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

The bull-horned dung beetle Onthophagus taurus has been introduced (intentionally or otherwise) to several continents, including Australasia and North America, where it provides key ecological functions. These introductions offer a unique system to study the rapid evolution of sexual traits during range expansion over the past 50 years. Males of this species exhibit exaggerated horns that are nutritionally plastic and mediated by a developmental threshold. Within populations, only males with access to high-quality diets surpass this threshold and develop large horns (“majors”), while those below it develop small or no horns (“minors”). This results in a distinct polyphenism with a bimodal distribution of horn length corresponding to alternative reproductive tactics. Previous research has shown that the body size threshold separating minors and majors evolves across populations: Australian populations exhibit higher thresholds than native populations, whereas Eastern U.S. populations show lower thresholds. This divergence has been linked to the intensity of sexual competition associated with differing population densities. Here, we test this hypothesis using the previously unstudied introduction in California. West Coast populations have comparatively low densities, and under the density linked competition hypothesis, we predicted a lower threshold relative to other populations. Contrary to our expectation, thresholds were higher, suggesting that sexual competition may not be the primary driver of horn divergence in these populations.