Effects of breeding ecology on parental care and family living in passalid beetles

Family-based societies occur in a variety of animal taxa; yet the selective factors that favor the evolution of family living are poorly understood. Here, I examine four species of log-dwelling passalid beetles to determine whether key habitat differences are associated with differences in family composition, parental care, and cooperative brood care. Specifically, I test the hypothesis that nutritionally poor resources are associated with greater levels of parental care and cooperation. Contrary to predictions, Verres hageni, a species that inhabits the lower-quality heartwood, did not invest more time in parental activities, spend more time in contact with their larvae, or experience more begging calls from larvae than did the three Passalus species that inhabit the higher-quality sapwood. Heartwood families also did not retain more adult offspring nor were they more socially monogamous than sapwood species. Although no direct cooperation in brood care by adult offspring was observed for any species, no aggression between parents and adult offspring occurred even when adult offspring overlapped with their parents’ second brood. Family living and parental behavior, thus, appear generally stable and conserved across species, despite habitat differences. These findings suggest that ecological selective factors might not always yield predicted effects on behavioral traits. In the case of passalids, alternative adaptations to poor-quality resources as well as phylogenetically conserved parental care might explain the lack of observed response of parental care to breeding resources.