Assessing mate size in the red swamp crayfish Procambarus clarkii: effects of chemical versus visual stimuli.

1. Mate assessment requires an efficient system of information exchange between sexes and often relies on a form of multimodal communication. Both sexes of the crayfish, Procambarus clarkii, select a mate on the basis of its body size, independently of chelar size/symmetry and dominance status, large dimension being a proxy of the high quality of the partner. Here, we explored the effects that visual and chemical stimuli, emitted alone or in combination, exert on the assessment of the size (i.e. the quality) of potential mates. 2. We followed a binary choice test paradigm, in which two male or female “targets”, of either a large or a small size, were simultaneously presented to a female or a male “chooser”. We recorded the first target visited, the total duration of the visits per target, and the behaviour of the chooser approaching the target’s area. 3. Our results show that females require the combination of visual and chemical stimuli to select the larger male. A more complex pattern was found in males, who seemed to rely on smell only when they had to make the first choice of the larger female and to persist in it, but used a combination of visual and chemical cues to modulate their behaviour towards the target. In P. clarkii’s mate assessment, visual and chemical information seems to act as ‘non-redundant signals’.