Snake-directed antipredator behavior of rock squirrels (Spermophilus variegatus): Population differences and snake-species discrimination

The purposes of this study were: (1) to describe the snake-directed antipre dator behavior of rock squirrels; (2) to assess whether rock squirrels dist inguish nonvenomous gopher snakes from venomous rattlesnakes; (3) to compar e antisnake behavior in a snake-rare urban site and a snake-abundant wilder ness site as a means of assessing whether natural selection or experience h as generated population differences in behavior; (4) to assess snake densit ies in the two study sites; (5) to compare the antisnake behavior of rock s quirrels with that of their closest relatives, California ground squirrels (Spermophilus beecheyi), a species that appears to differ from rock squirre ls in exhibiting marked sexual-size dimorphism and (6) to gather additional data on sexual size dimorphism in these two ground squirrel species. We te thered nonvenomous gopher snakes (Pituophis melanoleucus) and venomous west ern diamondback rattlesnakes (Crotalus atrox) in the field near burrows of marked squirrels and videotaped the ensuing interactions. Rock squirrels fr om both urban and wilderness populations confronted snakes while waving the ir fluffed tails from side to side, throwing substrate at the snakes, and e ven attacking snakes on occasion. Survey data confirmed large differences i n snake densities between the two sites. Squirrels from the snake-abundant wilderness site distinguished rattlesnakes from gopher snakes, but squirrel s from the snake-rare urban site did not. Since these squirrels show simila r evidence of selection from snakes, as revealed by their equivalent physio logical resistance to rattlesnake venom, we attributed these behavioral dif ferences to the effects of snake experience. Rock squirrel antisnake behavi or was very similar to that of California ground squirrels. Where the two s pecies' behavior was dissimilar, the differences may be due in part to the interspecies variation in sexual size dimorphism confirmed in this study, a nd to the greater number of rattlesnake species that rock squirrels encount er.