Female choice and the relatedness of mates in the field cricket, Gryllus bimaculatus

Anim. Behav., 1991,41,493-501

Female choice and the relatedness of mates in the field cricket,

Gryllus bimaculatus L. W. S I M M O N S * Department of Environmental and Evolutionary Biology, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, U.K.

(Received 25 April 1990; initial acceptance 5 June 1990; final acceptance 24 August 1990; MS. number."3567)

Abstract. Female choice for degree of relatedness of males in the field cricket, Gryllus bimacuiatus, was

investigated under simulated field conditions. Both the time spent by females with courting males and the probability of mating increased with decreasing degree of relatedness. After mating, the time spent by females with a guarding male and the duration of spermatophore attachment similarly increased with decreasing degree of relatedness. The differential behaviour of females towards males of different degrees of relatedness resulted in unrelated males having a greater probability of fertilizing eggs. Dominance status influenced a male's opportunity to mate but had no influence over female choice. Female behaviour was dependent on the location of courtship and mating; females mated sooner and were indiscriminate when they were courted away from burrows. While there was a weak tendency for females to prefer an intermediate degree of relatedness (cousin) over unrelated males, most evidence for this population suggests that female choice is simply a mechanism of inbreeding avoidance. Female choice for males carrying 'good genes' is the subject for continuing debate (e.g. Maynard Smith 1978; Thornhill 1980; Bradbury & Andersson 1987). One subtle argument of the 'good genes' hypothesis is that of genetic complementarity (e.g. Halliday 1983); females should mate with males whose genotypes complement their own in producing viable offspring. Both inbreeding and outbreeding are thought to incur fitness costs for females and/or their offspring. It is well established that crosses between individuals of very similar genotypes can result in an increasing homozygosity of deleterious mutations that reduce viability (inbreeding depression; Falconer 1989). Reduced viability can also result from crosses between individuals of very different genotypes (Templeton 1986; Partridge 1983 reviews the cost and benefits of inbreeding and outbreeding). Thus, an ability to recognize kin, and therefore genetic similarity, may allow individuals to mate with conspecifics whose genotypes best complement their own.

Bateson (1983) argued that animals may be selected to mate optimally with regard to relatedness, thereby balancing the costs of mating with close kin or unrelated individuals (optimal out*Present address: Department of Zoology, University of Western Australia, Nedlands, Perth, WA 6009, Australia. 0003-3472/91/030493+09 $03.00/0

breeding). A number of studies support the prediction that animals recognize kin and avoid inbreeding (e.g. Maynard Smith 1956; Dewsbury 1982; Smith & Ayasse 1987). However, few studies have revealed an optimal balance in outbreeding. Bateson (1983) was able to demonstrate a choice for first cousins rather than more closely or distantly related mates in Japanese quail, Coturnix coturnix. Two studies of rodents have revealed a similar pattern. Male laboratory mice, Mus domesticus, prefer females of an intermediate degree of relatedness and this preference is associated with increased reproductive output (Barnard & Fitzsimons 1988, 1989). Essentially identical results were obtained in a study of field-caught whitefooted mice, Peromyscus leucopus (Keane 1990). Female field crickets, Gryllus bimaculatus, discriminate among potential mates before and after mating. Females orient to aggregations of calling males and then move among them in bouts of repeated mating (Simmons 1986a, b). They choose large males as mates, removing the spermatophores of those males rejected before sperm have transferred, but remain with preferred males to allow full and repeated insemination (Simmons 1986a). This behaviour appears to bias subsequent paternity in favour of chosen males (Simmons 1987). When I confined females with a single male their response

9 1991 The Association for the Study of Animal Behaviour 493

Animal Behaviour, 41, 3

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Figure 1. Mating scheme for the generation of individualsof varyingdegree of relatedness.Male and femaleparents with the same letter were full siblings;those with different letters were unrelated. FS: full siblings;TS: three-quarter siblings; HS: half siblings;CS: cousins; NK: unrelated. to courtship varied with the relatedness of the male (Simmons 1989). Females evaded and even attacked courting full siblings but readily mated with unrelated males. Furthermore, after mating, females persistently tried to escape from the guarding attempts of full siblings. Although females appeared to respond differently to kin and non-kin, they did mate with all males. Thus these data can only be evidence for a 'preference' for unrelated mates (sensu Heisler et al. 1987). Whether females actually choose unrelated males (i.e. differential mating by females as a result of their preference) under natural conditions remains to be tested. Here I describe an experiment in which females were allowed to interact freely with males in an aggregation where the males differed in their degree of relatedness to the females. Males of a broad range of relatedness were provided to determine whether females have an optimal choice of mate (sensu Bateson 1983). Since males were able to compete within the aggregation, the relative importance of inter-male competition and female choice for degree of relatedness in generating variation in male mating success could be assessed.

METHODS Experimental Animals I obtained crickets ranging in genetic relatedness from a two-generation breeding programme.

Parents for the first generation were taken from a laboratory stock collected from a site in southern Spain. I took five males and five females as final instar larvae and reared them to sexual maturity in individual cages l0 cm in diameter. Food (laboratory rodent pellets) and water were provided ad libitum. The animals were taken from different stock cages such that there were no sibling or half sibling relationships between them. Six days after the adult moult, I transferred one male into each of the female's cages and left the pairs to mate. After 24 h the males were removed. Females were provided with a moist pad of cotton wool for oviposition and left for a further 6 days. The cotton wool pads were incubated at 29~ until the eggs began to hatch. Approximately 25 larvae from each female were placed into separate 13-1itre cages and reared to adulthood. Thus animals derived from a single female would have been full siblings but unrelated to animals derived from any of the other females. Parents for the second generation were taken from the 13-1itre cages as final instar larvae and housed individually in cages 10 cm in diameter. Six days after the adult moult, the females were provided with males according to the scheme in Fig. 1. Mating, egg collection and incubation proceeded as for the first generation. Two full sibling females were mated with a single male who was unrelated to themselves. Thus, the offspring derived from each of these females would be related through

Simmons: Female choice and male relatedness

their common father and through the full sibling relationship of their mothers (i.e. r-- 0.25 + 0-125 = 0'375); termed here three-quarter siblings (TS). The male was also allowed to mate with a female who was unrelated to his original mates. This female's offspring would therefore be half siblings (HS), related to the other groups through their common father alone (i.e. r=0.25). A second male, which was a full sibling of the first, was allowed to mate with a female that was unrelated to any of the first male's mates. Her offspring would thus be cousins (CS) to those in the other groups, related through the full sibling relationship of their fathers (i.e. r = 0"125). Finally, a male and female that were unrelated to any other individual were paired to provide offspring that were themselves unrelated (NK) to those in the other groups. Because there were no full sibling or half sibling relationships between parents or grandparents, the degree of relatedness for these individuals would have been close to zero. The larvae derived from each female were reared in full sibling (FS) groups in cages (37 x 37 x 55 cm) provided with food and water ad libitum. Five sheets of crumpled paper (28 • 37 era) were placed in each cage as shelter. Kin recognition in this species appears to be mediated through the recognition of characteristics associated with contact cuticular pheromones (Simmons 1990a). Recognition does not depend on prior experience with conspecifics although it is enhanced when individuals have the opportunity to learn the characteristics of unrelated individuals (Simmons 1989). To provide animals with some experience of the cuticular pheromones of individuals of varying degrees of relatedness during development, I removed four sheets of the soiled paper from each cage and placed one piece into each of the other four cages containing larvae of different degrees of relatedness to those in the source cage. This was repeated every 7 days throughout the developmental period. The sexes were separated during the final instar and maintained in full sibling groups prior to testing. Individuals were not used in experiments until 6 days after the adult moult. Behavioural Observations

Artificial aggregations of calling males are readily established in the laboratory by providing males with artificial burrows in the form of inverted sections from an egg box (Simmons 1986b). I released five males into an arena (100 x 100 cm) containing a

495

substrate of fine dry sand and six artificial burrows; burrows were not a limited resource. The density of males used was sufficiently low (5 males/m 2) to ensure high levels of aggressive and acoustic behaviour typical of the densities found in nature (see Simmons 1986b). The males were matched for size (pronotum width) and age but varied in their degree of relatedness to one another and to the test females (i.e. they were either FS, TS, HS, CS or NK). The males were marked on the pronotum with enamel paint to allow recognition. The arena was placed in a laboratory maintained at ca 29~ with a 12:12 h light:dark reversed cycle. The males were allowed to establish themselves for 2 days. This experiment was not designed to investigate kin recognition abilities per se. Rather, the aim was to determine the relative importance of sexual competition and the discriminatory abilities of females (established elsewhere, Simmons 1989) in generating variance in the long-term mating success of members of an established aggregation (i.e. simulated field conditions). Thus the same five males were monitored throughout the experiment. On each day of the experiment all five males produced calling song and readily engaged in sexual activity. Each day, at the start of the dark cycle, a single virgin female was introduced into the centre of the arena and observed for 3 h. I recorded all malefemale interactions. Females either encountered males as they moved about the arena or oriented to them as they called from the burrows. I noted the location of an encounter (i.e. open arena or burrow) together with the identity of the male. Encounters were divided into pre- and post-mating phases. During the pre-mating phase males courted females (Simmons 1986a, b provides detailed description of courtship and mating). The time spent by the female in the presence of the male was recorded as the time from first antennal contact until the female either moved away from the male by more than 20 cm (unsuccessful courtships) or mounted the male for spermatophore transfer (successful courtships). During the post-mating phase, the time spent by the female with the male in the so called guarding phase (Simmons 1990b) was recorded as the time from spermatophore transfer until the female had moved away by more than 20 cm and the male had failed to regain contact with her for 60 s. After mating the female removes and consumes the spermatophore before remating (Simmons 1986a). The duration of spermatophore attachment was recorded.

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Animal Behaviour, 41, 3

Competition within aggregations is intense (males compete for burrows from which to attract females) and success in competition can influence a male's mating success (Simmons 1986b). I recorded all aggressive interactions between males during the 3-h period, noting the outcome and the identity of the males involved. At the end of the 3-h period, I removed the female. A total of 18 females were tested giving a total of 54 h observations over 3 weeks. Data were thus available for a varying number of encounters between the 18 females and the five males that differed in their degree of relatedness. Since females frequently interacted with each male more than once, more than one measure of each female with the five different males was available. When this occurred, mean values for interactions with each of the males were calculated and used in the analysis. Data were analysed using a non-parametric A N O V A for related samples and repeated measures (Meddis 1984). This test allows for unequal cell frequencies where they arise. Means are presented

( + 1 SE). RESULTS

Male Competitive Ability Males varied significantly in their competitive abilities as measured by the percentage of disputes won per day over the 18-day period. The TS won an average of 64.9 _ 7.1% of its disputes per day, the CS 63.1 +6.6%, the HS 50.2-F 6.3%, the FS 29.6__ 7.9% and the N K 25-3__+6.0~ (F=6.12, df=4, 89, P<0.005; analysis performed on arcsine transformed values). Males were assigned a dominance rank of 1-5 depending on their competitive abilities. Thus the TS male received a rank of 1, being the most successful competitor in the group, and the N K male received a rank of 5. I used these ranks to examine the influence of male competitive ability on female behaviour.

Female Pre-mating Behaviour The time spent by females with courting males was dependent on the location of encounter. Females spent longer with courting males when they oriented to them at their burrows (summing across all degrees of relatedness -Y__SE: burrows 42.1+6.3 s; open 13.2_1.4s; t=2.64, df=16, P<0.01; matched pairs t-test performed on the mean values for each of

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Figure 2. The mean (_+SE) time spent by females with males of different degrees of relatedness prior to mating for encounters in the open arena ( 9 and at burrows (0). 17 females that had encounters both at burrows and in the open). Furthermore, there was a tendency, albeit short of significance, for females to show a greater probability of mating with burrow residents (39/94 (41'5%) of courtships at burrows resulted in mating versus 52/167 (31.1%) in the open, %2= 2.84, P < 0.1). I obtained a similar but significant result previously (see Simmons 1986b). There was no significant heterogeneity in the percentage of each male's courtships that were performed at burrows (FS=19/47 (40.4%), TS=16/66 (24.2%), H S = 21/49 (42.9%), CS = 26/66 (39-4%) and N K = 12/33 (36"4%); %2= 5.70, NS) despite the fact that they varied in competitive ability. This is not surprising since burrows were provided in excess. There was an increase in the duration of the pre-mating encounter with decreasing degree of relatedness, significant for encounters at burrows (specific non-parametric ANOVA (Meddis 1984): L=404, z=2.863, P<0.03) but not so in the open arena (L = 622, z = 0.928, Ns). At burrows, females moved away from courting males that were closely related to themselves (see Fig. 2). There was a tendency for females to remain with their CS for longer than the N K male. This difference was not significant (pair-wise comparison, Meddis 1984; Z = 0.096, Q = 1, MS). The differential response of females to the courtship attempts of males of varying degrees of relatedness resulted in a differential mating success for males (Table I). For burrow residents, the probability of females mating with a courting male increased with decreasing

Simmons." Femalechoiceandmalerelatedness

497

Table I. The proportion of courtship attempts by males of different degrees of relatedness that resulted in mating

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3/19 (15.8%) 5/16(31.3%) 9/21 (42.9%) 13/26 (50.0%) 9/12 (75.0%) 12'20"

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*P<0.05. degree of relatedness. The same trend was not evident for courtships in the open arena. There was no significant relationship between the dominance rank of males and the time spent by females with males during courtship, either for encounters in the open arena (L= 567, z = 1.890, NS) or for those at burrows ( L = 377, z=0"135, Ns). Neither was there a significant tendency for dominant males to have a higher courtship success (Fig. 3). There was, however, a significant correlation between dominance ranking and the percentage of total courtships that were delivered by each of the males (Fig. 3); males successful in competition courted more but success in courtship depended on the degree of relatedness between the male and female, not the male's dominance status.

Female Post-mating Behavioar The behaviour of females after mating was similarly dependent on the location of the encounter; females remained with males during the guarding phase significantly longer when matings were at burrows (.~_+SE duration of guarding: open arena, 1.55+_0.65 min; burrows 8.77+_2.77 min; t=2.04, dr= 14, P<0.05; matched-pairs t-test performed on the mean values for each of 15 females that mated with males both in the open arena and at burrows). The time spent by females with guarding males increased with decreasing degree of relatedness both at burrows (L=211, z=2.289, P<0.02) and in the open (L=329, z=2.780, P<0.01). Females left close kin sooner than unrelated males (Fig. 4). There was no significant difference between the time spent with the CS or NK males (pair-wise comparisons; burrows Z = 0.517, Q = 1, Ns; open Z = 0"954, Q = l, Ns). Females did not remain longer with dominant males after mating in the open arena ( L =

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Figure 3. Percentage of the total courtships delivered by males relative to their dominance rank (O; rs = -0.975, N= 5, P<0.01) and the percentage of those courtships that were successful((3; rs = 0.700, N= 5, NS).

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Figure 4. The mean (+ SE)time spent with guarding males of differentdegrees of relatednessafter mating in the open arena (9 and at burrows (O). 314, z=0.579, Ns). However, there was a relationship between dominance rank and the time females remained with guarding males at their burrows (L = 181, z = 2.108, P<0.01). The time spent with males

Animal Behaviour , 41, 3

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Figure 5. The mean (_+SE) duration of spermatophore attachment for matings with males of different degrees of relatednessin the open arena ((3) and at burrows (0). increased with decreasing dominance rank. This is opposite to the expected trend if females favoured dominant males. The trend can be explained by the fact that the preferred N K mate was the most subordinate and the non-preferred TS mate was the most dominant (this relationship similarly generated the non-significant trend between courtship success and decreasing dominance status in Fig. 3). There was a positive and significant correlation between the time females spent with guarding males and the duration of spermatophore attachment, both for open arena and burrow matings (open, r = 0.291, df=40, P<0.05; burrows r=0.619, df= 32, P<0'001). Thus, females that remained with guarding males at burrows had longer durations of spermatophore attachment (_~-t-SE duration of spermatophore attachment: open arena 12.1 + 1.5 min; burrows 22.3 -1-3.3 min; t=2.17, df=14, P<0"05; matched-pairs t-test on the mean values for each of 15 females that mated in the open arena and at burrows). When females mated at burrows, the duration of spermatophore attachment increased as the degree of relatedness between a female and her mate decreased (L = 202, z = 2.574, P < 0.01). This trend was not significantfor matings in the open arena (L=307, z=1-381, P<0.1). Thus the spermatophores of burrow residents that were unrelated remained attached for longer than those of close kin or males encountered in the open (Fig. 5). At burrows, there was a tendency for the N K male's spermatophores to be removed earlier than the CS although again this was not significant (pair-wise comparison Z = 0"707, Q = 1, Ns).

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Figure 6. The mean (_+SE) expected fertilization success for males of different degrees of relatedness (18 females).

Fertilization Success The mechanism of sperm competition in G. bimaculatus is one of random sperm mixing (Simmons 1987; Parker et al. 1990). A male's fertilization success depends on the relative proportions of his own and previously stored sperm. I converted spermatophore attachment times to sperm numbers (using the relationship S=ll.711og(t)+2-39, where t is the spermatophore attachment duration and S the number of sperm; from data in Simmons 1986a) for all matings by each of the females during the observation periods. Sperm transfer rates are assumed equivalent for all matings; males do not appear to vary in the number of sperm transferred from their spermatophores (Simmons 1988a). The expected proportion of each female's eggs that would be fertilized by each of the males was estimated using the 'fair raffle' model of Parker et al. (1990) where 1/P~= (SJSi) + 1 and i relates to the male of interest and j the other males that mated with the female. The proportion of eggs to be fertilized by the male of interest is then 1 - Pj. The behaviour of females was translated directly into fertilization advantages for preferred males (Fig. 6). The expected fertilization success increased with decreasing degree of relatedness (L = 867, z = 2'795, P < 0'01). A post-hoc comparison showed no significant difference between the expected fertilization success of CS and N K males (Z=0.498, Q = 1, Ns) although again, clearly, the CS had the greatest chance of fertilizing eggs.

Simmons: Female choice and male relatedness

DISCUSSION These results demonstrate that the apparent preference among female G. bimaculatus for unrelated males during courtship (Simmons 1989) results in differential mating under simulated field conditions. In isolation, these results should perhaps be viewed with some degree of caution because the same five males were used throughout the experiment; females may have been responding to some other characteristic of the preferred male who happened to be unrelated. Males were matched for physical characteristics including age. Furthermore, the gradation in response of females to the males of varying degrees of relatedness in this experiment was consistent with results obtained during experiments designed specifically to test the ability of G. bimaculatus to recognize kin and discriminate against different degrees of relatedness during intersexual encounters (see Simmons 1989, 1990a). The probability that these results are confounded with other characteristics therefore seems unlikely. With this caveat in mind, a male's degree of relatedness did appear to be an important cue for female choice. When females oriented to calling males at their burrows, they spent longer with an unrelated male and had a greater probability of mating. Bateson (1983) argued that an animal might be expected to have some optimally related mate choice because of the costs associated with outbreeding as well as inbreeding. In this study, females did tend to stay longer with their cousin although not significantly longer than with an unrelated male. Furthermore, there was a linear trend in the probability of mating with decreasing degree of relatedness; females were more likely to mate with the unrelated mate than with their cousin. These data, together with the fact that females prefer to associate with the cuticular pheromones of unrelated males rather than with those of cousins (Simmons 1990a), suggest that for G. bimaculatus there is not a pre-mating choice for an intermediate degree of mate relatedness. Rather, female choice may be a mechanism for inbreeding avoidance. Female G. bimaeulatus also adopt a mechanism of post-copulatory choice; females influence the number of sperm transferred, and therefore paternity, by removal of spermatophores (Simmons 1986a, 1987). Thus, male mating success does not necessarily correlate with reproductive success. During the guarding phase males will dissuade

499

females from removing spermatophores (Simmons 1990b). However, conflict over the fate of the spermatophore appears to be resolved in favour of the female since females can readily escape from guarding males (Simmons 1986a, 1990b). In this study, females that mated with close kin left the guarding male sooner than when they mated with a cousin or an unrelated male and removed spermatophores relatively quickly. Therefore, even those females that did mate with close kin failed to accept sperm and effectively reduced the probability of close kin fertilizing their eggs. These data are in agreement with my previous results (Simmons 1989) which showed how females confined with closely related males after mating, persistently tried to escape from their guarding attempts. The data on post-copulatory female choice presented here may suggest that females might choose optimally with regard to relatedness. The behaviour of females tended to result in the cousin having the greatest probability of fertilizing eggs albeit not significantly so. Female choice within a mating system characterized by intense inter-male competition is often difficult to detect (e.g. Halliday 1983; Partridge & Halliday 1984). The behaviour of female G. bimaculatus, however, is a relatively clear example of how a female's behaviour can influence male reproductive success regardless of petition (see also Simmons 1986a, b). Success in competition did influence a male's opportunity to mate; dominant males delivered more of the courtship attempts than subordinates. However, dominance status did not influence courtship success or post-copulatory discrimination by the female. While competition resulted in certain males being more sexually active, female choice for degree of relatedness determined a male's reproductive success. The behaviour of females was dependent on the location of courtship and mating. Females tended to mate relatively quickly when they encountered males in the open arena. There was no influence of kinship on the probability of mating when courted in the open; female preference was apparent only at burrows. Furthermore, after open arena matings, females left males quickly and removed their spermatophores, regardless of kinship (a similar preference for burrow residents was demonstrated by Simmons 1986b). There are at least two possible explanations for the apparent lack of discrimination in the open. First, females may choose burrow residents as mates over males that they

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Animal Behaviour , 41, 3

encounter in the open because, where burrows are a limited resource, the possession of a burrow may be indicative of a male's competitive ability. Males encountered en route to burrows are likely to be adopting the alternative reproductive strategy of 'satellite' because of their inability to compete successfully (Cade 1980; Simmons 1986b). However, female choice in this study was not otherwise influenced by the dominance status of males. Alternatively, the costs of choice may be greater when females encounter males away from the security of a burrow. Parker (1983) demonstrated theoretically that, when the search costs are high relative to the benefits of choice, females should be indiscriminate. Parker (1983) envisaged search costs in terms of the time to find another mate. For G. bimaculatus one of the major costs for females is likely to be exposure to predation. Female gryllids and tettigoniids do appear to suffer predation as a direct consequence of seeking out males (e.g. Gwynne & Dodson 1983; Sakaluk & Belwood 1984; Simmons 1986c). The risk of predation is therefore likely to be an increasing function of time spent in the open environment. The time required to assess males accurately prior to mating may outweigh the benefits of choice when females encounter males in the open. Predation risk will be greatly reduced when females are in the shelter of a male's burrow, reducing the costs of choice such that females are discriminating. This hypothesis is supported by the behaviour of females in this study. The fact that females bother to mate at all in the open may reflect the benefits obtained from consuming spermatophores (Simmons 1988b) and/or the greater time costs which would be associated with avoiding persistently courting males. While the costs of choice could account for the lack ofpre-mating discrimination in the open arena, it does not explain the indiscriminate removal of spermatophores. Females could, without risk of predation, leave spermatophores of chosen males attached after they reach shelter. However, the recognition of kin requires the perception of contact cuticular pheromones (Simmons 1990a) which may not be present on the spermatophore. If a female fails to determine male relatedness during courtship, once away from the male she may have no means of assessing her mate and may therefore remove spermatophores indiscriminately. The avoidance of inbreeding can have important fitness implications for females and/or their offspring. The effects ofinbreedingin laboratory popu-

lations of Drosophila are well documented. Female fecundity and the competitive mating ability of male D. melanogaster are reduced by 50% after only one generation of sibling mating (Latter & Robertson 1962; Sharp 1984). Male infertility and decreased development rate occur in inbred lines of D. subobscura (Hollingsworth & Maynard Smith 1955). Maynard Smith (1956) was able to demonstrate female choice for outbred males in D. subobscura; inbred males were apparently inferior in the quantity and quality of sperm, thereby reducing female reproductive success. There is good evidence from field populations of vertebrates that inbreeding reduces reproductive success (Greenwood et al. 1978; Rails et al. 1979; Partridge 1983 and Falconer 1989 review the genetic consequences of inbreeding). There is therefore good reason to believe that inbreeding avoidance in G. bimaculatus should be adaptive. The reason that females failed to show a significant preference for an intermediate degree of relatedness in their outbreeding tendency may either be that, for G. bimaculatus, outbreeding costs are negligible or that the unrelated males provided to females in these and previous experiments (Simmons 1990a) were not genetically different enough to detect an optimal choice of mate. While the unrelated males in these studies were known to have a coefficient of relatedness close to zero, individuals of varying degrees of relatedness were derived from the same local population. It is conceivable that, within a population the best mate will be unrelated but that females may avoid matings with individuals from different populations. Such outcrossings can reduce viability and fertility in Drosophila (Prakash 1972; Dobzhansky 1974) and G. bimaculatus (unpublished data). Whether females are faced with these types of choices in reality will largely depend on the extent to which they disperse as adults.

ACKNOWLEDGMENTS My thanks to D. J. Thompson for comments on the manuscript. This research was supported by an SERC post-doctoral fellowship.

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