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In some species, especially those that form breeding aggregations, males form groups around females vying for fertilizations. These mating groups are often highly variable in size even during a single breeding event. We examined this variation in the American horseshoe crab, Limulus polyphemus, a species in which some females (nesting in pairs) attract additional males (satellites) and spawn in groups whereas others spawn only with their attached male. The observed distribution of group sizes often consists of an overabundance of pairs with no satellite males and groups with large numbers of satellites. We characterized this variation using well-known models of count data, such as the Poisson and negative binomial distributions, and evaluated how operational sex ratio (OSR) and pair density contribute to the observed variation. We complemented this descriptive approach with spatially explicit simulations of the group formation process. By comparing our simulation results to the observed breeding aggregation data, we identified how simple behavioural rules might contribute to the variation we observe. Those rules amount to hypotheses about pair and satellite arrival at the beach and satellite sampling of and choice among pairs. We found that the observed variation could be explained by a consistent high frequency of females that attract satellites in combination with males that sample different fractions of the beach as pair density and OSR vary. Furthermore, our simulations suggested that males are not choosy in joining pairs on the beach, despite variation among pairs in the fertilization success satellites can achieve. This suggests that new insights might come from investigating the costs and benefits of male choice and the sensory mechanisms satellite males use to assess the breeding aggregation before coming ashore.

Why do females mate with multiple males? Since females often acquire all the sperm they need in one mating and since mating is usually costly, the evolution of polyandry is puzzling. Females may gain from mating multiply by ensuring fertilization, increasing offspring diversity, improving mate quality (‘good genes’) or increasing genetic compatibility. Alternatively, multiple mating may result in a net cost to females as a by-product of male–male competition. We report on field experiments in which we evaluated whether spawning with multiple males is costly or beneficial for female horseshoe crabs, Limulus polyphemus . Unpaired male horseshoe crabs crowd around some spawning pairs (polyandrous) as satellites while ignoring others (monandrous). We conducted three field experiments in which we compared the success of (1) unmanipulated polyandrous and monandrous females, (2) polyandrous females with and without satellites (satellites removed) and (3) monandrous females without and with satellites (forced to be polyandrous by adding satellites). When polyandrous females lost their satellite males, they laid more eggs than did polyandrous females that retained their satellites. When a satellite male arrives at a monandrous pair, some females are tolerant and will nest as a polyandrous female, but many females are intolerant of satellite males and leave immediately, thus laying fewer eggs. These results indicate that spawning with multiple males is costly in terms of nesting success for female horseshoe crabs. Yet, natural monandrous and polyandrous females did equally well, suggesting that there are compensating benefits from multiple males for polyandrous females. We suggest that monandry and polyandry may be alternative reproductive tactics for female horseshoe crabs.

Unpaired or satellite male horseshoe crabs, Limulus polyphemus, are attracted to and often form a group around a pair (a female with an attached male) that is nesting in the high intertidal zone. These males are engaged in sperm competition. We observed nesting pairs and their associated satellites in the wild, collected and reared their eggs and used genetic markers to examine paternity. We found that the unpaired, satellite males are highly successful at fertilizing eggs; two satellites can leave the attached male with few fertilizations. Two satellites together are each as successful as one spawning with a pair. A satellite's location around the female greatly affects his success, and males compete for access to a position over the dorsal canal between the prosoma and opisthosoma of the female and under the front margin of the paired male where they are most likely to fertilize eggs. Although eggs and sperm retain their viability for some time after spawning, nearly all eggs are fertilized by the satellites that are around the nesting pair at the time of egg laying and by the attached male. A number of factors including beach current, female size and male behaviour affect the outcome of sperm competition in this externally fertilizing species. Copyright 2000 The Association for the Study of Animal Behaviour.

Male horseshoe crabs have two mating tactics: some come ashore attached to females, whereas others remain unattached and crowd around the pairs that are nesting on the beach. The benefits are clear because unattached males release sperm and gain fertilizations by their satellite behaviour, but there are also costs associated with this alternative mating tactic. Horseshoe crabs sometimes become overturned and stranded on beaches during spawning, which can result in death by desiccation or predation. In some populations, such as in the Delaware Bay where this study was conducted, large numbers of horseshoe crabs may be stranded on the beach after spawning. Most overturned crabs right themselves quickly using their telsons, but stranded individuals are unable to do so. The results of this study show that stranding is due to two interacting factors, mating tactic and righting ability. First, unattached males were more likely to strand than attached males. Second, unattached males were, on average, older than attached males. Experiments reveal that once overturned, older crabs were less likely to right themselves than younger ones. Ultimately, the reason that older males are more likely to strand is probably related to senescence and parasitism.

This study asks whether male-male competition and female choice are important in the mating system of sexually dimorphic, adult horseshoe crabs (Chelicerata: Limulidae, Limulus polyphemus L.). Amplexed pairs approaching a beach for spawning are generally accompanied by a number of unattached, sexually mature suitor males. For the amplexed pair, the average pairwise ratio of male to female carapace width remains constant at 0·78 throughout the spawning season. Morphological characters that relate to mating in amplexed males were compared with unpaired males. Mated and suitor male crabs could not be distinguished on the basis of morphology or body size. Male clasper dimensions, relative to the point of amplexus on the female opisthosoma, suggest that there are no size-related constraints on the point of amplexus on the female opisthosoma, suggest that there are no size-related constraints on the ability of males to amplex with females. Although multiple males participate in the mating process, via the formation of spawning clusters of up to 15 males to one female, there is no evidence of successful male-male displacement. Size-assortative mating does not occur, and large females do not attract either larger males or larger numbers of suitor males in a cluster. A simulation model, using data from 1800 individual horseshoe crabs, generated mated pairs based on the assumption that amplexus was a completely random process with respect to prosoma width. The frequency distribution of actual mated pairs was indistinguishable from the output of the simulation model. The role of size dimorphism and the male-biased sex ratio in the mating system of horseshoe crabs are discussed.

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.