When most people think of tentacles they don't think of snakes: but not Ken Catania from Vanderbilt University. He is fascinated by the strange tentacle structures on the upper lip of tentacled snakes. ‘When I first saw them I thought what the heck are those tentacles for,’ recalls Catania. He is also impressd by the snake's remarkable ability to outsmart fish. They startle unsuspecting victims into fleeing straight into their hungry jaws, and they also have voracious appetites, consuming large numbers of fish. ‘I am absolutely amazed by this behavioural specialisation for detecting and catching fish,’ says Catania. Having previously worked with another tentacled animal, the star nosed mole, Catania wondered whether the snake's tentacles could be used to detect the subtle water movements generated by fish. ‘Other people have thought this before, but nobody had done any experiments to confirm the idea,’ says Catania, so he set out to solve the mystery of the serpent's tentacles (p. 359).
Teaming up with Duncan Leitch and Danielle Gauthier, Catania decided to investigate the nerves in the tentacles. Using microscopy, the trio could see tiny nerve branches travelling across the middle of the tentacle. ‘This is interesting because it is a clue to the tentacle's function’ says Catania. He explains that nerves involved in taste or touch sensations usually travel close to the surface, but by crossing the tentacle they are better placed to detect when water currents deflect the tentacle. But this was just a smoking gun; Catania needed real evidence that the tentacles were sensitive to deflection.
Using von Frey hairs to gently deflect the tentacles and then measuring the tentacle's responses in the trigeminal ganglion, the team found that the tentacles were remarkable sensitive, responding to even the tiniest displacement produced by the finest hair. They also mapped the tentacle's inputs to the snake's optic tectum (the region of the brain that receives sensory inputs and then directs the behavioural responses) and found that the region of the optic tectum that received signals from the tentacles was close to regions that responded strongly to visual inputs. The two senses must be highly integrated, which made Catania wonder how well the snakes could hunt by one sense alone.
Fooling the snakes into hunting by vision, the team showed the snakes a movie of a cartoon fish swimming beneath the tank and waited to see if the fish would strike. Sure enough, they did, although the snakes soon became bored of striking at an unrewarding computer screen. Next, the team switched off the lights and filmed a snake in infrared light as a hapless goldfish swam within range. The snake struck successfully. It could hunt by tentacle-sense alone, although Catania suspects that it is less accurate.
Catania admits that he is a little surprised at how good the snakes are at hunting by vision alone, given their remarkable adaptation to detecting fish-generated eddies. ‘I think they are using every sense that they have to the maximum,’ says Catania, and adds ‘because the snakes eat so much I suspect there is strong selective pressure for them to use every trick in the book, which is why they have evolved this extra [tentacle] sense for detecting fish.’
- © 2010.