Female preferences for timing in a fiddler crab with synchronous courtship waving displays

• We used robotic males to investigate female preferences in a fiddler crab.
• We examine the effect of male signal timing on attractiveness.
• Males who signal just before a synchronous group are favoured.
• Males who signal in alternation with the group are also favoured.
• These preferences could drive synchronous signalling in wild males.

Studies of sexual communication typically focus on the design and information content of a signal of interest, but the timing of signal production relative to nearby competitors can be crucial. Male fiddler crabs, Uca mjoebergi, court females with a stereotyped claw-waving display, and males are often observed waving in synchrony with nearby claw-waving males. Using female mate preference experiments with robots that imitate male claw waves, we found evidence that females are more attracted to males whose waves immediately precede a synchronous group of waves (leaders); females also favoured males that waved in opposite phase to a synchronous group (alternators). By contrast, males whose waves lagged behind a group of synchronous wavers (laggards) were no more attractive. We discuss a simple sensory process that could explain how this female preference arises. Our results agree with past findings suggesting that synchrony in fiddler crabs occurs as an epiphenomenon of adaptive male responses to female preferences.