Influence of seeing a red face during the male–male encounters of mosquito-specialist spiders

Males of Evarcha culicivora, an East African jumping spider (Salticidae), have bright red faces. Here, we investigated how seeing a red face might influence a male’s behaviour during encounters with another male. We applied black eyeliner to conceal the red on a male’s face and measured the spectral properties of male faces with and without the eyeliner. Only the faces without eyeliner reflected in the long-wavelength range corresponding to red. In experiments over 2 days, where eyeliner was absent on the first day and present on the second, we compared how two groups of males responded to their mirror images. Face Group: eyeliner concealed their faces. Head Group: eyeliner was applied to the tops of the males’ heads instead of on their faces. The males from both groups displayed to their mirror image as if it were a living same-sex conspecific. However, when they could see a red face, males in the face group escalated to higher levels of aggression to their mirror image and initiated displaying from farther away than when the red had been concealed. We also found that the influence of eyeliner was stronger for the face group than for the head group. These findings suggest that, when seeing a red face, E. culicivora males become more confident that the individual in view is another male.