Social organization and spatial relationships in coastal river otters: assessing form and function of social groups, sex-biased dispersal, and gene flow

Abstract

River otters (Lontra canadensis) inhabiting marine environments are top-level predators foraging in the nearshore ecosystem and recently have been recognized as indicators of environmental health. Otters were extirpated from much of their historic distribution because of exposure to pollution and urbanization, resulting in expansive reintroduction programs that continue today. Without an understanding of the influence of factors such as social structure, mating system, or sex-biased dispersal on genetic variation and gene flow among populations, effects of local extirpation and the potential for natural recolonization (i.e., the need for reintroductions) cannot be determined. The objective of this study was to assess social organization and evaluate the importance of factors such as prey availability and kinship on formation of social groups and dispersal of individuals. Fifty-five otters were radio-tracked in three study areas in Prince William Sound, Alaska, from 1996 to 1999, to determine social organization and dispersal rates. Data from 111 individual otters (seven study areas) were obtained to assess relatedness and gene flow (with microsatellite DNA) and diet (with stable isotope analysis of ð¹³C and ð¹⁵N). DNA analysis indicated that kinship had no effect on social organization or spatial relationships among otters. Analyses of diet and home-range size indicated that social groups may be formed to facilitate cooperative foraging, enabling social otters to obtain a better-quality diet more efficiently (i.e., social otters had diets higher in schooling pelagic fishes and had smaller home ranges, compared to nonsocial otters). Male otters were more social than females, but reproductive constraints likely limited opportunities for sociality among females. Both telemetry and genetic data indicated that male and female otters had an equal, low probability of natal dispersal and male otters also exhibited breeding dispersal resulting in gene flow to nearby populations. Genetic data indicated distances for natal dispersal were bimodal; most males and some females settled nearby (within 16-30 km), but some females dispersed 60-90 km. Despite lack of geographic barriers to dispersal in a marine system, dispersal distances were relatively short, indicating that extirpation of local populations would be difficult to correct via natural recolonization unless viable otter populations were available nearby.