Visual mutual assessment of size in male Lyssomanes viridis jumping spider contests

Numerous animals are known to assess the resource holding potential of their opponents using conventional signals and other correlates of resource holding potential. Although body and weapon size generally correlate with resource holding potential and are often presumed to be visually evaluated in animal contests, no one has demonstrated visual assessment of opponent size while controlling for all potential correlates of size. To this end, we presented male Lyssomanes viridis jumping spiders with computer-animated opponents 1) of 3 different overall sizes and 2) with different weapon and nonweapon appendages elongated by the amount that would normally accompany a 20% increase in body size. Male L. viridis have strikingly colored, exaggerated chelicerae and forelegs, which are used as weapons in contests, and the forelegs are waved during visual agonistic displays. We scored 4 levels of escalation in males’ responses to animations. Using generalized linear mixed modeling, we assessed the relative predictive power of the following variables on escalation intensity: 1) focal male size, 2) animated opponent size, and 3) the difference in size between the focal male and his animated opponent. When we presented males with animations scaled to different sizes, we found that size difference was the best predictor of escalation intensity, followed by opponent size. The effect of opponent size disappeared when size difference was included in the same model. Focal male size did not significantly predict escalation intensity. This suggests that males employ a mutual assessment strategy. Surprisingly, males did not respond differently to animations with versus without elongated weaponry.