Alternative reproductive tactics in female horseshoe crabs

Consistent differences among females in mating with one (monandrous) or multiple males (polyandrous) may be a product of male behavior or may reveal the existence of female alternative reproductive tactics (ARTs). The distinction is an important one for understanding the evolution of sexually selected behavior. We evaluated whether ARTs exist in female horseshoe crabs (Limulus polyphemus), a species in which male alternative reproductive tactics are well known. In this species, attached pairs migrate to shore and spawn on high tides; the male fertilizes the female's eggs externally with free-swimming sperm as the eggs are being laid. Unattached males are attracted to pairs by visual and chemical cues and become satellites of some females while ignoring others. We used multiple lines of evidence, including mark/resighting; measurements of size, physical condition, and eggs laid; and field manipulations of female response to satellite males. We show that even at high nesting densities with intense male–male competition, some females mated only with their one attached male, and females were consistently monandrous or polyandrous across multiple nestings. Monandrous females did not attract satellites but when males were experimentally manipulated to join monandrous pairs, some females stopped nesting and left rather than nest with a satellite male. These females tended to be smaller than polyandrous females. Our results suggest condition-dependent differences between monandrous and polyandrous females that result in different context-dependent mating decisions by monandrous and polyandrous females to cope with sexual conflict.