William D. Brown

FEMALE CHOICE FOR AN INDICATOR OF MALE SIZE IN THE SONG OF THE BLACK-HORNED TREE CRICKET, OECANTHUS NIGRICORNIS (ORTHOPTERA: GRYLLIDAE: OECANTHINAE)

The calling song of male crickets, including Oecanthus nigricornis (Walker), attracts females for mating and provides a model system of sexual communication. We give the first conclusive identification of a feature of cricket song that is both attractive to females and indicates a phenotypic feature (body size) that determines male mating success and female reproductive benefits. We do this by first testing for correlations between song characteristics and aspects of male phenotype that are hypothesized to indicate male quality. We show that song is a reliable indicator of male size and male age, and that large male size is associated with increased female fecundity. We then use playbacks of synthetic songs that mimic natural variation in song parameters to study song preferences and we compare preferences under different presentation regimes to determine whether choices are based on relative song quality or some fixed criterion. Females show a preference for the lower frequency songs produced by large males, but only during simultaneous playbacks. Thus female choice is based on the relative quality of calls that can be sampled simultaneously. These results provide strong support for the hypothesis that females use variation in calling song to assess male mate quality.

CRITERIA FOR DEMONSTRATING FEMALE SPERM CHOICE

Females may base their choice of sperm on male attractiveness or on genetic (in)compatibility with the male. Evidence is accumulating that male and female genotype both affect fertilization success (Wedekind et al. 1996, Clark et al. 1999). If female sperm choice is a mechanism to avoid the negative effects of inbreeding (as the studies reviewed in Birkhead 1998 suggest), one could perform P2-experiments using groups of related versus unrelated females. Such an example is provided by Wilson et al. (1997) and discussed in Birkhead (1998). A recent study on decorated field crickets (Gryllodes supplicans) used a different approach (Stockley 1999). In this study, each female of a sibling pair was allowed to copulate first with a sibling male, followed by either another sibling male or a nonsibling. The paternity obtained by the first sibling male (P1 value) was lower when the second male was a nonsibling, providing some evidence for female sperm choice based on genetic similarity (Stockley 1999).

Aggressive contests in house crickets: size, motivation and the information content of aggressive songs

The function of aggressive signals, including both their information content and the circumstances under which they cause resolution or escalation of conflict, continues to be controversial. We studied the information content of aggressive songs in the house cricket, Acheta domesticus, to test their ability to inform opponents about fighting ability and motivation. We show that songs produced by individual males are highly repeatable and contain information about male resource holding potential, the ability of a male to win an aggressive contest. Temporal components of aggressive song were correlated with male body size, and size affected both the intensity and outcome of fights. In contrast, we found no components of song that signal motivation to fight. Although males with restricted access to mates were significantly more aggressive and won fights more often, their songs did not reflect the asymmetry in motivation to fight. We discuss possible reasons for the absence of signals of motivation during animal contests.

Evolution of female remating behaviour following experimental removal of sexual selection

The relatively small number of ova produced by a female can be fertilized by a single ejaculate in most species. Why females of many species mate with multiple males is therefore enigmatic, especially given that costs associated with remating have been well documented. Recently, it has been argued that females may remate at a maladaptive rate as an outcome of sexually antagonistic coevolution: the evolutionary tug–of–war between manipulation by one sex and resistance to being manipulated by the other sex. We tested this hypothesis experimentally for the evolution of the female remating interval in a naturally promiscuous species, Drosophila melanogaster. In two replicate populations, sexual selection was removed through enforced monogamous mating with random mate assignment, or retained in polyandrous controls. Monogamy constrains the reproductive success of mates to be identical, thereby converting prior conflicts between mates into opportunities for mutualism. Under these experimental conditions, the sexually antagonistic coevolution hypothesis generates explicit predictions regarding the direction of evolutionary change in female remating behaviour. These predictions are contingent upon the mechanism of male manipulation, which may be mediated biochemically by seminal fluids or behaviourally by courtship. Levels of divergence in female remating interval across lines, and in male ejaculatory and courtship effects on female remating, were quantified after 84 generations of selection. Data refute the hypothesis that the evolutionary change in female remating behaviour was due to sexually antagonistic coevolution of courtship signal and receiver traits. The data were, however, consistent with a hypothesis of sexual conflict mediated through ejaculate manipulation. Monogamy–line males evolved ejaculates that were less effective in inducing female non–receptivity and monogamy–line females evolved to remate less frequently, symptomatic of lowered resistance to ejaculate manipulation. The consistency of the results with alternative hypotheses to explain female promiscuity are discussed.

Colony sex ratios vary with queen number but not relatedness asymmetry in the ant Formica exsecta.

Split–sex–ratio theory assumes that conflict over whether to produce predominately males or female reproductives (gynes) is won by the workers in haplodiploid insect societies and the outcome is determined by colony kin structure. Tests of the theory have the potential to provide support for kin–selection theory and evidence of social conflict. We use natural variation in kinship among polygynous (multiple–queen) colonies of the ant Formica exsecta to study the associations between sex ratios and the relatedness of workers to female versus male brood (relatedness asymmetry). The population showed split sex ratios with about 89% of the colonies producing only males, resulting in an extremely male–biased investment ratio in the population. We make two important points with our data. First, we show that queen number may affect sex ratio independently of relatedness asymmetry. Colonies producing only males had greater genetic effective queen number but did not have greater relatedness asymmetry from the perspective of the adult workers that rear the brood. This lack of a difference in relatedness asymmetry between colonies producing females and those producing only males was associated with a generally low relatedness between workers and brood. Second, studies that suggest support for the relatedness–asymmetry hypothesis based on indirect measures of relatedness asymmetry (e.g. queen number estimated from relatedness data taken from the brood only) should be considered with caution. We propose a new hypothesis that explains split sex ratios in polygynous social insects based on the value of producing replacement queens.

Courtship feeding in tree crickets increases insemination and female reproductive life span

Courtship feeding in the majority of insects may influence female reproductive patterns either directly, through effects of the gift material, or indirectly, through correlated effects of prolonged copulation and larger ejaculates. This distinction is important because the cause of changes in fecundity may influence patterns of the allocation of resources between the sexes, with implications for the intensity of sexual selection and magnitude of sexual conflict. I show that post-copulatory feeding on the secretions of a gland on the metanotum of male Oecanthus nigricornis. Walker correlates with oviposition and affects the number of sperm remaining within the spermatophore. Manipulations of gland feeding and insemination duration showed that changes in fecundity are due to the gift rather than the ejaculate. Metanotal gland feeding increased female fecundity by increasing reproductive life span without significantly increasing oviposition rate. These changes in reproduction were directly due to the gift itself. Although gland feeding was positively correlated with the duration of insemination and thus the number of sperm transferred from the spermatophore to the female, experimentally prolonging or reducing insemination had no significant effect on reproductive life span. Male phenotype was also associated with female fecundity but in this case the effect was caused by an increase in the oviposition rate of females that mated with relatively large males. Male size had no significant effect on female reproductive life span, suggesting that its effect is not simply due to a quantitative increase in gift size. Three other measures of male phenotype, fluctuating asymmetry, condition (i.e. size-standardized wet body mass) and age, had no significant effects on female reproduction.Copyright 1997 The Association for the Study of Animal BehaviourCopyright 1997The Association for the Study of Animal Behaviour.

NO EVIDENCE THAT POLYANDRY BENEFITS FEMALES IN DROSOPHILA MELANOGASTER

Abstract Understanding the evolution of polyandry (mating with multiple males) is a major issue in the study of animal breeding systems. We examined the adaptive significance of polyandry in Drosophila melanogaster, a species with well‐documented costs of mating in which males generally cannot force copulations. We found no direct fitness advantages of polyandry. Females that mated with multiple males had no greater mean fitness and no different variance in fitness than females that mated repeatedly with the same male. Subcomponents of reproductive success, including fecundity, egg hatch rate, larval viability, and larval development time, also did not differ between polyandrous and monogamous females. Polyandry had no affect on progeny sex ratios, suggesting that polyandry does not function against costly sex‐ratio distorters. We also found no evidence that polyandry functions to favor the paternity of males successful in precopulatory sexual selection. Experimentally controlled opportunities for precopulatory sexual selection had no effect on postcopulatory sperm precedence. Although these results were generally negative, they are supported with substantial statistical power and they help narrow the list of evolutionary explanations for polyandry in an important model species.

The cause of size-assortative mating in the leaf beetle Trirhabda canadensis (Coleoptera : Chrysomelidae)

SummaryExamples of positive assortative mating by body size are abundant but its causes remain controversial. I show that size-assortative mating occurs in the chrysomelid beetle Trirhabda canadensis and I test a series of alternative hypotheses to explain how this mating pattern comes about. Results suggest that assortative mating in this beetle is due to the greater ease with which size-matched pairs can achieve intromission, and not due to size-biased skews in the availability of mates or mate choice favoring large individuals. There was no correlation between male and female elytron length (a measure of body size) at the initiation of courtship, but pairs assorted positively by size at the onset of intromission. Moreover, in the laboratory, there was a negative correlation between male and female size for pairs engaged in courtship that terminated without mating. Assortative mating was not associated with a large-male mating advantage and there was no evidence of female choice of large males. Nor was there unequivocal evidence for male choice of large females; although mating females were slightly larger and considerably heavier than solitary females, males did not differ in the frequency with which they rejected large and small females. Assortative mating in T. canadensis appeared to be caused by the lower ability of mismatched pairs to achieve intromission after an encounter, both when males were larger and when they were smaller than the female.

Size-assortative mating in the blister beetle Lytta magister (Coleoptera: Meloidae) is due to male and female preference for larger mates.

Abstract Although positive assortative mating has been the subject of much theoretical analysis, it causes are still poorly understood. Results of a study of the blister beetle Lytta magister suggest that assortative mating by size occurs because both males and females prefer large mates. Males were less likely to terminate courtships with larger females, resulting in a mating advantage for larger females in the field. In captivity, males provided with a size-matched and a larger female courted and mated more frequently with the larger of the two females. However, in experiments with a female and two males, small males courted and mated with relatively small females more often than did large males. Even though size-related male-male aggression was absent, large males showed a mating advantage in the field. This finding suggests that large males are preferred by females. Because large size is the universally preferred phenotype, these individuals are more able to express their preference and tend to pair off. Small beetles are then left no choice but to mate among themselves. Preference for large mates has presumably evolved because large females produce more eggs per clutch and large males are more capable of producing the large spermatophore passed during mating.

Complicity or Conflict over Sexual Cannibalism? Male Risk Taking in the Praying Mantis Tenodera aridifolia sinensis

Male complicity versus conflict over sexual cannibalism in mantids remains extremely controversial, yet few studies have attempted to establish a causal relationship between risk of cannibalism and male reproductive behavior. We studied male risk‐taking behavior in the praying mantid Tenodera aridifolia sinensis by altering the risk imposed by females and measuring changes in male behavior. We show that males were less likely to approach hungrier, more rapacious females, and when they did approach, they moved more slowly, courted with greater intensity, and mounted from a greater distance. Similarly, when forced to approach females head‐on, within better view and better reach of females, males also approached more slowly and courted with greater intensity. Thus, males behaved in a manner clearly indicative of risk avoidance, and we support the hypothesis of sexual conflict over sexual cannibalism.