Sex differences in immunity and rapid upregulation of immune defence during parental care in the burying beetle, Nicrophorus orbicollis

1. Immunity may trade-off against other important life history traits, with recent work suggesting that reproduction and parental care in particular impinge on immune defence. However, whereas the effect of parental care on immunocompetence has been intensively studied in birds and mammals, virtually nothing is known about how it affects insect immunity. 2. Burying beetles provide extensive biparental care that includes the burial, preparation and defence of a carcass, as well as the subsequent feeding of the larvae. In addition, they cover the carcass with anal exudates that have been shown to serve an antimicrobial function ( social immunity sensu Behavioral Ecology, 21, 663-668). We examined the effect of sex, mating and parental care on measurements of individual and social immunity in the burying beetle Nicrophorus orbicollis. 3. Both males and females showed a rapid upregulation of the encapsulation response upon discovery of a carcass. The high encapsulation rate was maintained during the entire period of parental care. Lytic activity in anal exudates, a measure of social immunity, likewise increased. Mating had no effect on individual or social immunity, but females generally exhibited higher individual immunity than male N. orbicollis. 4. Our results suggest that the unusual breeding environment of burying beetles - a microbe-rich carcass - has selected for an atypical pattern of immune defence, with a significant upregulation of individual and social immunity during the physically demanding period of reproduction and parental care. The simultaneous investment in two life history traits that normally compete for resources may be an adaptive response in species that breed in environments with high densities of micro-organisms.