Reproductive competition promotes the evolution of female weaponry

Abstract

Secondary sexual traits in females are a relatively rare phenomenon. Empirical studies have focused on the role of male mate choice in their evolution; however, recently it has been suggested that secondary sexual traits in females are more likely to be under selection via reproductive competition. We investigated female competition and the influence of female phenotype on fitness in Onthophagus sagittarius, a species of dung beetle that exhibits female-specific horns. We compared reproductive fitness when females were breeding in competition versus breeding alone and found that competition for breeding resources reduced fitness for all females, but that smaller individuals suffered a greater fitness reduction than larger individuals. When females were matched for body size, those with the longest horns gained higher reproductive fitness. The fitness function was positive and linear, favouring increased horn expression. Thus, we present evidence that female body size and horn size in O. sagittarius are under directional selection via competition for reproductive resources. Our study is a rare example of female contest competition selecting for female weaponry.

Figure 1.

Relationship between female body size (pronotum width) and horn size (head horn length). Symbol size represents from 1 to 3 overlapping data points (R² = 0.87, p < 0.0001).

Figure 2.

Focal female fitness (average number of brood balls produced ± 1 s.e.) when breeding alone (black bars) and in competition (white bars) for all female size categories.

Figure 3.

Relationship between relative horn length and relative fitness of competing O. sagittarius females. Positive values on the x- and y-axes represent cases where, between two competing females that had been matched for body size, the focal female had a larger horn and produced more broods relative to her competitor.