The role of movement in orientation to and selection of prey in the garter snake, Thamnophis sirtalis

The role of prey movement in the feeding behavior of garter snakes (Thamnophis sirtalis) was investigated in several experiments. Twelve snakes were repeatedly presented with paired (nonmoving and rotating) sections of earthworm (Lumbricus terrestris). Additionally, intensity of odor of earthworm was varied (assuming intensity of earthworm odor increases with mass), and the performance of a tongueless snake compared to controls.

Preliminary tests indicated that garter snakes will select (by open-mouthed attack) rotating over nonmoving sections of earthworm across a wide range of speeds, with an optimum between 16 and 64 revs/minute (as measured by frequency of and latency to attack). However, it was possible that chemical or vibratory cues from the rotating sections were attracting the snakes in some way. Experiment 1 demonstrated that both blocking the odor from the sections and presentation of speeds greater than 500 revs/minute decreased response to moving sections; a second experiment showed that at 22-32 revs/minute moving sections were selected (as measured by approach) over nonmoving sections when odor from both was blocked, and that the snakes exhibited no preference under these conditions when visual cues were blocked.

Experiment 3 assessed the effects of varying odor conditions upon selection of moving and stationary prey, when a new section of the cage, complete with moving or nonmoving plastic earthworms, was made available to the snakes. Strong earthworm odor resulted in a sustained high rate of tongue-flicking while, with no odor present, snakes initially showed a high rate of tongue-flicking, but returned to baseline within several minutes.

Experiments 4 and 5 studied the effects of tongue removal upon the selection of moving and nonmoving prey. Gross changes in the feeding sequence were noted. A tongueless adult fed by opening its mouth and thrashing about its cage when presented with earthworm odor; a control showed the normal stalk-and-strike sequence. The tongueless snake showed a weaker attraction to the moving earthworm than did the controls.

Ordinarily, tongue contact with prey seems to precede attack. However, when presented with rapidly moving sections of earthworm (over 100 revs/minute or its linear equivalent), snakes probably do not touch the sections with their tongues before striking. The omission of a phase of the stalk-and-strike sequence may enable the garter snake to capture rapidly-moving quarry which might otherwise escape.

Selective attention to visual cues, in this instance, might lead to less dependence upon chemical cues. Several instances of snakes attending to one sensory modality while "ignoring" another are discussed.