# Heterospecific interaction in two beetle species: Males with weapons decrease the reproductive success of species with weaponless males

## TL;DR

Male beetles with weapons reduce the reproductive success of weaponless beetles when they interact, showing how traits evolved for same-species competition can affect other species.

## Contribution

Demonstrates that male weapon traits can influence reproductive success in heterospecific interactions, beyond same-species competition.

## Key findings

- G. cornutus males reduced T. castaneum's resource acquisition and mating opportunities.
- T. castaneum's reproductive success decreased when cohabiting with G. cornutus males.
- Male weapon traits can impact reproduction in closely related species.

## Abstract

Many species often show male–male combat for mating opportunities and resources within the species. Sexual selection through this radical combat leads to the evolution of males with exaggerated traits used as weapons, such as horns or mandibles, that often result in victory during combat. However, heterospecific interaction due to errors in species identification has often been observed, which results in decreased mating opportunities within the same species and fewer fertilized eggs. Males with exaggerated weapons may show dominance in resource acquisition over males without weapons and may decrease the reproductive success of the latter due to competition between the two. However, few studies have examined heterospecific interaction focusing on males with or without weapons. In this study, we investigated the effects of the male weapon on reproductive traits in heterospecific interaction in two species: the broad‐horned flour beetle (Gnatocerus cornutus), in which males have exaggerated weapon traits; and the red flour beetle (Tribolium castaneum), in which males have no weapon traits. Both species are closely related and use the same food resources. G. cornutus males interfered with the resource acquisition and reproductive opportunities of T. castaneum by attacking T. castaneum. The reproductive success of T. castaneum decreased when they cohabited with G. cornutus males. These findings show that male weapon traits, which are important for sexual selection within the same species, can also greatly influence reproduction in other species.

Weapon traits possessed by males of many species play an important role in male–male combat for reproduction, but it was unclear whether they affect reproduction in other species. We compared the reproductive success of each other using Gnatocerus cornutus, whose males have the weapon, and its close relative, Tribolium castaneum, whose males do not have weapon. We found that the presence of G. cornutus males reduced the reproductive success of T. castaneum, but not vice versa.