The effect of condition on mate searching speed and copulation frequency in the Cook Strait giant weta

Abstract

Nutritional condition is important to the function and performance of several facets of male reproduction such as searching for mates and producing ejaculates. Nutritional status should be important to male success in the Cook Strait giant weta (Deinacrida rugosa), a flightless orthopteran insect, in which males invest considerable effort in mate searching during scrambles and transfer multiple ejaculates to female partners during a single mating bout. In this study, we hypothesize that success in scrambles for mates is related to nutritional status and thus predict that males in better nutritional condition will: (1) be more successful at finding a female, (2) find her more quickly, and (3) cover more distance per unit time during their search (i.e., have greater speed). We also predict that males in better nutritional condition will transfer more spermatophores than starved males. Although we found that critical facets of mate searching (e.g., speed) were not significantly affected by body condition, we did discover that spermatophore transfer to females was significantly related to female size and the interaction between male search effort and male body condition. We posit that perhaps males transfer spermatophores based on their current body condition and perceived mate availability. Nutritional conditon is an important factor in determining the expression of sexually selected traits in males; well-fed males produce better sexual ornaments and larger ejaculates than poorly fed males. Most studies that address this hypothesis, however, focus on species in which males display to potential female partners and then, if successful, transfer a single ejaculate to their mate. In contrast, condition could be more critical to male success in species in which males search for mates over long distances. Such scramble competition is shown by Cook Strait giant weta, where male leg length is a sexually selected trait. In this enormous insect (of conservation importance), males pass multiple ejaculates (spermatophores) to each mate; so condition in this species is also expected to be important to copulation success. Our study supports this hypothesis, but we found no evidence that mate searching speed is condition-dependent.