Post-rut aggression in caribou: limited evidence for interference competition but strong evidence for male harassment

Abstract Social competition for resources is widespread among animals. Females of many species compete for foraging resources through interference competition, which is a behavioural interaction between animals that reduces an individual’s access to shared resources. In addition to competition from other females, females also often face harassment from males during and after the breeding season. The female competition hypothesis predicts that female-initiated aggression associated with foraging competition increases as a function of group size, but we expected this effect to be more pronounced in higher quality foraging habitat. The male harassment hypothesis predicts that male harassment of females should also increase as a function of group size as well as a function of the ratio of males to other group members. Here, we tested the female competition and male harassment hypotheses for caribou ( Rangifer tarandus ) within the context of variation in the social environment (i.e., group size and sex ratio) and the physical environment (i.e., variation in habitat quality). We conducted focal observations of caribou on Fogo Island, Newfoundland, over three years and quantified aggressive social interactions in different habitat types to test our hypotheses. Specifically, we took advantage of natural variation in caribou behaviour in our system and observed caribou foraging in an enhanced habitat (i.e., recreational sports complex field) and natural habitat (i.e., lichen barrens). We found limited support for the female competition hypothesis, possibly because of females’ propensity to increase foraging rates in enhanced habitats. By contrast, we found strong support for the male harassment hypothesis, where males increase the frequency of harassment as group size increased. Our results suggest costs of grouping for female caribou in the post-rut time period may be related to the presence of males potentially seeking mating opportunities.