FEMALE BEHAVIOR, SEXUAL COMPETITION AND MATE GUARDING IN THE BLUE-CRAB, CALLINECTES-SAPIDUS

Blue crabs mate immediately after the female's final moult. We tested the influence of female moult stage, sex ratio and male size on the pr e-mating behaviour of both sexes, and the ability of males to pair wit h females and aggressively compete for access to females. We observed crabs in field enclosures and surveyed pre-copulatory mate-guarding pa tterns in the field. Female behaviour changed as they progressed throu gh the final moult cycle, such that early moult-stage females avoided males, but late moult-stage females initiated pair formation. The chan ges in female behaviour influenced both the behaviour and pairing capa bility of males. Males courted and paired with late moult-stage female s on their first attempt, but pursued early moult-stage females becaus e their first attempts to pair often failed. In the field, early moult -stage females were paired less often than late moult-stage females. T he pre-mating behaviour of both sexes also varied with sex ratio; when males were abundant, males traded courtship for forced capture and fe males courted less. Large males were more successful at take-overs, bu t did not pair more often with late moult-stage females, suggesting th at large males do not consistently guard for less time than small male s. The changes in female behaviour are consistent with the female's ne ed to avoid the costs of guarding and suggest that females influence h ow pre-copulatory mate guarding occurs in this species. (C) 1998 The A ssociation for the Study of Animal Behaviour.