Cobreeding in the burying beetle, <em>Nicrophorus vespilloides</em>:Tolerance rather than cooperation

Under intra- and interspecific competition, cooperative behaviour can provide direct fitness benefits if individuals work together to expel intruders. In the burying beetle Nicrophorus vespilloides, a relatively small species, multiple unrelated pairs can breed together, and individuals are weak competitors in interactions when competing with larger individuals of the same species and with larger species of the same genus. In field and laboratory studies, we found that two pairs together attracted significantly fewer congeneric and non-congeneric competitors compared with single pairs. No other benefits were found. Communal breeding had large negative effects on fitness, as there were fewer offspring per pair and a higher chance of injuries. The higher chance of injuries reflected pairs fighting among themselves and not against competitors. These costs are much greater than the small benefit of fewer intruders. Why should a N.vespilloides breeding pair eventually allow another pair to join? A potential partial explanation is that these are not cooperative pairs in the traditional sense, but rather pairs have a higher tolerance for each other. From the resident's perspective, joining pairs are not expelled because the chance of injury makes the cost of fighting high. Joining others may have unusually low costs in this species because reproductive opportunities are rare, dependent on a resource that is unpredictable in time and space, and residents should be inclined to tolerating new pairs because of the cost of fighting. Thus, for N.vespilloides, communal breeding appears to be making the best of a bad job', providing some reproduction rather than none.