Infanticide, sexual selection and task specialization in a biparental burying beetle

Abstract

Burying beetles (Nicrophorus spp.) compete for access to small vertebrate carrion, a highly valued resource. Intruders that take over a carcass will kill young of residents and use the carcass for a replacement brood. To examine whether sexual selection alters interactions with intruders, I staged encounters in which resource competition was the only important motivator for infanticide and then compared outcomes to those in which both sexual selection and resource competition might operate. On carcasses with first-instar larvae, a single resident male or female N. orbicollis was confronted with either a heterospecific or conspecific intruder of either sex (at this stage, a carcass retains 44–75% of its original value if used for a replacement brood). Single males defended their brood significantly better than did single females. Males appear to be efficient task specialists, having both a greater tendency and greater ability to guard the brood. When intruders were heterospecifics, there was no interaction between the two independent experimental variables of sex of defender and sex of intruder. When intruders were conspecifics, however, there was a significant interaction such that infanticide was more common when a defender confronted an intruder of the opposite sex. That is, when a defender had the opportunity to recoup some of its losses from infanticide by participating in a replacement brood with the intruder (opportunity for sexual selection), the probability of infanticide increased. A follow-up experiment staged at the second and early third instar indicated that infanticidal take-overs are quite common when single females defend second instars, but are infrequent when single females defend third instars, or when pairs defend second or third instars. Other measures of reproductive success (number and mass of broods in trials not including take-overs) were similar for single females and pairs. I hypothesize that the threat of infanticide and the inability of a parent to fully compensate for the absence of a partner that is a task specialist promote extended biparental care in burying beetles.