Assessment of rattlesnake dangerousness by California ground squirrels: exploitation of cues from rattling sounds

We propose that the predator-prey relationship between California ground sq uirrels, Spermophilus beecheyi beecheyi, and northern Pacific rattlesnakes, Crotalus viridis oreganus, offers a compelling analogy with the well-studi ed case of intraspecific fighting and assessment. Because ground squirrels frequently place themselves at risk by harassing rattlesnakes, they stand t o benefit from assessment strategies which serve to mediate risk. For examp le, larger and warmer snakes are more dangerous than smaller and cooler one s. These determinants of dangerousness covary with acoustic characteristics of the rattling sounds elicited by squirrel harassment. To determine wheth er squirrels use these acoustic cues regarding rattlesnake body size and bo dy temperature, we played back rattling and control sounds to individuals i n a population of free-living squirrels. The squirrels clearly associated r attling sounds with rattlesnakes and proved capable of assessing both deter minants of rattlesnake dangerousness on the basis of acoustic cues. Several features of squirrel behaviour covaried with these acoustic cues, includin g tail flagging, bipedal posture, and hesitancy to reapproach the area wher e the rattle was heard. Many of these behavioural differences were sustaine d for up to 10 min postplayback. (C) 1999 The Association for the Study of Animal Behaviour.