In this study we investigated ecological determinants of sociality in burying beetles (Nicrophorus spp.), potential conflicts of interest among reproductive females, and the effects of nesting failure and costs of fighting on cooperation. Burying beetles are known to form monogamous pairs when exploiting small vertebrates carcasses. More complex social behavior in this group is poorly understood. We conducted experiments in which one or two females (N. defodiens, N. orbicollis) were provided small or large carcasses on which to breed. On large but not on small carcasses, two females often formed cooperative breeding associations (jointly prepared a carcass and fed young). In N. defodiens, but not N. orbicollis, two females produced a larger brood than single females on large carcasses. In both species, the reproductive output per female was less for two than for one female. The presence of a second female did not decrease the preparation time of a carcass (discovery of resource to egg hatch). Conflict was evident between females. Trials employing females of similar size were more likely to result in injury than trials using females of dissimilar size (N. tomentosus, N. defodiens, N. orbicollis). In N. tomentosus, those associations that persisted the longest resulted in the fewest injuries. After care of young was initiated, conflict among familiar nest mates was not observed. There was no evidence that breeding females could discriminate between brood; use of a genetic marker (N. orbicollis) demonstrated that females fed related and unrelated young alike. Females of similar size (high potential cost of fighting for the dominant individual) were not more likely to form cooperative breeding associations than females of dissimilar size (low cost of fighting for dominant). Females of a species subject to a high rate of nest failure (N. defodiens) were more likely to cooperate than females of a species with a low rate of nest failure (N. orbicollis). It is argued that limited reproductive opportunities, difficulty in controlling rivals’ access to a large carcass, and the superabundant larval food supply represented by a large carcass, but not kin selection, have contributed to the evolution of cooperative behavior in this group. In addition, we hypothesize that beetles might initially tolerate consexual rivals on large carcasses when there is a high likelihood of nesting failure, therby avoiding potentially costly conflicts.