their prey with every tail-slap, what matters is the amount of food they could acquire every time they did. Sharks without a weaponised tail generally have to find and pursue one prey animal at a time, which can be both time-consuming and tiring, especially if the prey is quick, elusive, and confusing, like a fish in a school. What the thresher sharks have been able to do is use the schooling defense against the fish – the tighter those sardines are packed in, the more casualties there are likely to be. “[The tail slapping is] less effective than you might imagine, with a 50% success rate, BUT when successful, sharks were able to consume several sardines at a time. So you could say that tail-slaps are efficient,” Oliver reasons.
He also noticed signs of possible social behaviour occurring between the tail-slapping thresher sharks. “There were occasions during in situ observations when two or more thresher sharks sideways tail-slapped different areas of the bait ball at approximately the same time, suggesting social behaviour,” Oliver and his colleagues report in today’s edition of PLOS One. They recall a study done by a different research team in 1996 where tail-slapping was described in white sharks (Carcharodon carcharias) as an aggressive signal between individuals competing for the same prey; one shark would only feed on dead prey nearby if its tail-slap was quicker and more vigorous than its opponent’s.
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“Other studies have shown that large marine predators will hunt socially, using their tails to control the shape and density of bait balls, and it has been speculated that thresher sharks use the technique to corral schooling prey. Since field reports of two or more thresher sharks deploying sideways tail-slaps concurrently at Pescador Island were anecdotal and unverified, proposals that the behaviour was an agonistic signal among conspecifics over resource competition, or that they hunted cooperatively, were treated with caution,” Oliver and his team concluded.
While the social aspect of the thresher shark’s tail-slap is yet to be explored, the technique can now be added once and for all to the vast repertoire of shark hunting strategies. “Some benthic [deep-sea] sharks employ an ambush strategy and conceal themselves by burrowing into the substrate to lie in wait for their prey. Stalking sharks may approach a prey item while concealed using tidal movements, stealth, and/or light attenuation, and then make a sudden assault,” says Oliver. ”Other more specialised species may lure their prey using luminescent tissue on their upper jaws or white patches on their fin tips. Many shark species will employ a combination of search and approach strategies, and most will scavenge food given the opportunity.”
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