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PubMed2005

Multiple paternity and postcopulatory sexual selection in a hermaphrodite: what influences sperm precedence in the garden snail Helix aspersa?

Sperm competition has been studied in many gonochoric animals but little is known about its occurrence in simultaneous hermaphrodites, especially in land snails. The reproductive behaviour of the land snail Helix aspersa involves several features, like multiple mating, long-term sperm storage and dart-shooting behaviour, which may promote sperm competition. Cryptic female choice may also occur through a spermatheca subdivided into tubules, which potentially allows compartmentalized sperm storage of successive mates. In order to determine the outcome of postcopulatory sexual selection in this species, we designed a cross-breeding experiment where a recipient ('female') mated with two sperm donors ('males'). Mates came from either the same population as the recipient or from a distinct one. To test for the influence a recipient can have on the paternity of its offspring, we excluded the effects of dart shooting by using only virgin snails as sperm donors because they do not shoot any dart before their first copulation. We measured the effects of size of mates as well as time to first and second mating on second mate sperm precedence (P2; established using microsatellite markers). Multiple paternity was detected in 62.5% of clutches and overall there was first-mate sperm precedence with a mean P2 of 0.24. Generalized linear modelling revealed that the best predictors of paternity were the time between matings and the time before first mating. Overall, both first and second mates that copulated quickly got greater parentage, which may suggest that postcopulatory events influence patterns of sperm precedence in the garden snail.

Animals↗

PubMedDate not supplied

The Wilhelmine E. Key 1996 Invitational Lecture. Sexual selection: a driver of genetic change in Hawaiian Drosophila.

Within the last five million years, the Hawaiian archipelago has successively added 14, spaced, subaerial volcanoes at its southeast end. Each has received endemic terrestrial colonists; established populations appear to stem mainly from the immediately adjacent older island. Chromosome markers were used to trace the evolution of a phylad of 106 unique and closely related Drosophila species. With one exception, each species is restricted to one of the four groups of islands; 57 species appear to be restricted to single volcanoes. Founding of new populations across each of the three major oceanic channels appears to be infrequent. Genetic and morphological analysis suggests that each founding event has resulted in a newly organized, highly specific gene pool, not simply a transplanted colony of the source species. Progressive volcanic forces have thus provided an instructive spatiotemporal environment for the formation of new species. In Drosophila genetic change involves primarily spectacular premating sexual behavior, marked by novel male characters that appear to have evolved under the driving force of sexual selection. Permanent genetic changes in reproductive biology are thus elicited by selection for intrademe Darwinian fitness of individuals and are not the result of ad hoc selection for interpopulation reproductive isolation. The facts reinforce the view that reproductive isolation from other species is largely an incidental by-product of selection imposed by both the ambient and sexual environments in local populations.

Animals↗

PubMed2002

Species divergence in sexually selected traits: increase in song elaboration is related to decrease in plumage ornamentation in finches.

Elaborate plumage and complex songs of male birds are two of the best-known examples of sexually selected traits, yet the interaction between these traits is poorly understood. Theory suggests that among a suite of potential displays, animals will emphasize traits that are most conspicuous, least costly, or best signal condition and reduce display of other traits. Here we examined the relationship between song and plumage elaborations in cardueline finches, songbirds that are highly variable in plumage displays and songs, but that share a similar mating system. We statistically controlled for body mass, habitat characteristics, and phylogenetic relationships and found that across species song complexity was strongly negatively related to elaboration of plumage ornamentation. When plumage coloration was partitioned into carotenoid-based and melanin-based components, song complexity was negatively related to elaboration of male carotenoid-based coloration but unrelated to elaboration of melanin-based coloration. These observations support the idea that, for condition-dependent traits, animal species trade off trait expression in response to changes in the costs or the information content of these traits. We discuss several alternative explanations for the observed pattern.

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PubMed2026

Accounting for recombination rate variation improves inference of barrier loci and reveals the role of both natural and sexual selection in an incipient bird radiation.

Examining genomic patterns of differentiation across lineage pairs at different stages of the speciation continuum, in combination with recombination maps, can help disentangle the effects of linked and divergent selection and identify lineage-specific targets of selection that may act as barrier loci during speciation. Here, we apply this framework to genomic data from African and Indian Ocean bird species of the genus Zosterops (Zosteropidae) to identify candidate barrier loci between ecologically, phenotypically, and genetically distinct Reunion gray white-eye (Zosterops borbonicus) parapatric geographic forms. Using analyses that account for recombination rate variation, we show that putative targets of divergent selection are primarily located on the Z chromosome, except in comparisons between geographic forms that differ in their ecologies. Functional annotation revealed that candidate barrier loci between forms with similar environmental niches are associated with genes involved in song formation and immune function, whereas those between forms with different environmental niches are associated with adaptation to altitude, morphology, and song behavior. Our results highlight the combined roles of natural and sexual selection in the evolution of reproductive barriers in this incipient species radiation.

Animals↗

PubMed2006

Costs of breeding and their effects on the direction of sexual selection.

A recent life-history model has challenged the importance of the operational sex ratio and the potential reproductive rates of males and females as the factors most important for the control of sexual selection, arguing that the cost of breeding, interpreted as the probability of dying as a consequence of the current breeding attempt, is the single most important factor that best predicts a mating system. In one species of bushcricket, the mating system can be reversed by resource manipulation. Here, we examine the costs of breeding in this system. Consistent with the model, increased costs of breeding can explain female competition and increased male choosiness under resource limitation. However, this is due to differences in the time required for a breeding attempt, rather than differences in breeding mortality which did not differ between the sexes. In general, males lived longer than females and we discuss the possible reasons behind this pattern of sex-biased non-breeding mortality.

Animals↗

PubMed2006

Intrasexual competition and sexual selection in cooperative mammals.

In most animals, the sex that invests least in its offspring competes more intensely for access to the opposite sex and shows greater development of secondary sexual characters than the sex that invests most. However, in some mammals where females are the primary care-givers, females compete more frequently or intensely with each other than males. A possible explanation is that, in these species, the resources necessary for successful female reproduction are heavily concentrated and intrasexual competition for breeding opportunities is more intense among females than among males. Intrasexual competition between females is likely to be particularly intense in cooperative breeders where a single female monopolizes reproduction in each group. Here, we use data from a twelve-year study of wild meerkats (Suricata suricatta), where females show high levels of reproductive skew, to show that females gain greater benefits from acquiring dominant status than males and traits that increase competitive ability exert a stronger influence on their breeding success. Females that acquire dominant status also develop a suite of morphological, physiological and behavioural characteristics that help them to control other group members. Our results show that sex differences in parental investment are not the only mechanism capable of generating sex differences in reproductive competition and emphasize the extent to which competition for breeding opportunities between females can affect the evolution of sex differences and the operation of sexual selection.

Aggression↗

PubMed1993

Honesty, perception and population divergence in sexually selected traits.

We investigate the evolution of female preference for one and two male ornaments, to address two issues in sexual selection: (i) what factors affect the evolution of female preferences; and (ii) how do preferences diverge between isolated populations, leading to speciation? We assume that the male traits are costly indicators of male condition ('handicaps'), and that females benefit directly from a high-condition mate. We find that optimal female preference for a single male trait equals (benefit of condition) x (detectability of male trait) x (honesty of male trait)/(costliness of preference). With two male traits to choose from, females should prefer the one with greatest honesty x detectability, and ignore the second. These results highlight the role of perception in the evolution of both male ornaments and female preferences, and provide a theoretical illustration of 'sensory drive'. They confirm that a less honest male trait can displace a more honest trait if its detectability is sufficiently high. Environmental differences can drive evolutionary divergence between populations in both the male trait and female preference. Even small differences between habitats in detectability of male traits can trigger dramatic change in the female preference. Finally, populations may drift apart in arbitrary directions if choice of different male traits yields equivalent benefits to females.

Animals↗

PubMed2003

Males influence maternal effects that promote sexual selection: a quantitative genetic experiment with dung beetles Onthophagus taurus.

Recently, doubt has been cast on studies supporting good genes sexual selection by the suggestion that observed genetic benefits for offspring may be confounded by differential maternal allocation. In traditional analyses, observed genetic sire effects on offspring phenotype may result from females allocating more resources to the offspring of attractive males. However, maternal effects such as differential allocation may represent a mechanism promoting genetic sire effects, rather than an alternative to them. Here we report results from an experiment on the horned dung beetle Onthophagus taurus, in which we directly compare genetic sire effects with maternal effects that are dependent on sire phenotype. We found strong evidence that mothers provide more resources to offspring when mated with large-horned males. There were significant heritabilities for both horn length and body size, but when differential maternal effects were controlled, the observed estimates of genetic variance were greatly reduced. Our experiment provides evidence that differential maternal effects may amplify genetic effects on offspring traits that are closely related to fitness. Thus, our results may partly explain the relatively high coefficients of additive genetic variation observed in fitness-related traits and provide empirical support for the theoretical argument that maternal effects can play an important role in evolution.

Animals↗

PubMed2003

Sexual selection in female perceptual space: how female túngara frogs perceive and respond to complex population variation in acoustic mating signals.

Female preferences for male mating signals are often evaluated on single parameters in isolation or small suites of characters. Most signals, however, are composites of many individual parameters. In this study we quantified multivariate traits in the advertisement call of the túngara frog, Physalaemus pustulosus. We represented the calls in multidimensional scaling space and chose nine test calls to represent the range of population variation. We then tested females for phonotactic preference between calls in each pair of the nine test calls. We used statistics developed for paired comparisons in such "round robin" competitions to evaluate the null hypothesis of equal attractiveness, and to examine the degree to which females responded to calls as being different from or similar to one another in attractiveness. We then examined the attractiveness of each test call relative to all other test calls as a function of their location in multivariate acoustic space (the acoustic landscape) to visualize sexual selection on calls. Finally, we used methods from cognitive psychology to illustrate the females' perception of call attractiveness in multivariate space, and compared this perceptual landscape to the acoustic landscape of quantitative call variation. We show that correlations between individual call characters are not strong and thus there are few biomechanical constraints on their independent evolution. Most call variables differed among males, and there was high repeatability of call characters within males. Females often discriminated between pairs of calls from the population, and there were significant differences among calls in their attractiveness. Female preferences for calls were not stabilizing. The region of the acoustic landscape that was most attractive to females included the mean call but was not centered around it. The females' perceptual or preference landscape did not correlate with the call's acoustic landscape, and female perception of calls decreased rather than enhanced call differences.

Animal Communication↗

PubMed2003

Sexual selection promotes hybridization between Pecos pupfish, Cyprinodon pecosensis and sheepshead minnow, C. variegatus.

Rapid and extensive genetic introgression has occurred between Pecos pupfish (Cyprinodon pecosensis) and sheepshead minnow (Cyprinodon variegatus) in the wild. We studied both female mate choice and male-male competition for mates among C. pecosensis, C. variegatus, and their F1 hybrids to determine what role these behaviours played in the formation of the hybrid swarm. Female C. pecosensis preferred male C. variegatus to conspecific males, C. variegatus females displayed no significant preference when given a choice between purebred males, and neither C. pecosensis nor C. variegatus females discriminated against F1 hybrid males. We found no evidence for female olfactory recognition of mates. Male F1 hybrids and C. variegatus were more aggressive than C. pecosensis males, achieving greater reproductive success under two different experimentally-induced mating systems. Hybrids were superior to C. variegatus when only two males competed (dominance interactions), but the two types were competitively equivalent in a territorial mating system. Our results indicate that active inter- and intra-sexual selection contributed to the accelerated hybridization between these two species. By including the possibility that some aspects of a hybridization and introgression event may be under positive selection, researchers may better understand the dynamics that lead to hybrid zone stability or the spread of introgressed genetic material.

Aggression↗

PubMed2004

Directional sexual selection on chroma and within-pattern colour contrast in Labeotropheus fuelleborni.

Speciation via intersexual selection on male nuptial colour pattern is thought to have been a major force in promoting the explosive speciation of African haplochromine cichlids, yet there is very little direct empirical evidence of directional preferences within populations. In this study, we used objective spectrophotometry and analyses based on visual physiology to determine whether females of the Katale population of Labeotropheus fuelleborni, a Lake Malawi haplochromine, prefer males that have higher chroma and more within-pattern colour contrast. In paired male preference tests, female Katale L. fuelleborni showed increasing preferences for males with more relatively saturated colours on their flanks. They also showed increasing preferences for males with relatively higher contrast levels among flank elements. This is the first empirical evidence, to our knowledge, for male colour as a directionally sexually selected trait within a haplochromine cichlid population.

Animals↗

PubMed2003

Darwinian aesthetics: sexual selection and the biology of beauty.

Current theoretical and empirical findings suggest that mate preferences are mainly cued on visual, vocal and chemical cues that reveal health including developmental health. Beautiful and irresistible features have evolved numerous times in plants and animals due to sexual selection, and such preferences and beauty standards provide evidence for the claim that human beauty and obsession with bodily beauty are mirrored in analogous traits and tendencies throughout the plant and animal kingdoms. Human beauty standards reflect our evolutionary distant and recent past and emphasize the role of health assessment in mate choice as reflected by analyses of the attractiveness of visual characters of the face and the body, but also of vocal and olfactory signals. Although beauty standards may vary between cultures and between times, we show in this review that the underlying selection pressures, which shaped the standards, are the same. Moreover we show that it is not the content of the standards that show evidence of convergence--it is the rules or how we construct beauty ideals that have universalities across cultures. These findings have implications for medical, social and biological sciences.

Animals↗

PubMed2004

Selective sexual harassment: differential treatment of similar groups of women workers.

If male workers categorize different groups of women coworkers and, subsequently, treat them differently, the experiences of women from one of these groups would not be indicative of the experiences of women from another group. When this different treatment involves hostile environment sexual harassment of one group, but not the other, then the law must recognize the possibility of "selective sexual harassment." Without this understanding of the nuances of the workplace dynamics, a court could mistake the women of the unharassed group as representing "reasonable women" and the women of the harassed group as simply oversensitive. This paper draws on empirical data to demonstrate such a situation and advocates for a version of the "reasonable victim" standard to facilitate a closer analysis of hostile environment sexual harassment suits.

Data Collection↗

PubMed2004

Sexual selection can constrain sympatric speciation.

Recent theory has suggested that sympatric speciation can occur quite easily when individuals that are ecologically similar mate assortatively. Although many of these models have assumed that individuals have equal mating success, in nature rare phenotypes may often suffer decreased mating success. Consequently, assortative mating may often generate stabilizing sexual selection. We show that this effect can substantially impede sympatric speciation. Our results emphasize the need for data on the strength of the stabilizing component of selection generated by mating in natural populations.

Animals↗

PubMed2003

Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus.

In crustacean species with precopulatory mate-guarding, sexual size dimorphism has most often been regarded as the consequence of a large male advantage in contest competition for access to females. However, large body size in males may also be favoured indirectly through scramble competition. This might partly be the case if the actual target of selection is a morphological character, closely correlated with body size, involved in the detection of receptive females. We studied sexual selection on body size and antennae length in natural populations of Asellus aquaticus, an isopod species with precopulatory mate guarding. In this species, males are larger than females and male pairing success is positively related to body size. However, males also have longer antennae, relative to body size, than females, suggesting that this character may also be favoured by sexual selection. We used multivariate analysis of selection to assess the relative influences of body size and antennae length in five different populations in the field. Selection gradients indicated that, overall, body size was a better predictor of male pairing success than antennae length, although some variation was observed between sites. We then manipulated male antennae length in a series of experiments conducted in the lab, and compared the pairing ability of males with short or long antennae. Males with short antennae were less likely to detect, orient to, and to pair with a receptive female compared with males with long antennae. We discuss the implications of our results for studies of male body size and sexual dimorphism in relation to sexual selection in crustaceans.

Animals↗

PubMed2003

Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus.

In crustacean species with precopulatory mate-guarding, sexual size dimorphism has most often been regarded as the consequence of a large male advantage in contest competition for access to females. However, large body size in males may also be favoured indirectly through scramble competition. This might partly be the case if the actual target of selection is a morphological character, closely correlated with body size, involved in the detection of receptive females. We studied sexual selection on body size and antennae length in natural populations of Asellus aquaticus, an isopod species with precopulatory mate guarding. In this species, males are larger than females and male pairing success is positively related to body size. However, males also have longer antennae, relative to body size, than females, suggesting that this character may also be favoured by sexual selection. We used multivariate analysis of selection to assess the relative influences of body size and antennae length in five different populations in the field. Selection gradients indicated that overall body size was a better predictor of male pairing success than antennae length, although some variation was observed between sites. We then manipulated male antennae length in a series of experiments conducted in the laboratory, and compared the pairing ability of males with short or long antennae. Males with short antennae were less likely to detect, orient to and to pair with a receptive female compared to males with long antennae. We discuss the implications of our results for studies of male body size and sexual dimorphism in relation to sexual selection in crustaceans.

Animals↗

PubMed1998

Sexual selection, receiver biases, and the evolution of sex differences.

REVIEW Recent approaches to analyzing the evolution of female mating preferences emphasize how historical influences on female receiver systems can bias the evolution of male traits that females find attractive. These studies combine animal behavior, sensory biology, phylogenetics, and artificial neural network models. They attempt to understand why specific phenotypes involved in sexual selection have evolved, rather than merely determining whether such traits and preferences are adaptive. It is now clear that traits and preferences often do not coevolve via genetic correlations, that female mating preferences for a given male trait are influenced by adaptations and constraints outside of the context of female responses to that particular trait, and that receiver biases can explain much of the diversity in male signaling phenotypes. It also appears that an understanding of historical effects will prove valuable in investigating why neural and cognitive systems respond to sensory stimuli as they do.

Animal Communication↗

PubMed2001

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.

Animals↗