Expression Of Sexual Ornaments Research Articles

Viability and expression of sexual ornaments in the barn swallow Hirundo rustica: a meta-analysis.

Sexual selection results in the evolution of exaggerated secondary sexual characters that can entail a viability cost. However, in species where sexual ornaments honestly reflect individual quality, the viability cost of secondary sexual characters may be overwhelmed by variation in individual quality, leading to expect that individuals with the largest secondary sexual characters show higher, rather than lower viability. Here, we used meta-analyses to test whether such expected positive relationship between sexual ornamentation and viability exists in the barn swallow Hirundo rustica, which is one of the most studied model species of sexual selection under field conditions. We found a mean positive effect size of viability in relation to the expression of secondary sexual characters of 0.181 (CI: 0.084-0.278), indicating that in this species the more ornamented individuals are more viable, and therefore of high quality. Analyses of moderator variables showed similar effects in males and females, the H.r.rustica subspecies rather than others and tail length rather than other secondary sexual characters. Future research emphasis on other subspecies than the European one and secondary sexual characters than tail length may help identify the sources of heterogeneity in effect sizes.

Unsociable juvenile male three-spined sticklebacks grow more attractive

An animal’s personality may be an adaptive behavioral response of individuals to consistent differences in physiology and life-history. Studying the relationship between behavior and other fitness-related traits within the pace-of-life syndrome (POLS) framework, which explains the integration of different traits using the concept of a fast-slow life-history continuum, may thus be a useful approach to understanding the evolution and maintenance of animal personality. According to the POLS hypothesis, for example, fast individuals should be behaviorally more active, bolder or less sociable, and potentially invest more in reproduction than slow individuals. Here, we examined whether the sociability of juvenile sticklebacks (Gasterosteus aculeatus) is linked with their key reproductive traits, carotenoid-based nuptial coloration of males and egg production of females. We also tested whether experimental manipulation of environmental conditions (control vs. warm winter) can change pace-of-life and the link between behavioral and reproductive traits. Male sticklebacks prone to shoaling expressed lower peak red coloration at the reproductive stage, during which they were solitary and territorial. In females, fecundity was not correlated with early social behavior. Experimental sticklebacks reared in warm winter conditions decreased their investment in reproduction, but the negative relationship between sociability and sexual signal of males was maintained. Our results suggest that the POLS could partly explain the maintenance of personality under predation pressure or severe mating competition. In natural environments, any benefit to sociable male sticklebacks may be counterbalanced by the cost of growing less attractive. Why do animals vary in personality, and how is such variation maintained within a population? An interesting approach for understanding this is studying how personality traits are correlated with other fitness-related traits within the pace-of-life framework. For the first time, our results show the link between sociability at the juvenile stage and the expression of sexual ornament during reproduction in male three-spined sticklebacks. Sociable juveniles may benefit from reduced predation risk, but they grow less attractive and this may reduce their mating success during reproduction. Thus, the trade-off between survival and reproduction may contribute to the maintenance of among-individual variation in behavior.

Expression of sexual ornaments in a polymorphic species: phenotypic variation in response to environmental risk.

Secondary sexual traits may evolve under the antagonistic context of sexual and natural selection. In some polymorphic species, these traits are only expressed during the breeding period and are differently expressed in alternative phenotypes. However, it is unknown whether such phenotypes exhibit phenotypic plasticity of seasonal ornamentations in response to environmental pressures such as in the presence of fish (predation risk). This is an important question to understand the evolution of polyphenisms. We used facultative paedomorphosis in newts as a model system because it involves the coexistence of paedomorphs that retain gills in the adult stage with metamorphs that have undergone metamorphosis, but also because newts exhibit seasonal sexual traits. Our aim was therefore to determine the influence of fish on the development of seasonal ornamentation in the two phenotypes of the palmate newt (Lissotriton helveticus). During the entire newt breeding period, we assessed the importance of phenotype and fish presence with an information-theoretic approach. Our results showed that paedomorphs presented much less developed ornamentation than metamorphs and those ornamentations varied over time. Fish inhibited the development of sexual traits but differently between phenotypes: in contrast to metamorphs, paedomorphs lack the phenotypic plasticity of sexual traits to environmental risk. This study points out that internal and external parameters act in complex ways in the expression of seasonal sexual ornamentations and that similar environmental pressure can induce a contrasted evolution in alternative phenotypes.

Antioxidant supplementation during early development reduces parasite load but does not affect sexual ornament expression in adult ring-necked pheasants

Summary 1. The ‘parasite-mediated sexual selection’ (PMSS) hypothesis predicts that exaggerated male ornamentation could provide a signal to females of a male’s ability to resist parasites. Empirical tests of the PMSS have been largely equivocal, however, which may be because most have not considered the role of early life-history effects. 2. Many sexually selected traits are carotenoid-based. Allocation of dietary-derived carotenoids to sexual ornaments may trade-off with allocation to pro-inflammatory immune response andUor antioxidant functions, mediated by the oxidative status of individuals. Exposure to parasites can increase oxidative stress, so under this scenario, sexually selected traits indicate ability to resist oxidative stress rather than ability to resist parasites per se . Such life-history trade-offs, mediated by oxidative status of individuals, are particularly acute during growth and development. 3. Here, we use ring-necked pheasants, Phasianus colchicus, a strongly sexually selected species, to test whether supplementation with dietary antioxidants (vitamin E) can mitigate the effects of early exposure to parasites (the nematode, Heterakis gallinarum ), via alteration of the oxidative status of individuals, and positively affect the expression of sexual ornaments at adulthood. 4. We found that vitamin E mediated the effect of early exposure to parasites on levels of oxidative damage at 8 weeks of age and reduced the parasite load of individuals at adulthood as predicted. However, the expression of sexual ornaments, immune function and growth were unaffected by either early vitamin E supplementation or manipulation of parasite load. In contrast to the predictions of the PMSS hypothesis, the intensity of sexual ornament expression was not related to either parasite load or oxidative status of individuals (current or long-term). Consequently, there was no evidence that the expression of sexual ornaments provided information on the ability of males to resist infection from parasites.

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.

Individual variation in plasma testosterone levels and its relation to badge size in House Sparrows Passer domesticus: It’s a night-and-day difference

The steroid hormone testosterone (T) plays a central role in the regulation of reproduction in animals. Although seasonal variation in T levels is well-studied, differences between day and night have only been described in relatively few species, and daily within-individual variation has been largely neglected when evaluating the relationship between T and the expression of sexual ornaments or behavior. We measured plasma T levels during day and night in a captive population of House Sparrows, and analyzed their relationship with an important male ornament – badge size. T levels were on average twice as high at night than during daytime. This was true in all seasons, and in both males and females. Disturbance of the birds at night, but not during the day, led to significantly lower T levels, suggesting a rapid drop after an individual wakes up. The relationship between T levels and badge size depended on the time when T was measured. During the breeding season, badge size was strongly positively correlated with night-time, but not with daytime T levels. This suggests that badge size signals information related to an individual’s maximum potential T level such as social dominance. Our study highlights that integrative research on the endocrine control of ornament expression needs to take diel variation in hormone levels into account.

Carotenoids, immune response and the expression of sexual ornaments in male greenfinches (Carduelis chloris)

Allocation trade-offs of carotenoids between their use in the immune system and production of sexual ornaments have been suggested as a proximate mechanism maintaining honesty of sexual signals. To test this idea, we experimentally examined whether carotenoid availability in the diet was related to variation in antibody response to novel antigens in male greenfinches (Carduelis chloris aurantiiventris), a species with extensive carotenoid-dependent plumage colouration. We also measured the cost of mounting a humoral response in terms of circulating carotenoids. Finally, we examined the relationship between plumage colour, immune response and circulating carotenoids. We found that males with carotenoid-supplemented diets showed stronger antibody response than non-supplemented birds. We also found that activation of the immune system significantly reduced circulating carotenoids (24.9% lower in immune-challenged birds than in control birds). Finally, intensity (chroma) of ventral plumage colouration of males, a character directly related to concentration of total carotenoids in feathers, was negatively correlated with the immune response and circulating carotenoids in winter. These results support the idea that carotenoids are a limiting resource and that males trade ornamental colouration against immune response.

Testosterone, immunocompetence, and honest sexual signaling in male red grouse

The expression of sexual ornaments has been suggested to reliably indicate individual quality, such as the ability to cope with parasites and diseases. The Immunocompetence Handicap Hypothesis (IHH) states that testosterone-dependent ornaments honestly signal such quality because of physiological costs associated with testosterone, such as impaired immune function. We tested predictions of the IHH both correlatively and experimentally in red grouse Lagopus lagopus scoticus. Male grouse exhibit supra-orbital red combs whose size is testosterone-dependent. We found that comb size was not correlated to infection intensity by two parasites (coccidia and the nematode Trichostrongylus tenuis), but it was significantly positively correlated with condition and T-cell-mediated immunity (the ability to mount a primary inflammatory response). We manipulated testosterone by means of implants and re-caught males after a month to investigate the effects on comb size, condition, immunity, and parasite load. Males implanted with testosterone had increased comb size, lost more condition, and had lower T-cell-mediated immunity than control males. Increased testosterone also resulted in a significant increase in coccidia infection intensity but had no effect on T. tenuis burden. The results are consistent with predictions of the IHH and suggest that comb size honestly indicates immunocompetence and males' ability to cope with certain parasites. Females could thus benefit from choosing mates based on the expression of this sexual trait.

Pheasant sexual ornaments reflect nutritional conditions during early growth.

Differences in growth conditions during early life have been suggested to cause long-lasting effects on morphology and quality of adult birds. We experimentally investigated the effect of early growth conditions on the expression of sexual ornaments later in life in male ring-necked pheasants (Phasianus colchicus). We also investigated the effects on immune function, as it could be a functional link between early nutrition and ornament expression. We manipulated the dietary protein intake during the first eight weeks post hatching. Males receiving fodder with 27% protein during the first three weeks of life grew larger and more colourful wattles when sexually mature than males receiving a low-protein diet (20.5% protein). Spur length was unaffected by diet treatment. Manipulation of food protein levels during weeks 4-8 after hatching had no effect on the development of ornaments. The different protein treatments had no long-term effect on either humoral or cell-mediated immune responses. There was, however, a positive relationship between spur length and cell-mediated immune responsiveness. Our study shows that expression of a sexual ornament in adult pheasants reflects nutritional conditions early in life. Because the expression of secondary sexual ornaments is affected by conditions during early growth, by selecting more ornamented males, females would choose mates that are superior at handling early nutritional stress. If the susceptibility to early nutritional stress also has a hereditary basis, females may benefit by obtaining 'good genes'.

A trade-off between reproduction and a condition-dependent sexually selected ornament in the house sparrow Passer domesticus.

Reproductive effort was manipulated in a free-living population of house sparrows (Passer domesticus) to investigate the trade-off between reproductive investment and the expression of a condition-dependent sexually selected ornament. Phenotypic plasticity in the expression of this trait was related to the experimentally manipulated size of the brood reared by a male. Males that invested more in current reproduction subsequently became more attractive to females in this population as they showed a preference for males with smaller badges. This supports the argument that direct benefits are a primary focus for mate choice by females. Trade-offs between reproductive effort and the expression of sexual ornaments are a potentially important source of phenotypic variation in both sexual ornaments and life-history traits.

Environmental determination of a sexually selected trait

Models of sexual selection usually assume that variation in the expression of sexual ornaments is determined largely by genetic, rather than environmental, factors1. However, empirical support for this assumption comes from studies of species with little parental care1,2, in which the influence of environmental factors may be limited3,4, and from studies of just two species5,6 with parental care, in both of which heritability estimates vary hugely between years or populations7,8. In the remaining studies of species with parental care, it is not known whether resemblance in sexual ornamentation between relatives was due to shared genes or shared patterns of care3,4,9,10,11,12,13,14,15. Here we use cross-fostering experiments in house sparrows, Passer domesticus, to examine the relative roles of these effects. We demonstrate that, although sons resemble their fathers with respect to sexual ornamentation, this resemblance is mainly due to post-hatching environmental effects rather than shared genes. We also show that sons hatching early in the year have the largest ornaments. These results support models that emphasize the importance of environmental sources of variation16,17,18, such as direct paternal effects4,19,20, on the expression of sexual ornaments, and agree with the general observation that sexually selected traits tend to be condition dependent1. We urge the incorporation of gene–environment interactions into future models of sexual selection.