Plasticity in sex roles and female competition in Neotropical dung roller beetles

Numerous studies show that sex roles can be plastic, and different eco-evolutionary scenarios yield sexual selection also acting on females. Plastic sex roles can occur in response to environmental cues, such as when food and mate resources are limited. In dung roller beetles, the encounter of sexes occurs at the food source; males are commonly reported cutting, forming balls and attracting females. Intrasexual competition has been particularly associated with fights for mate acquisition among males, while females heavily invest in parental care. Here, we test the hypothesis that females have high motivation to compete for balls and mates due to the striking relevance of gaining fertilizations before nesting. This competition is expected to be more intense when the operational sex ratio is biased, affecting behaviors related to ball acquisition (ball production and fights for ball possession) and access to mates (male-female pair formation, courtship and mating). We used the Neotropical dung roller beetle Canthon bispinus and exposed males and females to two experimental groups differing in the operational sex ratio: Female and Male Bias group. We examined the proportion of successful trials for females and males. We found that males were very active in food search in both groups, while fighting with each other in the Male Bias group. In contrast, females became highly competitive, actively engaging in ball rolling and fights in the Female Bias group. Our findings revealed an elevated competition for mates at the food source and that sexual selection may also operate on females in dung roller beetles.