Population differences in the strength of sexual selection match relative weapon size in the Japanese rhinoceros beetle,Trypoxylus dichotomus(Coleoptera: Scarabaeidae)†

Abstract

Exaggerated weapons of sexual selection often diverge more rapidly and dramatically than other body parts, suggesting that relevant agents of selection may be discernible in contemporary populations. We examined the ecology, reproductive behavior, and strength of sexual selection on horn length in five recently diverged rhinoceros beetle (Trypoxylus dichotomus) populations that differ in relative horn size. Males with longer horns were better at winning fights in all locations, but the link between winning fights and mating success differed such that selection favored large males with long horns at the two long‐horned populations, but was relaxed or nonexistent at the populations with relatively shorter horns. Observations of local habitat conditions and breeding ecology point to shifts in the relative abundance of feeding territories as the most likely cause of population differences in selection on male weapon size in this species. Comparisons of ecological conditions and selection strength across populations offer critical first steps toward meaningfully linking mating system dynamics, selection patterns, and diversity in sexually selected traits.

Digest: Sexually selected weapons: Winning the fight, but losing the war for reproduction on a changing battlefield*

“…In this issue, Del Sol et al. (2020) propose that changes to the location and number of feeding sites in T. dichotomus beetles may explain why horn sizes have diverged in this species. To test their hypothesis, the authors combined pre‐existing data with their own observations of male mating success, behavior, and ecology for five T. dichotomus populations across the islands of Japan and Taiwan.…”

“…This study by Del Sol et al. (2020) comparing selection strength across populations offers an important first look into the complex interaction between mating systems, patterns of selection, and sexually selected traits.…”

“…Graphical representation of the patterns described by Del Sol et al. (2020) that illustrates that female habitat and resource use have the potential to influence evolution of male weapon size through sexual selection, and that this process could initiate or maintain divergence of weapon size.…”

Sizing Up Swords: Correlated Evolution of Antlers and Tusks in Ungulates

“…Sexual weapons are used in physical combat to secure mating opportunities; victorious males typically have larger body size and larger sexual weapons as a direct result of intrasexual selection (Geist 1999;Emlen 2008;McCullough et al 2016;Sol et al 2020). Structures that serve as intrasexual signals and weapons (e.g., ungulate antlers and horns) typically exhibit positive allometry in both developmental growth and evolution: As individuals grow and species evolve larger body sizes, they grow and evolve disproportionately larger ornaments than smaller individuals and species (Gould 1974;McCullough et al 2015; Rico-Guevara and Hurme 2018; Somjee et al 2021).…”

“…Larger, stronger individuals invest more energy into these presumably honest signals of strength in order to avoid costly or injurious ghts (Barrette 1977;Emlen 2008). Pure sexual weapons (e.g., tusks, claws, rhinoceros beetle horns), however, have evolved to provide advantages during intrasexual ghts, are only used in combat, and thus provide little signaling value (McCullough et al 2016; Rico-Guevara and Hurme 2018; Sol et al 2020). Few studies, though, have examined static allometric growth or allometric evolution of these pure, non-signaling, sexual weapons, and even fewer examine these allometric relationships in species with multiple sexual weapons.…”