Benefit of polyandry in a highly monandrous species when females mate with already mated males

Female mating frequency varies among animal taxa. A benefit to females of remating has usually been found, but almost all tests have been with polyandrous species. A species being monandrous does not guarantee that mating only once benefits the female, instead the monandry may result from sexual conflict, where her failure to remate benefits her mate, but not her. The parasitoid wasp Spalangia endius (Hymenoptera: Pteromalidae) is highly monandrous. Females do not benefit from either immediate or delayed remating when their first mate is virgin. However, some females are likely to mate with already mated males because sex ratios are female-biased. Here, the effect of experimentally induced polyandry on female fitness was examined for females whose first mate had already mated four times, i.e., for fifth females. Fifth female S. endius produce significantly fewer daughters than first females. Production of daughters, but not sons, requires sperm in hymenopterans. Fifth females were experimentally induced to mate with a second male, by preventing such females' first mate from providing postcopulatory courtship. The proportion of female offspring produced by these polyandrous fifth females was greater than by monandrous fifth females and not significantly different than by monandrous first females. Total number of offspring did not differ among the three treatments. These results show that there are conditions under which females benefit from polyandry in this highly monandrous species and that the benefit is through effects on offspring sex ratio, not fecundity. Mating frequency varies widely among animals. In most of the more than 100 past studies of insect species in which females mate multiply, females benefit from remating. However, this same question has been addressed in few species in which females mate just once. In the tiny parasitic wasp studied here, females were tricked into remating. Females benefited from remating if their first mate had previously mated multiple times. The benefit was not producing more offspring, but rather being able to produce a greater proportion of daughters. Sons can be produced without sperm in wasps and wasp relatives. Explanations for why females do not normally remate include that they have been manipulated by their first mate and that waiting for a male means sacrificing searching for hosts.