Infanticide, sexual selection and task specialization in a biparental burying beetle STEPHEN T. TRUMBO Department of Ecology and Evolutionary Biology, University of Connecticut (Received 7 October 2005; initial acceptance 17 January 2006; final acceptance 11 May 2006; published online 26 September 2006; MS. number: A10264) 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 in- truder of either sex (at this stage, a carcass retains 44e75% 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 sig- nificant 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 infanti- cide by participating in a replacement brood with the intruder (opportunity for sexual selection), the prob- ability 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 bipa- rental care in burying beetles. Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. There are few behaviours for which our perspective has changed so much as for infanticide. Once considered rare and pathologically maladaptive, most biologists today view infanticide as a product of natural selection. Al- though accepted as adaptive, there is disagreement in many systems over what particular factors select for infanticide. Hrdy (1979) proposed a number of hypothe- ses to explain infanticide by unrelated adults, including resource competition, cannibalism and sexual selection. These hypotheses are nonexclusive. Infanticide by males, for example, may increase access to both breeding females and to food resources for young (Hrdy 1979; Agoramoor- thy & Rudran 1995). Sexually selected infanticide has been found in many vertebrate taxa and is usually com- mitted by conspecific males that gain earlier access to breeding females (reviews in van Schaik & Janson 2000). The threat of infanticide is thought to select for female af- filiation with friendly males (Palombit 1999; Prenschoft & van Schaik 2000), or monogamy (Freed 1986; van Schaik & Dunbar 1990). Experimental manipulation will help to separate the effects of multiple causes of infanticide but this is difficult with most of our model systems. Iden- tification of the factors that promote infanticide will help to answer long-standing questions of how the threat of in- fanticide shapes social systems (Wrangham 1979; Packer 1986; van Schaik 2004). Task efficiency (a subset of group or colony efficiency) is defined here as the linkage of a greater tendency to perform a task with a greater ability to perform the same task by an individual. All individuals are unique because Correspondence: S. T. Trumbo, Department of EEB, University of Connecticut, 99 East Main Street, Waterbury, CT 06702, U.S.A. (email: trumbo@uconn.edu). 1159 0003e 3472/06/$30.00/0 Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2006, 72, 1159e1167 doi:10.1016/j.anbehav.2006.05.004