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BORN TO HUNT

Review of Kenneth Catania, ‘Born to hunt’. In: Scientific American, May-June 2011, pag. 28-31.

The May-June issue of Scientific American contains a very interesting article by Kenneth Catania, Professor of Biology at the Vanderbilt University in Nashville, U.S.A. Catania studied the hunting behaviour of the relatively unknown tentacle snake, Erpeton tentaculatum, and came to some amazing conclusions.

Erpeton tentaculatum, a small snake (50-90 cm) occurs in Thailand, Cambodia and Vietnam and spends her whole life in the water, varying from lakes, rice fields and slow streaming streams and rivers. They can be found in sweet, brackish and salt water. The animals can stay under water for up to half an hour. At night and during dry times they sometimes dig themselves in the mud. Erpeton tentaculatum is ovoviviparous and the young are born under water.

This snake got her name from the two characteristic tentacles on both sides of her snout. Already for some time it was suspected that these tentacles helped the snake in locating fish but this hypothesis was never tested. Catania took it upon himself to solve the mystery of the strange tentacles once and for all. But during this quest he discovered quit a lot more.

As it turned out Erpeton tentaculatum has an impressive complex of attack strategies at her disposal; strategies that she already knows at birth. This shows that behaviour is not only shaped by the environment but also that behaviour is genetically predisposed through the genes. To study the function of the tentacles of Catania had to look into detail into the hunting behaviour of Erpeton tentaculatum. This could only be done by replaying in slow mation images that were taken by a high-speed camera. The battle between snake and fish lasts for no more than 40 milliseconds. 1/25ste of a second and only a camera that takes 500 to 2000 images per second allows to observe the hunting behaviour in detail.

The recorded images, replayed in slow-motion appeared to show suicide behaviour of the prey; in many attacks the fish actually turned towards the approaching jaws of the snake. Some even swan straight into its mouth. This is especially strange since fish have fast nerve circuits and body reactions to detect enemies and escape them: they rarely need more than six to seven milliseconds to start an escape. Such an escape reflex in fish is called a C-start because the fish bends its body in a C-shape and can quickly swim away from an enemy.

That during the research of Catania the fish sometimes fled straight into the mouth of the snake turned out to be caused by the unusual J-shape of the snake when hunting. That lures the prey straight into the trap. Erpeton tentaculatum preferably hunts for prey that swims into the J-shaped curl between its head and body. If then the snake moves her body she alarms the fish that notes the pressure difference in the water and automatically swims in the wrong direction, directly into the mouth of the snake.

Catania explains how the nervous system of fish works. Located on both sides of the fish brain there are two neurons that normally works to the fish's advantage. These Mauther-neurons send signals to the other side of the body. A race between the two extremely fast neurons determines in which direction the fish will try to escape. However, the movement of the mid part of the snake’s body puts the fish on the wrong fin and sends the fleeing animal straight towards its predator.

The posture of the fleeing fish also contributes to the fact that devouring the prey happens in incredibly fast. I will swim head first into the mouth of the snake, the ideal way for a snake to eat a fish. This not only allows the snake to eat lots of fish, she will also stay more or less unnoticed. The latter is extra bonus because tentacle snakes also have enemies and they will get noticed while struggling to eat their prey.

Catania had to postpone his research on the function of the tentacles because he encountered even more tricks of Erpeton tentaculatum. Ideally a prey will be in striking distance between head and neck, parallel to the jaws. But there are fish that are not in this ideal position. As it turns out, the tentacle snake can predict the behaviour of its prey: she first uses a feint movement to send the prey away from her back body, parallel to her jaws and even before thh fish moves, she strikes at the spot where the head of the fish will appear
Catania discovered that this all happens so quickly that the visual information for the snake is unreliable and that he has to think in advance. He also discovered variants to this predictive behaviour whereby snakes really had to tie themselves in a knot. Apparently they have a whole repertoire of strategies to be able to strikein any scenario.

Also new-born Erpeton tentaculatum already have the ability to predict prey behaviour: in experiments they also struck in future locations of fleeing fish. Catania thus proofed, he believes, that the tentacle snake already knows at birth how fish move and, more importantly, how to anticipate to this.

Catania lifts his findings on Erpeton tentaculatum to a higher level: inborn talent testifies of a long evolutionary history of this snake species. And that is another important finding for one of the most fundamental questions in biology; the relative importance of nature and nurture in the development of behaviour.

Fish have not yet learned evasive behaviour to this way of being hunted because tentacle snakes are rare enemies that exploit behaviour that has proven its effectiveness under normal circumstances.

Finally Catania touches on the tentacles. These turn out to be very sensitive tactile organs that can sense water movements of nearby objects. They enable the owner to detect and catch prey even at night or in very murky waters.

If you search Google for video’s with the name ‘Kenneth Catania’, you will get a beautiful illustration of the above.

Translated from Dutch by dr. René van der Vlugt.
English corrections: Mark Wootten.

First published in Litteratura Serpentium 31 (2011), 200-205.