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.

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.