Effects of relatedness on social interaction rates in a solitary marmot

Kinship often influences social interactions, such that animals behave more amicably towards close kin than towards distant kin or unrelated individuals. However, competition also may affect interactions among kin and the degree of sociality in a population. I used microsatellite DNA markers and behavioural observations to examine effects of kinship on social interactions among woodchucks, Marmota monax, which generally are considered asocial or aggressive. Rates of agonistic interactions did not change with relatedness, but rates of amicable interactions increased with increasing relatedness. I observed more amicable interactions than expected between mother–offspring and littermate sibling dyads, but I observed fewer amicable interactions between nonlittermate siblings and more distant kin. Instances of physical aggression were less frequent than expected in mother–offspring, nonlittermate siblings and distant kin dyads, but more frequent than expected among littermate siblings. Interactions between mother–offspring dyads also changed with offspring age, with mothers behaving more amicably towards younger offspring and more agonistically towards older offspring, especially females. Among littermate siblings, rates of amicable interactions were highest between juveniles and yearlings, but then decreased between adult siblings. Social interactions reflected a balance between cooperation and competition. Although woodchucks in this population lived near kin and biased interactions based on kinship, many interactions were agonistic, and animals did not show greater degrees of sociality. Ecological factors, such as predation risk, resource distribution and climate, may favour solitary living in this population.