LAUSR

Abstract Rising sea level is reducing the inter-tidal zone in many mangrove forests. This breaks down the normal species distribution patterns of fiddler crabs, with an increasing number of heterospecifics moving from adjacent zones into an area normally occupied by a single species. Here we examine the interspecific social and sexual interactions that have resulted. We show that male Austruca mjoebergi are just as likely to help their small conspecific neighbor fight off an intruder when the intruder is a conspecific or heterospecific male. It appears that keeping a known neighbor is preferable to having any new neighbor (even a heterospecific neighbor that would not compete for receptive females) since the costs of renegotiating territory boundaries would be the same whatever the species of the new neighbor. We also show that males court females of their own species just as vigorously as those of two heterospecific species. Courtship is costly, so the time and energy spent courting heterospecific females is wasted: a potentially high cost of living among heterospecifics.