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

The potential impact of interspecific competition on mating systems ha s received limited attention. In this study, I examine the ability of single females and pairs of burying beetles (Nicrophorus spp.) to secu re and prepare carrion resources for their brood when matched against two very different types of competitors. In the first set of experimen ts. I presented N. defodiens small and large rodent carcasses which pr eviously had been exposed in the field to oviposition by carrion flies . Pairs experienced significantly fewer brood failures than single fem ales (25% vs 51%). In successful reproductive attempts (at least one b eetle larva produced), the presence of a male marginally increased the number and mass of the brood on small, but not large, carcasses. Carc asses prepared by pairs also attracted fewer free-flying congeners tha n those prepared by single females. In a second set of experiments. I examined direct contests between single females or pairs of N. orbicol lis and single females of N. pustulatus. The presence of a conspecific male increased the probability that a N. orbicollis female would cont rol the carcass. As a result, paired females experienced four times th e reproductive success of single females. In addition. the injury rate of paired N. orbicollis females was 40% lower than that of single fem ales. 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 fema le with heterospecific first instar larvae at one of two stages in the parental cycle. it was determined that a female will accept heterospe cific young that arrive on the carcass at the 'expected' time. but wil l cannibalize young that arrive more than 20 h before her own eggs hat ch. These results suggest that interspecific competition can promote t he origin of biparental cooperation in organisms which prepare a discr ete and valuable resource for young.