Fossils and phylogeny uncover the evolutionary history of a unique antipredator behaviour

Recently, two squirrel species (Spermophilus spp.) were discovered to anoint their bodies with rattlesnake scent as a means of concealing their odour from these chemosensory predators. In this study, we tested multiple species with predator scents (rattlesnake and weasel) to determine the prevalence of scent application across the squirrel phylogeny. We reconstructed the evolutionary history of the behaviour using a phylogenetic analysis and fossil records of historic predator co-occurrence. Squirrels with historical and current rattlesnake co-occurrence all applied rattlesnake scent, whereas no relationship existed between weasel scent application and either weasel or rattlesnake co-occurrence. This was surprising because experimental tests confirmed rattlesnake and weasel scent were both effective at masking prey odour from hunting rattlesnakes (the primary predator of squirrels). Ancestral reconstructions and fossil data suggest predator scent application in squirrels is ancient in origin, arising before co-occurrences with rattlesnakes or weasels in response to some other, now extinct, chemosensory predator.

Do California ground squirrels (Spermophilus beecheyi) use ritualized syntactic cephalocaudal grooming as an agonistic signal?

Animal communication theory holds that many signals have evolved from nonsignal precursors. This field and laboratory study of California ground squirrels (Spermophilus beecheyi) provides evidence for the coexistence of such a precursor with its derived display. The precursor is an ancient, endogenously sequenced (syntactic) pattern of cephalocaudal grooming movements (CCGs) shared by all rodent suborders. The following evidence supports the hypothesis that a supernormal version of this pattern has been selected for signal function. Syntactic CCGs in the field (a) were more rigidly stereotyped than ordinary syntactic CCGs in the laboratory; (b) differed from laboratory syntactic CCGs in other ways that enhanced their conspicuousness, in part through exaggeration of the syntactic cephalocaudal pattern; (c) were associated with scent marking and social staring; and (d) were associated with intrasexual agonistic encounters that did not escalate to fighting.

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

We propose that the predator-prey relationship between California ground squirrels, Spermophilus beecheyi beecheyi, and northern Pacific rattlesnakes, Crotalus viridis oreganus, offers a compelling analogy with the well-studied case of intraspecific fighting and assessment. Because ground squirrels frequently place themselves at risk by harassing rattlesnakes, they stand to benefit from assessment strategies which serve to mediate risk. For example, larger and warmer snakes are more dangerous than smaller and cooler ones. These determinants of dangerousness covary with acoustic characteristics of the rattling sounds elicited by squirrel harassment. To determine whether squirrels use these acoustic cues regarding rattlesnake body size and body temperature, we played back rattling and control sounds to individuals in a population of free-living squirrels. The squirrels clearly associated rattling sounds with rattlesnakes and proved capable of assessing both determinants of rattlesnake dangerousness on the basis of acoustic cues. Several features of squirrel behaviour covaried with these acoustic cues, including tail flagging, bipedal posture, and hesitancy to reapproach the area where the rattle was heard. Many of these behavioural differences were sustained for up to 10 min postplayback. Copyright 1999 The Association for the Study of Animal Behaviour.

Conflict and assessment in a predator-prey system: ground squirrels versus rattlesnakes

Adult California ground squirrels, Spermophilus beecheyi beecheyi, actively confront and harass northern Pacific rattlesnakes, Crotalus viridis oreganus, which are the principal predator of ground squirrel pups. In this report we examine the roles of risk (snake size) and context (location of encounter and squirrel reproductive category) in rattlesnake assessment by ground squirrels. In interpreting the results, we borrow heavily from the well-developed conceptual framework applied in the analogous case of intraspecific conflict. Large and small snakes were tethered near the home burrows of male ground squirrels, and maternal and nonmaternal female ground squirrels. Ground squirrels appeared to employ assessment strategies which served to mediate the level of risk associated with confronting larger snakes. The results suggest that ground squirrels exercise greater caution when dealing with large snakes and invest more in monitoring the snake from a safe distance. Maternal squirrels, which have more at stake reproductively, spent more time and effort in snake-directed activities than did squirrels from other reproductive categories. Mothers also differentiated more strongly between large and small snakes, perhaps reflecting the greater vulnerability of their pups to larger snakes. Finally, ground squirrels discriminated between snakes found close to their home burrow and those encountered further abroad. At the home burrow, squirrels monitored the snake from a closer distance, displayed a greater willingness to confront the snake, and escalated to more dangerous levels of harassment. This assessment strategy may reflect a higher payoff to squirrels that persist in driving snakes out of the home area, thereby reducing the risk of future ambush. Copyright 1999 The Association for the Study of Animal Behaviour.

Ranging of male Oropsylla montana fleas via male California ground squirrel (Spermophilus beecheyi) juveniles

We asked if fleas more frequently remain on those California ground squirrels (Spermophilus beecheyi) that are likely to emigrate from their natal nest. In Camp Ohlone, Alameda County, California, juvenile male squirrels were infested with more fleas (Oropsylla montana) than were juvenile females, and juveniles of both sexes were infested with more fleas (O. montana and Hoplopsyllus anomalus) than adults. There was no difference between the adult sexes in the number of fleas. The disproportionate infestation of male juveniles was accounted for almost exclusively by male O. montana. Greater activity on the part of juvenile males did not account for this difference; the activity of male and female juveniles was very similar. As yearlings, male squirrels established home ranges at greater distances from the natal burrow than did females. Remaining on ranging male squirrels might help male fleas find nonsibling mates in new nests, whereas female fleas might tend to stay in the natal nest in order to assure their progeny of its resources. Flea behavior, modified by characteristics of the host that are sex-specific and predictive of future traits, such as the tendency to range, may thus determine the nature and extent of infestations in juvenile squirrels.