MALE MATING TACTICS IN THE HORSESHOE-CRAB, LIMULUS-POLYPHEMUS

Horseshoe crabs nest on beaches of the eastern U.S. at the new and full moon high tides during spring and summer. Some males arrive attached to females, whereas others arrive unattached. The unattached males crowd around the nesting couples where they are thought to gain fertilizations (fertilization is external). Because males return to the beach more frequently than females (and most females arrive attached), the operational sex ratio during one tide can be strongly male biased. Attached and unattached males did not differ in size but attached males were less likely to have their prosomas and eyes encrusted with epibionts, and they were lighter in colour, suggesting that they were on average either younger than unattached males or that they had moulted more recently. When attached males were experimentally detached from females, they were more likely to return with a female on the following tide than males that had arrived on the beach unattached. When detached males and unattached males were allowed to pair with females in wading pools and were then released at sea while still attached, the detached males were more likely to return the next day with the same partner than the unattached males. Although detached and unattached males were equally likely to attach while in the pools if given enough time, unattached males took longer to attach and were more likely to let go of the female during the release process than the detached males. These results suggest that attached and unattached male mating tactics are the result of condition-dependent differences. © 1992 The Association for the Study of Animal Behaviour.