Component Of Sexual Selection Research Articles

Aging modulates cuticular hydrocarbons and sexual attractiveness inDrosophila melanogaster

Attractiveness is a major component of sexual selection that is dependent on sexual characteristics, such as pheromone production, which often reflect an individual's fitness and reproductive potential. Aging is a process that results in a steady decline in survival and reproductive output, yet little is known about its effect on specific aspects of attractiveness. In this report we asked how aging impacts pheromone production and sexual attractiveness in Drosophila melanogaster. Evidence suggests that key pheromones in Drosophila are produced as cuticular hydrocarbons (CHC), whose functions in attracting mates and influencing behavior have been widely studied. We employed gas chromatography/mass spectrometry and laser desorption/ionization mass spectrometry to show that the composition of D. melanogaster CHC is significantly affected by aging in both sexes and that these changes are robust to different genetic backgrounds. Aging affected the relative levels of many individual CHC, and it shifted overall hydrocarbon profiles to favor compounds with longer chain lengths. We also show that the observed aging-related changes in CHC profiles are responsible for a significant reduction in sexual attractiveness. These studies illuminate causal links among pheromones, aging and attractiveness and suggest that CHC production may be an honest indicator of animal health and fertility.

Partitioning sexual selection into its mating success and fertilization success components.

Postcopulatory sexual selection due to sperm competition and/or cryptic female choice has been documented in a diversity of taxonomic groups and is considered a pivotal component of sexual selection. Despite this apparent importance, the relative contribution of postcopulatory fertilization success to overall sexual selection has not yet been measured in any species. Here, we used a laboratory-adapted population of the promiscuous fruit fly Drosophila melanogaster to partition the variance in male reproductive success into mating success (a major component of precopulatory sexual selection) and fertilization success (a major component of postcopulatory sexual selection). We found that fertilization success contributed nearly as strongly as mating success to a male's net performance in sexual selection, but that most of this postcopulatory component was attributable to variation in male mating order (the tendency to be the last male to mate a female). After adjusting for mating order, only ≈2% of the residual variation in male reproductive success was attributable to differential fertilization success. We found no correlation between male mating success and fertilization success in this system. Unlike natural populations of Drosophila, our laboratory population is adapted to a semelparous lifecycle, so our findings will be most applicable to other promiscuous species with strong sperm precedence and one short breeding period per year or lifetime. In these species, fertilization success may have as much influence on male reproductive success as mating success, but the timing of mating (mating order) may be the predominant factor contributing to variation in fertilization success.

Understanding intrasexual competition and sexual selection requires an evolutionary ecology framework

Across most species, sexually selected traits have a dual role in both gaining mates and in intrasexual competition (Hunt et al. 2009). As suggested by Rosvall in this review, most definitions of sexual selection restrict it to the evolutionary dynamics of competition for mates, yet in females, results point to the fact that intrasexual competitions between females often occur for access to high-quality mates and resources rather than for the number of mates. As the review highlights, both males and females may compete for resources necessary to raise offspring, and in many cases where it appears that females are competing for access to males, they are probably competing for resources in the form of direct fitness benefits. Therefore, distinguishing between components of selection is seldom easy in either sex, particularly when natural and sexual selection operate in the same direction. I realize that there is debate to be had and this review makes suggestions as to how to move forwards, but in my opinion instead of formally distinguishing components of sexual selection from natural selection and arguing about the true nature of sexual selection, we will gain greater insight into the evolutionary dynamics of male and female phenotypes if we instead concentrate on quantifying the important components of selection within an evolutionary ecology framework. As Rosvall suggests, examining contrasting fitness components and quantifying the intensity and targets of selection in males and females can reveal important insights. Studies that have examined both the factors that influence the fitness component of gaining mates (i.e., the number of matings), and the fitness outcomes of those matings (i.e. the number/ quality of the offspring produced), have revealed that selection components may not always act in the same direction, if competitively superior individuals do not always gain increased fitness or convey increased fitness to their mates. Allocating resources to competitive intrasexual interactions for mates may trade-off against survival (Robinson et al. 2008), sperm competition (Preston et al. 2001), and parental care in males (Duckworth 2006), and there is no reason why females should not also face resource allocation trade-offs under certain conditions. Furthermore, if either sex face trade-offs then the fitness benefits gained from intrasexual competition or intersexual mate choice will be dependent on these, although at present, there are few estimates of selection acting on these components. Therefore, rather than formally defining which components are under sexual versus natural selection, we need to quantify individual differences in breeding success and determine its relationship with various underlying components in both sexes. Only in this way, will we understand the potential factors that maintain variation that we often see in natural populations or drive the differences that we observe between the sexes. Although research over the past decade has greatly extended our knowledge of the operation of sexual selection in females (LeBas 2006; and this review), quantifying the selection is only one side of the coin. As pointed out in this review, understanding the evolutionary dynamics of a trait will also require estimates of the genetic basis in males and females, the cross-sex genetic correlation, of the traits under selection. Furthermore, the dynamics of sexual selection will vary across species as mating systems vary but will also vary within species or within populations as social and environmental factors change. As described in this review, density, resource availability, and many other social and environmental factors will drive competitive interactions, and the strength of selection acting on phenotype. As the genetic basis of traits under selection is also likely to change across similar abiotic and biotic environments as a result of genotype–environment interactions (for a review see Ingleby et al. 2010), we really require simultaneous estimation of the genetic basis of characters in males and females, and the selection pressures acting on each sex. In my opinion, this should also be a priority for future research. Applying an evolutionary ecology framework to the study of sexual selection in both sexes will enable us to better understand the evolutionary dynamics of sexual selection within the natural world.

UNDERSTANDING WHAT WE SEE IN NATURE: HOW TO SPEND YOUR LIFE AS AN EVOLUTIONARY ECOLOGIST

Review of: Losos, J. B (ed.). 2011. In the Light of Evolution. Essays from the Laboratory and Field. Roberts and Company Publishers. 330 pp. ISBN: 9780981519494; $59.95 HB. Have you ever wondered what evolutionary ecologists do? What sorts of questions drive them and how do they approach the answers to these questions? Are you interested in understanding patterns and phenotypes seen in nature? Are you wondering what to do with the next 20 years of your life? If you answered yes to at least two of these questions, then I recommend that you read “In the Light of Evolution,” a volume of 17 essays edited by Jonathan Losos. He has brought together a loosely connected set of individuals, many from Harvard, where he works, together with others who he has collaborated with, shares interests with, or simply admires, to write about their obsessions. Yes, for the most part, these are people who obsess and in doing so they have illuminated aspects of nature that were in the dark prior to their probing. They have helped us understand their small part of the world in the light of evolution and thereby provide us with parts of a framework that can be applied to other systems. As I tell my students in my evolution course, it is worth trying to see the world through the eyes of an evolutionary biologist, which is what this volume is all about and something I hope my students can learn to do. Many of the essays in this book differ from traditional chapters on any given scientific topic, because the focus is on how answers are found as much as on what the answers are. On display are the inspiration, perspiration, and motivation of the individual researchers. This includes the four essays that discuss Darwin and/or Wallace, two eccentric individuals who profoundly changed how we view the natural world. In the essay on Darwin by Janet Browne, she weaves aspects of Darwin’s upbringing, his travels, his home life, and his beliefs throughout, and thereby brings the humanity of the man and his writing to the fore. Importantly, both she and Carl Zimmer (in his essay on researchers of microbes) note that Darwin was not only an observer, but also a lover of experiments and their interpretation, and hence the “theory” of evolution by natural selection was the product of a naturalist/experimentalist. The importance of observation is also highlighted in the essay by Marlene Zuk and Teri Orr, who point to the disagreement between Darwin and Wallace on whether female choice, as a component of sexual selection, existed. Apparently Wallace felt that ornamentation of males was a consequence of their relatively heightened vigor compared to females. All Darwin had to go on at the time were patterns of ornamentation, rather than experiments, but as this essay points out, he remained convinced that female choice was involved in male ornamentation until the day he died. Michael Ryan’s essay, which is devoted to the question of whether female choice has influenced the mating calls of tungara frogs, shows how far we have come. It outlines an impressive set of field observations rich in the ecology of the frogs, experiments, and knowledge of the phylogeny of the genus that allows him to say yes, female choice has heavily influenced the evolution of male mating calls.

Female Choice for Males with Greater Fertilization Success in the Swedish Moor Frog, Rana arvalis

BackgroundStudies of mate choice in anuran amphibians have shown female preference for a wide range of male traits despite females gaining no direct resources from males (i.e. non-resource based mating system). Nevertheless, theoretical and empirical studies have shown that females may still gain indirect genetic benefits from choosing males of higher genetic quality and thereby increase their reproductive success.Methodology/Principal FindingsWe investigated two components of sexual selection in the Moor frog (Rana arvalis), pre-copulatory female choice between two males of different size (‘large’ vs. ‘small’), and their fertilization success in sperm competition and in isolation. Females' showed no significant preference for male size (13 small and six large male preferences) but associated preferentially with the male that subsequently was the most successful at fertilizing her eggs in isolation. Siring success of males in competitive fertilizations was unrelated to genetic similarity with the female and we detected no effect of sperm viability on fertilization success. There was, however, a strong positive association between a male's innate fertilization ability with a female and his siring success in sperm competition. We also detected a strong negative effect of a male's thumb length on his competitive siring success.Conclusions/SignificanceOur results show that females show no preference for male size but are still able to choose males which have greater fertilization success. Genetic similarity and differences in the proportion of viable sperm within a males ejaculate do not appear to affect siring success. These results could be explained through pre- and/or postcopulatory choice for genetic benefits and suggest that females are able to perceive the genetic quality of males, possibly basing their choice on multiple phenotypic male traits.

Make Love Not War: When Should Less Competitive Males Choose Low‐Quality but Defendable Females?

Male choosiness for mates is an underexplored mechanism of sexual selection. A few theoretical studies suggest that males may exhibit--but only under rare circumstances--a reversed male mate choice (RMMC; i.e., highly competitive males focus on the most fecund females, while the low-quality males exclusively pair with less fecund mates to avoid being outcompeted by stronger rivals). Here we propose a new model to explore RMMC by relaxing some of the restrictive assumptions of the previous models and by considering an extended range of factors known to alter the strength of sexual selection (males' investment in reproduction, difference of quality between females, operational sex ratio). Unexpectedly, we found that males exhibited a reversed mate choice under a wide range of circumstances. RMMC mostly occurs when the female encounter rate is high and males devote much of their time to breeding. This condition-dependent strategy occurs even if there is no risk of injury during the male-male contest or when the difference in quality between females is small. RMMC should thus be a widespread yet underestimated component of sexual selection and should largely contribute to the assortative pairing patterns observed in numerous taxa.

Intrasexual competition facilitates the evolution of alternative mating strategies in a colour polymorphic fish

BackgroundIntense competition for access to females can lead to males exploiting different components of sexual selection, and result in the evolution of alternative mating strategies (AMSs). Males of Poecilia parae, a colour polymorphic fish, exhibit five distinct phenotypes: drab-coloured (immaculata), striped (parae), structural-coloured (blue) and carotenoid-based red and yellow morphs. Previous work indicates that immaculata males employ a sneaker strategy, whereas the red and yellow morphs exploit female preferences for carotenoid-based colours. Mating strategies favouring the maintenance of the other morphs remain to be determined. Here, we report the role of agonistic male-male interactions in influencing female mating preferences and male mating success, and in facilitating the evolution of AMSs.ResultsOur study reveals variation in aggressiveness among P. parae morphs during indirect and direct interactions with sexually receptive females. Two morphs, parae and yellow, use aggression to enhance their mating success (i.e., number of copulations) by 1) directly monopolizing access to females, and 2) modifying female preferences after winning agonistic encounters. Conversely, we found that the success of the drab-coloured immaculata morph, which specializes in a sneak copulation strategy, relies in its ability to circumvent both male aggression and female choice when facing all but yellow males.ConclusionsStrong directional selection is expected to deplete genetic variation, yet many species show striking genetically-based polymorphisms. Most studies evoke frequency dependent selection to explain the persistence of such variation. Consistent with a growing body of evidence, our findings suggest that a complex form of balancing selection may alternatively explain the evolution and maintenance of AMSs in a colour polymorphic fish. In particular, this study demonstrates that intrasexual competition results in phenotypically distinct males exhibiting clear differences in their levels of aggression to exclude potential sexual rivals. By being dominant, the more aggressive males are able to circumvent female mating preferences for attractive males, whereas another male type incorporates subordinate behaviours that allow them to circumvent male aggression and female mating preferences. Together, these and previous results indicate that exploiting different aspects of social interactions may allow males to evolve distinct mating strategies and thus the long term maintenance of polymorphisms within populations.

A Cost of Sexual Attractiveness to High-Fitness Females

Author SummaryIn many species, females are frequently subject to harassing courtship from males attempting to mate with them. These persistent male behaviors can result in females incurring substantial direct fitness costs. We set out to examine how these costs may influence adaptive potential in a species that also exhibits male mate choice, i.e., a preference by males for females exhibiting certain traits. We found that harmful courtship behaviors were directed predominantly towards females of greater reproductive potential (and away from females of lesser potential), resulting in a reduction in the variation of lifetime reproductive successes among females in the population. This change in distribution of realized fitnesses represents a previously unappreciated consequence of sexual conflict–adaptive male mate preference can slow the rate of accumulation of beneficial mutations and speed the rate of accumulation of harmful mutations, thereby creating a “sexual conflict adaptive load” within a species.

Spouse selection by health status and physical traits. Sardinia, 1856–1925

Military medical information and data from civil registers of death and marriage have been used to study the role of physical characteristics and health conditions in explaining access to marriage for the male population of Alghero, a small city located in Sardinia Island (Italy), at the turn of 19th century. Literature data about contemporary populations have already demonstrated the influence of somatic traits in the mate choice. The results presented here show that men with low height and poor health status at the age of 20 were negatively selected for marriage. This holds true also in a society where families often arranged marriages for their children. This pattern of male selection on marriage was found to be particularly marked among the richest and wealthiest SES groups. Our hypothesis is that this social group carefully selected for marriage those individuals who were apparently healthier and therefore more likely to guarantee good health status and better life conditions to offspring. In evolutionary terms, the mate choice component of sexual selection suggests that the height of prospective partners could be claimed as one of the determinants, along with other environmental causes, of the observed higher stature of men belonging to the wealthiest social strata of the Alghero population.

SEXUAL SELECTION AND THE GENETIC MATING SYSTEM OF WILD TURKEYS

Molecular measures of parentage provide important insights into the opportunity for sexual selection; in birds, such studies have been conducted almost exclusively on pair-bonded passerines. Here I employ a multitiered parentage analysis involving 10-locus microsatellite genotypes to characterize the genetic mating system of a population of Wild Turkeys (Meleagris gallopavo), a promiscuous species in which males and potentially females have been thought to mate multiply. Young in almost half of nests (48%, n = 15) were each apparently the product of a single male and female parent. Fourteen broods (45%) resulted from multiple paternity; seven contained eggs from multiple females, four of which appeared to be cases of quasi parasitism, in which the offspring of the brood parasitic female were fathered by the same male that sired at least one of the host females' offspring. Bateman gradients for males and females indicate that males experience a significantly greater gain in reproductive success from additional mates; the trend for females to benefit from multiple matings disappeared when the small “clutches” of parasitic females were excluded from the analysis. Of the components of variance in male fitness, number of mates was the most important determinant of male reproductive success. Somewhat surprisingly, when considering only reproductively successful males, the proportion of a female's offspring that a male sired also explained a substantial proportion of the total variance in male reproductive success. Incomplete sampling of offspring could mean that these estimates, particularly the importance of mate number, may be underestimated. Regardless, these results suggest that multiple mating by females may be an important and overlooked component of sexual selection in species with lek-like mating systems, and that selection may act independently on males to attract more mates and increase their share of paternity with those mates.

Patterns of Parental Investment and Sexual Selection in Teleost Fishes: Do They Support Bateman's Principles?

Bateman demonstrated differences in variance for fertility and mating success between the sexes, with males usually having a greater variance than females. Thus in general, male reproductive success increases with number of mates acquired. These results have been referred to as "Bateman's principles" and taken together with other parameters (e.g., relative parental investment) have been proposed to estimate a component of sexual selection. For this review I examine patterns of parental care and sexual selection in teleost fishes (substrate brooding and with internal fertilization). I present data for the pumpkinseed sunfish Lepomis gibbosus, in which I estimated cost of paternal care and compared direct measures of the intensity of selection on possible sexually selected traits to measures of sexual selection based on Bateman's principles.Despite high levels of paternal care in substrate brooding fishes, sexual selection tends to act more strongly on males than on females, which suggests that maternal investment is higher than paternal investment and that parental care does not limit the reproductive rate for males. In pumpkinseed sunfish, selection favors parents with high levels of defense that may exclude predators more effectively and, as suggested by Bateman's measures, alternative reproductive strategies may decrease the opportunity for sexual selection within the parental strategy. In teleost fishes with internal fertilization, patterns of parental investment and intensity of sexual selection seem to support Bateman's principles, but further studies using these systems and these measures of selection will improve the understanding of factors affecting the intensity of sexual selection and its relation to mating systems.

Sex, mutation and fitness: asymmetric costs and routes to recovery through compensatory evolution

A wild-type population of Drosophila melanogaster was used to assess the impact of a known deleterious mutation, nub(1), when it had (1) evolved for up to 180 generations with the mutation or (2) recently had the same mutant allele introgressed into it. Relative to this benchmark, we observed much stronger initial fitness depression in males (-74%) than in females (-38%) and also relatively greater fitness recovery by evolved males (+55%) than females (+17%). Experimental assays revealed amelioration in both juvenile and adult fitness and suggested that the greater relative recovery of male fitness was from gains through sexual selection. These evolutionary changes in male fertility depended on pairing with their coevolved mates for both mate choice and post-copulatory components of sexual selection. Without replication at the population level, these results are used to motivate a general hypothesis rather than definitively test it: Differences in reproductive optima may generally skew mutational effects towards the more strongly sexually-selected sex due to genic capture and condition dependence.

Sexual selection for male dominance reduces opportunities for female mate choice in the European bitterling (Rhodeus sericeus)

Sexual selection involves two main mechanisms: intrasexual competition for mates and intersexual mate choice. We experimentally separated intrasexual (male-male interference competition) and intersexual (female choice) components of sexual selection in a freshwater fish, the European bitterling (Rhodeus sericeus). We compared the roles of multiple morphological and behavioural traits in male success in both components of sexual competition, and their relation to male reproductive success, measured as paternity of offspring. Body size was important for both female choice and male-male competition, though females also preferred males that courted more vigorously. However, dominant males often monopolized females regardless of female preference. Subordinate males were not excluded from reproduction and sired some offspring, possibly through sneaked ejaculations. Male dominance and a greater intensity of carotenoid-based red colouration in their iris were the best predictors of male reproductive success. The extent of red iris colouration and parasite load did not have significant effects on female choice, male dominance or male reproductive success. No effect of parasite load on the expression of red eye colouration was detected, though this may have been due to low parasite prevalence in males overall. In conclusion, we showed that even though larger body size was favoured in both intersexual and intrasexual selection, male-male interference competition reduced opportunities for female choice. Females, despite being choosy, had limited control over the paternity of their offspring. Our study highlights the need for reliable measures of male reproductive success in studies of sexual selection.

Sexual selection explains Rensch's rule of size dimorphism in shorebirds.

Sexual size dimorphism shows a remarkably widespread relationship to body size in the animal kingdom: within lineages, it decreases with size when females are the larger sex, but it increases with size when males are the larger sex. Here we demonstrate that this pattern, termed Rensch's rule, exists in shorebirds and allies (Charadriides), and it is determined by two components of sexual selection: the intensity of sexual selection acting on males and the agility of the males' display. These effects are interactive so that the effect of sexual selection on size dimorphism depends on male agility. As a control, we also examine dimorphism in bill length, which is a functionally selected trait. As such, dimorphism in bill length neither exhibits Rensch's rule nor is associated with sexual selection and display. Our results show that variation among taxa in the direction and magnitude of sexual size dimorphism, as manifested as Rensch's rule, can be explained by the interaction between the form and strength of sexual selection acting on each sex in relation to body size.

Extrapair paternity and the evolution of bird song

Bird song is usually considered to have evolved in the context of sexual selection. Because extrapair paternity is a major component of sexual selection, mating advantages at the social level for males that produce songs of high quality may be transformed into higher success in extrapair paternity. Therefore, males with longer and more complex songs should suffer less from extrapair paternity intraspecifically, whereas species with high rates of extrapair paternity, reflecting intense sperm competition, should produce more elaborate songs. Although some intraspecific studies demonstrated a negative link between features of songs and extrapair paternity in own nest, others failed to detect such a relationship. Contrary to expectation, a metaanalysis of all studies revealed no significant intraspecific evidence for songs being associated with extrapair paternity. In addition, in comparative analyses based on generalized least squares (GLS) models, we found that no measures of song complexity and temporal output were significantly related to extrapair paternity interspecifically, even when potentially confounding factors such as social mating system, life history, migration, habitat, or sexual dichromatism were held constant. Only plumage dichromatism was significantly related to extrapair paternity. The absence of both intra- and interspecific relationships between measures of song variability and extrapair paternity suggests that factors other than postmating sexual selection have been the important evolutionary forces shaping differences in song. Key words: bird song, evolution, extrapair paternity, generalized least squares, meta-analysis, repertoire size, sexual selection. [Behav Ecol 15:508–519 (2004)]

Sexual selection, germline mutation rate and sperm competition

BackgroundAn important component of sexual selection arises because females obtain viability benefits for their offspring from their mate choice. Females choosing extra-pair fertilization generally favor males with exaggerated secondary sexual characters, and extra-pair paternity increases the variance in male reproductive success. Furthermore, females are assumed to benefit from 'good genes' from extra-pair sires. How additive genetic variance in such viability genes is maintained despite strong directional selection remains an evolutionary enigma. We propose that sexual selection is associated with elevated mutation rates, changing the balance between mutation and selection, thereby increasing variance in fitness and hence the benefits to be obtained from good genes sexual selection. Two hypotheses may account for such elevated mutation: (1) Increased sperm production associated with sperm competition may increase mutation rate. (2) Mutator alleles increase mutation rates that are revealed by the expression of condition-dependent secondary sexual characters used by choosy females during their mate choice. M Petrie has independently developed the idea that mutator alleles may account for the maintenance of genetic variation in viability despite strong directional selection.ResultsA comparative study of birds revealed a positive correlation between mutation rate at minisatellite loci and extra-pair paternity, but not between mutation rate and relative testes mass which is a measure of relative sperm production. Minisatellite mutation rates were not related to longevity, suggesting a meiotic rather than a mitotic origin of mutations.ConclusionWe found evidence of increased mutation rate in species with more intense sexual selection. Increased mutation was not associated with increased sperm production, and we suggest that species with intense sexual selection may maintain elevated mutation rates because sexual selection continuously benefits viability alleles expressed in condition-dependent characters. Sexual selection may increase mutational input, which in turn feeds back on sexual selection because of increased variance in viability traits.

How well do mating frequency and duration predict paternity success in the polygynandrous water strider Aquarius remigis?

The relationship between mating success and paternity success is a key component of sexual selection but has seldom been estimated for species in which both sexes mate with many partners (polygynandry). We used a modification of Parker's sterile male technique to measure this relationship for the water strider Aquarius remigis in 47 laboratory populations simulating natural conditions of polygynandry. We also tested the hypothesis that prolonged copulation, a characteristic of this species, enhances paternity success. Mating behavior and paternity success were assayed for four days while males and females freely interacted. Paternity success was also assayed for an additional 7 days when females were isolated from males. Mating success significantly predicted paternity success and accounted for < or = 36% of the variance. Copulation duration was negatively related to both mating success and paternity success and did not explain any of the residual variance in paternity success. Thus, we found no evidence that prolonged copulation functions as a paternity assurance strategy in this species. Comparisons of sterile and fertile males suggested that paternity success is directly influenced by the quantity of sperm transferred. Our results support previous studies that have used mating success to estimate sexual selection, but also highlight the potential importance of sperm competition and other postinsemination processes.

Sexual coercion in a territorial salamander: males punish socially polyandrous female partners

Clutton-Brock & Parker (1995, Animal Behaviour, 49, 1345–1365) proposed that sexual coercion is a third component of sexual selection, along with intrasexual competition for mates and intersexual mate choice. They stated that sexual coercion occurs when males subject females to forced copulation, harassment, or intimidation, and that aggression towards females may occur even when females merely associate (not mate) with other males. We tested this proposition in laboratory experiments with red-backed salamanders, Plethodon cinereus, that had been found in male–female pairs (we call members of a pair ‘partners’) and as single (nonpaired) individuals in the forest. We manipulated paired females so that they either had associated with another male (socially polyandrous partners) or had not associated with another male (socially monogamous partners) before returning to their partners' territories. We also manipulated single females so that, upon encountering a single male for the first time, they either had associated with another male (socially polyandrous strangers) or had not associated with another male (socially naı̈ve strangers). Males were significantly more aggressive towards and stayed farther away from female partners that were socially polyandrous relative to those that were socially monogamous, during both the summer noncourtship season and autumn courtship season. Socially polyandrous females increased escape behaviour relative to socially monogamous females. However, males that met females for the first time did not differ significantly in aggression towards or distance from them when those females were either socially polyandrous or naı̈ve, during both the noncourtship and courtship seasons. Thus, male aggressive behaviour was context dependent, with enhanced aggression only towards partners that were socially polyandrous. We infer that males attempted to manipulate female partners into social monogamy by increasing aggression towards socially polyandrous partners. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

Sexual selection fails to promote adaptation to a new environment.

Selection can be divided into sexual and nonsexual components. Some work finds that a component of sexual selection, adaptive female selection for good genes, can promote nonsexual fitness. Less studied is the benefit from sexual selection in toto, that is, when intra- and intersexual selection are both present and able to affect females directly and indirectly. Here an upper bound for the net benefit of sexual selection is estimated for Drosophila melanogaster. Replicate populations were allowed to adapt to low-grade thermal stress, with or with out the operation of sexual selection. Because proteins and lipids are highly sensitive to temperature, low-grade thermal stress will select broadly across the genome for alternative alleles. Such broad, directional selection for thermal tolerance should increase the measurable benefits of sexual selection far beyond that available under stabilizing selection. Sexual selection was removed by enforced monogamy without mate choice and retained by enforced polyandry (four males per female). After 36 generations of thermal stress exposure, there was substantial adaptation to the new environment (the net reproductive rate increased six standard deviations relative to thermal controls). However, sexual selection did not affect the rate of adaptation. Therefore, adaptive female selection for thermal tolerance either was insignificant or negated by other aspects of sexual selection, for example, male-induced female harm, which has been shown to diminish under monogamy. This experiment employed two parameters that reduced the opportunity for divergence in such harm: a truncated intersexual interaction period and strong directional selection for thermal tolerance. No divergence in male-induced harm was observed.

HOW WELL DO MATING FREQUENCY AND DURATION PREDICT PATERNITY SUCCESS IN THE POLYGYNANDROUS WATER STRIDER AQUARIUS REMIGIS?

The relationship between mating success and paternity success is a key component of sexual selection but has seldom been estimated for species in which both sexes mate with many partners (polygynandry). We used a modification of Parker's sterile male technique to measure this relationship for the water strider Aquarius remigis in 47 laboratory populations simulating natural conditions of polygynandry. We also tested the hypothesis that prolonged copulation, a characteristic of this species, enhances paternity success. Mating behavior and paternity success were assayed for four days while males and females freely interacted. Paternity success was also assayed for an additional 7 days when females were isolated from males. Mating success significantly predicted paternity success and accounted for ≤ 36% of the variance. Copulation duration was negatively related to both mating success and paternity success and did not explain any of the residual variance in paternity success. Thus, we found no evidence that prolonged copulation functions as a paternity assurance strategy in this species. Comparisons of sterile and fertile males suggested that paternity success is directly influenced by the quantity of sperm transferred. Our results support previous studies that have used mating success to estimate sexual selection, but also highlight the potential importance of sperm competition and other postinsemination processes.