Reproductive behavior and ecology of the horned rainbow scarab beetle Phanaeus difformis (Coleoptera: Scarabaeidae)

The morphology, reproductive behavior, and ecology of the horned rainbow scarab beetle Phanaeus difformis was studied at the Rob and Bessie Welder Wildlife Refuge near Sinton, Texas to determine the influence of morphological and ecological traits on reproductive success. Phanaeus difformis is a sexually dimorphic dung beetle in which males possess much larger horns than females. Horn size correlates with body size in both sexes; however, horn size is distributed discontinuously in males resulting in major and minor male morph categories. Beetle abundance, sex ratio, and major:minor male morph ratio varies under natural conditions. Thus, the social conditions at dung pats where males and females construct feeding or nesting burrows and search for mates can result in intense competition for dung and mates. Pair formation occurs when males locate females in burrows. Copulation takes place in burrows and males sometime assist females in provisioning burrows. Under natural conditions, larger individuals of both sexes pair more frequently than smaller individuals. However, field and laboratory experiments reveal that variation in sex ratio and male morph ratio only influences male pairing success. Larger females are more successful in competitive encounters and have greater pairing success than smaller females under all experimental sex ratios. For males, large body and large horn size confer greater success in male-male encounters; however, major males obtain a pairing advantage when the sex ratio is male-biased or when major males outnumber minor males. Small males employ an alternate mating tactic more often than larger males, particularly when male-male competition is intense. The tactic involves sneaking into a pair\u27s burrow and mating with the paired female. Minor males, can therefore, maximize their mating opportunities by either sneaking or guarding females depending on prevailing environmental and social conditions