ALTERNATIVE REPRODUCTIVE TACTICS AND MALE-DIMORPHISM IN THE HORNED BEETLE ONTHOPHAGUS-ACUMINATUS (COLEOPTERA, SCARABAEIDAE)

Adult dung beetles (Onthophagus acuminatus) exhibit continuous variati on in body size resulting from differential nutritional conditions exp erienced during larval development. Males of this species have a pair of horns that protrude from the base of the head, and the lengths of t hese horns are bimodally distributed in natural populations. Males gro wing larger than a threshold body size develop long horns, and males t hat do not achieve this size grow only rudimentary horns or no horns a t all. Previous studies of other horned beetle species have shown that horned and hornless males often have different types of reproductive behavior. Here I describe the mating behaviors of the two male morphs of O. acuminatus during encounters with females, Females excavate tunn els beneath dung, where they feed, mate and provision eggs. Large, hor ned males were found to guard entrances to tunnels containing females. These males fought with all other males that attempted to enter these tunnels. In contrast, small, hornless males encountered females by sn eaking into tunnels guarded by other males. In many instances, this wa s accomplished by digging new tunnels that intercepted the guarded tun nels below ground. Side-tunneling behavior allowed sneaking males to e nter tunnels beneath the guarding male, and mate with females undetect ed. Both overall body size and relative horn length significantly affe cted the outcome of fights over tunnel ownership. These results sugges t that alternative reproductive tactics may favor divergence in male h orn morphology, with long horns favored in males large enough to guard tunnels, and hornlessness favored in smaller males that adopt the ''s neaking'' behavioral alternative.