Colonization in a pika population: dispersal vs philopatry

Relative tendencies toward dispersal or philopatry in a marked population of alpine mammals, the pika (Ochotona princeps), were investigated over a 3-year period in the Rocky Mountains of Colorado. Home range size, distances between centers of activity of dyads, and weighted overlaps of home ranges of dyads were used to define space use patterns. Disappearance and establishment of individuals reflected the temporal component of space use. Social relationships among conspecifics were defined by agonistic and affiliative behaviors. Home ranges of resident adult males and females were of equal size on talus, the obligate habitat of pikas. Adjacent home ranges were normally occupied by members of the opposite sex, and this spacing apparently results from the balance of agonistic and affiliative behaviors exhibited by nearest-neighbor heterosexual pairs. Most juveniles were philopatric. Throughout the summer they remained on their natal home range where they were involved in both agonistic and affiliative behaviors with their mother and putative father. Most animals that established residency were juveniles, and of these almost all settled within 50 m of their natal home range center. This pattern was independent of population density. Immigrants were met with extreme aggression by resident adults that was not balanced by affiliative behaviors. Few immigrants of either sex successflly established on the study area. Adults occasionally changed home ranges, probably to enhance their chances of mating. Intense aggression directed at unfamiliar animals coupled with the acceptance of spatial overlap of related young throughout the summer apparently promote philopatry in pikas. Philopatric settlement in pikas may lead to incestuous matings and contribute to their low intrapopulation genetic variability.