It is well documented that sexual selection acts on morphological differences between individuals and can lead to sexual dimorphism in species with male combat and female choice. However, the effect of sexual selection on the evolution of morphological traits is poorly understood in species with scramble competition in which males race for access to females during a brief pulse of breeding activity. Because male access to females may be more random in scramble mating systems, male morphological traits may not strongly influence mating success. To investigate the influence of sexual selection on male morphology in a species with a scramble mating system, we collected detailed morphological measurements from male and female Columbia Spotted Frogs (Rana luteiventris), which have an extreme scramble mating system. Male Columbia Spotted Frogs have enlarged thumb (nuptial) pads and muscular forearms, which may help them grasp females. As predicted, we discovered that males found in amplexus with females had proportionately larger nuptial pads and thicker forearms than did unpaired males, but mating success was not related to overall body size. Although many single males attempted to dislodge males already in amplexus with females, no attempted takeovers were successful. Therefore, we suggest that the advantage of thick forearms and large nuptial pads occurs during the initial phase of securing a female when she first enters the breeding area. These findings suggest that sexual selection may influence morphology even in a species with scramble mating.
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