Indicator Models Of Sexual Selection Research Articles

Ultraviolet signaling in a butterfly is preferred by females and conveys male genetic quality.

Indicator models of sexual selection posit that females chose males on the basis of traits that reveal male genetic quality and thereby enjoy increased offspring production. Here we report that females of the butterfly Eurema hecabe receive indirect benefits from choosing males based on their UV wing coloration, a heritable and condition-dependent trait in this species. We first used a large laboratory-bred pedigree to demonstrate a per-family association between inbreeding and male UV trait value. Females exerted choice for UV-bright males within this protocol, and average male UV trait value increased over six consecutive generations presumably due to such selection and despite an increasing rate of pedigree-wide inbreeding. We then experimentally imposed a standard strength of inbreeding upon lines of divergent male UV trait value. Inbreeding depressed the siring performance of low-UV treatment males more severely and resulted in a marginal reduction of their UV brightness, which rebounded sharply following subsequent outcrossing. These findings are consistent with the ornament-based signaling of genetic quality as a function of underlying individual-level mutational load.

Pace-of-life syndrome: linking personality, metabolism and colour ornamentation in male guppies

Within populations, there commonly exists consistent among-individual differences in behaviour. As a potential mechanism maintaining this variation, the pace-of-life-syndrome (POLS) hypothesis posits that consistent differences in the behaviour of individuals (i.e. ‘personality’) are intercorrelated and have coevolved with consistent individual differences in metabolism and life-history traits. Here, using adult male guppies, Poecilia reticulata , we tested under laboratory conditions (1) whether behavioural and metabolic traits vary consistently among individuals over time (≤7 days) and (2) the POLS prediction that repeatable behavioural traits should be intercorrelated with each other and with repeatable metabolic rate at the among-individual level. Furthermore, based on indicator models of sexual selection, we expected that sexually selected male colour ornamentation would predict individual personality and metabolic rate. We repeatedly assayed three behavioural traits (sociability, boldness, exploration) and three metabolic traits (resting metabolic rate, maximal metabolic rate, aerobic scope) in a set of males that varied in body size and colour ornamentation. All behavioural trait measures were repeatable, consistent with individual personality. Of the three metabolic traits, only resting metabolic rate (RMR) was repeatable. Behavioural trait measures were significantly intercorrelated and thus integrated in a behavioural syndrome. More ‘proactive’ males were bolder, more exploratory and more sociable than ‘reactive’ ones. However, contrary to the POLS hypothesis, RMR was not significantly correlated with any of the behavioural trait measures, suggesting that consistent individual differences in RMR do not drive or support differences in personality, or vice versa. Male colour ornamentation did not covary with behaviour or RMR and thus does not appear to be a reliable predictor of either behavioural or metabolic phenotypes. We therefore did not find any compelling support for the POLS hypothesis, suggesting that individual differences in metabolism do not underlie the evolution and maintenance of among-individual behavioural variation in our study population. • We tested for behaviour–metabolism covariation in guppies (pace-of-life syndrome, POLS). • Behaviour and resting metabolic rate (RMR) differed consistently among males. • Behavioural differences between males, however, did not covary with their RMR. • Male colour ornamentation did not predict either their personality or metabolism. • Overall, our results do not support the central prediction of the POLS hypothesis.

Influence of dietary nutrient balance on aggression and signalling in male field crickets

Indicator models of sexual selection predict that sexually selected trait elaboration should covary positively with condition. However, nutrition might influence the expression of multiple traits, where high-quality diets may result in positive trait correlations and low-quality diets in trade-offs. Although previous studies have examined how diet quantity or single nutrients, such as protein, carbohydrate and phosphorus, influence sexual traits, few have examined how dietary nutrient balance affects sexual trait expression. We therefore investigated how dietary protein:carbohydrate ratio and percentage of phosphorus influence the relationship between investment in mate attraction signalling and aggression. We fought groups of six adult male Gryllus veletis crickets, each consuming a unique diet, while recording their prefight and postfight signalling parameters. We found no evidence that diet influenced aggression or prefight signalling, with the exception that high-phosphorus diets had a negative influence on several signalling effort parameters. Body size was an important predictor of aggressive behaviour and most signalling parameters, suggesting that developmental diet may have a greater influence on these sexual traits. Several prefight signalling parameters were weakly related to aggression, suggesting that signalling may advertise competitive abilities. Males consuming high-carbohydrate and equal protein:carbohydrate diets experienced changes in signalling parameters that represented an overall increase in signalling effort following aggressive contests compared to males consuming high-protein diets, suggesting that dietary effects on signalling may only become apparent following periods of highly energetic activity. Changes in signalling following aggressive contests were also related to aggression levels modulated by dietary phosphorus content, such that only males consuming low-phosphorus diets were able to invest heavily in signalling after investing heavily in aggression. Our findings highlight the importance of research on the interplay between multiple sexually selected traits, and how dietary nutrient balance influences these relationships.

Differential genotypic effects of sexual trait size on offspring mating success and viability

Indicator models of sexual selection predict that females mating with the most ornamented males should produce offspring with enhanced expression of fitness-related traits, such as overall vigor and viability. Empirical support for this prediction, however, is limited. We quantified the effects of a heritable and condition-dependent secondary sexual trait on offspring performance traits in Drosophila bipectinata Duda (Diptera: Drosophilidae). Forty-eight genetic (isofemale) lines were extracted from a natural population, reared in a common environment, and characterized in terms of sex comb size. We measured pupal viability and adult mating success among the progeny of the 5 lines with the largest combs (high line category) and the 5 lines with the smallest combs (low line category). The high line category produced offspring that were significantly more viable than the low line category, and this advantage held across 2 developmental temperatures. In contrast, there was no effect of line category on male mating success, although at the individual-level, comb size was significantly positively correlated with mating success. Our results indicate that the relative size of the D. bipectinata sex comb taps genotypic properties that enhance offspring fitness in a trait-specific manner. Thus, distinct proximate mechanisms likely underlie relationships between secondary sexual trait expression and different performance traits in offspring, offering a possible explanation for inconsistent support for the existence of indirect benefits in sexual selection.

Beyond species recognition: somatic state affects long-distance sex pheromone communication.

Long-range sex pheromones have been subjected to substantial research with a particular focus on their biosynthesis, peripheral perception, central processing and the resulting orientation behaviour of perceivers. Fundamental to the research on sex attractants was the assumption that they primarily coordinate species recognition. However, especially when they are produced by the less limiting sex (usually males), the evolution of heightened condition dependence might be expected and long-range sex pheromones might, therefore, also inform about a signaller's quality. Here we provide, to our knowledge, the first comprehensive study of the role of a male's long-range pheromone in mate choice that combines chemical analyses, video observations and field experiments with a multifactorial manipulation of males' condition. We show that the emission of the long-distance sex pheromone of the burying beetle, Nicrophorus vespilloides is highly condition-dependent and reliably reflects nutritional state, age, body size and parasite load--key components of an individual's somatic state. Both, the quantity and ratio of the pheromone components were affected but the time invested in pheromone emission was largely unaffected by a male's condition. Moreover, the variation in pheromone emission caused by the variation in condition had a strong effect on the attractiveness of males in the field, with males in better nutritional condition, of older age, larger body size and bearing less parasites being more attractive. That a single pheromone is influenced by so many aspects of the somatic state and causes such variation in a male's attractiveness under field conditions was hitherto unknown and highlights the need to integrate indicator models of sexual selection into pheromone research.

Natural variation in condition-dependent display colour does not predict male courtship success in a jumping spider

In many animals, males display costly, condition-dependent ornaments to choosy females. Indicator models of sexual selection predict that females should choose mates based on natural variation in such traits. In Habronattus pyrrithrix jumping spiders, males have conspicuous, condition-dependent red faces and green legs that they display to cryptically coloured females during courtship. In a correlational study using field-collected spiders, we paired individual males with virgin females in the laboratory and found that natural variation in male coloration did not predict mating success (likelihood of copulation) or levels of female aggression. Rather, mating success was best predicted by male body condition. We then conducted an outdoor experiment under natural sunlight where we gave both virgin and mated females the choice between two simultaneously courting males, one with his facial coloration experimentally reduced and the other that received a sham treatment. Again, we found no relationship between male coloration and courtship success. Our previous studies have shown that the presence (versus absence) of red facial coloration improves courtship success, but here we found no evidence that more subtle natural variation in this trait has any effect on success. We discuss these findings in the context of complex signalling and species recognition in Habronattus. While many studies report negative results for condition-dependent quality signalling alongside positive ones, few discuss the implications of negative results. We argue that, as empirical data accumulate, we should move towards asking why some condition-dependent traits are consistently important to females while other equally condition-dependent traits are either ignored, or only matter in certain contexts.

Carotenoid-based plumage coloration reflects feather corticosterone levels in male house finches (Haemorhous mexicanus)

Indicator models of sexual selection predict that exaggerated traits communicate information about sender con- dition or quality to conspecific receivers. Environmental chal- lenges have often been considered as one such condition that could be encoded in an ornamental trait, and there is now extensive evidence showing how different stressors (e.g., nutri- tional, parasitological, and environmental) impact sexual signal elaboration. One of the primary means of assessing stress is by quantifying glucocorticoid (corticosterone or cortisol (CORT)) levels. For many ornaments, CORT impairs trait expression; however, the evidence is limited and mixed for one of the classic honest signals in animals, ornamental carotenoid color- ation. In a model species for studies of carotenoid ornamenta- tion (the house finch, Haemorhous mexicanus), we examined the relationship between male plumage redness and feather CORT levels, which serve as an integrated measure of hormone concentration during feather growth. We measured CORT in both tail (melanin-containing) and breast (carotenoid-contain- ing) feathers and found that CORT levels were not different between body regions, but they were negatively correlated with plumage hue, with redder birds having more CORT in feathers. Despiteopposingtraditionalviews onstressandornamentation, our results actually corroborate three other studies showing positive relationships betwee nc arotenoid coloration and CORT levels. Though the molecular mechanisms underlying such a relationship are still unclear, our results suggest that CORT should not be considered as a simple indicator of indi- vidual quality but rather as a mediator of complex allocation decisions or signals of metabolic activity that could link up with more elaborate expression of ornamental traits.

Male great bowerbirds accumulate decorations to reduce the annual costs of signal production

Indicator models of sexual selection suggest that honest mating signals are costly to produce or maintain. Animals that use extended phenotypes as courtship signals may be able to reduce these costs in a given year by reusing nonbodily ornaments acquired during previous years, perhaps affecting signal honesty. I examined whether male great bowerbirds, Ptilonorhynchus nuchalis , are able to reduce the annual costs of signal production by reusing bower decorations acquired during previous years. I paired males that had similar numbers of bower decorations and removed decorations from one member of the pair. Males that had decorations removed from their bowers collected more decorations from the environment the following year than did control males, and they tended to steal more decorations. However, they had fewer decorations at their bowers than did control males, most likely because control males reused almost 50% of the decorations that they had acquired during previous years. These results suggest that individuals that accumulate nonbodily ornaments can reduce the costs of signal production in subsequent years; and that resource accumulation can lead to differences in both behaviour and display quality between individuals that do and do not accumulate resources. I also found a positive correlation between the number of decorations at bowers before removal and the number of decorations acquired by males after removal, suggesting that decoration numbers indicate a male's past condition and present performance. ► Male great bowerbirds reuse about 50% of decorations from previous years. ► Males that reuse decorations have higher-quality displays than males that do not. ► Males that reuse decorations collect fewer decorations and tend to steal less. ► Animals can reduce signalling costs by accumulating ‘wealth’ over several years.

The condition dependence of a secondary sexual trait is stronger under high parasite infection level

Indicator models of sexual selection predict that the expression of sexual ornaments should be condition dependent. This is only partly supported by data, as many studies do not find positive associations between ornaments and condition. The reason for this inconsistency remains poorly understood. It has been hypothesized that environmental context may explain variation in the condition dependence of sexual traits, with stronger relationships between ornaments and condition expected in harsher environments. However, field tests of this idea are scarce. We studied 9 populations of wild red grouse Lagopus lagopus scoticus over 11 years, and compared the relationship between ornamentation and body mass (an index of body condition) in relation to environmental variability. We used the abundance of a key parasite in this system, Trichostrongylus tenuis, as an index of environmental conditions. We found that both ornament expression and body mass negatively correlated with parasite infection at both population and individual levels. More interestingly, we found that the relationship between ornamentation and body mass was stronger in populations with high parasite infection levels. Our findings support the idea that the condition dependence of secondary sexual ornaments varies in relation to environmental context. In sites and years when parasites are abundant, sexual ornaments provide better signals of condition.

Environmental heterogeneity influences the reliability of secondary sexual traits as condition indicators

Numerous studies have shown positive associations between ornaments and condition, as predicted by indicator models of sexual selection. However, this idea is continuously challenged by opposite results, which reveal our lack of full understanding of how sexual selection works. Environmental heterogeneity may explain such inconsistencies, but valid field tests of this idea are currently lacking. We first analysed the relationship between condition and ornament expression from nine populations over 7 years in a wild bird, the red grouse Lagopus lagopus scoticus. We then manipulated male aggressiveness at the population level by means of testosterone implants in a replicated field experiment. We found that the relationship between condition and ornamentation varied greatly between environments and became stronger when environmental conditions (ECs) were worse or when aggressiveness in the population was experimentally increased. Some ornaments may therefore reliably advertise a better condition only in adverse ECs. Considering environmental heterogeneity can help reconcile conflicting findings regarding the reliability of ornaments as indicators of condition and will help our understanding of sexual selection processes.

Pteridine, not carotenoid, pigments underlie the female-specific orange ornament of striped plateau lizards (Sceloporus virgatus)

Indicator models of sexual selection suggest that signal honesty is maintained via costs of ornament expression. Carotenoid-based visual signals are a well-studied example, as carotenoids may be environmentally limited and impact signaler health. However, not all bright yellow, orange and red ornaments found in vertebrates are carotenoid-based; pteridine pigments may also produce these colors. We examine the contribution of carotenoid and pteridine pigments to the orange reproductive color of female striped plateau lizards (Sceloporus virgatus). This color ornament reliably indicates female mate quality, yet costs maintaining signal honesty are currently unknown. Dietary carotenoid manipulations did not affect orange color, and orange skin differed from surrounding white skin in drosopterin, not carotenoid, content. Further, orange color positively correlated with drosopterin, not carotenoid, concentration. Drosopterin-based female ornaments avoid the direct trade-offs of using carotenoids for ornament production vs egg production, thus may relax counter-selection against color ornament exaggeration in females. Direct experimentation is needed to determine the actual costs of pteridine-based ornaments. Like carotenoids, pteridines influence important biological processes, including immune and antioxidant function; predation and social costs may also be relevant.

Male health status, signalled by courtship display, reveals ejaculate quality and hatching success in a lekking species

1. The information content of secondary sexual traits and the benefits gathered by choosy females are at the heart of sexual selection theory. Indicator models of sexual selection assume that secondary sexual traits reflect the phenotypic/genetic quality of their bearers and that females gather benefits from choosing these high-quality males. 2. Here, we tested the idea that courtship display reflects the health status in a bird species with a lek-based mating system, the houbara bustard (Chlamydotis undulata undulata). A group of males was treated with the lipopolysaccharide (LPS) of the cell wall of the bacterium Escherichia coli during the seasonal peak of courtship display, while another group of males was injected with a phosphate-buffered saline solution (PBS) as a control. We then monitored the effect of the treatment on both courtship display and ejaculate quality. Finally, females were artificially inseminated with semen from LPS and PBS males, which allowed us to assess the effect of the immunological treatment on reproduction. 3. We found that the inflammatory challenge reduced courtship display and semen quality compared to controls. Interestingly, males that better resisted to the immune challenge in terms of courtship display also better resisted in terms of ejaculate quality. Early reproductive failure was increased when females were artificially inseminated with semen from immune-activated males. Failure of eggs laid by females inseminated with LPS semen was due to a reduced fertilization power of sperm of LPS males or to increased embryo mortality in the very early stage of embryo development. As a consequence, hatching rate was reduced for females inseminated with semen collected from LPS males. 4. These results show that by assessing male courtship display, females may gain insight into the current phenotypic/genetic quality of mates and gather fitness benefits in terms of reproductive success.

Do male Great Bowerbirds (Ptilonorhynchus nuchalis) minimise the costs of acquiring bower decorations by reusing decorations acquired in previous breeding seasons?

In many species, males must acquire resources from the environment in order to produce the sexual signals attractive to females, and indicator models of sexual selection suggest that the costs of resource acquisition may contribute to honest sexual signalling. I used data collected during a 5-year field study of the Great Bowerbird (Ptilonorhynchus nuchalis) to test the hypothesis thatacquiring decorations is costly to males in time and energy, and that malesminimise costs by reusing decorations acquired in previous breeding seasons. Males reused 240 ± 30 decorations, which represents 30% of the decorations at their bowers. Males that reused a higher proportion of their decorations were more likely to equal or exceed the numbers of decorations they displayed at their bowers during the previous breeding season than males that reused a lower proportion. Males that inherited bower-sites, where they were able to reuse decorations acquired by the previous owner, had more decorations than males that built at new sites, where decorations were not available to reuse. Males only reused a high proportion of rocks, a common decoration type, when the distance over which they had to carry the rocks was small, but they reused less common decoration types regardless of distance. These data suggest there are costs associated with acquiring decorations, and males reuse decorations to minimise costs.

A functional approach to sexual selection

Summary Sexual selection theory is a robust and dynamic field within evolutionary biology, yet despite decades of research, remarkably little is known of the mechanistic bases of mate choice and male competition. Because many aspects of sexual selection involve dynamic movements, and are physically challenging, the limits of sexual selection may be defined by key functional and physiological variables (i.e. the ‘functional approach’). We advocate the functional approach for providing mechanistic resolution on the adaptive basis of sexual structures and signals, the nature of mate choice and how males compete, among other issues. An overview of recent work, highlighted in this special issue, shows that many features of sexual selection, including the outcome of male fights, the morphology and design of sexual signals, and the nature of mate choice, all appear to be dictated in large part by functional and physiological parameters. We argue that the functional approach provides a potent tool for resolving several unresolved issues in sexual selection theory, particularly the nature of male quality, the handicap and indicator models of sexual selection, and the basis of mate choice.

Foraging ability in the scorpionfly Panorpa vulgaris: individual differences and heritability

According to indicator models of sexual selection, mates may obtain indirect, i.e. genetic, benefits from choosing partners indicating high overall genetic quality by honest signals. In the scorpionfly Panorpa vulgaris, both sexes show mating preferences on the basis of the condition of the potential partners. Females prefer males that produce nuptial gifts (i.e. salivary secretions) during copulation, while males invest more nuptial gifts in females of high nutritional status. Both characters, males' ability to produce nuptial gifts and high nutritional status of females, are known to be reliable indicators of foraging ability. Thus, besides possible direct benefits, both sexes might also obtain indirect benefits in terms of “good foraging genes” by their choice and thereby increase the fitness of their offspring. A prerequisite for this possibility is the heritability of the respective trait. In the present study, we estimated the repeatability and the heritability of foraging ability. Our results indicate (1) a significant repeatability of individual foraging efficiencies in males and females and (2) a heritable component of this trait by a significant parent–offspring regression. These findings suggest that genetic benefits in terms of increased offspring foraging ability might contribute to selection for mating preferences in both sexes.

Multiple Receivers, Multiple Ornaments, and a Trade‐off between Agonistic and Epigamic Signaling in a Widowbird

Sexual displays often involve several different ornamental traits. Yet most indicator models of sexual selection based on a single receiver (usually a choosy female) find that multiple handicap signals should be unstable. Here we study reasons for this contradiction, analyzing signal function, signal content, and trade-offs between signals in the polygynous red-collared widowbird Euplectes ardens. Males have both a long, graduated tail and a red carotenoid collar badge. Territory-holding "residents" have slightly shorter tails than the nonbreeding "floaters," but their carotenoid collars are 40% larger, and they have (on the basis of reflectance spectrometry and objective colorimetry) a 23-nm more long-wave ("redder") hue than floaters. This corroborates experimental evidence that the red collar is selected by male contest competition, whereas female choice is based almost exclusively on male tail length. Tail length is negatively correlated with the carotenoid signal, which together with body size and condition explains 55% of the variation in tail length. The trade-off in tail length and carotenoid investment is steeper among residents, suggesting an interaction with costs of territory defense. We propose that the "multiple receiver hypothesis" can explain the coexistence of multiple handicap signals. Furthermore, the trade-off between signal expressions might contribute to the inverse relation between nuptial tail elongation and coloration in the genus Euplectes (bishops and widowbirds).

Sexual selection and condition dependence of courtship display in three species of horned dung beetles

Sexual selection has traditionally been divided into competition over mates and mate choice. Currently, models of sexual selection predict that sexual traits are expressed in proportion to the condition of their bearer. In horned beetles, male contest competition is well established, but studies on female preferences are scarce. Here I present data on male mating success and condition dependence of courtship rate in three species of horn-dimorphic dung beetles, Onthophagus taurus, Onthophagus binodis, and Onthophagus australis. I found that in the absence of male contest competition, mating success of O. taurus and O. australis was unrelated to their horn length and body size, whereas in O. binodis horn size had a negative effect but body size had a positive effect on male mating success. Overall, in O. binodis major morph males had greater mating success than minor morph males. In all three species male mating success was affected by courtship rate, and the courtship rate was condition dependent such that when males were manipulated to be in poor condition they had lower courtship rates than males that were manipulated to be in good condition. My findings provide new insight into the mating systems of horned dung beetles and support an important assumption in indicator models of sexual selection.

Multiple Receivers, Multiple Ornaments, and a Trade-off between Agonistic and Epigamic Signaling in a Widowbird

Sexual displays often involve several different ornamental traits. Yet most indicator models of sexual selection based on a single receiver (usually a choosy female) find that multiple handicap signals should be unstable. Here we study reasons for this contradiction, analyzing signal function, signal content, and trade‐offs between signals in the polygynous red‐collared widowbird Euplectes ardens. Males have both a long, graduated tail and a red carotenoid collar badge. Territory‐holding “residents” have slightly shorter tails than the nonbreeding “floaters,” but their carotenoid collars are 40% larger, and they have (on the basis of reflectance spectrometry and objective colorimetry) a 23‐nm more long‐wave (“redder”) hue than floaters. This corroborates experimental evidence that the red collar is selected by male contest competition, whereas female choice is based almost exclusively on male tail length. Tail length is negatively correlated with the carotenoid signal, which together with body size and condition explains 55% of the variation in tail length. The trade‐off in tail length and carotenoid investment is steeper among residents, suggesting an interaction with costs of territory defense. We propose that the “multiple receiver hypothesis” can explain the coexistence of multiple handicap signals. Furthermore, the trade‐off between signal expressions might contribute to the inverse relation between nuptial tail elongation and coloration in the genus Euplectes (bishops and widowbirds).

Components of phenotypic variation in avian ornamental and non-ornamental feathers

Phenotypic variation, measured as the coefficient of variation (CV), is usually larger in secondary sexual characters than in ordinary morphological traits. We tested if intraspecific differences in the CV between ornamental and non-ornamental feather traits in 67 evolutionary events of feather ornamentation in birds were due to differences in (1) the allometric pattern (slope of the regression line when regressing trait size on an indicator of body size), or (2) the dispersion of observations around the regression line. We found that only dispersion of observations around the regression line contributed significantly to total variation. A large dispersion of observations around the regression line for ornamental feathers is consistent with these characters showing condition-dependence, supporting indicator models of sexual selection more strongly than a pure Fisher process. Ornamental feathers in males demonstrated negative allometry when regressed on tarsus length, which is a measure of skeletal body size. This finding is consistent with ornamental feather traits being subject to directional selection due to female mate preferences, where large body size is less important than in male–male competition. This pattern of phenotypic variation for avian secondary sexual characters contrasts with patterns of variation for insect genitalia, supposedly subject to sexual selection, since the latter traits only differ from ordinary morphology traits in allometry coefficient. The prevailing regime of selection (directional or stabilizing) and the effects of environmental factors are proposed to account for these differences among traits.

Sex-specific nest defense in house sparrows ( Passer domesticus ) varies with badge size of males

According to indicator models of sexual selection, females can benefit from choosing males with above average epigamic traits, but empirical evidence for such benefits is scarce. Here, we report results from an experiment with 29 pairs of house sparrows (Passer domesticus) where the intensity of nest defense against a mounted mustelid predator was related to the size of the black throat and breast patch (“badge”) in males. Using principal components analysis (PCA), original response variables of both sexes were reduced to two factors: “Approach” to the predator and “Distant warning”. “Approach”, the more risky behavior, increased from small- through medium- to large-badged males and decreased in their females. Since large-badged males have a higher certainty of paternity (i.e. greater benefits from defense) and may be older and more experienced (i.e. incur lower costs), the most likely explanation for male defense intensity increasing with badge size is an improving benefit/cost ratio. The resulting optimal response of their females and evolutionarily stable participation in joint parental care is illustrated by a graphical model. It shows that females would, indeed, benefit directly from choosing large-badged males. This, however, is no proof of a direct evolutionary tie between badge size and paternal behavior, as assumed by indicator models of sexual selection. It may simply represent a spurious relationship, originating from the correlation of badge size and defense with confidence of paternity.