Contrasting sex roles in two species with stable biparental care

Biparental care involves multiple forms of care and is organized between males and females in diverse ways. When this organization varies among closely related species, there are potential insights into both the evolutionary lability of sex roles and the factors that stabilize biparental care. Recent work on burying beetles suggests stark differences among species in which parent, if either, is specialized for defending the nest and brood. I investigated the relative ability of single male and female parents to both raise and defend offspring in two additional species. In contests, male Nicrophorus marginatus were more successful than females, both as resident defenders and as intruders. Defenders also had more success against same-sex intruders than opposite-sex intruders, and injuries were more common in same-sex encounters, evidence that sexually selected infanticide influenced decisions about defense. When residents excluded intruders from the nest, males and females produced broods of similar size. In N. defodiens, males and females were also equally successful in rearing offspring, but, in contrast to N. marginatus, neither sex was more successful in contests. These findings and a review of earlier work suggest that despite many similarities in caregiving within this genus, species have diverged in sex-role specialization of brood defense (male-biased, female-biased, and sex-neutral). I hypothesize that variation in competitive environments alters selection on body size and defense specialization, but that this does not have qualitative effects on the stability of biparental care in this genus.