Female choice in the field cricket

L. W. SIMMONS Animal Behaviour Research Group, Department of Zoology, University of Nottingham, University Park, Nottingham NG7 2RD, U.K. Abstract. Females of the field cricket G. birnaculatus show multiple mating. Since such a strategy is likely to incur costs such as time and energy spent in orientation to calling males or increased susceptibility to predation, it must have certain selective advantages. This study shows that multiple mating is a potential mechanism of mate choice. Females removed the spermatophores of some males before complete insemination was achieved. Females also remained with large males in order to mate a second or third time. Since spermathecal morphology may limit last-male sperm precedence in this species, a function of multiple mating may be to dilute the sperm stored from previous matings with that of the current male, so increasing his representation in offspring production. Inter-male competition and female choice have often been viewed as alternative means by which a mating system may evolve. It is more likely, however, that both processes work simultaneously in influencing the reproductive behaviour of a species (Halliday 1983). For example, in mating rookeries of the elephant seal, Mirounga angustir- ostris, where inter-male competition is intense, females may make effective mate-choice decisions by inciting male aggression (Cox & LeBoeuf 1977; Cox 1981). Similarly, female dung files, Scatophaga stercoraria, adopt a rocking motion when mounted which increases the attack rate by nearby males and hence the probability of male replacement. Since competitive ability is correlated with body size, females effectively ensure matings with only the largest males (Borgia 1979). Because inter-male competition and female choice are likely to operate in close unison and because inter-male competition is often conspicuous and intense, more subtle instances of female choice may prove difficult to discern. Consequently, whilst inter-male competi- tion has readily been accepted as an evolutionary driving force, the role of female choice in the evolution of animal mating systems has become the subject of considerable debate (e.g. Partridge & Halliday 1984). Inter-male competition in the field cricket Gryl- lus bimaculatus is typically intense. Males show highly stylized aggressive behaviour and compete for access to shelters, such as gaps under stones, which are used as calling stations for the attraction of females. Large males tend to win aggressive disputes, are more likely to occupy calling sites and thus ultimately achieve a greater mating success (Simmons 1986). Females of G. bimaculatus show multiple mating. Repeated mating is common in many insect species despite the fact that females store sperm internally and a single mating is often sufficient to fertilize all the eggs a female will produce in her lifetime (Wigglesworth 1972; Sivinski 1980). Mating pat- terns of this kind should be selected against if the time and energy costs of mate searching courtship are high relative to the benefits gained from mating (Parker 1970). Since females of G. bimaculatus must orientate to sedentary calling males, time and energy costs are likely to be high. Mate searching may make females susceptible to predation. Sakaluk & Belwood (1984) have demon- strated how 'satellite predation' may impose sex- biased mortality on females of the decorated cricket, Gryllus supplicans (see also Daly 1978 for a review of the potential costs of mating). To offset such costs certain selective advantages must be attributable to female multiple mating in insects. Females may remate in order to increase de- pleted sperm stores (Gromko & Pyle 1978; Gromko et al. 1984; but see Fuerst et al. 1973). In G. birnaculatus, females accept many more sperm than are necessary to maintain fertility (see below). Multiple mating may also prove advantageous when males provide more than sperm at copula- tion. In some insect species, males provide nutritio- nal gifts either in the form of prey items (Thornhill 1976) or glandular secretions (Gwynne 1983). 1463