Strength Of Sexual Selection Research Articles

Population structure, growth rates, and seasonal abundance of twoSyngnathus pipefish species

Northern pipefish,Syngnathus fuscus, and dusky pipefish,Syngnathus floridae, are among the most abundant ichthyofauna components of the Chesapeake Bay, USA, eelgrass beds,Zostera marina, but population structure and many life history traits remain uncharacterized. We conducted monthly collections from May through September 2003–2005 in Chincoteague Bay, Virginia, to investigate seasonal migration and spawning, sex ratios, size at maturity, sexual dimorphism in length, and growth rates. BothS. fuscus andS. floridae spawned from May through September. Water temperature was significantly correlated withS. fuscus catches, whereasS. floridae abundance peaked after maximum water temperatures. Sex ratio data indicatedS. floridae populations are balanced, whileS. fuscus populations are strongly female-biased. Both species can quickly reach reproductive maturity, potentially within one season, becauseS. fuscus andS. floridae population growth rates average 1.0 mm d−1 and minimum standard length at maturity measures 125 and 103 mm, respectively, for females and 99 and 91 mm, respectively, for males. ForS. fuscus, females were significantly longer than conspecific males during time periods when juveniles were not rapidly maturing. Size sexual dimorphism in this species coincides with reports of extensive paternal care and supports the hypothesis that the strength of sexual selection differs in these species.

The Evolution of Condition‐Dependent Sexual Dimorphism

Theory suggests that the net benefit of allocating resources to a sexual trait depends both on the strength of sexual selection on that trait and on individual condition. This predicts a tight coevolution between sexual dimorphism and condition dependence and suggests that these patterns of within-sex and between-sex variation may share a common genetic and developmental basis. Although condition-dependent expression of sexual traits is widely documented, the extent of covariation between condition dependence and sexual dimorphism remains poorly known. I investigated the effects of condition (larval diet quality) on multivariate sexual dimorphism in the fly Telostylinus angusticollis (Neriidae). Condition determined the direction of sexual size dimorphism and modulated sexual shape dimorphism by affecting allometric slopes and/or intercepts of sexually homologous traits in both sexes. Although the greatest responses to condition manipulation were observed in male sexual traits, both sexual and nonsexual traits exhibited substantial variation in the nature and magnitude of condition effects. Nonetheless, condition dependence and sexual dimorphism were remarkably congruent: variation in the strength of condition effects on male traits explained more than 90% of the variation in the magnitude of sexual dimorphism, whether quantified in terms of trait size or allometric slope. The genetic mechanisms that give rise to multivariate sexual dimorphism in body shape thus function in a strongly condition-dependent manner in this species, suggesting a common genetic basis for body shape variation within and between sexes.

Effect of Dietary Carotenoid Supplementation on Food Intake and Immune Function in a Songbird with no Carotenoid Coloration

AbstractStudies of ornamental carotenoid coloration suggest that animals may have evolved specialized mechanisms for maximizing color expression and advertising their potential worth as a mate. For example, when given a choice of foods, many carotenoid‐pigmented fishes and birds select the more colorful, presumably carotenoid‐rich foods, and then accumulate these pigments at high levels in both the integument and systemically, in order to boost their immune system and hence directly advertise their health state with their colors. The majority of animals, however, do not exhibit sexually selected carotenoid coloration, which raises the question of whether they still optimize pigment intake and allocation in ways that boost endogenous accumulation and health. We tested the effect of carotenoid supplementation on food intake, carotenoid accumulation in blood, and both innate and adaptive immunity in male society finches (Lonchura domestica) – a non‐carotenoid‐colored estrildid finch relative of the zebra finch (Taeniopygia guttata; a species in which males do display sexually attractive and health‐revealing carotenoid color). Males fed a carotenoid‐rich diet for 2 wk did not consume more food than control males. Still, consumption of the carotenoid‐rich diet for 2 wk significantly elevated circulating levels of carotenoids in blood in male society finches, yielding the potential for immune enhancement. In fact, carotenoid‐enriched finches performed significantly better than control birds in our assay of constitutive innate immunity (bacterial‐killing activity of whole blood), although not in our test of inducible adaptive immunity (response to a mitogenic challenge with phytohemagglutinin). These results suggest that affinities for carotenoid‐rich foods may be particular to species with sexually selected carotenoid pigmentation, but that, as in humans and other mammals (e.g. mice, rats) without carotenoid color, the immune‐boosting action of carotenoids is conserved regardless of the strength of sexual selection on pigment use.

Can sexual selection drive female life histories? A comparative study on Galliform birds

Sexual selection has been identified as a major evolutionary force shaping male life history traits but its impact on female life history evolution is less clear. Here we examine the impact of sexual selection on three key female traits (body size, egg size and clutch size) in Galliform birds. Using comparative independent contrast analyses and directional discrete analyses, based on published data and a new genera-level supertree phylogeny of Galliform birds, we investigated how sexual selection [quantified as sexual size dimorphism (SSD) and social mating system (MS)] affects these three important female traits. We found that female body mass was strongly and positively correlated with egg size but not with clutch size, and that clutch size decreased as egg size increased. We established that SSD was related to MS, and then used SSD as a proxy of the strength of sexual selection. We found both a positive relationship between SSD and female body mass and egg size and that increases in female body mass and egg size tend to occur following increases in SSD in this bird order. This pattern of female body mass increases lagging behind changes in SSD, established using our directional discrete analysis, suggests that female body mass increases as a response to increases in the level of sexual selection and not simply through a strong genetic relationship with male body mass. This suggests that sexual selection is linked to changes in female life history traits in Galliformes and we discuss how this link may shape patterns of life history variation among species.

Thermodynamic behavior of a phase transition in a model for sympatric speciation

We investigate the macroscopic effects of the ingredients that drive the origin of species through sympatric speciation. In our model, sympatric speciation is obtained as we tune up the strength of competition between individuals with different phenotypes. As a function of this control parameter, we can characterize, through the behavior of a macroscopic order parameter, a phase transition from a nonspeciation to a speciation state of the system. The behavior of the first derivative of the order parameter with respect to the control parameter is consistent with a phase transition and exhibits a sharp peak at the transition point. For different resources distribution, the transition point is shifted, an effect similar to pressure in a PVT system. The inverse of the parameter related to a sexual selection strength behaves like an external field in the system and, as thus, is also a control parameter. The macroscopic effects of the biological parameters used in our model are a reminiscent of the behavior of thermodynamic quantities in a phase transition of an equilibrium physical system.

Independent effects of familiarity and mating preferences for ornamental traits on mating decisions in guppies

The avoidance of familiar individuals as mates can act to maximize the benefits of polyandry or might help to minimize inbreeding in small or highly philopatric populations. As previous mates are also familiar, the effects of familiarity and mating history can often be confounded. Here, we disentangle these effects on mating decisions in the guppy, Poecilia reticulata, and examine their influence on sexual selection. In 3 experiments, males and females were 1) able to mate, 2) had visual and olfactory contact, or 3) had visual contact only. Familiarity was successfully acquired via visual cues, and females were in all cases more likely to mate with unfamiliar than with familiar males, indicating that familiarity is a more important determinant of mating outcome than mating history. Males did not court unfamiliar females any more than familiar females and did not differentially allocate sperm. Familiarity did not alter the strength of sexual selection on male coloration: we found overall positive selection for bright, large males. Female preferences for unfamiliar males and ornamental traits may therefore be largely independent. Key words: mate choice, mating history, polymorphism, polyandry, sexual selection, sperm competition. [Behav Ecol]

Does a lack of mating opportunities explain monandry in the green‐veined white butterfly (Pieris napi)?

In the nuptial gift‐giving butterfly, Pieris napi, multiple mating corresponds to higher lifetime fecundity than monandry does. Yet, female mating frequency, which is genetically determined and heritable, varies from strict monandry to a high degree of polyandry. Polyandrous females are known to suffer from reduced longevity when denied the opportunity to mate according to their intrinsic mating frequency. Here we test if monandry is likely to be maintained due to a lack of mating opportunities in the wild. We also explore longitudinal variation in female mating frequency (i.e. lifetime number of matings) and remating rate (i.e. time interval between successive matings). The latter was explored by comparing the mating patterns of females with varying origins in a laboratory, and the former by comparing the mating frequencies of wild and laboratory females. We were able to reveal spatial variation in female mating frequency, with monandry being a clearly more common mating tactic in the north than in the south. However, we did not find direct evidence that either the maintenance of monandry or spatial variation in female mating frequency is promoted by a lack of mating opportunities. Still, our findings may have profound implications, since decreasing mating frequency towards the north generates spatial variation in the strength of sexual selection.

Reproductive migrations of the sex role reversed pipefish Nerophis lumbriciformis(Pisces; Syngnathidae)

Using an individual identification technique, a population of worm pipefish Nerophis lumbriciformis was followed during 19 months, in order to determine the exact use of the intertidal and, considering the specific movement patterns of males and females, the mating system exhibited by this population. Field observations showed that the number of adults increased during the breeding season, with males arriving 1 month earlier than females. Furthermore, males and females presented distinct permanence periods, showing that the intertidal is used as a mating arena. It was also observed that both male and female worm pipefish mated repeatedly over the span of a reproductive season, but females exhibited shorter remating intervals. Also, females stayed for longer periods on the mating grounds, the intertidal zone, whereas males typically left for the subtidal after mating, usually returning within 2 months. These inter‐sexual differences in the occupation of the intertidal suggest that females breed with different males but also that males accept eggs from various females since, on their return, a new group of mating partners was now available. Thus, N. lumbriciformis might be considered polygynandric. It is a clearly dimorphic species in spite of the observed polygynandry, suggesting that differences in remating intervals may be influential in determining the strength of sexual selection along with what may be expected from the polygynandrous mating system alone.

Sexual dimorphism in immunocompetence: what does life-history theory predict?

Sexual dimorphism in immunocompetence, usually in the direction of inferior male immunocompetence, has historically been explained as the result of proximate physiological mechanisms such as the immunosuppressive effects of the male hormone testosterone. More recently, it has been argued that this pattern is best understood as a result of resource-based trade-offs between male mating effort and immune defense, a trade-off that females do not make. The central prediction of this hypothesis is that as the strength of sexual selection on males increases, the magnitude of the sex differences in immunocompetence will increase. Two implicit assumptions of this argument are that 1) longevity is of more importance for female than for male fitness and 2) that the primary benefit of immunocompetence is increased longevity. However, both of these assumptions may not be as broadly applicable as has been argued. We have modeled the optimal allocation to immunocompetence for males and females without making these assumptions. We find that the optimal allocation to immune defense for males decreases as the strength of sexual selection increases, as predicted. However, males may still invest more, relative to females, into immunocompetence if the impact of parasites on condition differs for the sexes and/or if the relationship between condition and reproduction differs for the sexes. We argue that these previously overlooked assumptions may be critical for predicting sex-specific patterns of immu- nocompetence. Key words: ecological immunology, immune defense, life-history theory, sexual selection. (Behav Ecol)

The Distribution of Male and Female Reproductive Success in a Broadcast Spawning Marine Invertebrate

Many studies have addressed sexual selection in animals, but few data are available on animals that release eggs and sperm into the environment for external fertilization. Although this reproductive mode represents the ancestral condition and is still a very common reproductive strategy, it is underrepresented in empirical studies and theoretical treatments. Here I present data on the pattern of reproductive success in male and female sea urchins. The results suggest that the strength of sexual selection and the differences between the sexes in the intensity of sexual selection depend on mate density. In general, despite the high degree of multiple paternity, the variance in reproductive success appears to be lower in males and higher in females than it is in polygamous species with internal fertilization. These results may provide insight into the patterns of effective population size in marine invertebrates and also more generally the evolutionary transition from sexual monomorphism to polymorphism in adult traits.

Reproductive ageing and sexual selection on male body size in a wild population of antler flies (Protopiophila litigata)

Little is known about the importance of trade-offs between ageing and other life history traits, or the effects of ageing on sexual selection, particularly in wild populations suffering high extrinsic mortality rates. Life history theory suggests that trade-offs between reproduction and somatic maintenance may constrain individuals with higher initial reproductive rates to deteriorate more rapidly, resulting in reduced sexual selection strength. However, this trade-off may be masked by increased condition dependence of reproductive effort in older individuals. We tested for this trade-off in males in a wild population of antler flies (Protopiophila litigata). High mating rate was associated with reduced longevity, as a result of increased short-term mortality risk or accelerated ageing in traits affecting viability. In contrast, large body size was associated with accelerated ageing in traits affecting mating success, resulting in reduced sexual selection for large body size. Thus, ageing can affect sexual selection and evolution in wild populations.

Intra‐ and Intersexual Selection for Multiple Traits in the Peacock (Pavo cristatus)

AbstractAnimal communication involves a multitude of signals ranging from morphological to behavioural traits. In spite of the diversity of traits used in animal signalling, most studies of sexual selection have focused on single male traits. Moreover, the two forces of sexual selection (male–male competition and female preference) may target different traits and favour the diversification of male signalling. Still, little is known on the combined effects of intra‐ and intersexual selection on the evolution of multiple signals. The peacock is often cited as one of the best examples of the strength of sexual selection in producing exaggerated traits. Here, we investigated traits under intra‐ and intersexual selection in a population of free‐ranging common peafowl. Peacocks with longer trains and tarsi were more likely to establish a display territory in a central position within the lek and had a higher number of intrusions and agonistic interactions. These traits appeared therefore to be under intrasexual selection. Female selection was assessed as the number of copulations. Mating success was positively correlated with behavioural traits (display activity) and with train ornamentation (number and density of ocelli) suggesting that females use multiple cues during mate selection. Therefore, intra‐ and intersexual selection seem to operate on different sets of traits. Overall, our results stress the role of multiple receivers on the evolution of multiple signals.

Socially transmitted mate preferences in a monogamous bird: a non-genetic mechanism of sexual selection

There is increasing evidence that animals can acquire mate preferences through the use of public information, notably by observing (and copying) the mate preferences of others in the population. If females acquire preferences through social mechanisms, sexual selection could act very rapidly to spread the preference and drive elaboration of the preferred trait(s). Although there are reports of 'mate-choice copying' in polygynous species, there is no clear evidence for this process in monogamous species. Here, we investigated whether adult female zebra finches Taeniopygia guttata can socially acquire sexual preferences for individual males and, in a separate study, for a generalized trait (coloured leg bands) of males. In both studies, test females observed males in two simultaneous conditions: a ('chosen') mixed-sex situation in which a male was paired with a (model) female, and a ('unchosen') same-sex situation in which a male was paired with another male. In the first experiment, after two weeks of females observing males, test females significantly preferred individual males who had been paired with another female (i.e. chosen males). In the second experiment, test females significantly preferred novel males that were wearing the same leg band colour as the apparently chosen males. Our findings are consistent with the conclusion that female zebra finches' mate preferences are altered by public information. Our study implies that mate preferences can spread rapidly through populations by social mechanisms, affecting the strength of sexual selection in a monogamous species.

Density‐dependent influence of male characters on mate‐locating efficiency and pairing success in the waterlouse Asellus aquaticus: an experimental study

Population density is likely to determine the form of competition in which males are engaged for access to females. At low density, scramble competition should be of paramount importance because of the low probability of encounter between males and females. Consequently, sexual selection should favour characters that allow rapid detection of females. Conversely, at high population density, sexual selection should favour attributes that raise the fighting ability of males because of the more frequent contacts between males. These general predictions were tested in this study for the precopulatory mate-guarding isopod, Asellus aquaticus. In this species, male-biased sexual dimorphisms are reported for body size and antennae length and are, respectively, ascribed to contest and scramble competition over females. Therefore, the relative strength of sexual selection on male body size and antennae length at two different densities was experimentally assessed. Multivariate analyses indicated that density affected morphological correlates of the mating success of males, with body size being the main determinant of pairing success in males at a high density, whereas only antennae size significantly affected access to females at a low density. The antennae length of males was manipulated to examine how antennae length affects the ability of males to detect females in three experimental conditions varying in the probability of random contacts with receptive females. An advantage of having long antennae was only observed when there was an intermediate difficulty in finding females. Our results are discussed in relation to the influence of density on selective regimes in A. aquaticus.

Evolution of Sexual Isolation during Secondary Contact: Genotypic versus Phenotypic Changes in Laboratory Populations

We monitored the phenotypic and genotypic changes that occur when two behavioral races come into contact in laboratory populations. Drosophila melanogaster from Zimbabwe and nearby regions (Z type) show strong but asymmetric sexual isolation from their cosmopolitan counterparts (M type). Crosses of Z females and M males do not take place readily when other choices are available. At least 15 loci are known to control Z-type mating preferences and performance. By thoroughly mixing the genomes of the two types in laboratory populations, we artificially created maximum secondary contact. Despite the strength of sexual selection favoring Z-type male characters, Z-type behavior is eliminated or greatly diminished in all 12 hybrid populations after only 60 generations. This trend is consistent with the spread of the M-type behavior throughout the world as well as a detailed analysis of fitness components. Surprisingly, in contrast with the phenotypic convergence toward the M-type, genotypic samples broadly covering the genomic regions of mapped behavioral loci show no such trend. The genome appears to be "fine grained," with adjacent loci having different evolutionary dynamics and genealogical histories.

Contrasting Mutual Sexual Selection on Homologous Signal Traits in Drosophila serrata

The nature of male mating preferences, and how they differ from female mating preferences in species with conventional sex roles, has received little attention in sexual selection studies. We estimated the form and strength of sexual selection as a consequence of male and female mating preferences in a laboratory-based population of Drosophila serrata. The differences between sexual selection on male and female signal traits (cuticular hydrocarbons [CHCs]) were evaluated within a formal framework of linear and nonlinear selection gradients. Females tended to exert linear sexual selection on male CHCs, whereas males preferred intermediate female CHC phenotypes leading to convex (stabilizing) selection gradients. Possible mechanisms determining the nonlinear nature of sexual selection on female CHCs are proposed.

Sexual selection: harem size and the variance in male reproductive success.

Sexual selection is potentially stronger than natural selection when the variance in male reproductive fitness exceeds all other components of fitness variance combined. However, measuring the variance in male reproductive fitness is difficult when nonmating males are absent, inconspicuous, or otherwise difficult to find. Omitting the nonmating males inflates estimates of average male reproductive success and diminishes the variance, leading to underestimates of the potential strength of sexual selection. We show that, in theory, the proportion of the total variance in male fitness owing to sexual selection is approximately equal to H, the mean harem size, as long as H is large and females are randomly distributed across mating males (i.e., Vharem=H). In this case, mean harem size not only provides an easy way to estimate the potential strength of sexual selection but also equals the opportunity for sexual selection, I(mates). In nature, however, females may be overdispersed with Vharem<H or more aggregated than random with Vharem>H. We show that H+(k-1) is a good measure of the opportunity for sexual selection, where k is the ratio Vharem/H. A review of mating system data reveals that in nature the median ratio for Vharem/H is 1.04, but as H increases, females tend to become more aggregated across mating males with V(harem) two to three times larger than H.

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.

Unusually dynamic sex roles in a fish.

Sex roles are typically thought of as being fixed for a given species. In most animals males compete for females, whereas the females are more reluctant to mate. Therefore sexual selection usually acts most strongly on males. This is explained by males having a higher potential reproductive rate than females, leading to more males being sexually active (a male-biased operational sex ratio). However, what determines sex roles and the strength of sexual selection is a controversial and much debated question. In this large-scale field study, we show a striking temporal plasticity in the mating competition of a fish (two-spotted goby, Gobiusculus flavescens). Over the short breeding season fierce male-male competition and intensive courtship behaviour in males were replaced by female-female competition and actively courting females. Hence, sex role reversal occurred rapidly. This is the first time that a shift in sex roles has been shown in a vertebrate. The shift might be explained by a large decline in male abundance, strongly skewing the sex ratio towards females. Notably, the sex role reversal did not occur at an equal operational sex ratio, contrary to established sex role theory.

Size Dimorphism, Intrasexual Competition, and Sexual Selection in Wattled Jacana (Jacana Jacana), A Sex-Role-Reversed Shorebird in Panama

AbstractWe studied sexual size dimorphism, intrasexual competition, and sexual selection in an individually marked population of Wattled Jacanas (Jacana jacana) in the Republic of Panama. Males are the sole incubators of eggs (28-day incubation) and primary providers of chick care (50–60 days). Females were 48% heavier than, and behaviorally dominant over, males. Females also showed greater development of secondary sexual characters (fleshy facial ornamentation and wing spurs) than males. Both sexes defended territories throughout the year against same-sex conspecifics. Competition for territorial space was intense, and many individuals of both sexes did not become breeders. Resident females further competed with one another to accumulate multiple mates, resulting in a mating system of simultaneous polyandry. Female and male residents (territory holders) were larger, heavier, and more ornamented than adult floaters of the same sex. Larger and heavier females also had more mates than smaller females. Body size was thus a critical predictor of success in intrasexual competition for territories (both sexes) and for mates (females). Three measures of sexual selection—(1) sex difference in the opportunity for sexual selection, (2) female-to-male ratio of potential reproductive rates, and (3) operational sex ratio—each indicated that sexual selection is currently operating more strongly on females than on males (female-to-male ratios ranged from 1.43:1 to 2.22:1). Values of 1.61:1 and 1.43:1 represent the first published quantitative estimates of the opportunity for sexual selection for any sex-role-reversed bird. Our study supports the theory that when increased parental care entails reduced opportunities for future reproduction, asymmetries in parental care behaviors of the sexes can influence the intensity of competition for mates and the direction and strength of sexual selection.