Does sexual competition predict horn polyphenism in the bull-horned dung beetle?

The Mediterranean dung beetle Onthophagus taurus has been introduced (intentionally or otherwise) to several continents including Australasia and North America, providing key ecological functions along the way. These recent introductions provide a unique system to study rapid evolution of sexual traits during range expansions. Males of this species exhibit exaggerated horns that are nutritionally plastic. Within populations, only large (‘major’) males with access to a high-quality diet develop exaggerated horns, while small (‘minor’) males only develop reduced horns. This leads to a distinct polyphenism with a bimodal distribution of horn length that coincides with two distinct reproductive tactics. Previous research on horn development has shown that the body size threshold separating ‘minor’ males from ‘major’ males evolves across populations. For example, in populations with higher population densities (e.g., Australia), O. taurus evolved a higher body size threshold separating ‘minors’ from ‘majors’. This divergence has been linked to the intensity of sexual competition imposed by varying population densities. Here, we test this hypothesis by investigating a previously unstudied introduction in California. Westcoast populations have comparatively low population densities and based on the hypothesis that population density shapes adaptive scenario, we expected this population to have evolved a lower threshold compared to other populations. In contrast to our prediction, thresholds were higher along the Westcoast, suggesting that sexual competition may not be the primary mechanism driving horn dynamics. We discuss alternative selective and neutral processes that could drive the evolution of secondary sexual traits during rapid range expansions.