Do longer genital spines in male seed beetles function as better anchors during mating?

In a wide variety of taxa, males are equipped with harmful structures on their genitalia such as hooks, barbs or spines. The proximate function of these structures and the evolutionary forces behind their evolution have been discussed and investigated during the last few decades. One model system in which these structures have attracted particular attention is the Callosobruchus seed beetle group. The main suggestion for the occurrence of genital spines in this group of species has been that their primary function is to act as an anchor during mating, to aid the male in staying attached to the female. This would prevent females terminating copulation prematurely, or would hinder take-overs by rival males. We used five populations of Callosobruchus seed beetles, with differing lengths of the male genital spines, to test whether longer spines provide males with an enhanced attachment during mating. This was tested both with and without male competition in the form of rival males present or not during focal copulations. We found that males from populations with longer spines did not stay in copula for longer than males from populations with shorter spines. In addition, females mating with males with longer genital spines suffered a fitness cost in terms of lower lifetime offspring production. In conclusion, we did not find any support for the hypothesis that the primary function of genital spines in seed beetles is to serve as an anchor.