The effects of adult sex ratio and density on parental care in Lethrus apterus (Coleoptera, Geotrupidae)

Abstract

Theoretical models suggest that adult sex ratio (ASR) and population density are expected to influence parental roles by reducing the mating opportunities of the commoner sex and by changing the intensity of sperm competition, although experimental evidence for these predictions is sparse. In biparental species with a high risk of extrapair paternity and consecutive egg laying over the breeding period, males are expected to reduce their parental investment and to spend more time on mate guarding if male density is high, to maximize their fitness. We conducted a field experiment to test this hypothesis in Lethrus apterus, a flightless biparental beetle species from the Geotrupidae family. Using seminatural enclosures, we assigned individuals to nine treatment groups differing in adult sex ratio (three levels) and individual density (three levels) using a full factorial experimental design. Nest attendance and parental provisioning (i.e. collecting and transporting leaves to the nest) were recorded as well as the number, size and sex ratio of the offspring. We found that as the level of male–male competition increased, generated either by the increased density of individuals or by the male-biased sex ratio, pairs showed higher nest attendance and collected fewer leaves. Male-biased groups also produced fewer offspring under high and low densities indicating a possible conflict of interest between the sexes over paternity and brood size. These results support the increased paternity assurance hypothesis under a high level of intrasexual competition.