Dominance status and sex influence nutritional state and immunity in burying beetles Nicrophorus orbicollis

Intrasexual competition for mates or resources important for reproduction often leads to the establishment of dominance relationships that influence an individual’s reproductive success. Although dominance can be an honest indicator of health or immunocompetence, the attainment and maintenance of dominance status can also influence an individual’s ability to invest in immunity, making it difficult to disentangle cause and effect. Here we examine the relationship between intrasexual competition and the nutritional condition and immunity of burying beetles, Nicrophorus orbicollis, insects that reproduce on small vertebrate carcasses that serve as a larval food source. We staged intrasexual contests on carcasses in both sexes and compared the nutritional state and immunity of dominant and subordinate individuals with those of beetles reproducing on a carcass without competitors. The nutritional state and immunity of dominant beetles were not significantly different from beetles without competitors, but subordinates were characterized by a lower weight gain and reduced encapsulation response. In addition, we found a clear sex effect, with females gaining more weight than males and having superior immunity. We conclude that the subordinate’s exclusion from the carcass plays an important role in mediating the difference in encapsulation. Our data suggest that this is not entirely a nutritional effect because better-fed subordinates did not exhibit higher immune responses. Rather, subordinates may have no need to invest in improved immunity because they do not participate in carcass maintenance and defense.