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Moving Prey Attracts Snakes (1977)

Moving Prey Attracts Snakes (1977)

 ©1977 by Dallas Denny

Source: Denny, Dallas, & Burghardt, G.M. (1977). Moving prey attracts snakes: The role of a visual cue in the feeding of the garter snake Thamnophis sirtalis. Paper presented at Animal Behavior Society 1977 National Meeting, University Park, PA, 1‑10 June, 1977.

This is the text of the talk. The graph has been lost.



Moving Prey Attracts Snakes

The Role of a Visual Cue in the Feeding of the Garter Snake Thamnophis sirtalis

By Dallas Denny

And Gordon M. Burghardt


Chemical cues are of great importance-in the food-finding behavior of snakes. However, visual cues may direct the attacks of foraging snakes, especially if the snakes have been alerted by olfactory cues. In the present experiments, testing chambers were saturated with prey odor, after which movement of prey objects of a constant size was varied quantitatively across a wide range of speeds. Results showed garter snakes choose moving over nonmoving prey items independently of chemical cues arising from prey items themselves. Moreover, moving prey items tend to decrease latency to attack.


Chemical cues seem to be important in several aspects of the lives of snakes. As John Kubie demonstrated Wednesday, they are essential for initiation of courtship in male garter snakes; chemical cues seem to be important also in the prey-seeking behavior of snakes. For example,’Kahmann, in 1932, found chemical cues are sufficient to elicit prey-seeking and trailing behavior, but usually fail to release the prey-attack response. Wilde, in 1938, abolished the prey-attack response of garter snakes by severing the vomeronasal nerves, and in 1966, Burghardt demonstrated that newborn garter snakes will not attack normally-eaten prey items in the absence of chemical cues.

Several studies have suggested the importance of movement in the location of prey by snakes. Recently, MacDonald found the common boa locates moving prey more quickly than non­moving prey, and Smith & Watson found activity of prey was important in the feeding of the corn snake. However, there have been no attempts made to quantify the effect of movement in a controlled situation when working with snakes.

In a series of experiments, we tested common garter snakes, Thamnohis sirtalis, in a free-choice test situation in which we did quantify movement. Snakes were allowed a choice of stationary or rotating pieces of prey. The prey used were 1.5 cm sections of earthworms, a normally-eaten prey item for garter snakes.

Before sectioning, earthworms were killed by holding them under hot running water. Earthworm sections were fastened onto hooks which were in turn attached to the armatures of variable-speed electric motors. Earthworm segments were then introduced through holes in the bottom of a normally closed-off section of the snakes’ cages.

Prior to trials, a cloth sack containing about a half-dozen earthworms was suspended over the test chamber. The odor from this sack released searching behavior, and increased the rate of tongue-flicks over a baseline rate; besides overt locomotion, tongue-flicks were used as a measure of appetitive behavior.

In the first experiment,, snakes were presented with a nonmoving piece of worm paired with pieces moving at speeds ranging from .5 to 256 rpm. Across speeds, moving pieces of worm were selected over nonmoving pieces (refer to table). Furthermore, most attacks of very short latency were to moving pieces of worm, whereas when latencies were long, there was no preference. Furthermore, most attacks of very short latency were to moving pieces of worm, whereas when latencies were long, there was no preference (refer to graph).

Our results indicate the snakes prefer the moving to the nonmoving pieces of worm, and moving pieces of worm hasten attack. However, we were moved to ask, “Are visual cues really responsible? What of chemical cues emanating from the worm pieces themselves? Could moving worms somehow fan their chemical cues towards the snakes?”

In a second experiment, two conditions: pieces of worm covered with inverted transparent drinking glasses, and pieces of worm uncovered by glasses were presented to garter snakes. In this experiment, speeds of up to 2048 rpm were used. For purposes of analysis, 1, 16, and 32 rpm were treated as “slow” speeds and 512, 1024, and 2048 rpm were treated as “fast” speeds. The results indicate an interaction between speed and odor. Moving pieces of worm were selected less often both at “high” speeds and when glasses covered the pieces of worm, and leastoften at “high” speeds with the pieces of worm covered.

In a third experiment, speeds of moving worms were held constant at 22 or 32 rpm, with pieces of worm presented covered or uncovered in a random fashion. Two of the snakes used in the previous experiment and a third animal which was naive to our experimental situation selected moving over nonmoving pieces of worm when glasses covered the pieces of worm. Visual cues, then, rather than chemical cues, were responsible for directing the attacks of the snakes.

Whereas, the lingering nature of chemical cues helps direct a snake towards prey, upon approach the snake needs a cue that will direct it unambiguously to its prey. We suggest visual cues, especially movement, play an important role in directing attack after searching behavior has been elicited by chemical cues. We must keep in mind, however, that chemical cues may play an important role in the actual elicitation of the attack.