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Inside JEB |
GONE FISHIN'
kathryn{at}biologists.com
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No one knows what tempted snakes back into the water, but Michael Alfaro explains that a few species from most snake lineages seem to have slid off the bank, and happily returned to the water. Having left the land, many of these snakes have adapted and become extremely agile swimmers. But hunting for food in their new environment doesn't appear as easy as striking at prey from solid ground; a sudden lunge forward might produce a bow wave that washed the prey away. Alfaro explains that most snakes were thought simply to wave their heads from side-to-side as they swam, snapping up any fish that happened to pass their way. But no one had ever compared aquatic snakes' hunting strategies. Alfaro recorded three aquatic snake species' movements, as they feasted on minnows, to discover which tactics they had opted for, and found that far from taking a leisurely approach to predation, aquatic snakes can strike just as fast as their terrestrial cousins (p. 2381).
Alfaro decided to test the reactions of three distantly related species. He headed out to Arkansas to collect diamond-backed water snakes, and also travelled to the Sierra Mountains to collect western aquatic, and western terrestrial, garter snakes. Alfaro remembers that the garter snakes were not at all aggressive, although the water snakes were extremely agile and difficult to catch. But it was a different matter with the diamond-backed water snake `they bit and bit, and they were really unpleasant' he says. Back in the lab, Alfaro painted white spots onto the backs of the snakes and placed them inside an escape-proof aquarium, stocked with tasty fat head minnows, ready to recorded the snakes' fishing techniques and track the animal's movements.
But instead of gently waving their heads from side-to-side, he realised that the snakes were definitely launching themselves at the minnows, and striking just as fast as land-based snakes! Both of the garter snakes were able to align themselves towards a tasty target, and lunge forwards, although the western terrestrial garter snake sometimes chose to open its mouth wide, and sweep its head back and forth picking up fish that strayed into its path.
When Alfaro turned his attention to the troublesome diamond-back water snakes, they seemed to have come up with a completely different solution to the problem. Instead of watching the fish and aligning themselves ready for a fast forward attack, the diamond-backed snakes waited until an unsuspecting fish brushed past, then they struck sideways, at an amazingly fast speed. But even though all of the snakes were capable of incredibly fast reactions, they also missed a lot of fish.
Alfaro admits that he was amazed that the water specialists were as fast as terrestrial snakes, and that all three species seem to have come up with completely different solutions to the fishing problem; proving that for water snakes, there's more then one way to catch a fish!
References
Alfaro, M. E. (2003). Sweeping and striking: a
kinematic study of the trunk during prey capture in three thamnophiine snakes.
J. Exp. Biol. 206,2381
-2392.
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