EXAMINATION OF SOCIALLY INDUCED DISPERSAL IN SIGMODON HISPIDUS

Aggressive behavior within small-mammal populations may lead to dispersal of subordinates. Enclosures were used to test if dispersal in cotton rats (Sigmodon hispidus) resulted from group pressure; animals that crossed a 3-m mowed strip in 24 h were classified as dispersers. Animals were tested under two density treatments to identify dispersers, once when no other animals were present (16/ha) and once when three other animals were present in the enclosure (64/ha). The proportion of dispersers from the high-density enclosures was less than that from the single-animal enclosures, indicating that group pressure did not cause dispersal. Dispersal was not related to sex, season, or body mass, but was related to the reproductive condition of females. Reproductive females tended to be nondispersers. Behavioral data were collected by observing paired animals. Males that dispersed from the high-density treatment were more dominant than male nondispersers, suggesting that subordinates were not forced to disperse by more dominant individuals. Female dominance was not related to dispersal, but was confounded by reproductive condition. Several hypotheses include the assumption that social behavior plays a central role in dispersal of small mammals (Gaines and McClenaghan, 1980). Howard (1960) proposed that dispersal results from two separate processes: movement at about the time an animal attains puberty and movement in response to crowded conditions. Lidicker (1975) further refined Howard's ideas and included the suggestion that dispersal results from social subordination (Christian, 1970) to produce the presaturation-saturation dispersal hypothesis. Saturation dispersal occurs when populations are at or near their carrying capacity and social subordinates (juveniles, old animals, and animals in poor condition) are forced to disperse by aggressive social interactions. Presaturation dispersal occurs when conditions are not crowded. Individuals that disperse under these conditions are predicted not to be social subordinates, to be in good physical condition, and to have higher fitness than saturation dispersers. Stenseth (1983) proposed that the term nonadaptive dispersal refer to dispersers forced out of the pop- ulation and adaptive dispersal refer to presaturation dispersal. Dispersal under the genetic- behavioral polymorphism hypothesis (Chitty, 1967; Krebs, 1978; Morris, 1984) also results from social subordination. The proportion of aggressive genotypes increases during the increase phase of a cycle resulting in increased dispersal of subordinates in response to aggression. Different types of dispersers are not components of the social-fence hypothesis (Hestbeck, 1982, 1986). This hypothesis contains the postulate that spacing behavior results in dispersal from central populations into low-density neighboring populations, but, as density increases, aggression within the neighboring populations prevents dispersal from the central population. Although not directly developed for small-mammal populations, the social-cohesion hypothesis (Bekoff, 1977) also incorporates social behavior. Under this hypothesis, dispersal patterns of social mammals (pri- marily canids) are related to the development of strong social ties with littermates. Dispersal resulting from weak social ties occurs among the most dominant and also the most subordinate individuals because dominant individuals are avoided whereas subordinates avoid their litter- mates. Tests of these alternative hypotheses for a species require empirical documentation of details of the relationship between social interactions and the process of dispersal, but such documentation is rare. When the importance of social interactions has been demonstrated in a population, their relation to dispersal can be explored. Several investigators suggested that aggression is an im- portant component of the population dynamics of the hispid cotton rat, Sigmodon hispidus