The Meaning of the Sound of Rattling By Rattlesnakes To California Ground Squirrels

California ground squirrels (Spermophilus beecheyi) approach, harass and even attack one of their important predators, the rattlesnake (Crotalus viridis). In some of these encounters, rattlesnakes respond with defensive behavior, including rattling. Although the rattle functions primarily as an aposematic signal, we hypothesized that the squirrels extract from the sound the following additional information which indicates the risk associated with interacting with the snake. 1. Rattling indicates that the snake is in a defensive motivational state, which appears to be associated with both a low probability of envenomation and a high probability of retreat by the snake. 2. The intensity and spectral composition of the rattling sound indicates the size of the snake, and risk to the squirrel is positively correlated with snake size. The purpose of these two experiments was to test these hypotheses by exaemining the influence of the rattling sound on the snake-directed behavior of California ground squirrels. In the first experiment we video taped the responses of squirrels to a rattlesnake when it could rattle normally, and to the same snake when its rattle was silenced. In the second experiment, we compared the reactions of squirrels to recordings of the rattling sounds of a large and a small rattlesnake. We found the following. 1. The rattling sound potentiated both contact-promoting and contact-reducing snake-directed behavior by the squirrels. Although the onset of a rattle episode startled the squirrel, the rattle's overall effect was to intensify harassment by the squirrel. Squirrels sand kicked more and at a higher rate and harassed the snake longer when the snake could rattle than when the rattles were silenced. 2. The nature of the rattle's potentiating effect on snake-directed behavior by the squirrel depended on the size related information in the rattling sound. Squirrels turned and ran away from a recording of a large rattlesnake, but approached and investigated a recording of a small rattlesnake. These results suggest both that the squirrels' snake-directed behavior is sensitive to risk and that squirrels extract information from the snake's rattling sounds about the potential costs and benefits of interacting with a rattlesnake.