Why Do River Otters Inhabiting Marine Environments Live in Groups?
By Gail Blundell, Institute of Arctic Biology, University of Alaska Fairbanks.

Photo by Eric Peterson©

Why animals are social is a question that has intrigued biologists for many years. Explanations for sociality generally fall into two categories: association with a group may offer some benefit in acquiring resources such as food and mates, or in avoiding predation. Nonetheless, for many animals (especially secretive predators) it has been difficult to determine the factors underlying group living. In previous studies (1989-1992) in Prince William Sound (PWS), Alaska, we observed high variability in sociality among river otters inhabiting coastal waters. Our preliminary observations indicated that males were more social than females, which is a unique phenomenon among mammals. To investigate this unique social organization of coastal river otters, I and several colleagues, live-captured 111 individuals in an area encompassing 4,800-km2 in PWS from 1996 to 1999. We obtained blood and hair samples from all otters for DNA fingerprinting and stable isotope analysis, and implanted radio-transmitters in 55 of them.

The first idea we explored was that river otters were social to avoid the risk of predation. If this was true, then females with young should be more social than males to protect the pups from their main predators (bald eagles, Orcas, and sea lions). Also, otters should be more social in winter when predators have no alternative foods in PWS and would be more likely to attack otters because salmon are unavailable in those months. Our telemetry data revealed the opposite! Group size for otters decreased in winter when predation risk was high and increased in summer when predation risk was low. Also, social otters did not forage further from shore where predation risk is greater. And similar to our previous observations, males were more social then females. So we had to conclude that predation risk was probably not a significant factor promoting sociality in coastal river otters.

Because males were more social than females, we wondered if sociality in river otters was related to the acquisition of mates. In other mammals, such as African lions, groups of sibling males take over groups of females and assist each other in mating. Using the telemetry data to determine association and group composition, and the DNA fingerprinting to determine relatedness among individuals, we found that relatedness of otters appeared to have little bearing on whom they interacted with or even who their neighbors were, because otters did not preferentially associate with their relatives. Social groups, ranging from two to nine otters in our study, were not composed of kin or family groups. Rather, otter groups included a mixture of individuals that were not related, as well as some that were highly related. Therefore, the relatedness of social groups was not different from a random sample of individuals from the area in which the otters resided. There also was no correlation between how related a pair of individuals was and the incidence of adjoining or overlapping home ranges. In addition, we found no indication that sociality resulted in increased reproductive opportunities - neither males nor females that were social had more offspring than did nonsocial otters, and social otters did not have more relatives in the population than non-social ones.

Photo courtesy of the Exxon Valdez Oil Spill Trustees Council

So we were left with the possibility that sociality in river otters was related to the acquisition of food. In PWS, otters can feed on intertidal fishes (such as gunnels and ronquils) and invertebrates (such as crabs and mussels), or they can feed on pelagic fishes (such as herring, sand lance, capelin, and salmon) that come to the near-shore environment to spawn. Those fishes, which constitute a better-quality diet compared with intertidal fishes, usually migrate to the near shore habitat in summer and are seasonally available, although the timing of their arrival and their location is unpredictable. To determine diets of otters, we conducted analysis of stable isotope ratios of carbon (C) and nitrogen (N) on hair samples. Because the stable isotope signatures (noted as ?13C and ?15N) of prey items are reflected in the tissues of the predator that consumes them, one can establish the diet of the predator by comparing those signatures of prey with that of the predator. For that reason, we also determined stable isotope signatures of the fishes commonly consumed by river otters in a marine environment.

Our analyses of stable isotopes and degree of sociality (with radio-telemetry) indicated that larger groups of otters coincided with availability of schooling fishes, and that social otters included more pelagic fishes in their diet compared with solitary ones. Studies of other species have shown that schooling fishes are more prone to predation from groups of predators because multiple predators can surround a school of fishes and prevent fishes from escaping. By cooperatively foraging in this manner, social predators can achieve a greater capture success (i.e., each predator in the group can capture more fishes) compared with the success of a solitary predator attempting to capture the same type of fishes. Although we were unable to directly observe cooperative foraging behavior in the wild, a companion study on captive otters at the Alaska Sealife Center in Seward, Alaska, demonstrated greater capture success for otters foraging in groups.

In further support for our conclusion that sociality in river otters was related to the acquisition of food, is the observation that social otters also had smaller home ranges than did less social otters, indicating greater foraging efficiency for social otters. When a solitary predator is unsuccessful in capturing prey, the animal must move on to a different location in hopes of encountering prey that are unaware of its presence, thus increasing the likelihood of a successful hunt by that predator. A solitary predator in a marine environment often expends a great deal of energy and covers a large distance before it is able to capture enough food to meet its energetic requirements. In contrast, a group of predators that are cooperatively foraging can capture enough fishes to become satiated by focusing their efforts on one or two large schools of fishes and thus they do not need to cover large areas to meet their energetic requirements.

Photo by Eric Peterson©

So why are males more gregarious than females? First we explored the possibility that because male otters were larger than females, they were perhaps stronger swimmers, and thus better at capturing the more rapidly swimming schooling fishes. We had to abandon this idea, however, because our data indicated that body size was less important in capturing pelagic fishes than was sociality, because both males and females that were more social had the better quality diets of pelagic fishes, compared with solitary males and females.

Our latest explanation for the differences in sociality between males and females in coastal river otters revolves around our observation that some females joined male groups for various lengths of time. We suspect that females were constrained in movements by their need to rear and provision young. For up to 6 months of the year and concurrent with the availability of schooling pelagic fishes in the near shore environment, reproductive female otters are limited in movements to the vicinity of the natal and maternal dens where they care for their young. We suspect that the females that join the male groups are those who failed to reproduce that year. For those females, joining male groups to take advantage of the high quality diet of schooling fishes is the best strategy. Although we have no data to support this explanation we hope to continue our studies in PWS and test this idea in the future.

So, why do river otters inhabiting marine environments live in groups? All lines of evidence lead us to conclude that sociality among coastal river otters, at least in Alaska, likely evolved primarily to enhance the capture of schooling pelagic fishes through cooperative foraging.