Condition-dependence and Sex Traits in the Male Great Bustard

Secondary sex traits involved in the courtship display of the male great bustard (Otis tarda) are analyzed in the light of condition-dependence theory. Heaviest males at the start of the mating period further gained about 30% in body mass. Both display performance and development of morphological traits involved are mainly determined by body mass. Males who developed sex traits were among those in good physical condition, although not the current physical condition but weight of the male correlates with the development of sex traits. We discuss the idea that weight may be a good indicator of overall physical condition during development up to maturity, rather than just the current condition at a given season. Our results show a discontinuity both in display performance and in the development of morphological traits — i.e. birds of weight below a certain threshold neither performed the display nor fully developed the secondary sex traits. This constitutes evidence of cost reduction by males when expected benefits from mating are low, and is consistent with the viability-conditioned expression of the traits.     

Limited condition dependence of male acoustic signals in the grasshopper Chorthippus biguttulus

In many animal species, male acoustic signals serve to attract a mate and therefore often play a major role for male mating success. Male body condition is likely to be correlated with male acoustic signal traits, which signal male quality and provide choosy females indirect benefits. Environmental factors such as food quantity or quality can influence male body condition and therefore possibly lead to condition-dependent changes in the attractiveness of acoustic signals. Here, we test whether stressing food plants influences acoustic signal traits of males via condition-dependent expression of these traits. We examined four male song characteristics, which are vital for mate choice in females of the grasshopper Chorthippus biguttulus. Only one of the examined acoustic traits, loudness, was significantly altered by changing body condition because of drought- and moisture-related stress of food plants. No condition dependence could be observed for syllable to pause ratio, gap duration within syllables, and onset accentuation. We suggest that food plant stress and therefore food plant quality led to shifts in loudness of male grasshopper songs via body condition changes. The other three examined acoustic traits of males do not reflect male body condition induced by food plant quality.     

Condition-dependent traits and the capture of genetic variance in male advertisement song

The occurrence of additive genetic variance (VA) for male sexual traits remains a major problem in evolutionary biology. Directional selection normally imposed by female choice is expected to reduce VA greatly, yet recent surveys indicate that a substantial amount remains in many species. We addressed this problem, also known as the 'lek paradox', in Achroia grisella (Lepidoptera: Pyralidae), an acoustic moth in which males advertise to females with a pulsed ultrasonic song. Using a standard half-sib/full-sib breeding design, we generated F1 progeny from whom we determined VA and genetic covariance (COVA) among seven traits: three song characters, an overall index of song attractiveness, nightly singing period, adult lifespan, and body mass at adult eclosion. Because A. grisella neither feed nor drink as adults, the last trait, eclosion body mass, is considered a measure of 'condition'. We found significant levels of VA and narrow-sense heritabilities (h2) for all seven traits and significant genetic correlations (= COVAi,j / radical (VA i x VA j)) between most pairs of traits (i, j). Male attractiveness was positively correlated with body mass (condition), adult lifespan, and nightly singing period, which we interpret as an energy constraint preventing males in poor condition from singing attractively, from singing many hours per night, and from surviving an extended lifespan. The positive genetic correlation (r = 0.79) between condition and attractiveness, combined with significant levels of VA for both traits, indicates that much of the variation in male song can be explained by VA for condition. Finally, we discuss the morphological and physiological links between condition and song attractiveness, and the ultimate factors that may maintain VA for condition.     

Sounds different: inbreeding depression in sexually selected traits in the cricket Teleogryllus commodus

If male sexual signalling is honest because it captures genetic variation in condition then traits that are important mate choice cues should be disproportionately affected by inbreeding relative to other traits. To test this, we investigated the effect of brother-sister mating on advertisement calling by male field crickets Teleogryllus commodus. We quantified the effect of one generation of inbreeding on nightly calling effort and five finer-scale aspects of call structure that have been shown to influence attractiveness. We also quantified inbreeding depression on six life history traits and one morphological trait. Inbreeding significantly reduced hatching success, nymph survival and adult lifespan but had no detectable effect on hatching rate, developmental rate or adult body mass. The effect of inbreeding on sexually selected traits was equivocal. There was no decline in calling effort (seconds of sound production/night) by inbred males, but there were highly significant changes in three of five finer-scale call parameters. Sexually selected traits clearly vary in their susceptibility to inbreeding depression.     

Condition-dependent signaling affects male sexual attractiveness in field crickets, Gryllus campestris

The condition dependence of sexually selected traits is an importantassumption of sexual selection theory. Several laboratory studieshave documented a positive relationship between food availability,body condition, and sexual display. However, these studies mightnot reflect the resource allocation between body maintenance,reserves, and the sexually selected trait under natural conditions.Further, the effect of condition-dependent signaling on femalemate choice has hardly been investigated experimentally in thefield. We therefore investigated the effect of food availabilityon body condition, calling behavior, and sexual attractivenessof male field crickets, Gryllus campestris, under field conditions.Food availability was manipulated for individual males by supplementingfood in a confined area close to the burrow. Food-supplementedmales showed a significant increase in body condition, whereasthe opposite was found in the control males. Males receivingextra food called more frequently, whereas the calling-songcharacteristics were not affected by the treatment. Further,food-supplemented males attracted more females than did controlmales, and their higher attractiveness was partly explainedby their superior calling rate. Our study thus indicates condition-dependentsignaling as an important determinant of the sexual attractivenessof males to females under natural condition.     

Physiological costs enforce the honesty of lek display in the black grouse (Tetrao tetrix)

Females may use condition-dependent sexual traits as reliable cues of male “quality” if the costs of the expression of such traits vary with male “quality”, and if there is positive genetic correlation between male traits and condition. However, there are multiple ways of measuring the changes in body condition which reflect physiological costs meaning that the multifaceted nature of the physiological costs associated with the expression of sexual traits has rarely been thoroughly examined. In the lekking black grouse (Tetrao tetrix), mating success is highly skewed towards males defending central territories and having high survival rates to the following year, but the mechanisms underpinning such superior performance remain unclear. In this study, we quantified the changes in five measures of body condition before and after the mating season and related these changes to male lek performance (fighting rate, territory centrality and mating success) to understand the physiological costs of male reproductive effort. Between the two capture sessions, male body mass decreased significantly, blood parasite counts and plasma carotenoid concentration increased substantially while the total immunoglobulin concentration tended to increase. There was no overall impairment of individual body condition as the changes in the five measures of body condition were unrelated. Male fighting rate was unrelated to changes in the condition measures but males losing more body mass defended central territories and had high mating success. Therefore, females preferring central, dominant males may select males better able to afford the energetic costs of lek performance thereby effectively enforcing the honesty of male display.     

Body Shape Preferences: Associations with Rater Body Shape and Sociosexuality

There is accumulating evidence of condition-dependent mate choice in many species, that is, individual preferences varying in strength according to the condition of the chooser. In humans, for example, people with more attractive faces/bodies, and who are higher in sociosexuality, exhibit stronger preferences for attractive traits in opposite-sex faces/bodies. However, previous studies have tended to use only relatively simple, isolated measures of rater attractiveness. Here we use 3D body scanning technology to examine associations between strength of rater preferences for attractive traits in opposite-sex bodies, and raters’ body shape, self-perceived attractiveness, and sociosexuality. For 118 raters and 80 stimuli models, we used a 3D scanner to extract body measurements associated with attractiveness (male waist-chest ratio [WCR], female waist-hip ratio [WHR], and volume-height index [VHI] in both sexes) and also measured rater self-perceived attractiveness and sociosexuality. As expected, WHR and VHI were important predictors of female body attractiveness, while WCR and VHI were important predictors of male body attractiveness. Results indicated that male rater sociosexuality scores were positively associated with strength of preference for attractive (low) VHI and attractive (low) WHR in female bodies. Moreover, male rater self-perceived attractiveness was positively associated with strength of preference for low VHI in female bodies. The only evidence of condition-dependent preferences in females was a positive association between attractive VHI in female raters and preferences for attractive (low) WCR in male bodies. No other significant associations were observed in either sex between aspects of rater body shape and strength of preferences for attractive opposite-sex body traits. These results suggest that among male raters, rater self-perceived attractiveness and sociosexuality are important predictors of preference strength for attractive opposite-sex body shapes, and that rater body traits –with the exception of VHI in female raters– may not be good predictors of these preferences in either sex.     

Much more than a ratio: multivariate selection on female bodies

Studies of the attractiveness of female bodies have focussed strongly on the waist, hips and bust, but sexual selection operates on whole phenotypes rather than the relative proportions of just two or three body parts. Here, we use body scanners to extract computer‐generated images of 96 Chinese women’s bodies with all traits unrelated to body shape removed. We first show that Chinese and Australian men and women rate the attractiveness of these bodies the same. We then statistically explore the roles of age, body weight and a range of length and girth measures on ratings of attractiveness. Last, we use nonlinear selection analysis, a statistical approach developed by evolutionary biologists to explore the interacting effects of suites of traits on fitness, to study how body traits interact to determine attractiveness. Established proxies of adiposity and reproductive value, including age, body mass index and waist‐to‐hip ratio, were all correlated with attractiveness. Nonlinear response surface methods using the original traits consistently outperform all of these indices and ratios, suggesting that indices of youth and abdominal adiposity tell only part of the story of body attractiveness. In particular, our findings draw attention to the importance of integration between abdominal measures, including the bust, and the length and girth of limbs. Our results provide the most comprehensive analysis to date of the effect of body shape and fat deposition on female attractiveness.     

Relative importance of chemical attractiveness to parasites for susceptibility to trematode infection

While the host immune system is often considered the most important physiological mechanism against parasites, precontact mechanisms determining exposure to parasites may also affect infection dynamics. For instance, chemical cues released by hosts can attract parasite transmission stages. We used the freshwater snail Lymnaea stagnalis and its trematode parasite Echinoparyphium aconiatum to examine the role of host chemical attractiveness, physiological condition, and immune function in determining its susceptibility to infection. We assessed host attractiveness through parasite chemo‐orientation behavior; physiological condition through host body size, food consumption, and respiration rate; and immune function through two immune parameters (phenoloxidase‐like and antibacterial activity of hemolymph) at an individual level. We found that, although snails showed high variation in chemical attractiveness to E. aconiatum cercariae, this did not determine their overall susceptibility to infection. This was because large body size increased attractiveness, but also increased metabolic activity that reduced overall susceptibility. High metabolic rate indicates fast physiological processes, including immune activity. The examined immune traits, however, showed no association with susceptibility to infection. Our results indicate that postcontact mechanisms were more likely to determine snail susceptibility to infection than variation in attractiveness to parasites. These may include localized immune responses in the target tissue of the parasite. The lack of a relationship between food consumption and attractiveness to parasites contradicts earlier findings that show food deprivation reducing snail attractiveness. This suggests that, although variation in resource level over space and time can alter infection dynamics, variation in chemical attractiveness may not contribute to parasite‐induced fitness variation within populations when individuals experience similar environmental conditions.     

Facial attractiveness signals different aspects of “quality” in women and men

We explored the relationships between facial attractiveness and several variables thought to be related to genotypic and phenotypic quality in humans (namely fluctuating asymmetry (FA), body mass index (BMI), health, age). To help resolve some controversy around previous studies, we used consistent measurement and statistical methods and relatively large samples of both female (n=94) and male (n=95) subjects (to be evaluated and measured), and female (n=226) and male (n=153) viewers (to rate attractiveness). We measured the asymmetry of 22 traits from three trait families (eight facial, nine body and five fingerprint traits) and constructed composite asymmetry indices of traits showing significant repeatability. Facial attractiveness was negatively related to an overall asymmetry index in both females and males, with almost identical slopes. Female facial attractiveness was best predicted by BMI and past health problems, whereas male facial attractiveness was best predicted by the socioeconomic status (SES) of their rearing environment. Composite FA indices accounted for a small (<4%) but usually significant percentage of the variation in facial attractiveness in both sexes, when factors related to asymmetry were controlled statistically. We conclude that, although facial attractiveness is negatively related to developmental instability (as measured by asymmetry), attractiveness also signals different aspects of “quality” in the two sexes, independent of FA.     

Sexual Traits as Quality Indicators in Lekking Male Great Bustards

We studied the development of two sexual traits, whiskers and neck plumage, in relation to sexual selection in 41 free‐living great bustard, Otis tarda, males radio‐tracked at nine leks in central Spain in 1998–2001. During the pre‐breeding male–male competition period (Feb.) prior to female arrival, number and length of whiskers correlated with weight, but not with body size or age. Whiskers may thus have evolved as an intrasexual indicator of weight, which in the absence of other weapons in this species is decisive in male–male combats. Signalling through whiskers contributes to minimizing dangerous aggressive interactions in the lek. During the mating period (Apr.), both whisker and neck development were correlated with weight and age. Males reaching higher expression of both traits exhibited higher display intensity, a more prolonged display period through the mating season, and a higher estimated mating success. Moreover, interannual changes in a male’s expression of both traits were associated with changes in its display intensity and estimated mating success. Our results resolve earlier debates and contradictory results from previous authors, suggesting that these two secondary sexual traits, whiskers and neck, may function as reliable indicators of age and weight, the two main factors determining social rank of males in great bustard leks, during both rival assessment and mate choice. Their dual functions provide support for the pre‐existing trait and redundant signal hypotheses and suggest that multiple ornaments functioning as redundant signals might be more widespread than previously acknowledged.     

Influence of nutrition on male development and reproduction in Tribolium castaneum

Tribolium beetles have evolved over several thousand years to colonize and exploit various food products that vary widely in their nutritional quality. Here Tribolium castaneum was used as a model organism to explore the effect of nutritional quality on male development and reproduction. The results showed, when tested across different qualities of nutritional diets, Tribolium males developed faster and their body size was larger on a high-quality diet, and there were significant correlations between male developmental traits. However, Tribolium males fed different nutritional diets did not show significant variation in olfactory attractiveness, mating rate, insemination rate, sperm defense (P1), sperm offense (P2), and reproductive success within a population context (RSPC). Moreover, there was no significant correlation of male reproductive performances except RSPC with developmental traits, and except for P2 and RSPC, no significant correlations between male reproductive performances. Therefore, although male developmental performance was significantly influenced by diet quality, reproductive performance was not. We discussed these findings and their sexual selection implications in light of its habitat.     

The Costs of Male Display and Delayed Plumage Maturation in the Satin Bowerbird (Ptilonorhynchus violaceus)

The costs associated with the evolution of male display traits has attracted much attention in regard to the type of traits that evolve and the timing of their expression. We investigate these costs by using testosterone implants to alter the development of male display traits in the satin bowerbird (Ptilonorhynchus violaceus). Testosterone implants advanced the development of adult display traits in males with juvenile plumage. We then measured the costs of early expression of these characters in treated males. Testosterone implants caused young males to (1) become more involved in behavior normally carried out by older males when visiting bowers, (2) build bowers, and (3) molt prematurely into an adult plumage. No difference was observed between the testosterone-treated birds and controls in return rate or condition the year following treatment, suggesting that the physical costs of display are not high. We found that males in adult plumage are tolerated less, and are not displayed to as frequently as juvenile plumage males at the bowers of established bower holders. The causes of delayed plumage maturation are most consistent with the facilitated-learning hypothesis. That is, early development of adult characteristics reduces the opportunity of young males to learn display behavior critical for later reproductive success.     

The relative importance of the face and body in judgments of human physical attractiveness

A number of traits have been proposed to be important in human mate choice decisions. However, relatively little work has been conducted to determine the relative importance of these traits. In this study, we assessed the relative importance of the face and body in judgments of human physical attractiveness. One hundred twenty-seven men and 133 women were shown images of 10 individuals of the opposite sex. Participants rated the images for their attractiveness for either a short-term relationship or a long-term relationship. Images of the face and the body were rated independently before participants were shown and asked to rate the combined face and body images. Face ratings were found to be the best predictor of the ratings of combined images for both sexes and for both relationship types. Females showed no difference in ratings between short- and long-term conditions, but male ratings of female bodies became relatively more important for a short-term relationship compared with a long-term relationship. Results suggest that faces and bodies may be signaling different information about potential mates.     

Early Developmental Conditions and Male Attractiveness in Zebra Finches

Traits used in mate choice are often costly to produce or maintain, and thus can reflect an individual’s current condition. Mate choice, however, might not only be influenced by the current condition of a potential partner, but also by the condition it had experienced during its early development which can have strong and long‐lasting effects on various traits. Here we studied the effects of different early developmental conditions, imposed by brood size manipulations (small, medium and large broods), on male attractiveness as measured by female choice experiments in zebra finches (Taeniopygia guttata). In three different experiments, we allowed females to choose between males that had been raised in different experimental brood sizes. In none of the experiments, females showed a significant preference for males which had experienced better developmental conditions, i.e. were raised in the relatively smaller experimental broods. Song rate was higher in males coming from small than large broods, but females did not prefer males that sang more. These results suggest that sexual attractiveness either was not affected by our experimental treatment or that males subsequently had compensated in their overall attractiveness for negative effects of early developmental conditions.     

Juvenile immune status affects the expression of a sexually selected trait in field crickets

Parasite-mediated sexual selection theory presumes that variation in sexual traits reliably reflects variation in parasite resistance among available mates. One mechanism that may warrant signal honesty involves costs of immune system activation in the case of a parasitic infection. We investigated this hypothesis in male field crickets Gryllus campestris, whose attractiveness to females depends on characteristics of the sound-producing harp that are essentially fixed following adult eclosion. During the nymphal stage, males subjected to one of two feeding regimes were challenged with bacterial lipopolysaccharides (LPS) to investigate condition-dependent effects on harp development as compared to other adult traits. Nymphal nutritional status positively affected adult body size, condition, and harp size. However, nymphal immune status affected harp size only, with LPS-males having smaller harps than control-injected males. In addition, the harps of LPS-males showed a lesser degree of melanization, indicating an enhanced substrate use by the melanin-producing enzyme cascade of the immune system. Thus, past immune status is specifically mirrored in sexual traits, suggesting a key role for deployment costs of immunity in parasite-mediated sexual selection.     

Foliar damage modifies floral attractiveness to pollinators in Alstroemeria exerens

Pollination is a requisite for sexual reproduction in plants and its success may determine the reproductive output of individuals. Pollinator preference for some floral designs or displays that are lacking or poorly developed in focal plants may constrain the pollination process. Foliar herbivory may affect the expression of floral traits, thus reducing pollinator attraction. Natural populations of the Andean species Alstroemeria exerens (Alstromeriaceae) in central Chile show high levels of foliar herbivory, and floral traits show phenotypic variation. In the present field study, we addressed the attractive role of floral traits in A. exerens and the effect of foliar damage on them. Particularly, we posed the following questions: (1) Is there an association between floral display and design traits and the number and duration of pollinator visits? and (2) Does foliar damage affect the floral traits associated with pollinator visitation? To assess the attractiveness of floral traits for pollinators, we recorded the number and duration of visits in 101 focal plants. To evaluate the effects of foliar damage on floral traits, 100 plants of similar size were randomly assigned to control or damage groups during early bud development. Damaged plants were clipped using scissors (50% of leaf area) and control plants were manually excluded from natural herbivores (<5% of leaf area eaten). During the peak of flowering, we recorded the number of open flowers, and estimated corolla and nectar guide areas. The number and duration of pollinator visits was statistically associated with floral design and display traits. Plants with larger displays, corollas and nectar guide areas received more visits. Visits lasted longer as display increases. In addition, foliar damage affected attractive traits. Damaged plants had fewer open flowers and smaller nectar guide areas. We conclude that foliar damage affects plant attractiveness for pollinators and hence may indirectly affect plant fitness.     

Genotype x environment interaction for male attractiveness in an acoustic moth: evidence for plasticity and canalization

The lek paradox arises when choosy females deplete the genetic variance for male display traits from a population, yet substantial additive genetic variation (V_A) in male traits persists. Thus, the lek paradox can be more generally stated as one of the most fundamental evolutionary questions: What maintains genetic variation in natural populations? One solution to this problem may be found in the condition‐dependent nature of many sexually selected traits. Genotype × environment (G × E) interactions can maintain V_A under conditions of environmental heterogeneity provided certain restrictions are met, although antagonistic pleiotropy has also been proposed as a mechanism. Here, we provide evidence for G × E interactions and against the role of antagonistic pleiotropy in the maintenance of V_A for sexually selected traits. Using inbred lines of the lesser waxmoth Achroia grisella, we measured V_A for song attractiveness, condition and development rate under different competitive environments and found that genotypes differed in their plasticity. We argue that variation persists in natural populations because G × E interactions prevent any one variant from producing the optimal phenotype across all environments.     

Facial attractiveness,developmental stability,and fluctuating asymmetry

Despite robust cross-cultural reliability of human facial attractiveness ratings, research on facial attractiveness has only superficially addressed the connection between facial attractiveness and the history of sexual selection in Homo sapiens. There are reasons to believe that developmental stability and phenotypic quality are related. Recent studies of nonhuman animals indicate that developmental stability, measured as fluctuating asymmetry in generally bilateral symmetrical traits, is predictive of performance in sexual selection: Relatively symmetrical males are advantaged under sexual selection. This pattern is suggested by our study of facial attractiveness and fluctuating asymmetry in seven bilateral body traits in a student population. Overall, facial attractiveness negatively correlated with fluctuating asymmetry; the relation for men, but not for women, was statistically reliable. Possible confounding factors were controlled for in the analysis.     

Male cockroaches prefer a high carbohydrate diet that makes them more attractive to females: implications for the study of condition dependence

Sexual selection is a major force driving the evolution of elaborate male sexual traits. Handicap models of sexual selection predict that male sexual traits should covary positively with condition, making them reliable indicators of male quality. However, most studies have either manipulated condition through varying diet quantity and/or caloric content without knowledge of specific nutrient effects or have correlated proxies of condition with sexual trait expression. We used nutritional geometry to quantify protein and carbohydrate intake by male cockroaches, Nauphoeta cinerea, and related this to sex pheromone expression, attractiveness, and dominance status. We found that carbohydrate, but not protein, intake is related to male sex pheromone expression and attractiveness but not dominance status. Additionally, we related two condition proxies (weight gain and lipid reserves) to protein and carbohydrate acquisition. Weight gain increased with the intake of both nutrients, whereas lipid reserves only increased with carbohydrate intake. Importantly, lipid accumulation was not as responsive to carbohydrate intake as attractiveness and thus was a less-accurate condition proxy. Moreover, males preferentially consumed high carbohydrate diets with little regard for protein content suggesting that they actively increase their carbohydrate intake thereby maximizing their reproductive fitness by being attractive.