Behavioural, microhabitat, and phylogenetic dimensions of intrasexual contest competition in combatant monkey beetles (Scarabaeidae: Hopliini)

Abstract

The importance of sexual selection as a driver of evolution, from microevolution to speciation, has overwhelmingly been studied in the context of female choice, but there is evidence that male-male competition can also drive evolution. Recent reviews of the intrasexual competition literature have developed several hypotheses of weapon divergence in both allopatry and sympatry and have suggested means by which weapon divergence may cause reproductive isolation and speciation, both alone and together with mate choice and ecological selection. Here, I assess the role of sexual selection, in the context of environmental variation at the level of the contest substrate and the developmental environment, in contributing to microevolution within the monkey beetles (Coleoptera: Scarabaeidae: Hopliini), a taxonomically and phenotypically diverse group of pollinating insects in the Greater Cape Floristic Region (GCFR) that shows a high degree of sexual dimorphism and mating behaviour driven by male-male competition. I build on previous observations of hind leg use in intrasexual male-male contest for reproductive access to females by showing that, in Heterochelus chiragricus, contests occur in the context of a significantly maleskewed sex-ratio and consist of vigorous wrestling and pushing between two males on the flower heads occupied by embedded, feeding females, who apparently exert no mate choice. Contest outcomes are influenced by hind femur size and residency effects, and I apply hypotheses informed by evolutionary game theory to assess how males make decisions regarding persistence versus retreat. I proceed to assess the evidence for the ‘divergent fighting contexts' hypothesis which predicts weapon divergence driven by intrasexual contest competition in the context of variation in the contest substrate. I find that hind leg size in another combatant monkey beetle, the species complex Scelophysa trimeni, varies across gradients of flower size among several spatially distributed populations, suggesting that variation in flower size (the contest substrate) mediates selection for weapon morphologies that maximise performance under different fighting styles necessitated by differences in the contest substrate. I also find that male elytral colour varies both across gradients in the developmental environment and with variation in flower colour, suggesting that this trait may function as an honest signal of male fitness, but also that it may be under selection to maximise signal transmission against variable backgrounds of contest substrates. Finally, I quantify the extent to which integration, modularity, multivariate allometry, and phylogenetic effects influence the evolutionary lability of male monkey beetle's hind legs, and so mediate the pace of their evolutionary diversification in response to these varying contest substrates. My findings support a two-module pattern of modularity at both static and evolutionary levels, and I find that allometric scaling relationships are conserved within S. trimeni. These findings indicate that monkey beetle weapons are relatively unconstrained in their evolutionary diversification across divergent fighting substrates. I conclude by discussing these findings within the broader field of sexual selection and monkey beetle ecology and suggest directions for further work. The findings presented here support a role for sexual selection, interacting with variation in the flower contest substrate, as being an important driver of the diversification of monkey beetles in the GCFR.