Investigating the role of a mud structure in a fiddler crab: do semidomes have a reproductive function?

Sensory traps occur when a signal evolves to mimic a cue to which the receiver responds favorably in another context. For example, if females are attracted to the noise a prey item makes, then a conspecific male can mimic that noise, thereby attracting females in the context of mating. Thus, sensory traps can be beneficial to receivers since attraction helps locating males more efficiently. But what happens when receivers are less attracted by the sensory trap outside of the original context (due to the cost of the response for instance)? Does this weaken and eventually “break” the sensory trap? Here, we show that a rare, small mud structure (a semidome) built by some male fiddler crabs (Austruca mjoebergi) at their burrow entrances are most likely relics of the sensory traps used by other fiddler crabs (the larger, far more abundant hoods of Leptuca species). Females of A. mjoebergi have retained their tendency to move towards vertical structures for protection when threatened by a predator—the original context thought to be important in the evolution of sensory traps in other fiddler crab species. However, females show no significant preference for males with semidomes. Additionally, males do not use the semidomes as landmarks to relocate their burrows when courting. The results suggest that in A. mjoebergi, semidomes do not function as sensory traps, despite the existence of a pre-existing bias in females. We suggest that semidomes have lost their attractiveness in the mating context due to ecological features, such as predation and environmental heterogeneity. Significance statement: There is growing evidence of sensory traps in many animal species, with an ever-widening range of contexts from nuptial gift giving in spiders to egg recognition in non-brood-parasitized birds. While we are starting to understand the significance of sensory traps, little thought has been given to what happens when they break down. If there is a change in the original context that favored the receiver’s response, then there is likely to be a subsequent change in the receiver’s bias and finally, in the sensory trap. If the mimic of the original stimulus does not induce a beneficial response in the receiver anymore, the sensory trap would become inefficient and stop being selected. The effect will be most profound when the sensory trap was costly to set up, since there will be selection against its retention. Here, we show that a sensory trap can become a relic when ecological conditions change and make it not functional anymore. To our knowledge, it is the first study providing such empirical evidence.The study was funded by an Australian Research Council Discovery grant to PRYB (DP160100316) and a Research School of Biology (Australian National University) grant to PRYB (R41060 52PB)