SOCIAL GROUP FISSION AND GENE DYNAMICS AMONG BLACK-TAILED PRAIRIE DOGS (CYNOMYS LUDOVICIANUS)

Social groups of philopatric female kin coupled with male-biased dispersal characterize many mammalian species. Such groups exhibit genetic properties, or gene dynamics (i.e., changes in genetic correlations), that potentially facilitate the evolution of group cohesion and cooperation. When groups fission, changes in gene dynamics occur that might affect cooperation. The distribution of females among coalitions newly formed via fission also may promote reproductive success by alleviating intraspecific competition. Families of black-tailed prairie dogs (Cynomys ludovicianus), called coteries, were observed during a 15-year study at Wind Cave National Park, South Dakota, and pedigrees of individuals were determined from livetrapping and electrophoretic analyses of paternity. We investigated the importance of coterie size, coterie density, and coancestry on the probability of fission; the effect of fissions on survivorship and territorial boundaries of coteries; and gene dynamics during and after fissions. Most new coteries occupied a subsection of the original territory. New coteries that contained a single female became extinct after about 1 year and their territories were usurped by adjacent coteries. Large coteries were more likely than smaller coteries to fission, but female coancestry within coteries usually did not influence the likelihood of fission, or the individuals that banded together in new groups. Members of new coteries interacted hostilely in the year after fission, but 1 or 2 generations elapsed before the new coteries became genetically distinct. We conclude that fission is probably driven by intraspecific competition within coteries rather than coancestry, and that coancestry does not explain the abrupt cessation of cooperation between individuals of the new coteries that result from fission.