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We examined multiple sources of selection on the multimodal courtship display of two sister species of wolf spiders, Schizocosa crassipalpata and Schizocosa bilineata. We first experimentally altered body condition in these two species by employing divergent diet treatments. We then tested for differences in adult male seismic courtship displays, visual courtship displays, and foreleg morphology; as well as adult female foreleg morphology. We then simultaneously examined both content and efficacy-based sources of selection on the visual and seismic multimodal courtship display of both species by conducting mate choice trials with high and low diet individuals across manipulated signaling environments and by conducting mate choice trials where the level of male ornamentation was experimentally manipulated.
Our diet treatments diverged body condition indices between HD and LD males and females of both species. We found condition dependent expression of male S. crassipalpata seismic courtship display and foreleg length as well as male S. bilineata foreleg length and brush size, and female S. bilineata foreleg length. We found no difference in the pattern of copulatory success for either species between males that had their level of foreleg ornamentation experimentally altered. However, we did find a difference in the signal efficacy function of male multimodal courtship displays between species. Male S. crassipalpata courtship displays were dominated by the seismic signal, across our treatments copulation success was highest when it was present. Conversely, male S. bilineata courtship displays were dominated by the visual signal, and across our environmental treatments, copulation success was highest when it was present. Male S. bilineata also courted significantly more in the visual positive environment. An ecological shift in activity patterns may have caused the initial split between S. crassipalpata and S. bilineata and a shift in female preference for a more visual display by S. bilineata females may have subsequently reinforced reproductive isolation between these two species.