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Snake Mobbing By California Ground Squirrels: Adaptive Variation and Ontogeny

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California ground squirrels (Spermophilus beecheyi) commonly live in association with and are the prey of snakes. Field observations indicate that encounters between the two may often include mobbing by the squirrel and defensive behavior by the snake. Our research had three goals : 1) to systematically observe and describe these mobbing interactions in both field and laboratory situations; 2) to compare in the laboratory the intensity of mobbing gopher, garter, and rattlesnakes by squirrels from two populations which encounter rattlesnakes (rattlesnake adapted), and one which does not (rattlesnake nonadapted) ; 3) to record and describe reactions to a garter snake by young snake-naive squirrels from rattlesnake adapted and nonadapted populations. We found the following. 1. Squirrels approached snakes, investigated them in elongate postures, bobbed their heads, flagged their tails, and sniffed snakes. Aggressively motivated squirrels kicked sand at the snakes, displayed lateral postures, pounced on, and bit them. 2. The stressful impact of this behavior upon the snake is documented by positive correlations between indices of mobbing intensity by squirrels and indices of defensive behavior by the snake. 3. Squirrels from both rattlesnake adapted populations mobbed snakes less intensely than the rattlesnake nonadapted squirrels. 4. The adult-like response of young snake-naive squirrels to a garter snake was significantly stronger than to a moving novel object, thus demonstrating that their response to the snake was based upon stimulus parameters other than novelty. 5. Snake-naive young from a rattlesnake adapted population mobbed less intensely than snake-naive young from a rattlesnake nonadapted population. 6. Snake-naive young mobbed more intensely than wild-caught adults. We proposed the following. 1. Snake mobbing may benefit ground squirrels by reducing the snake's hunting efforts in the area in which mobbing occurred. 2. The presence of venomous snakes increases the risk to the mobber, thereby favoring an adaptive attenuation in mobbing intensity. 3. Because young squirrels are often left at the burrow by foraging mothers, it is likely that first encounters with snakes often occur in the absence of adults. Under these conditions natural selection might favor predetermined epigenesis of the ability to recognize and respond appropriately to snakes. 4. Young snake-naive squirrels may not be as capable as adults of recognizing immobile snakes. Their enhanced response, relative to adults, to moving snakes may be an adaptive mechanism which permits the young to learn to recognize static snake form.

Affiliations: 1: Department of Psychology, University of California, Davis, Calif., U.S.A.

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