30 June 2009

"Tonic immobility" demonstrated in a shark

This documentary was produced by a pro-shark group, presumably to counteract the senseless murder of sharks for the dorsal fins. The idea is to show that sharks can be gentle, nonaggressive creatures, but what interests me is the process by which the diver induces a state of tonic immobility in the shark by stroking its head.
Sharks may not always respond to tonic immobility by physical inversion of the animal, as has been done with lemon and reef sharks. With tiger sharks 10 to 15 feet in length, tonic immobility may be achieved by placing hands lightly on the sides of the animal's snout approximate to the general area surrounding its eyes. Great White sharks have been shown to be not as responsive as other species whenever tonic immobility has been attempted. Scientists believe that tonic, displayed by sharks, may be linked with defence, because female sharks seem more responsive than others. During tonic immobility, the dorsal fin(s) straighten, and both breathing and muscle contractions become more steady and relaxed.
It's reminiscent of the process of "hypnotizing" a chicken. The video is not really proof that sharks are gentle, but the content is interesting nevertheless. Via Presurfer.

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