Individual variation in social behavior is one of the most striking features of cooperative animal societies. In a new study from the June issue of American Naturalist, Michael A. Cant (University of Cambridge), Justine B. Llop (University of Cambridge), and Jeremy Field (University College London) investigate the extent to which differences in aggressive behavior within a cooperative society can be explained by "inheritance rank"--the likelihood that an individual will get to mate successfully in that society based on their rank--or place in the social hierarchy. They can only pass on their genes when they reach the top of the hierarchy, usually after those ahead of them in the rank have died and they have inherited the right to reproduce.

"Certain group members inflict or receive many more acts of aggression than others. In some cases, these acts (which include bites, shoves, mounts, and charges) appear to regulate cooperative activity in the group by activating lazy workers, for example, or punishing defectors," write the researchers.

The researchers developed two simple mathematical models that predicted that, if inheritance rank mattered in a cooperative society, then the rates of aggression would be highest toward the front of the queue and that the aggression would increase as the time available to inherit the ability to breed ran out in seasonal animals. These predictions were tested on field colonies of the paper wasp Polistes dominulus by recording aggression between all group members and then repeatedly removing the dominant wasps.

When individuals were experimentally promoted in rank, their aggression toward subordinates increased radically, suggesting that aggression depends on rank rather than vice versa and that particularly high levels of aggression are used by newly promoted dominants to establish their status. "We found that rates of both aggressive 'displays' (aimed at individuals of lower rank) and aggressive 'tests' (aimed at individuals of higher rank) decreased down the hierarchy, as predicted by our models," write the authors. Cant et al. conclude that variation in future fitness due to inheritance rank is the hidden factor accounting for much of the variation in aggression among apparently equivalent individuals.

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Michael A. Cant, Justine B. Llop, and Jeremy Field. "Individual variation in social aggression and the probability of inheritance: theory and a field test," The American Naturalist 167:6.