Female choice, male interference, and sperm precedence in the red-spotted newt

Abstract

Darwin first identified female choice and male-male competition as forms of sexual selection resulting in the evolution of conspicuous sexual dimorphism, but it has proven challenging to separate their effects. Their effects on sexual selection become even more complicated when sperm competition occurs because sperm precedence may be either a form of cryptic female choice or a form of male-male competition. We examined the effects of tail height on male-male competition and female choice using the sexually dimorphic red-spotted newt (Notophthalmus viridescens viridescens). Experiment 1 examined whether male tail height influenced male mating success. Males with deep tails were more successful at mating with females than those with shallow tails. Successful, deep-tailed males also were bigger (snout-vent length; SVL) than unsuccessful, shallow-tailed males, but they did not vary in tail length or body condition. Of these, only tail height and tail length are sexually dimorphic traits. Experiment 2 tested the hypothesis that the differential success of males with deeper tails was due to female choice by examining both simultaneous female preference for association and sequential female choice. We found no evidence of female choice. When males were not competing to mate with females, tail height did not influence male mating success. Successful males did not have different SVL and tail lengths than unsuccessful males. Thus, tail height in male red-spotted newts appears to be an intrasexually selected secondary sexual characteristic. Experiment 3 used paternity exclusion analyses based on molecular genetic markers to examine the effect of sperm precedence on sperm competition in doubly-mated females. Sperm precedence likely does not have a pervasive and consistent effect on fertilization success because we found evidence of first, last, and mixed sperm usage.