Anyone who has been on the receiving end of a truly hard slap knows just how jarring forceful impacts can be. In the animals world, slapping can be used to disorient and stun prey, making them easy pickings for an intelligent predator. Creating a slap with such force can be tough, though, especially in a liquid environment. Killer whales can do it. But while scientists have long hypothesized that thresher sharks might use tail-slapping to stun prey, none had actually studied the kinematics of their tail-waiving behavior to determine if these sharks actually slap or just herd fish with their tails like dolphins do.
Photo by Flickr user Raven_Denmark
Thresher sharks are found worldwide, and are known for their particularly stunning rears. The tails of these sharks can represent an impressive 50% their total length! A fact which is even more daunting when you consider that the common thresher shark (Alopias vulpinus) can grow up to 20 feet long and weigh in at over 1,000 pounds. But these giant ocean predators don’t tend to hang around our shores; lucky for us, they prefer the open ocean where they travel long distances to feed on schools of fish.
Given their extraordinary tails, scientists have long wondered exactly what the sharks use these long appendages for. They’re often seen whipping their tails near schools of fish and get caught by them on baited lines, but it was unclear whether they actually hit their prey to stun them, or simply whirl their tails around around in the water to herd, confuse, or break up the school. To unravel this biological mystery, Simon Oliver and his team of collaborators from Bangor University and The Thresher Shark Research and Conservation Project hopped in the water with these formidable predators while they fed. Using handheld video cameras, they filmed the sharks feeding on large schools of sardines, then analyzed the footage to determine exactly what the sharks were doing with their tails.
Of the 25 times the scientists recorded thresher sharks feeding, 22 of them involved overhead tail slaps, diagrammed above. Three times, the sharks slapped sideways instead. The scientists were able to break down slapping into four phases: the windup, strike, and recovery of the tail, and if successful, the collection of prey. Despite the complex kinetics, each slap lasted less than a few seconds. They clocked one tail at a speed of around 35 meters per second at its peak—close to 80 miles per hour! Furthermore, the scientists discovered that the larger the shark and its tail, the faster the slap. The sharks can swing their tails with such force that dissolved gasses to bubble out of the water. The net result of these posterior swings was that when the slaps hit, the sharks were able to eat an average of 3.5 fish. Considering most oceanic shark species hope to grab one fish at a time, the authors concluded that “tail-slapping is an efficient strategy for hunting schooling prey since thresher sharks are able to consume more than one prey item at a time.”
“This extraordinary story highlights the diversity of shark hunting strategies in an ocean where top predators are forced to adapt to the complex evasion behaviors of their ever declining prey,” said Oliver.
Most of us are more concerned with the front end of a thresher shark than what follows. But the small sardines and other schooling fish that these large, open-water sharks hunt have to worry about both ends. As the authors explain, “the evidence is now clear; thresher sharks really do hunt with their tails.”
VIDEO: Slap! A thresher shark strikes its prey, then collects the stunned fish. Video provided by Jan Acosta
Citation: Oliver S., Turner J., Gann K., Silvosa M. & D’urban Jackson T. (2013). Thresher Sharks Use Tail-Slaps as a Hunting Strategy, PLoS ONE, 8 (7) e67380. DOI: 10.1371/journal.pone.0067380
UPDATE: Ed Yong has another great video:
