The cost of aggregation: juvenile salmon avoid sharing winter refuges with siblings

Kin selection advantages are usually accrued by individuals that associate with close relatives. But aggregation may also be costly, by increasing the risk of predation or resource competition, for example. As a result, individuals should increase their inclusive fitness by trading the costs and benefits of kin association and aggregation. Studies of kin selection to date have focused on situations where there is ample opportunity for kin-biased behavior and therefore for the formation of kin groups. Here we used juvenile Atlantic salmon to test an alternative strategy: that under conditions where the potential for kin-biased behavior is negligible, individuals should, when aggregating, avoid rather than associate with kin to avoid imposing the costs of aggregation upon close relatives. By testing salmon during winter, when juveniles shelter inactively in streambed refuges, we tested whether individuals associate with or avoid their siblings at a time when the opportunity for kin-directed behaviors is restricted. Our results provide the first evidence of kin avoidance in nonreproductive animals studied under semi-natural conditions. Copyright 2003.