African Queens find mates when males are rare

The Cost of Mating

Male-killing Wolbachia in a live-bearing arthropod: Brood abortion as a constraint on the spread of a selfish microbe

Male–male competition and female mate choice act contemporaneously in the cockroach Nauphoeta cinerea and the social pheromone of males influences the outcome of both forms of sexual selection. We therefore examined the joint and separate effects of male–male competition and female mate choice to determine if the selective optima for the pheromone were the same or different. Dominant males in a newly established hierarchy mated more frequently, but not exclusively. Manipulations of the multi-component social pheromone produced by males of N. cinerea showed that both long- and close-range attraction of females by males were influenced by the quantity and composition of the pheromone. The most attractive composition, however, differed from that which was most likely to confer high status to males. Since the outcome of male–male competition can conflict with mating preferences exhibited by females, there is balancing sexual selection on the social pheromone of N. cinerea. Such balancing selection might act to maintain genetic variation in sexually selected traits. We suggest that the different forms of sexual selection conflict in N. cinerea because females prefer a blend different to that which is most effective in male–male competition in order to avoid mating with overly aggressive males.

Mating behaviour of the orange mud crab, Scylla olivacea: The effect of sex ratio and stocking density on mating success

Chance plays a critical but underappreciated role in determining mating success. In many cases, we tend to think of chance as background noise that can be ignored in studies of mating dynamics. When the influence of chance is consistent across contexts, chance can be thought of as background noise; in other cases, however, the impact of chance on mating success can influence our understanding of how mates are acquired and how sexual selection operates. In particular, when the importance of chance covaries with biological or ecological factors in a systematic manner—that is, when chance becomes consistently more or less important under certain conditions—then chance is important to consider if we want to fully understand the operation of mate acquisition and sexual selection. Here, we present a model that explores how chance covaries with factors such as sex ratio, adult population size, and mating regime in determining variation in mating success. We find that in some cases, chance covaries with adult population size and the operational sex ratio to create variation in mating success. We discuss how chance can influence our more general understanding of the operation of mating dynamics and sexual selection.

We assessed the extent to which traits related to ejaculate investment have evolved in lines of Drosophila melanogaster that had an evolutionary history of maintenance at biased sex ratios. Measures of ejaculate investment were made in males that had been maintained at male-biased (MB) and female-biased (FB) adult sex ratios, in which levels of sperm competition were high and low, respectively. Theory predicts that when the risk of sperm competition is high and mating opportunities are rare (as they are for males in the MB populations), males should increase investment in their few matings. We therefore predicted that males from the MB lines would (1) exhibit increased investment in their first mating opportunities and (2) deplete their ejaculates at a faster rate when mating multiply, in comparison to FB males. To investigate these predictions we measured the single mating productivity of males from three replicates each of MB and FB lines mated to five wild-type virgin females in succession. In contrast to the first prediction, there was no evidence for differences in productivity between MB and FB line males in their first matings. The second prediction was upheld: mates of MB and FB males suffered increasingly reduced productivity with successive matings, but the decline was significantly more pronounced for MB than for FB males. There was a significant reduction in the size of the accessory glands and testes of males from the MB and FB regimes after five successive matings. However, the accessory glands, but not testes, of MB males became depleted at a significantly faster rate than those of FB males. The results show that male reproductive traits evolved in response to the level of sperm competition and suggest that the ability to maintain fertility over successive matings is associated with the rate of ejaculate, and particularly accessory gland, depletion.

The effects on mating success of four X-linked mutations controlling locomotor activity are examined. The mutation hypoE has no effect on female mating success. The mutations hypoC, iav and iav2 reduce female mating propensity and extend courtship durations. Unlike wild type females these mutant females fail to reach their maximum propensity for mating on the second day after eclosion. The iav females have normal wild type compositions of cuticular hydrocarbons and are highly attractive to courting males. Attenuated perception or aberrant processing of male courtship stimuli, and delayed sexual maturation are likely contributors to the poor mating propensity of inactive females. The iav mutation also causes decreased mating success and extended courtship duration in male mutants. This is largely a consequence of their inactivity. The duration of courtship latency, which is related to the number of phenotypically inactive flies present in the pairing, is positively correlated with age. Copulation duration, though primarily a male determined trait, is also affected by age. Some inferences regarding possible modes of action of the mutations are discussed.

Reproduction is costly and individual decisions about when and how much to invest in reproduction will relate to potential benefits in terms of offspring survivorship or mating success. For those organisms that can reproduce more than once, individuals should invest relatively more in reproduction when the potential benefits are high. If females gain directly or indirectly from choosing attractive, highly ornamented males, then it could be predicted that females mated to these males should invest more in reproduction. Here we report the first test of this prediction for a lekking species, peafowl (Pavo cristatus), in which males do not provide resources for offspring, and females prefer to mate with those males that possess the most elaborate trains. We find that peahens randomly mated to males that vary in the degree of ornament produce more eggs for those peacocks with more elaborate trains. We could find no evidence to support the possibility that this difference arises as a result of difference between males in their ability to fertilize eggs.

Abstract1 Mating disruption is the primary tactic used to reduce rates of gypsy moth population spread in the United States Department of Agriculture’s Slow‐the‐Spread of the gypsy moth programme (STS). Because STS targets very low‐density gypsy moth populations within which it is extremely difficult to collect females or egg masses, mating success in native populations cannot be determined. Therefore, the evaluation of mating disruption treatments in field experiments such as those designed to test new formulations and application methods requires deploying and recovering laboratory‐reared female moths to determine mating success.2 Five methods of deploying females were evaluated for cost, rates of female and egg mass recovery, and female mating success. The deployment methods tested were: modified delta trap, square barrier, single and double trunk bands, and tethered females.3 Deployment of tethered females had the highest cost and mating success rate, but it did not yield the highest rates of female and egg mass recovery. Deployment of females in delta traps produced the lowest cost and mating success rate, but yielded the highest recovery rate. Neither of these deployment methods is recommended because of unacceptably high cost (tethered female) or low mating success (delta trap).4 There were no significant differences in cost or mating success among the other three deployment methods.5 The differences among the square barrier, single trunk band, and double trunk band methods in cost, female and egg mass recovery, and mating success are too small to recommend any one over the others.

Although sex ratio and investment theory have been recently refined and made more mathematically rigorous (Hamilton, 1967; Leigh, 1970; Trivers, 1972; Trivers and Hare, 1976; Charnov, 1978; MacNair, 1978), Fisher's (1930) original arguments remain essentially unchanged. In a panmictic population we expect equal amounts of time and energy to be allocated to the production of each sex. A disproportionate investment in the offspring of one sex will be corrected in subsequent generations by selection for parents that produce the sex in short supply. Because investment must be divided equally between the sexes, the optimal sex ratio is expected to be unity only when the parental effort expended on an average individual of each sex is also equal. If production of a female requires twice the effort as production of a male, the sex ratio should be 2 d :1 9 rather than unity. This study is, in part, an empirical test of sex ratio theory using a solitary bee, Osmia lignaria propinqua Cresson. It was prompted by the paucity of field data currently available to test the accuracy of theoretical formulations. Additionally, observations by Torchio (unpubl.) suggest that the sex ratio of offspring of 0. lignaria varies seasonally. Indeed, seasonal variation in the sex ratio is not uncommon in bees (Michener, 1974), although the reasons for such variability require clarification. Here we document seasonal variability and offer an explanation for it. Until recently (Trivers and Hare, 1976), the interrelationship between the ratio of investment in an average male and female and the sex ratio was not clearly perceived. It is now apparent that sex ratio is related to investment ratio and any study of sex ratio naturally extends to questions of optimal allocation of investment within each sex. Investment expended upon an average individual of each sex in many species is unequal, although total investment in each sex must be equal. Such differences are most clearly manifest in those species that exhibit sexual dimorphism in size. In Osmia lignaria females are distinctly larger than males. Of the explanations proposed for sexual dimorphism, two are frequently cited. Trivers (1972), in elaborating on Darwin (1871), hypothesized that members of the sex that invest most in offspring are subjects of intrasexual competition between members of the other sex. As a result of intrasexual selection (probably sexual selection also), members of the competing sex are expected to be larger and fewer because of the usual positive relation between size and mating success. This explanation is not applicable to 0. lignaria because females are responsible for all but sperm investment in offspring. The second hypothesis (Amadon, 1959; Selander, 1972) argues that sexual dimorphism is frequently due to intersexual competition for resources; a divergence in size reduces overlap in resource use, thereby increasing the overall efficiency of each sex. This hypothesis is not appropriate to 0. lignaria because the sexes do not differ in distribution across the limited number of flower species used for nectar (P. F. T., pers. observ.). We will demonstrate that the asymmetry in investment between individual males and females is probably due to a complicated process involving seasonal changes in resource availability.

Ecological and social factors underpinning the inequality of male mating success in animal societies can be related to sex ratio, sexual conflict between breeders, effects of nonbreeders, resource dispersion, climatic conditions, and the various sequential stages of mating competition that constitute the sexual selection process. Here, we conducted an individual‐based study to investigate how local resource availability and demography interact with annual climate conditions to determine the degree of male mating inequality, and thus opportunity for sexual selection across two sequential reproductive episodes (harem and subsequent mate acquisition) in a naturally regulated (feral) horse population in Sable Island National Park Preserve, Canada. Using a 5‐year, spatially explicit, mark‐resight dataset and hierarchical mixed‐effects linear modeling, we evaluated the influence of adult sex ratio (ASR) on mating success and then tested for effects of freshwater availability, density, unpaired male abundance, and precipitation during each breeding season. Unpaired male abundance, freshwater availability, and ASR differed in their effects on male mating success according to year and selection episode. Opportunity for sexual selection in males associated with harem acquisition increased with ASR, and unpaired male abundance further explained weather‐related interannual variation after accounting for ASR. In contrast, once a harem was secured, ASR had little effect on male mating inequality in regard to acquiring additional females, while interannual variation in mating inequality increased with decreasing freshwater availability. Our findings show that local demography, resource availability, and weather effect opportunity for sexual selection in males differently depending on selection episode, and can attenuate or accentuate effects of ASR.

Predation Risk Affects Mate Choice in a Gobiid Fish

Same-sex sexual behaviour as a by-product of reproductive strategy under male–male scramble competition

Paternally transmitted parasites

Male courtship signals are generally a product of strong selection on their capacity to attract mates and convey signaller quality to achieve mating success. Given the costly nature of courtship signals, their expression may also be influenced by the quality of the courted female. However, such effects are rarely considered. I tested how male and female nutritional conditions affect male drumming intensity, female abdomen lifting behaviour and mating success in the jumping spider Saitis barbipes , known for its elaborate drumming displays. As expected from a condition‐dependent signal, well‐fed males drummed more intensely than hungry males and thereby increased their mating success. Yet, contrary to expectations, males often reduced drumming towards well‐fed females. Well‐fed females were observed to frequently lift their abdomens, which strongly reduced male drumming and mating success. Curiously, males that achieved mating despite female abdomen lifting did so with lower drumming intensity than in nonabdomen lifting trials. These results illustrate that condition‐dependent behaviours in both sexes can interact in unexpected ways, shaping male investment and influencing courtship outcomes.