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.