Polyandry occurs because females initially trade sex for protection

In many species, females mate with multiple males, suggesting that polyandry confers fitness-enhancing benefits. The benefits of polyandry are usually attributed to either the cumulative acquisition of direct material benefits from consecutive mates or genetic benefits resulting from access to greater sperm diversity that facilitates cryptic female choice and sperm competition or simply elevates genetic diversity among offspring. With the notable exception of studies in birds that contrast within-pair and extrapair copulations, the alternative explanation that females receive different benefits from different types of mates is rarely explored. In the fiddler crab Uca mjoebergi, females mate multiply using two distinct mating tactics: surface and burrow mating. We found that females gained a different benefit from each type of mating. Females that initially mated on the mudflat surface secured the help of their mate in burrow defence. Male neighbours were significantly more likely to help after mating. In contrast, a female's final mating allowed her to choose the mate that would sire most of her offspring and gain access to a burrow that she then used for breeding. Together, these benefits provide a strong incentive for females to mate multiply.

► Male fiddler crabs will help a neighbouring female defend her territory.
► Males are more willing to assist if they have mated with the neighbour.
► Females surface mate with neighbours, but move away to burrow mate.
► Females are choosy about the size and claw type of burrow mated males.
► Burrow mated males were larger than average, and never had regenerated claws.