Population density, body size, and phenotypic plasticity of brood size in a burying beetle

Abstract

Theory predicts that organisms living in heterogeneous environments will exhibit phenotypic plasticity. One trait that may be particularly important in this context is the clutch or brood size because it is simultaneously a maternal and offspring characteristic. In this paper, I test the hypothesis that the burying beetle, Nicrophorus orbicollis, adjusts brood size, in part, in anticipation of the reproductive environment of its adult offspring. N. orbicollis use a small vertebrate carcass as a food resource for their young. Both parents provide parental care and actively regulate brood size through filial cannibalism. The result is a positive correlation between brood size and carcass size. Adult body size is an important determinant of reproductive success for both sexes, but only at higher population densities. I test three predictions generated by the hypothesis that beetles adjust brood size in response to population density. First, average adult body size should vary positively with population density. Second, brood size on a given-sized carcass should be larger (producing more but smaller young) in low-density populations than in high-density populations. Third, females should respond adaptively to changes in local population density by producing larger broods when population density is low and small broods when population density is high. All three predictions were supported using a combination of field and laboratory experiments. These results (1) show that brood size is a phenotypically plastic trait and (2) support the idea that brood size decisions are an intergenerational phenomenon that varies with the anticipated competitive environment of the offspring.