A sexual conflict in collared flycatchers,Ficedula albicollis: early male moult reduces female fitness

A sexual conflict over levels of parental care occurs in most animals with biparental care, and studies of sexual differences in levels of parental care have usually focused on its intra-annual fitness consequences. We investigated inter-annual fitness consequences of a sexual difference in timing of feather replacement (moult) in collared flycatchers (Ficedula albicollis). In this study, males overlapped reproduction and moult more often than females, they also initiated their moult at an earlier stage of breeding than females. Females mated to males with a moult-breeding overlap had significantly lowered survival chances than females mated with males initiating moult after breeding. Furthermore, females mated with moulting males risked a lowered future fecundity in terms of a delayed start to breeding in the following season. However, early moulting males achieved a similar reproductive success as males initiating moult after breeding. Likewise, male survival probability to the following breeding season did not differ between early and late moulting individuals, nor was there any evidence that males gained or lost in future mating advantages by moulting early. These results show not only that a sexual conflict over timing of moult may operate, but also that it can impose severe fitness consequences, in terms of reduced future fecundity and survival probability, upon the ''losing'' sex.     

黄腹山鹪莺稳定的配偶关系限制雄性欺骗者

多数鸟类通过性特征限制在同性竞争和配偶选择中的“欺骗者”存在,与此相反,雀形目扇尾莺科部分物种表现出繁殖季节性特征消退的身体特征变化模式.在广州市南沙区通过“目字笼”对黄腹山鹪莺配偶关系稳定性的限制机制进行研究,发现虽然雌性个体到访原配个体和对照个体的次数几乎相同,但是雌性个体对原配雄性的单次选择时间明显长于对照雄性个体,总计选择时间也明显长于对照雄性个体.选择实验过程中,原配雄性的跳动次数明显高于对照个体雄性,以竖尾扑哧和鸣声恐吓等为代表的威慑行为次数也明显高于对照雄性个体.结果说明,雌性更青睐于原配个体,配对时间越长,忠诚度越高,而且原配雄性比入侵雄性个体表现出更高的活跃度和威慑行为.繁殖季节性特征消退的物种可以通过保持稳定的配偶关系以限制“欺骗者”存在.可以推测繁殖的巨大投入和雌性之间的同性竞争可能是产生这种配偶稳定性的主要原因.     

Sexual selection in a lekking bird: the relative opportunity for selection by female choice and male competition

Leks are classic models for studies of sexual selection due to extreme variance in male reproductive success, but the relative influence of intrasexual competition and female mate choice in creating this skew is debatable. In the lekking lance-tailed manakin (Chiroxiphia lanceolata), these selective episodes are temporally separated into intrasexual competition for alpha status and female mate choice among alpha males that rarely interact. Variance in reproductive success between status classes of adult males (alpha versus non-alpha) can therefore be attributed to male-male competition whereas that within status largely reflects female mate choice. This provides an excellent opportunity for quantifying the relative contribution of each of these mechanisms of sexual selection to the overall opportunity for sexual selection on males (I males). To calculate variance in actual reproductive success, we assigned genetic paternity to 92.3% of 447 chicks sampled in seven years. Reproduction by non-alphas was rare and apparently reflected status misclassifications or opportunistic copulations en route to attaining alpha status rather than alternative mating strategies. On average 31% (range 7-44%, n=6 years) of the total I males was due to variance in reproductive success between alphas and non-alphas. Similarly, in a cohort of same-aged males followed for six years, 44-58% of the total I males was attributed to variance between males of different status. Thus, both intrasexual competition for status and female mate choice among lekking alpha males contribute substantially to the potential for sexual selection in this species.     

Sexual differences in moult–breeding overlap and female reproductive costs in pied flycatchers, Ficedula hypoleuca

1. In this study I show that a sexual difference in timing of the post-nuptial moult frequently occurs in a sub-arctic population of the pied flycatcher.
2. Most pairs started to moult after fledging of their young, but an overlap between moult and nestling feeding was more common among males than females. This sexual difference in moult–breeding overlap increased as the season progressed.
3. Females with moult–breeding overlap laid smaller clutches than non-moulting females. In addition to many other factors explaining the seasonal decline in clutch size that has been found for many bird species, it is possible that females adjust their clutch size according to their own risk of having to start moulting while still feeding the nestlings.
4. Nearly 24% of the females were deserted by their mate before the young fledged. Desertion imposed no fitness costs to males in terms of fledgling number or quality, suggesting that their females managed to adjust their care for the loss of male care.
5. Deserted females started moulting later than aided females, which may be a result of their increased reproductive investment.
6. Deserted females and females aided by moulting males had lower survival rate than females aided by non-moulting males.
7. These findings suggests that delayed moult may be one mechanism causing inter-annual reproductive costs in birds, and the relationship between a sexual difference in timing of moult and its fitness consequences may be widespread among passerine birds.     

Annual variation in the timing of breeding and moulting in male and female Pied Flycatchers Ficedula hypoleuca

During five breeding seasons, the timing of breeding and moulting was studied in the Pied Flycatcher Ficedula hypoleuca. In central Sweden, on average 67% of the males and 41% of the females started moulting before the young fledged. The proportion of individuals with an overlap between breeding and moulting varied considerably between years, with the highest proportion of moulting males being recorded in the year when the females started egg-laying on the latest date. Despite a large annual variation in the proportion of individuals showing a moult/breeding overlap, the duration of this overlap varied insignificantly between years. The onset of moult in males seemed to be related to both calendar date and timing of the current breeding attempt. Most females postponed their moult until just before or just after the fledging of their young, independent of calendar date. There was no significant relationship between male and female moult scores and nestling weight at fledging or fledging success of their brood. Thus, in long-distance migrants such as Pied Flycatchers, it may be adaptive to have some overlap between reproduction and moult, but there seems to be a limit to how early in the breeding cycle they are able to start moulting.     

Sexual selection resulting from extrapair paternity in collared flycatchers

Extrapair paternity has been suggested to represent a potentially important source of sexual selection on male secondary sexual characters, particularly in birds with predominantly socially monogamous mating systems. However, relatively few studies have demonstrated sexual selection within single species by this mechanism, and there have been few attempts to assess the importance of extrapair paternity in relation to other mechanisms of sexual selection. We report estimates of sexual selection gradients on male secondary sexual plumage characters resulting from extrapair paternity in the collared flycatcher Ficedula albicollis, and compare the importance of this form of sexual selection with that resulting from variation in mate fecundity. Microsatellite genotyping revealed that 15% of nestlings, distributed nonrandomly among 33% of broods (N=79), were the result of extrapair copulations. Multivariate selection analyses revealed significant positive directional sexual selection on two uncorrelated secondary sexual characters in males (forehead and wing patch size) when fledgling number was used as the measure of fitness. When number of offspring recruiting to the breeding population was used as the measure of male fitness, selection on these traits appeared to be directional and stabilizing, respectively. Pairwise comparisons of cuckolded and cuckolding males revealed that males that sired young through extrapair copulations had wider forehead patches, and were paired to females that bred earlier, than the males that they cuckolded. Path analysis was used to partition selection on these traits into pathways via mate fecundity and sperm competition, and suggested that the sperm competition pathway accounted for between 64 and 90% of the total sexual selection via the two paths. The selection revealed in these analyses is relatively weak in comparison with many other measures of selection in natural populations. We offer some explanations for the relatively weak selection detected. Copyright 1999 The Association for the Study of Animal Behaviour.     

Fluctuations in population composition dampen the impact of phenotypic plasticity on trait dynamics in superb fairy-wrens

1. In structured populations, phenotypic change can result from changes throughout an individual's lifetime (phenotypic plasticity, age-related changes), selection and changes in population composition (environment- or density-driven fluctuations in age-structure). 2. The contribution of population dynamics to phenotypic change has often been ignored. However, for understanding trait dynamics, it is important to identify both the individual- and population-level mechanisms responsible for trait change, because they potentially reinforce or counteract each other. 3. We use 22 years of field data to investigate the dynamics of a sexually selected phenological trait, the timing of nuptial moult in superb fairy-wrens Malurus cyaneus. 4. We show that trait expression is both climate- and age-dependent, but that phenotypic plasticity in response to climate variability also varies with age. Old males can acquire nuptial plumage very early after high rainfall, but 1- to 2-year-olds cannot. However, males of all ages that defer moult to later in the year acquire nuptial plumage earlier when conditions are warmer. 5. The underlying mechanism appears to be that old males may risk moulting in the most challenging period of the year: in autumn, when drought restricts food abundance and during the cold winter. By contrast, young males always moult during the spring transition to benign - warmer and generally wetter - conditions. Temperature changes dominate this transition that heralds the breeding season, thereby causing both young and late-moulting older birds to be temperature sensitive. 6. Climate and age also affect trait dynamics via a population dynamical pathway. The same high rainfall that triggers early moulting in old males concurrently increases offspring recruitment and thereby reduces the average age of males in the population. Consequently, effects of rainfall on trait dynamics through phenotypic plasticity of old males are dampened by synchronous rejuvenation of the age-structure. 7. A long-term trend towards drier environments prompted phenotypic change because of plasticity, but this was masked by climate-driven demographic change (causing apparent stasis). This suggests a novel explanation for why trait change may fail to reflect the observed pattern of directional selection or phenotypic plasticity.     

Extra-pair paternity and song characteristics in the willow warbler Phylloscopus trochilus

Complexity in bird song is often argued to be advantageous in processes of sexual selection, and numerous studies show that characteristics of song are associated with increased success in territory defence or mate attraction. Less evidence exists on the relationship between bird song characteristics and patterns of extra-pair paternity. We tested whether males that suffered from extra-pair paternity differed in song characteristics from males with no extra pair paternity in the willow warbler Phylloscopus trochilus . In the Scottish population that we studied, we found that 23.5% of young were not related to the social father, and that 47% of nests contained at least one extra pair young. Older males were less likely to suffer paternity loss. While song repertoire size was not related to loss of paternity, males with short songs suffered higher paternity loss than males with long songs. Although arrival date is a good correlate of social mate choice in this population, it was not related to extra-pair paternity. These results suggest that females use song length, or a trait correlated with this song characteristic, as a cue for choosing extra-pair partners in this species, or alternatively that variance in the success of mate guarding or female coercion is related to this song variable.     

Male cognitive performance declines in the absence of sexual selection

Sexual selection is responsible for the evolution of male ornaments and armaments, but its role in the evolution of cognition—the ability to process, retain and use information—is largely unexplored. Because successful courtship is likely to involve processing information in complex, competitive sexual environments, we hypothesized that sexual selection contributes to the evolution and maintenance of cognitive abilities in males. To test this, we removed mate choice and mate competition from experimental populations of Drosophila melanogaster by enforcing monogamy for over 100 generations. Males evolved under monogamy became less proficient than polygamous control males at relatively complex cognitive tasks. When faced with one receptive and several unreceptive females, polygamous males quickly focused on receptive females, whereas monogamous males continued to direct substantial courtship effort towards unreceptive females. As a result, monogamous males were less successful in this complex setting, despite being as quick to mate as their polygamous counterparts with only one receptive female. This diminished ability to use past information was not limited to the courtship context: monogamous males (but not females) also showed reduced aversive olfactory learning ability. Our results provide direct experimental evidence that the intensity of sexual selection is an important factor in the evolution of male cognitive ability.     

Conflict over multiple-partner mating between males and females of the polygynandrous common lizards

The optimal number of mate partners for females rarely coincides with that for males, leading to a potential sexual conflict over multiple-partner mating. This suggests that the population sex ratio may affect multiple-partner mating and thus multiple paternity. We investigate the relationship between multiple paternity and the population sex ratio in the polygynandrous common lizard (Lacerta vivipara). In six populations the adult sex ratio was biased toward males, and in another six populations the adult sex ratio was biased toward females, the latter corresponding to the average adult sex ratio encountered in natural populations. In males the frequency and the degree of polygyny were lower in male-biased populations, as expected if competition among males determines polygyny. In females the frequency of polyandry was not different between treatments, and polyandrous females produced larger clutches, suggesting that polyandry might be adaptive. However, in male-biased populations females suffered from reduced reproductive success compared to female-biased populations, and the number of mate partners increased with female body size in polyandrous females. Polyandrous females of male-biased populations showed disproportionately more mating scars, indicating that polyandrous females of male-biased populations had more interactions with males and suggesting that the degree of multiple paternity is controlled by male sexual harassment. Our results thus imply that polyandry may be hierarchically controlled, with females controlling when to mate with multiple partners and male sexual harassment being a proximate determinant of the degree of multiple paternity. The results are also consistent with a sexual conflict in which male behaviors are harmful to females.     

Determinants of reproductive success across sequential episodes of sexual selection in a firefly

Because females often mate with multiple males, it is critical to expand our view of sexual selection to encompass pre-, peri- and post-copulatory episodes to understand how selection drives trait evolution. In Photinus fireflies, females preferentially respond to males based on their bioluminescent courtship signals, but previous work has shown that male paternity success is negatively correlated with flash attractiveness. Here, we experimentally manipulated both the attractiveness of the courtship signal visible to female Photinus greeni fireflies before mating and male nuptial gift size to determine how these traits might each influence mate acceptance and paternity share. We also measured pericopulatory behaviours to examine their influence on male reproductive success. Firefly males with larger spermatophores experienced dual benefits in terms of both higher mate acceptance and increased paternity share. We found no effect of courtship signal attractiveness or pericopulatory behaviour on male reproductive success. Taken together with previous results, this suggests a possible trade-off for males between producing an attractive courtship signal and investing in nuptial gifts. By integrating multiple episodes of sexual selection, this study extends our understanding of sexual selection in Photinus fireflies and provides insight into the evolution of male traits in other polyandrous species.     

Females prefer to associate with males with longer intromittent organs in mosquitofish

Sexual selection is a major force behind the rapid evolution of male genital morphology among species. Most within-species studies have focused on sexual selection on male genital traits owing to events during or after copulation that increase a male''s share of paternity. Very little attention has been given to whether genitalia are visual signals that cause males to vary in their attractiveness to females and are therefore under pre-copulatory sexual selection. Here we show that, on average, female eastern mosquitofish Gambusia holbrooki spent more time in association with males who received only a slight reduction in the length of the intromittent organ (‘gonopodium’) than males that received a greater reduction. This preference was, however, only expressed when females chose between two large males; for small males, there was no effect of genital size on female association time.     

The effect of an inversion system and the time interval between matings on postcopulatory sexual selection in the seaweed fly, Coelopa frigida

The seaweed fly, Coelopa frigida, exhibits LMSP. A large chromosomal inversion system affects many traits including egg-to-adult viability via heterosis. Consequently, there is also considerable potential for cryptic female mate choice to operate on the basis of sperm karyotype. Here, we investigated the effect of time interval and chromosomal inversion karyotype on postcopulatory sexual selection. Homokaryotypic females were mated with a male of the same and a male of the opposite homokaryotype. The order of the matings was varied so cryptic female mate choice could operate either in concert or antagonistically with LMSP. LMSP was found when there was a 24 h time interval between matings, irrespective of the order in which the males were mated. However, when the males were mated in quick succession the order of mating was important. When LMSP and cryptic female mate choice work in concert a high level of LMSP was found. However, when the male of opposite homokaryotype mated first, then first male sperm precedence was observed. This suggests that polyandrous females might be able to bias paternity but only when matings occur in quick succession. Consequently, population density is likely to affect the operation of postcopulatory sexual selection.     

THE EVOLUTION OF PLUMAGE BRIGHTNESS IN BIRDS IS RELATED TO EXTRAPAIR PATERNITY

A positive association between plumage brightness of male birds and the degree of polygyny may be the result of sexual selection. Although most birds have a socially monogamous mating system, recent paternity analyses show that many offspring are fathered by nonmates. Extrapair paternity arises from extrapair copulations which are frequently initiated by females. Not all females will be able to mate with a male of the preferred phenotype, because of the mating decisions of earlier paired females; extrapair copulations may be a means for females to adjust their precopulation mate choice. We use two comparative analyses (standardized linear contrasts and pairwise comparisons between closely related taxa) to test the idea that male plumage brightness is related to extrapair paternity. Brightness of male plumage and sexual dimorphism in brightness were positively associated with high levels of extrapair paternity, even when potentially confounding variables were controlled statistically. This association between male brightness and extrapair paternity was considerably stronger than the association between male brightness and the degree of polygyny. Cuckoldry thus forms an important component of sexual selection in birds.     

Does kin selection moderate sexual conflict in Drosophila?

Two recent studies provide provocative experimental findings about the potential influence of kin recognition and cooperation on the level of sexual conflict in Drosophila melanogaster. In both studies, male fruit flies apparently curbed their mate-harming behaviours in the presence of a few familiar or related males, suggesting some form of cooperation mediated by kin selection. In one study, the reduction in agonistic behaviour by brothers apparently rendered them vulnerable to dramatic loss of paternity share when competing with an unrelated male. If these results are robust and generalizable, fruit flies could be a major new focus for the experimental study of kin selection and social evolution. In our opinion, however, the restrictive conditions required for male cooperation to be adaptive in this species make it unlikely to evolve. We investigated these phenomena in two different populations of D. melanogaster using protocols very similar to those in the two previous studies. Our experiments show no evidence for a reduction in mate harm based upon either relatedness or familiarity between males, and no reduction in male reproductive success when two brothers are in the presence of an unfamiliar, unrelated, ‘foreign’ male. Thus, the reduction of sexual conflict owing to male cooperation does not appear to be a general feature of the species, at least under domestication, and these contrasting results call for further investigation: in new populations, in the field and in the laboratory populations in which these phenomena have been reported.     

Number of mating males and mating interval affect last-male sperm precedence in Tenebrio molitor L.

Polyandrous females often mate with more than two males, and yet most studies of postcopulatory sexual selection involve only two males. In insects, second-male sperm precedence is usually taken as evidence of overall last-male sperm precedence despite some studies to the contrary. Furthermore, the processes or mechanisms causing the patterns are often unknown and yet are important when estimating how postcopulatory sexual selection might act on males. Whether the patterns and processes change in normal sperm competitive situations and the effects of other factors besides mating order need to be examined to better assess the evolutionary potential of postcopulatory sexual selection. In this study, I assessed the effects of mating interval and number of mating males on sperm precedence patterns and their causal mechanisms in the mealworm beetle, Tenebrio molitor. Last-male sperm precedence was the same when two or three males mated, but also depended on mating intervals and hence mechanisms of paternity bias. However, when females mated with many males, one of the mechanisms no longer created last-male sperm precedence. This example illustrates the importance of knowing both the patterns and mechanisms of paternity bias and whether they change depending on female mating frequency to make reasonable inferences about the potential for postcopulatory sexual selection on males. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.      

Why do female migratory birds arrive later than males?

1. In migratory birds males tend to arrive first on breeding grounds, except in sex-role reversed species. The two most common explanations are the rank advantage hypothesis, in which male-male competition for breeding sites drives stronger selection for early arrival in males than females, and the mate opportunity hypothesis, which relies on sexual selection, as early arrival improves prospects of mate acquisition more for males than for females. 2. To date, theoretical work has focused on selection for early arrival within a single sex, usually male. However, if fitness depends on territory quality, selection for early arrival should operate on both sexes. Here we use two independent modelling approaches to explore the evolution of protandry (male-first arrival) and protogyny (female-first arrival) under the rank advantage and mate opportunity hypotheses. 3. The rank advantage hypothesis, when operating alone, fails to produce consistent patterns of protandry, despite our assumption that males must occupy territories before females. This is because an individual of either sex benefits if it out-competes same-sex competitors. Rather than promoting protandry, the rank advantage mechanism can sometimes result in protogyny. Female-female competition is stronger than male-male competition early in the season, if females compete for a resource (territories occupied by males) that is initially less common than the resource of interest to males (unoccupied territories). 4. Our results support the mate opportunity hypothesis as an explanation of why protandry is the norm in migratory systems. Male-biased adult sex ratios and high levels of sperm competition (modelled as extra-pair young: EPY) both produce protandry as a result of sexual selection. Protogyny is only observed in our models with female-biased sex ratios and low EPY production. 5. We also show that the effects of sex ratio biases are much stronger than those of EPY production, explore the evidence for sex ratio biases and extra-pair paternity in migratory species and suggest future research directions.     

Context-dependent development of sexual ornamentation: implications for a trade-off between current and future breeding efforts

Allocation of resources into the development of sexual displays is determined by a trade-off between the competing demands of current reproduction and self-maintenance. When reproduction overlaps with acquisition of sexual ornamentation, such as in birds with a yearly post-breeding moult, such a trade-off can be expressed in elaboration of sexual traits used in subsequent matings. In turn, selection for elaboration of sexual ornaments should favour resolution of this trade-off through a modification of the ornaments' development, resulting in variable and life history-dependent development of sexual displays. Here we examined a novel hypothesis that the trade-off between current reproduction and development of sexual ornamentation in the house finch (Carpodacus mexicanus) can be mediated by the shared effects of prolactin - a pituitary hormone that regulates both parental care and moult in this species. We compared developmental variation in sexual ornamentation between breeding, nonbreeding, and juvenile males and examined the relative contribution of residual levels of prolactin and individual condition during moult to the acquisition of sexual ornamentation. Males that invested heavily in parental care entered post-breeding moult in lower condition and later in the season, but their higher plasma prolactin was associated with shorter and more intense moult ultimately resulting in equal or greater elaboration of sexual ornamentation compared with nonparental males. Elaboration of sexual ornamentation of nonparental males that entered moult in greater condition, but with lower prolactin, was produced by longer and earlier moult and by lesser overlap in moult between sexual ornaments. Ornamentation of juvenile males that acquire sexual ornamentation for the first time was closely associated with physiological condition during moult. We discuss the implications of such context-dependent ontogenies of sexual ornamentation and resulting differences in condition-dependence of sexual traits across life history stages on the evolution of female preference for elaborated sexual displays.     

An integrative view of sexual selection in Tribolium flour beetles

Sexual selection is a major force driving the evolution of diverse reproductive traits. This evolutionary process is based on individual reproductive advantages that arise either through intrasexual competition or through intersexual choice and conflict. While classical studies of sexual selection focused mainly on differences in male mating success, more recent work has focused on the differences in paternity share that may arise through sperm competition or cryptic female choice whenever females mate with multiple males. Thus, an integrative view of sexual selection needs to encompass processes that occur not only before copulation (pre-mating), but also during copulation (peri-mating), as well as after copulation (post-mating), all of which can generate differences in reproductive success. By encompassing mechanisms of sexual selection across all of these sequential reproductive stages this review takes an integrative approach to sexual selection in Tribolium flour beetles (Coleoptera: Tenebrionidae), a particularly well-studied and economically important model organism. Tribolium flour beetles colonize patchily distributed grain stores, and juvenile and adult stages share the same food resources. Adults are highly promiscuous and female reproduction is distributed across an adult lifespan lasting approximately 1 year. While Tribolium males produce an aggregation pheromone that attracts both sexes, there appears to be little pre-mating discrimination among potential mates by either sex. However, recent work has revealed several peri-mating and post-mating mechanisms that determine how offspring paternity is apportioned among a female's mates. During mating, Tribolium females reject spermatophore transfer and limit sperm numbers transferred by males with low phenotypic quality. Although there is some conflicting evidence, male copulatory leg-rubbing appears to be associated with overcoming female resistance to insemination and does not influence a male's subsequent paternity share. Evidence suggests that Tribolium beetles have several possible post-mating mechanisms that they may use to bias paternity. Male sperm precedence has been extensively studied in Tribolium spp. and the related Tenebrio molitor, and several factors influencing male paternity share among a female's progeny have been identified. These include oviposition time, inter-mating interval, male strain/genotype, the mating regimen of a male's mother, male starvation, and tapeworm infection. Females exert muscular control over sperm storage, although there is no evidence to date that females use this to differentiate among mates. Females could also influence offspring paternity by re-mating with additional males, and T. castaneum females more readily accept spermatophores when they are re-mating with more attractive males. Additional work is needed to examine the possible roles played by both male and female accessory gland products in determining male paternity share. Sexual selection during pre-mating episodes may be reinforced or counteracted by peri- and post-copulatory selection, and antagonistic coevolution between the sexes may be played out across reproductive stages. In Tribolium, males' olfactory attractiveness is positively correlated with both insemination success and paternity share, suggesting consistent selection across different reproductive stages. Similar studies across sequential reproductive stages are needed in other taxa to provide a more integrative view of sexual selection.     

Competing dwarf males: sexual selection in an orb-weaving spider

Hypotheses for the adaptive significance of extreme female-biased sexual size dimorphism (SSD) generally assume that in dimorphic species males rarely interfere with each other. Here we provide the first multivariate examination of sexual selection because of male-male competition over access to females in a species with 'dwarf' males, the orb-weaving spider Argiope aurantia. Male A. aurantia typically try to mate opportunistically during the female's final moult when she is defenceless. We show that, contrary to previous hypotheses, the local operational sex ratio (males per female on the web) is male-biased most of the season. Both interference and scramble competition occur during opportunistic mating, the former leading to significant selection for large male body size. Male condition and leg length had no effect on mating success independent of size. We discuss these findings in the context of the evolution of extreme female-biased SSD in this clade.