Sociality in river otters: cooperative foraging or reproductive strategies?

We evaluated factors influencing social organization in coastal river otters (Lontra canadensis) to test two hypotheses: group formation is an antipredation strategy, or, alternatively, group information is related to cooperative foraging. Data on group size, group composition, and sociality were obtained through radiotracking 55 otters in Prince William Sound, Alaska, USA, from 1996 through 1998. For males, larger groups occurred after the mating season and concurrent with availability of schooling pelagic fishes. Stable isotope analysis revealed that otters social in >10% of their locations had diets significantly higher in rapidly swimming pelagic fishes than did less social otters, regardless of gender. In addition, otters that were more social had significantly smaller home ranges than did less social otters, an observation consistent with increased foraging efficiency through cooperative foraging. Discounting associations of females with young of the year, approximately 47% of females and only 24% of males were asocial. Among social otters, males were social in 46% of their locations and 63% of that time occurred in all-male groups. Females were only social in 26% of locations and were in mixed-sex groups 78% of that time. We hypothesize that the time-consuming task of raising offspring prevents females from joining foraging groups. When not raising young, females may join males to cooperatively forage for better-quality prey (pelagic fishes), which would be more difficult to acquire as a solitary forager. Copyright 2002.