Familiar Neighbors, but Not Relatives, Enhance Fitness in a Territorial Mammal

• Published in: Current biology 2021-01, Vol.31 (2), p.438-445.e3
• Main Authors: Siracusa, Erin R., Boutin, Stan, Dantzer, Ben, Lane, Jeffrey E., Coltman, David W., McAdam, Andrew G.
• Format: Article
• Language: English
• Subjects: dear-enemy; familiarity; kin selection; mutualism; red squirrel; reproductive success; senescence; social behavior; survival; territoriality
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2020.10.072

One of the outstanding questions in evolutionary biology is the extent to which mutually beneficial interactions and kin selection can facilitate the evolution of cooperation by mitigating conflict between interacting organisms. The indirect fitness benefits gained from associating with kin are an important pathway to conflict resolution,1 but conflict can also be resolved if individuals gain direct benefits from cooperating with one another (e.g., mutualism or reciprocity).2 Because of the kin-structured nature of many animal societies, it has been difficult for previous research to assess the relative importance of these mechanisms.3–5 However, one area that might allow for the relative roles of kin selection and mutualistic benefits to be disentangled is in the resolution of conflict over territorial space.6 Although much research has focused on group-living species, the question of how cooperation can first be favored in solitary, territorial species remains a key question. Using 22 years of data from a population of North American red squirrels, we assessed how kinship and familiarity with neighbors affected fitness in a territorial mammal. Although living near kin did not enhance fitness, familiarity with neighbors increased survival and annual reproductive success. These fitness benefits were strong enough to compensate for the effects of aging later in life and have potential consequences for the evolution of senescence. We suggest that such substantial fitness benefits provide the opportunity for the evolution of cooperation between adversarial neighbors, offering insight into the role that mutually beneficial behaviors might play in facilitating and stabilizing social systems. [Display omitted] •In red squirrels, fitness is affected by relationships with territory neighbors•Neighbor familiarity enhanced reproduction and survival whereas living near kin did not•The fitness benefits of familiarity were strong enough to offset the effects of aging•Mutually beneficial interactions might play a central role in conflict resolution Siracusa et al. investigate whether neighbors affect the fitness of territorial red squirrels and show that familiarity with neighbors, not kinship, increases reproduction and survival. They find these effects are strongest in the senescent period, with potential to offset age-related declines in survivial and reproductive success.