Relations between allometry, male–male interactions and dispersal in a sap beetle, Librodor japonicus

Status-dependent tactics of males trying to gain access to females were examined in relation to morphology, male–male interactions and dispersal in a sap beetle, Librodor japonicus. Males of this species have sexually dimorphic enlarged mandibles which they use in fights for mates on a sap site. Morphological analyses revealed a switchpoint at which the linear slopes of mandible and hindwing width to body length increased and decreased, respectively. Larger males frequently fought with other males in an experimental arena, whereas smaller males often showed sneaking behaviour without fighting. Mark–release–recapture experiments showed a size-dependent difference in dispersal ability of males in the field. The longest dispersal distance was seen in intermediate-sized males, and the shortest dispersal distances were observed in the largest and smallest males. In females, no relation was found between body size and dispersal distance. The largest males thus remain on sap sites and fight with other males, the smallest males also remain on sap sites and try to sneak access to females, and intermediate-sized males move among sap sites and settle on one without a larger male. The results suggest a nondichotomous difference in optimal behavioural tactics of males even though male dimorphism has been detected in armed beetles.