Snakes trick prey for easy meal

Water snakes trick their fish prey into swimming directly into their waiting jaws, according to a linkurl:study published in PNAS;http://www.pnas.org/content/early/2009/06/18/0905183106.abstract last week (June 19). With a subtle body movement, the tentacled snakes trigger a preprogrammed escape response in fish, causing them to flee in a predictable direction so that the snakes know just where to positions their heads for an easy meal. Image: Ryan Somma, Wikimedia commons"It's a very clever ma

Jef Akst

| 3 min read

Water snakes trick their fish prey into swimming directly into their waiting jaws, according to a linkurl:study published in PNAS;http://www.pnas.org/content/early/2009/06/18/0905183106.abstract last week (June 19). With a subtle body movement, the tentacled snakes trigger a preprogrammed escape response in fish, causing them to flee in a predictable direction so that the snakes know just where to positions their heads for an easy meal.

Image: Ryan Somma,
Wikimedia commons

"It's a very clever maneuver," said neurobiologist linkurl:Nick Strausfeld;http://neurobio.arizona.edu/faculty/strausfeld.html of the University of Arizona, who didn't participate in the research. "It's really quite a remarkable thing that the snake has evolved this behavior to exploit this escape behavior." Neuroethologist linkurl:Kenneth Catania;https://medschool.mc.vanderbilt.edu/biosci/bio_fac.php?id3=9129 noticed fish repeatedly swimming straight into the mouths of the snakes he kept in his laboratory at Vanderbilt University in Nashville, Tenn. Using high speed video, he captured this behavior and slowed it down to reveal the snakes' secrets: The animals gives "a little twitch of the body just before the strike begins," he described. "When you watch this in real time you couldn't see anything." That little twitch is all it takes to initiate the fish's escape response, called the C-start -- one of the most well studied neural circuits in vertebrates. Two large nerve cells, known as Mauthner cells, run along either side of the fish's body and detect water disturbances. The cell closest to the signal will fire action potentials that stimulate trunk muscles on the opposite side of the body while simultaneously inhibiting the muscles on the near side. As a result, the fish turns away from the disturbance and flees. This whole process takes less than a tenth of a second. The tentacled snake takes advantage of this escape response by positioning its neck in a J position, with its head turned back towards its body. In Catania's experiment, as soon as the fish swam into position, the snake simply shook its side to elicit the fish's escape response. Seventy-eight percent of the time, the fish fled away from the snake's body and towards its head instead. The snakes captured their prey 65% of the times when the fish turned towards them, but had only a 50% success rate when they turned away.

"[It's] a very interesting example of parallel evolution of a very specialized behavior evolved to exploit an escape circuit," Strausfeld said. In order for this type of exploitation to work, the predator must be uncommon -- this is known as the "rare predator" effect. Otherwise, the prey would adapt to evade the predator's new hunting strategy. In this case, "most other predators are not using the snake's strategy [of surrounding the fish]," Strausfeld explained. "They're coming from one side to the other." Similarly, the painted redstart -- a small, North American songbird -- fans its contrasting tail plumage to evoke the aerial escape response of its insect prey and then "picks them off one by one as they fly up into the air," said Strausfeld, who has studied that organism. People have also been characterized as rare predators, Catania said. Worm grunters -- people who gather earthworms as bait -- generate vibrations in the soil that the worms mistake for their more common mole predators and escape to the surface for easy collecting. Like the redstarts and worm grunters, tentacled snakes don't just set their prey's escape response in motion and wait. To the contrary, the snakes appeared to compensate for the fish's escape attempt by striking not where the fish was at the start of the attack, but several millimeters to the side towards which the fish most often fled. This study "just scratches the surface of what the snakes are probably able to do," Catania said. "They [could] have a lot of different ways of efficiently outsmarting the fish when the fish is in different positions."
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