Testing for Differences in Metabolism Among Females and Dimorphic Males of Four Dung Beetle Species (Coloeoptera: Scarabaeinae).

Abstract

Both sexual and male dimorphism are common in nature, yet we have limited understanding of how different developmental pathways and reproductive strategies of morphs shape energetics. To address this gap, we examined metabolic rates of four species of dung beetle (Onthophagus taurus, Onthophagus hecate, Oxysternon silenus, and Phanaeus vindex) with both sexual and male dimorphism. In these species, males have horn length dimorphism, including larger-horned ("major") males and smaller-horned ("minor") males. The gene doublesex, dsx, drives both sexual dimorphism and, by mediating nutrition-dependent horn growth in some species, male dimorphism. Because females and minor males share developmental pathways and have greater investment in reproductive organs than major males, we hypothesized energetic costs would be similar and higher in females and minor males compared to major males. To test this hypothesis, we examined metabolic rates of morphs using flow-through respirometry to record CO₂ output. After accounting for body size and activity level, we found that in two species, Onthophagus taurus and Phanaeus vindex, females had higher CO₂ production compared to major males, and in O. taurus, females also had higher CO₂ production than minor males. We detected no differences between sexes for O. hecate and O. silenus. We also found no significant difference in metabolic rates between major and minor males of any species. Our results suggest that, for these species of dung beetles, any energetic tradeoffs due to reproductive strategies occur between females and males, but not between male morphs. The lack of a general trend in metabolic rates suggests energetic costs are decoupled from sex and male morph across dung beetle species, which runs counter to evolutionary explanations for the maintenance of alternative reproductive tactics.