INTERSPECIFIC COMPETITION, BROOD PARASITISM, AND THE EVOLUTION OF BIPARENTAL COOPERATION IN BURYING BEETLES

The potential impact of interspecific competition on mating systems has received limited attention. In this study, I examine the ability of single females and pairs of burying beetles (Nicrophorus spp.) to secure and prepare carrion resources for their brood when matched against two very different types of competitors. In the first set of experiments. I presented N. defodiens small and large rodent carcasses which previously had been exposed in the field to oviposition by carrion flies. Pairs experienced significantly fewer brood failures than single females (25% vs 51%). In successful reproductive attempts (at least one beetle larva produced), the presence of a male marginally increased the number and mass of the brood on small, but not large, carcasses. Carcasses prepared by pairs also attracted fewer free-flying congeners than those prepared by single females. In a second set of experiments. I examined direct contests between single females or pairs of N. orbicollis and single females of N. pustulatus. The presence of a conspecific male increased the probability that a N. orbicollis female would control the carcass. As a result, paired females experienced four times the reproductive success of single females. In addition. the injury rate of paired N. orbicollis females was 40% lower than that of single females. Nicrophorus pustulatus females that were excluded from a carcass were successful brood parasites on 31% of carcasses controlled by N. orbicollis. Parasitism by N. pustulatus resulted in a decrease in the total mass of N. orbicollis broods. By presenting a N. orbicollis female with heterospecific first instar larvae at one of two stages in the parental cycle. it was determined that a female will accept heterospecific young that arrive on the carcass at the 'expected' time. but will cannibalize young that arrive more than 20 h before her own eggs hatch. These results suggest that interspecific competition can promote the origin of biparental cooperation in organisms which prepare a discrete and valuable resource for young.