Sexual differences in immunity across developmental stages in rhinoceros beetles (Coleoptera: Scarabaeidae: Dynastinae)

Abstract

Life history theory suggests that females should allocate greater resources to immune function than males to increase their lifetime fitness. Although higher immune responses in adult females compared to males have been reported across various insect species, sexual differences in immunity during juvenile stages have been less explored. We investigated sexual differences in haemocyte density and phenoloxidase (PO) activiy in two species of rhinoceros beetles, Trypoxylus dichotomus (Linnaeus) (Coleoptera: Scarabaeidae) and Oryctes rhinoceros (Linnaeus) (Coleoptera: Scarabaeidae), during their larval stages. We found that male T. dichotomus larvae consistently displayed lower haemocyte densities than females, and this sexual difference persisted until adulthood. The sexual difference in immunity across life stages may be associated with sex-specific resource allocation patterns throughout lifetime. However, the less sexually-dimorphic species, O. rhinoceros, did not exhibit sexual differences in haemocyte densities during immature stages. PO activities did not exhibit sexual differences in larvae of either species. Additionally, we observed an increase in haemocyte densities with larval developmental stages in both species. However, while T. dichotomus exhibited lower PO activities in later stages, PO activities in O. rhinoceros did not largely change throughout development. Path analysis revealed that the ontogenetic changes in constitutive immune parameters were primarily driven by direct effects of body size rather than age. The species- and parameter-specific ontogenetic trends in immunity could be associated with allocation to other life history traits or tradeoffs among immunity parameters.