Target Of Sexual Selection Research Articles

Condition‐dependent expression of red colour differs between stickleback species

Sexual isolation may arise when male mating traits and female preferences differ between species. Such divergence in mating traits is likely to occur when the strength or targets of sexual selection differ. Therefore, by comparing the traits under sexual selection in closely related species and the nature of preference for those traits, we can gain insight into when sexual selection contributes to sexual isolation and how it does so. Collecting these data is no easy undertaking. To simplify this comparison, I use the presence and extent of condition dependence in traits to determine whether directional sexual selection is acting on them. Condition dependence thus serves as a signature of sexual selection. I investigate differences in sexual selection on red nuptial colour in limnetic-benthic species pairs of three-spined sticklebacks. I evaluate condition dependence by comparing the strength of the relationship between colour and condition, and the magnitude of variance in red nuptial colour to other colour traits and to nonsexual traits. I find that limnetic males have strong condition-dependent expression of red nuptial colour whereas benthic males have at most weak condition-dependent expression. Ancestral anadromous males show no condition dependence. This suggests that colour is under strong directional sexual selection only in limnetics and that this is the derived state. Moreover, I find that the strength of female preference for red is related to the extent of condition dependence. The extent of condition dependence is also associated with the importance of colour differences to mate recognition. These results show that differences between these species in the action of sexual selection underlie their sexual isolation.

SEXUAL SELECTION ON MULTIVARIATE PHENOTYPES IN ANASTREPHA FRATERCULUS (DIPTERA: TEPHRITIDAE) FROM ARGENTINA

Despite the interest in applying environmentally friendly control methods such as sterile insect technique (SIT) against Anastrepha fraterculus (Wiedemann) (Diptera: Tephritidae), information about its biology, taxonomy, and behavior is still insufficient. To increase this information, the present study aims to evaluate the performance of wild flies under field cage conditions through the study of sexual competitiveness among males (sexual selection). A wild population from Horco Molle, Tucumán, Argentina was sampled. Mature virgin males and females were released into outdoor field cages to compete for mating. Morphometric analyses were applied to determine the relationship between the multivariate phenotype and copulatory success. Successful and unsuccessful males were measured for 8 traits: head width (HW), face width (FW), eye length (EL), thorax length (THL), wing length (WL), wing width (WW), femur length (FL), and tibia length (TIL). Combinations of different multivariate statistical methods and graphical analyses were used to evaluate sexual selection on male phenotype. The results indicated that wing width and thorax length would be the most probable targets of sexual selection. They describe a nonlinear association between expected fitness and each of these 2 traits. This nonlinear relation suggests that observed selection could maintain the diversity related to body size.

Increase in song frequency decreases spermatophore size: correlative evidence of a macroevolutionary trade‐off in katydids (Orthoptera: Tettigoniidae)

In many katydids, the male feeds his mate with a large gelatinous spermatophore. Males of most species also produce elaborate calling songs. We predicted a negative relationship between spermatophore size and call frequency because of trade-offs between these two costly traits. Our comparative analysis controlling phylogeny and body size supported this prediction. Although call frequency is expected to decrease with increasing body size, after controlling for phylogeny, both variables were not related. Finally, given that song frequency and spermatophore size are likely targets of sexual selection, we examined the relationship between these variables and sexual size dimorphism (SSD) which can be influenced by sexual selection on body size. We found that only female body size was positively related to SSD, suggesting that natural and/or sexual selection on female body size may be stronger than sexual selection on male and spermatophore size.

Contrasting evolution of expression differences in the testis between species and subspecies of the house mouse

Regulatory changes in genes involved in reproduction are thought to be prime targets for divergence during speciation, since they are expected to play an important role in sexual selection and sexual conflict. We used microarray analysis of RNA from different wild populations of house mouse subspecies (including Mus m. musculus, Mus m. domesticus, and Mus m. castaneus) and from the sister species Mus spretus to test this assumption. A comparison of expression divergence in brain, liver/kidney, and testis shows a major difference in the evolutionary dynamics of testis-related genes. While the comparison between species confirms an excess in divergence in testis genes, we find that all comparisons between subspecies yield only a very small number of genes with significantly different expression levels in the testis. These results suggest that the early phase of the speciation process may not be driven by regulatory changes in genes that are potential targets of sexual selection, and that the divergence in these genes is only established during a later phase of the speciation process.

Ornament evolution in dragon lizards: multiple gains and widespread losses reveal a complex history of evolutionary change

The expression in females of ornaments thought to be the target of sexual selection in males is a long-standing puzzle. Two main hypotheses are proposed to account for the existence of conspicuous ornaments in both sexes (mutual ornamentation): genetic correlation between the sexes and sexual selection on females as well as males. We examined the pattern of ornament gains and losses in 240 species of dragon lizards (Agamidae) in order to elucidate the relative contribution of these two factors in the evolution of mutual ornamentation. In addition, we tested whether the type of shelter used by lizards to avoid predators predicts the evolutionary loss or constraint of ornament expression. We found evidence that the origin of female ornaments is broadly consistent with the predictions of the genetic correlation hypothesis. Ornaments appear congruently in both sexes with some lineages subsequently evolving male biased sexual dimorphism, apparently through the process of natural selection for reduced ornamentation in females. Nevertheless, ornaments have also frequently evolved in both sexes independently. This suggests that genetic correlations are potentially weak for several lineages and sexual selection on females is responsible for at least some evolutionary change in this group. Unexpectedly, we found that the evolutionary loss of some ornaments is concentrated more in males than females and this trend cannot be fully explained by our measures of natural selection.

Allometry for Sexual Size Dimorphism: Testing Two Hypotheses for Rensch’s Rule in the Water StriderAquarius remigis

Within any given clade, male size and female size typically covary, but male size often varies more than female size. This generates a pattern of allometry for sexual size dimorphism (SSD) known as Rensch's rule. I use allometry for SSD among populations of the water strider Aquarius remigis (Hemiptera, Gerridae) to test the hypothesis that Rensch's rule evolves in response to sexual selection on male secondary sexual traits and an alternative hypothesis that it is caused by greater phenotypic plasticity of body size in males. Comparisons of three populations reared under two temperature regimes are combined with an analysis of allometry for genital and somatic components of body size among 25 field populations. Contrary to the sexual-selection hypothesis, genital length, the target of sexual selection, shows the lowest allometric slope of all the assayed traits. Instead, the results support a novel interpretation of the differential-plasticity hypothesis: that the traits most closely associated with reproductive fitness (abdomen length in females and genital length in males) are "adaptively canalized." While this hypothesis is unlikely to explain Rensch's rule among species or higher clades, it may explain widespread patterns of intraspecific variation in SSD recently documented for many insect species.

Variation, but no covariance, in female preference functions and male song in a natural population of Drosophila montana

Studies of the variance and covariance between female mating preferences and sexually selected male traits in natural populations are rare. In D. montana, male courtship song is an important target of sexual selection. We analysed the variance in components of song and preferences among F1 families from a natural population. All song traits varied substantially, with among-family variance components ranging from 30 to 65%. The greatest variation was in carrier frequency, which is the most important predictor of mating success. This is compatible with the trait capturing mutational and other components of genetic variance in condition because of condition-dependent expression. There was also variation for some components of preference variation, with significant variation among sisters within families, and among families. Females varied in overall responsiveness, but not in the slope of the linear female preference function for male song carrier frequency. Such variation might be expected to generate assortative mating, with more choosy females mating with higher quality males, but there was no covariance across families between female responsiveness and male carrier frequency. Substantial variation in the level of responsiveness might allow low-quality males to achieve some mating success and counteract the build-up of a strong genetic covariance between preferences and traits.

Sex‐Specific Spawning Behavior and Its Consequences in an External Fertilizer

Identifying the target of sexual selection in externally fertilizing taxa has been problematic because species in these taxa often lack sexual dimorphism. However, these species often show sex differences in spawning behavior; males spawn before females. I investigated the consequences of spawning order and time intervals between male and female spawning in two field experiments. The first involved releasing one female sea urchin's eggs and one or two males' sperm in discrete puffs from syringes; the second involved inducing males to spawn at different intervals in situ within a population of spawning females. In both, fertilization success was measured as the fraction of eggs fertilized and the paternity share of each male. The results indicate that spawning after females imposes a cost on males but only during sperm competition. Further, the optimal interval between the initiations of male and female spawning depends on degree of sperm competition, distance between males and females, and water velocity. The results show that sex differences in spawning timing of marine invertebrates can be explained on the basis of the differential costs and benefits of spawning out of synchrony with the other sex and that the result of sexual selection on external fertilizers may be behavioral rather than morphological differentiation of the sexes.

Sex-Specific Spawning Behavior and Its Consequences in an External Fertilizer

Identifying the target of sexual selection in externally fertilizing taxa has been problematic because species in these taxa often lack sexual dimorphism. However, these species often show sex differences in spawning behavior; males spawn before females. I investigated the consequences of spawning order and time intervals between male and female spawning in two field experiments. The first involved releasing one female sea urchin’s eggs and one or two males’ sperm in discrete puffs from syringes; the second involved inducing males to spawn at different intervals in situ within a population of spawning females. In both, fertilization success was measured as the fraction of eggs fertilized and the paternity share of each male. The results indicate that spawning after females imposes a cost on males but only during sperm competition. Further, the optimal interval between the initiations of male and female spawning depends on degree of sperm competition, distance between males and females, and water velocity. The results show that sex differences in spawning timing of marine invertebrates can be explained on the basis of the differential costs and benefits of spawning out of synchrony with the other sex and that the result of sexual selection on external fertilizers may be behavioral rather than morphological differentiation of the sexes.

Chromosomal polymorphism, morphometric traits and mating success in Leptysma argentina Bruner (Orthoptera).

The South-American species Leptysma argentina Bruner is polymorphic for a centric fusion between chromosomal pairs 3 and 6 (fusion 3/6). Cytogenetic and morphological studies revealed that fusion 3/6 significantly increases some morphometric variables in males. A selection components study showed that the fusion is positively selected for longevity, the direct effect being exerted on thorax height. Moreover, a unisexual approach revealed that sexual male selection acts by increasing third femur length. In the present study the effect of fusion 3/6 on morphometric variables and the effect of body size and karyotype on mating success in both males and females were analysed through a bisexual approach. Total body length (TL) and third femur length (FL) were significantly larger amongst the fusion carrier females. Besides, the largest females and the carriers of the fusion have increased mating success. The selection differential suggests that directional sexual selection favours the females with increased TL and fusion dosage; however, the selection gradient revealed that none of the variables here analysed is a direct target of sexual selection. The male sexual selection study agrees with previous results, showing the importance of the third pair of legs in male mating success among grasshoppers.

A dynamic trait affects continuous pair assessment in the blue-footed booby, Sula nebouxii

Bright colours of male birds have often been shown to be the target of sexual selection through female choice, yet few studies have looked at the role of colour expressed after pairing on female motivation and behaviour. Here we analyse the role of an integumentary colour in the spectral range of 400–700 nm, the foot colour in male blue-footed boobies, Sula nebouxii, which is prominently displayed during pair courtship. Measurements early in the breeding season showed that foot colour of courting males is pale (high values of brightness) and has an aqua-blue chroma, and females in better body condition were mated to males with brighter feet. We carried out an experimental manipulation which modified the foot colour of males in courtship, making it closer to the foot colour of males in low nutritional state. We found that females paired to experimental males courted less and were less likely to copulate than females in the control group. Male behaviour was apparently unaffected by the manipulation; thus the change in female behaviour can be attributed exclusively to foot colour manipulation. These results strongly support the hypothesis of female preference for an integumentary colour and suggest that this dynamic trait is used as a male ornament after pairing.

DIETARY AND SEXUAL CORRELATES OF CAROTENOID PIGMENT EXPRESSION IN DOVE PLUMAGE

Carotenoid pigmentation in birds' plumage is considered an honest indicator of phenotypic quality, and thus a target of sexual selection. But carotenoids also fulfill essential physiological functions, and therefore, carotenoids should only appear in plumage if they are in excess of those needed physiologically. We explored the presence of carotenoid-based plumage coloration in columbids and its association with diet and sexual dichromatism using a comparative analysis. We found that carotenoid plumage pigmentation appeared three times independently in doves, and that these events were always associated with frugivorous feeding habits. This suggests that expression of carotenoid-based plumage color in granivorous species may be constrained by the scarcity of carotenoids in their diet. However, more than half of the frugivorous species lack carotenoid-pigmented plumage, indicating that rich dietary sources of these compounds are a necessary but not sufficient cause for their expression in plumage. Analyzing 12 pairs of sister taxa, we found that plumage dichromatism was neither associated with the amount of carotenoid pigment present in the plumage nor with the sexual dimorphism in carotenoid-pigmented plumage. Although the presence of carotenoid-based plumage coloration has been related to sexual selection in several taxa, we failed to show such an association in columbids. Correlación de la Expresión de Pigmentos Carotenoides en el Plumaje de Palomas con la Dieta y la Selección Sexual Resumen. Los pigmentos carotenoides en el plumaje de las aves son considerados indicadores honestos de la calidad fenotípica y, por lo tanto, objetos de selección sexual. Sin embargo, los carotenoides también cumplen funciones fisiológicas esenciales, por lo cual aquellos que se expresan en el plumaje deberían estar en exceso de los utilizados a nivel fisiológico. Exploramos la presencia de carotenoides en el plumaje de las palomas y su asociación con la dieta y el dicromatismo sexual usando un análisis comparativo. Encontramos que el plumaje carotenoide apareció tres veces independientemente en palomas, y que estos eventos estaban siempre asociados a frugivoría. Esto sugiere que la expresión de carotenoides en el plumaje de especies granívoras puede estar restringida por la escasez de estos pigmentos en la dieta. Sin embargo, más de la mitad de las especies frugívoras carecen de plumaje carotenoide, indicando que alimentos ricos en este compuesto son una causa necesaria pero no suficiente para su expresión en el plumaje. Analizando 12 pares de taxa hermanos, encontramos que el dicromatismo no estaba asociado ni a la cantidad de pigmentos carotenoides presentes en el plumaje ni al dimorfismo sexual de plumaje carotenoide. Aunque la presencia de plumaje carotenoide ha sido relacionada con la selección sexual en numerosos grupos, no encontramos una asociación similar en palomas.

Sexual selection on multivariate phenotype in wild and mass-reared Ceratitis capitata (Diptera: Tephritidae).

Males with a larger thorax and narrower face were found to be favoured by directional sexual selection in a previous field cage experiment where Ceratitis capitata males from the Seib 6-96 genetic sexing strain competed with wild males from Alto Valle (Patagonia) for the possession of wild females. Targets of sexual selection, however, might differ between wild and laboratory females as a response to adaptation to mass-rearing conditions. To evaluate possible divergences on the targets of sexual selection as a by-product of adaptation to mass-rearing conditions, field cage tests were performed with both wild and laboratory females. To avoid possible bias due to correlation among the measured traits (eye length [EL], face width [FW], head width [HW], and thorax length [TL]), a multivariate analysis was applied. Consistent with the previous experiment, the results indicated that TL and FW are probable targets of directional sexual selection independently of female strain. However, laboratory females were less selective than wild ones. Additionally, correlational sexual selection was detected acting on the multivariate phenotype. The effects of correlational selection overlap with those of directional selection on each single trait. The analysis of mating pair characteristics showed patterns that do not match the expectations for a random mating system. The current analysis indicates that during mating pair formation two processes overlap. On the one hand, sexual selection favours males with larger size (TL) and narrower faces (FW). This effect occurs in both wild and laboratory females. In addition, assortative mating based on both phenotype and origin was also observed.

To persuade or be persuaded: which sex controls mating in a plethodontid salamander?

AbstractIn the dynamic process of courtship, males often must perform behaviour patterns that function to persuade females that are initially unresponsive to mate. But, to what extent is the outcome of a sexual encounter determined by a male's persuasiveness versus a female's responsiveness? We addressed this question in three laboratory experiments using a plethodontid salamander, Desmognathus ocoee, for which persuasion by males appears to be obligatory. We found considerable variation in mating success among individuals of both sexes, indicating variation in both male persuasiveness and female responsiveness (Experiment 1). In addition, males of high persuasiveness mated more often than males of lower persuasiveness, regardless of the levels of responsiveness of the females that they encountered (Experiment 2). Finally, we found that males of high persuasiveness performed certain sexual behaviour patterns more frequently and more rapidly than males of lower persuasiveness (Experiment 3). Our results suggest that male D. ocoee of high persuasiveness can mate successfully even with unresponsive females, which may be especially choosy about which males they accept. Thus, those male behaviour patterns that increase female motivation to mate likely are targets of sexual selection by female choice. Whether females that mate with persuasive males enjoy increased fitness awaits further study.

Variation in the volume of zebra finch song control nuclei is heritable: developmental and evolutionary implications.

In many songbird species, females prefer males that sing a larger repertoire of syllables. Males with more elaborate songs have a larger high vocal centre (HVC) nucleus, the highest structure in the song production pathway. HVC size is thus a potential target of sexual selection. Here we provide evidence that the size of the HVC and other song production nuclei are heritable across individual males within a species. In contrast, we find that heritabilities of other nuclei in a song-learning pathway are lower, suggesting that variation in the sizes of these structures is more closely tied to developmental and environmental differences between individuals. We find that evolvability, a statistical measure that predicts response to selection, is higher for the HVC and its target for song production, the robustus archistriatalis (RA), than for all other brain volumes measured. This suggests that selection based on the functions of these two structures would result in rapid major shifts in their anatomy. We also show that the size of each song control nucleus is significantly correlated with the song related nuclei to which it is monosynaptically connected. Finally, we find that the volume of the telencephalon is larger in males than in females. These findings begin to join theoretical analyses of the role of female choice in the evolution of bird song to neurobiological mechanisms by which the evolutionary changes in behaviour are expressed.

SONG ORGANISATION AND SINGING PATTERNS OF THE WILLOW WARBLER, PHYLLOSCOPUS TROCHILUS

[In this paper we describe the organisation of the song of the willow warbler and its pattern of singing, with the objective of providing robust measurements of song complexity that can be related to the functional and evolutionary aspects of song. Songs are composed of distinct individual elements, which can be reliably identified. These elements are not randomly assembled within each song: most songs start always with the same element, and thereafter elements are produced in a highly predictable way. The organisation of elements corresponds to a hierarchically branching pattern, where elements either connect predictably to a second one, or constitute a decision point where two or more transitions are possible. This branching pattern pervades the whole length of the sequence of elements, and suggests that the concept of song type is not appropriate in this species. The singing pattern could be characterised by a random delivery: consecutive songs are not more similar from each other than if they were delivered at random. Individuals differ greatly in several song characteristics. A principal component analysis performed in a comprehensive set of song complexity measurements produced 5 meaningful components: Song repertoire size (PC1); Song length (PC2); Song versatility (PC3); Element rate (PC4); and Repertoire fixation (PC5). The first component - song repertoire size — showed the highest variation between individuals and a high sameyear repeatability. This makes of song repertoire size a likely target for sexual selection in this species., In this paper we describe the organisation of the song of the willow warbler and its pattern of singing, with the objective of providing robust measurements of song complexity that can be related to the functional and evolutionary aspects of song. Songs are composed of distinct individual elements, which can be reliably identified. These elements are not randomly assembled within each song: most songs start always with the same element, and thereafter elements are produced in a highly predictable way. The organisation of elements corresponds to a hierarchically branching pattern, where elements either connect predictably to a second one, or constitute a decision point where two or more transitions are possible. This branching pattern pervades the whole length of the sequence of elements, and suggests that the concept of song type is not appropriate in this species. The singing pattern could be characterised by a random delivery: consecutive songs are not more similar from each other than if they were delivered at random. Individuals differ greatly in several song characteristics. A principal component analysis performed in a comprehensive set of song complexity measurements produced 5 meaningful components: Song repertoire size (PC1); Song length (PC2); Song versatility (PC3); Element rate (PC4); and Repertoire fixation (PC5). The first component - song repertoire size — showed the highest variation between individuals and a high sameyear repeatability. This makes of song repertoire size a likely target for sexual selection in this species.]

Song versus plumage in some North American Oscines: Testing Darwin’s hypothesis

Sexual selection is thought to have favored the development of both song and plumage, but other selective pressures may impose constraints. Darwin suggested that there is a negative correlation between song and plumage complexity, but this hypothesis, which we dubbed the trade-off hypothesis, has seldom been tested. We examined the trade-off hypothesis on 123 species of North American Oscines. Canonical correlation analyses were used to assess the relationship between a set of variables measuring plumage conspicuousness and another measuring song complexity. Data were corrected to take phylogenetic autocorrelation into account. A weak positive correlation was found between plumage conspicuousness and song complexity. The positive correlation can be explained by the evolution of traits that may help a male to mate rapidly in regions where the breeding season is short. Factors such as the influence of habitat structure on the evolution of songs and plumages, predation rates, differences in mating systems and in targets of sexual selection, and experimental errors may explain the weakness of this correlation.

EFFECT OF ANDROGENS ON STRUCTURE AND RATE OF CROWING IN THE JAPANESE QUAIL (COTURNIX JAPONICA)

To study the influence of testosterone (T) on crowing in male Japanese quails, we combined hormonal and photoperiod manipulations of circulating androgens with a detailed analysis of the acoustic parameters of crows and of male courtship patterns. The structure of crowing was directly affected by T: high-androgen males uttered longer and lower-pitched calls than the others, call rate at dawn being related to androgens by a minimum threshold function. High- and low-callers, two consistent categories over many days, differed in their sexual performance: high-callers performed more courtship than low-callers, but there was no difference in attempted mating. Crowing is discussed as a potential target of sexual selection because, like a song, it is androgen-dependent, highly variable among individuals, and plastic in its structure at the beginning of the reproductive season.

Host sexual selection and cuckoo parasitism: an analysis of nest size in sympatric and allopatric magpie Pica pica populations parasitized by the great spotted cuckoo Clamator glandarius

Magpies (Pica pica) build large nests that are the target of sexual selection, since males of early breeding pairs provide many sticks for nests and females mated to such males enjoy a material fitness benefit in terms of better quality territory and parental care of superior quality. Great spotted cuckoos (Clamator glandarius) preferentially parasitize large magpie nests and sexual selection for large nests is thus opposed by natural selection due to brood parasitism. Consistent with the hypothesized opposing selection pressures, in a comparative analysis of 14 magpie populations in Europe we found that nest volume was consistently smaller in sympatry than in allopatry with the great spotted cuckoo, in particular in areas with a high parasitism rate and high rates of rejection of mimetic model cuckoo eggs. These observations are consistent with the suggestion that magpies have evolved a smaller nest size in areas where cuckoos have exerted strong selection pressures on them in the recent past.

Female preference for fly song: playback experiments confirm the targets of sexual selection

The courtship song ofDrosophilais thought to be involved in sexual selection and species recognition. Because of the mating system of flies, however, directly demonstrating that song influences female preference is difficult. The majority of previous studies have used an experimental design that potentially confounds male and female reactions to song. InD.montana, correlational evidence has suggested that males that produce short sound pulses consisting of a high number of sound cycles (i.e. a high carrier frequency) have a higher mating success than other males. In this study, we played synthetic song that varied in pulse length and carrier frequency to individual females in the laboratory, both alone and in the presence of mute males. We scored female preference via an acceptance posture, ‘wing spreading’, which the females of this species usually display prior to mounting by a male. Females responded to synthetic song in the absence of males. The presence of mute males significantly increased their overall responsiveness, but the relative effectiveness of the songs did not change, eliminating male reaction to song as a possible confounding factor in the results. The interaction between pulse length and carrier frequency determined the discrimination between song types, with females responding most readily to song consisting of short pulses with a high carrier frequency. Thus, direct examination of female preferences supports the previous studies of male mating success, and confirms female song preference as a likely determinant of male mating success.