Do carotenoid-based sexual traits signal the availability of non-pigmentary antioxidants?

Passive and active parental food allocation in a songbird

Parent-offspring conflict over food allocation can be modeled using two theoretical frameworks: passive (scramble competition) and active choice (signaling) resolution models. However, differentiating between these models empirically can be challenging. One possibility involves investigating details of decision-making by feeding parents. Different nestling traits, related to competitive prowess or signaling cryptic condition, may interact additively or non-additively as predictors of parental feeding responses. To explore this, we experimentally created even-sized, small broods of pied flycatchers and manipulated nestling cryptic quality, independently of size, by vitamin E supplementation. We explored how interactions between nestling cryptic condition, size, signals, and spatial location predicted food allocation and prey-testing by parents. Parents created the potential for spatial scramble competition between nestlings by feeding from and to a narrow range of nest locations. Heavier supplemented nestlings grew faster and were more likely to access profitable nest locations. However, the most profitable locations were not more contested, and nestling turnover did not vary in relation to spatial predictability or food supply. Postural begging was only predicted by nestling hunger and body mass, but parents did not favor heavier nestlings. This suggests that size-mediated and spatial competition in experimental broods was mild. Pied flycatcher fathers allocated food in response to nestling position and begging order, while mothers seemingly followed an active choice mechanism involving assessment of more complex traits, including postural intensity interacting with order, position, and treatment, and perhaps other stimuli when performing prey-testings. Differences in time constraints may underlie sex differences in food allocation rules.

Made-up mouths with preen oil reveal genetic and phenotypic conditions of starling nestlings

Animal coloration results from pigments, nanostructures, or the cosmetic use of natural products, and plays a central role in social communication. The role of cosmetic coloration has traditionally been focused in scenarios of sexual selection, but it could also take place in other contexts. Here, by using spotless starlings (Sturnus unicolor) as a model system, we explore the possibility that nestlings cosmetically use their intensely yellow-colored uropygial secretion to signal their genetic and/or phenotypic quality. In agreement with the hypothetical cosmetic use of the uropygial secretion, (i) video recorded nestlings collected secretion with the bill at the age of feathering, (ii) cotton swabs turned to the color of secretion after rubbing with them nestlings' gape, and (iii) gape and skin colorations correlated positively with that of secretion. Furthermore, we found that (iv) secretion coloration has a genetic component, and (v) associated positively with Vitamin E supplementation and (vi) with plasma carotenoid concentration, which highlights the informative value of nestling secretion. Finally, (vii) coloration of begging-related traits and of secretion of nestlings predicted parental feeding preferences. Consequently, all these results strongly suggest that the cosmetic use of colored uropygial secretion might also play a role in parent-offspring communication, complementing or amplifying information provided by the flamboyant colored gapes and skin of nestlings. The use of makeups by offspring for communication with relatives has been scarcely explored and we hope that these results will encourage further investigations in birds and other taxa with parental care.

Ecological effects on female bill colour explain plastic sexual dichromatism in a mutually-ornamented bird

Sex differences in ornamentation are common and, in species with conventional sex roles, are generally thought of as stable, due to stronger sexual selection on males. Yet, especially in gregarious species, ornaments can also have non-sexual social functions, raising the possibility that observed sex differences in ornamentation are plastic. For example, females may invest in costly ornamentation more plastically, to protect body and reproductive ability in more adverse ecological conditions. We tested this hypothesis with experimental work on the mutually-ornamented common waxbill (Estrilda astrild), supplementing their diets either with pigmentary (lutein, a carotenoid) or non-pigmentary (vitamin E) antioxidants, or alleviating winter cold temperature. We found that both lutein and vitamin E supplementation increased red bill colour saturation in females, reaching the same mean saturation as males, which supports the hypothesis that female bill colour is more sensitive to environmental or physiological conditions. The effect of vitamin E, a non-pigment antioxidant, suggests that carotenoids were released from their antioxidant functions. Alleviating winter cold did not increase bill colour saturation in either sex, but increased the stability of female bill colour over time, suggesting that female investment in bill colour is sensitive to cold-mediated stress. Together, results show that waxbill bill sexual dichromatism is not stable. Instead, sexual dichromatism can be modulated, and even disappear completely, due to ecology-mediated plastic adjustments in female bill colour.

Circulating nutrient concentrations in free-ranging Humboldt penguins (Spheniscus humboldti) in Punta San Juan, Peru

Free-ranging Humboldt penguin (HP, Spheniscus humboldti) populations are under pressure from resource competition with industrial fisheries, habitat loss, and El Niño Southern Oscillation events. Foraging patterns for this top marine predator change during periods of aberrant oceanographic conditions and scarce fish stock numbers. These radical dietary fluctuations can lead to poor fertility, early embryonic death, poor hatchability, suppressed immune function, high chick mortality, and illness. To understand the variability of nutrient status in reproductive seasons, we measured select circulating nutrient concentrations (fat-soluble vitamins A, D, E, and carotenoids, fatty acids, amino acids, minerals, and electrolytes) of 105 HP at Punta San Juan, Peru during the first reproductive seasons of 2007 and 2008. We determined significant differences in nutrient status between sexes, years of sampling, and reproductive stages. Males (4.5 ± 0.38 kg) weighed more than females (4.0 ± 0.29 kg) and exhibited higher concentrations of vitamin A (0.71 ± 0.11 vs. 0.61 ± 0.12 µg/ml) and docosahexaenoic acid (6.70 ± 1.61 vs. 5.65 ± 1.59%). Males also displayed lower concentrations of β-carotene (0.01 ± 0.01 vs. 0.012 ± 0.001 µg/ml) and phosphorus (3.43 ± 0.83 vs. 4.40 ± 1.66 mg/dl). Comparison between the 2 years showed most circulating amino acid concentrations were higher in 2007. Significant differences in circulating amino acids and vitamins were also noted between different reproductive stages. These results demonstrate concentrations of nutrients can vary due to the physiological state of the animal, as well as the overall dynamics of their marine ecosystem habitat.

No evidence that carotenoid pigments boost either immune or antioxidant defenses in a songbird

Dietary carotenoids have been proposed to boost immune system and antioxidant functions in vertebrate animals, but studies aimed at testing these physiological functions of carotenoids have often failed to find support. Here we subject yellow canaries (Serinus canaria), which possess high levels of carotenoids in their tissue, and white recessive canaries, which possess a knockdown mutation that results in very low levels of tissue carotenoids, to oxidative and pathogen challenges. Across diverse measures of physiological performance, we detect no differences between carotenoid-rich yellow and carotenoid-deficient white canaries. These results add further challenge to the assumption that carotenoids are directly involved in supporting physiological function in vertebrate animals. While some dietary carotenoids provide indirect benefits as retinoid precursors, our observations suggest that carotenoids themselves may play little to no direct role in key physiological processes in birds.

Dietary carotenoids have been proposed to have physiological benefits in addition to contributing to coloration. Here, Koch et al. compare immune and antioxidant functions in yellow, carotenoid-rich vs. white, carotenoid-deficient canaries and find no difference, suggesting a limited physiological role of carotenoids.

Predictors of facial attractiveness and health in humans

Facial attractiveness has been suggested to provide signals of biological quality, particularly health, in humans. The attractive traits that have been implicated as signals of biological quality include sexual dimorphism, symmetry, averageness, adiposity, and carotenoid-based skin colour. In this study, we first provide a comprehensive examination of the traits that predict attractiveness. In men, attractiveness was predicted positively by masculinity, symmetry, averageness, and negatively by adiposity. In women, attractiveness was predicted positively by femininity and negatively by adiposity. Skin colour did not predict attractiveness in either sex, suggesting that, despite recent interest in the literature, colour may play limited role in determining attractiveness. Male perceived health was predicted positively by averageness, symmetry, and skin yellowness, and negatively by adiposity. Female perceived health was predicted by femininity. We then examined whether appearance predicted actual health using measures that have been theoretically linked to sexual selection, including immune function, oxidative stress, and semen quality. In women, there was little evidence that female appearance predicted health. In men, we found support for the phenotype-linked fertility hypothesis that male masculinity signalled semen quality. However, we also found a negative relationship between averageness and semen quality. Overall, these results indicate weak links between attractive facial traits and health.

Carotenoids, birdsong and oxidative status: administration of dietary lutein is associated with an increase in song rate and circulating antioxidants (albumin and cholesterol) and a decrease in oxidative damage

Despite the appealing hypothesis that carotenoid-based colouration signals oxidative status, evidence supporting the antioxidant function of these pigments is scarce. Recent studies have shown that lutein, the most common carotenoid used by birds, can enhance the expression of non-visual traits, such as birdsong. Nevertheless, the underlying physiological mechanisms remain unclear. In this study we hypothesized that male European starlings (Sturnus vulgaris) fed extra lutein increase their song rate as a consequence of an improved oxidative status. Although birdsong may be especially sensitive to the redox status, this has, to the best of our knowledge, never been tested. Together with the determination of circulating oxidative damage (ROMs, reactive oxygen metabolites), we quantified uric acid, albumin, total proteins, cholesterol, and testosterone, which are physiological parameters potentially sensitive to oxidation and/or related to both carotenoid functions and birdsong expression. We found that the birds fed extra lutein sang more frequently than control birds and showed an increase of albumin and cholesterol together with a decrease of oxidative damage. Moreover, we could show that song rate was associated with high levels of albumin and cholesterol and low levels of oxidative damage, independently from testosterone levels. Our study shows for the first time that song rate honestly signals the oxidative status of males and that dietary lutein is associated with the circulation of albumin and cholesterol in birds, providing a novel insight to the theoretical framework related to the honest signalling of carotenoid-based traits.

Honest sexual signaling in turtles: experimental evidence of a trade-off between immune response and coloration in red-eared sliders Trachemys scripta elegans

Sexual signals can be evolutionarily stable if they are honest and condition dependent or costly to the signaler. One possible cost is the existence of a trade-off between maintaining the immune system and the elaboration of ornaments. This hypothesis has been experimentally tested in some groups of animals but not in others such as turtles. We experimentally challenged the immune system of female red-eared sliders Trachemys scripta elegans, with a bacterial antigen (lipopolysaccharide (LPS)) without pathogenic effects to explore whether the immune activation affected visual colorful ornaments of the head. The LPS injection altered the reflectance patterns of color ornaments. In comparison to the control animals, the yellow chin stripes of injected animals exhibited (1) reduced brightness, (2) lower long wavelength (>470 nm) reflectance, and (3) lower values for carotenoid chroma. The postorbital patches of injected individuals also showed reduced very long wavelength (>570 nm) reflectance but did not change in carotenoid chroma. Thus, experimental turtles showed darker and less "yellowish" chin stripes and less "reddish" postorbital patches at the end of the experiment, whereas control turtles did not change their coloration. This is the first experimental evidence supporting the existence of a trade-off between the immune system and the expression of visual ornaments in turtles. We suggest that this trade-off may allow turtles to honestly signal individual quality via characteristics of coloration, which may have an important role in intersexual selection processes.

An experimental test of the testosterone mediated oxidation handicap hypothesis in a wild bird

The oxidation handicap hypothesis (OHH) proposed that honesty in sexual signals is maintained when testosterone simultaneously promotes the development of elaborate signals and imposes an oxidative cost. Although there is evidence that testosterone enhances display traits in some cases, relatively few studies have tested the prediction that testosterone generates oxidative costs. We tested this prediction experimentally by administering testosterone (n=14) and control (n=14) implants to free-living common yellowthroat warblers (Geothlypis trichas) and quantifying testosterone and oxidative state before and 35±15days after implantation. We interpreted our experimental results in the context of a larger database of 83 unmanipulated males observed over five breeding seasons. In our observational data, testosterone was related to aspects of the carotenoid-based bib, but these relationships were age-dependent. Bib coloration was related to testosterone only for first time breeders, while bib size was positively and negatively associated with testosterone among experienced and inexperienced breeders, respectively. Two measures of oxidative metabolism-damage to DNA and total antioxidant capacity (TAC)-were unrelated to endogenous testosterone. Despite the correlation between endogenous testosterone and plumage, our experimental results failed to support the key prediction of the OHH. Testosterone treated males had higher levels of TAC upon recapture, but oxidative damage to DNA did not differ from controls. Because antioxidants can protect against the harmful effects of oxidative stress, one interpretation of our results is that males physiologically compensated for elevated testosterone, avoiding the honesty enforcing mechanism of the OHH. Taken together, our results suggest that testosterone is not a direct mediator of honest signaling in yellowthroats via its effects on oxidative stress.

Effects of carotenoid and vitamin E supplementation on oxidative stress and plumage coloration in house finches (Haemorhous mexicanus)

There has been much recent interest from both applied and basic scientists in the broad series of benefits that animals reap from acquiring high concentrations of dietary antioxidants, such as carotenoids and vitamins (e.g., vitamin E, or tocopherol). Most attention has been paid to separate effects of these compounds on, for example, coloration, health state, development, and vision, but because of possible interactions between these lipid-soluble molecules, we are in need of more studies that co-manipulate these substances and examine their possible synergistic impacts on animal physiology and phenotype. We capitalized on a model avian system (the house finch, Haemorhous mexicanus), where extensive information is available on the fitness roles of carotenoids, to test how variation in carotenoid and/or vitamin E concentrations in the diet impacts body accumulation of these compounds, factors related to oxidative damage (e.g., breast muscle and plasma oxidative-stress susceptibility, plasma nitric-oxide levels), and plumage color development. As in a previous study of ours on carotenoids and health in finches, we employed a 2×2 factorial experimental design on birds in both molting and non-molting conditions, to understand how seasonal shifts in carotenoid use (i.e., pigment incorporation into plumage) might alter the accumulation and roles of carotenoids and vitamins. As expected, lutein supplementation increased the level of circulating carotenoids in both experiments and the color of newly molted plumage. By contrast, vitamin E provisioning did not significantly affect plasma carotenoid levels or plumage coloration in either experiment. Interestingly, carotenoid provisioning decreased circulating vitamin E levels during molt, which suggests either molecular competition between carotenoids and tocopherol at the absorption/transport stages or that vitamin E serves as an antioxidant to offset harmful actions that carotenoids may have at very high concentrations. Finally, in both experiments, we found a reduction in breast-muscle oxidative damage for tocopherol-supplemented birds, which constitutes the first demonstration of a protective effect of vitamin E against oxidative stress in wild birds. Taken together, these findings provide an interesting contrast with our earlier work on season-specific physiological benefits of carotenoids in finches and point to complex associations between indicators of antioxidant and oxidative state in wild-caught animals.

New developments of the effect of melatonin on reproduction

In the past decades, a lot of advances in understanding the biochemistry and physiology of the pineal gland have been made. There is evidence that it interacts with many endocrine as well as non-endocrine tissues to influence their metabolic activity modulating many organs and functions. Melatonin is secreted by the pineal gland in the brain and plays an important role in regulating the neuroendocrine system. This hormone is one of the major role players in the regulation of the circadian sleep-wake cycle. It is normally released from the pineal gland during the night in response to environmental changes in light. Studies have shown that melatonin plays a role in the regulation of many reproductive processes such as puberty, gonadal function, and pregnancy. Beside these, melatonin has been shown to be able to directly neutralize a number of free radicals and reactive oxygen and nitrogen species. The main objective of this review is to provide comprehensive information about the new developments in melatonin research regarding its role in reproduction. A review of international scientific literature was done and a question-and-answer format was used in an attempt to convey comprehensive information in a simple manner. This review discusses evidence currently available relating to the effect of melatonin on reproductive processes. It deliberates the mechanism of action of melatonin, its effect on puberty, testicular and ova function, pregnancy, and oxidative stress. A growing body of scientific evidence is suggesting that melatonin plays an important role in reproductive function. It is therefore imperative to highlight the beneficial effects of this hormone in improving the reproductive processes.

Immune evasion, immunopathology and the regulation of the immune system

Costs and benefits of the immune response have attracted considerable attention in the last years among evolutionary biologists. Given the cost of parasitism, natural selection should favor individuals with the most effective immune defenses. Nevertheless, there exists huge variation in the expression of immune effectors among individuals. To explain this apparent paradox, it has been suggested that an over-reactive immune system might be too costly, both in terms of metabolic resources and risks of immune-mediated diseases, setting a limit to the investment into immune defenses. Here, we argue that this view neglects one important aspect of the interaction: the role played by evolving pathogens. We suggest that taking into account the co-evolutionary interactions between the host immune system and the parasitic strategies to overcome the immune response might provide a better picture of the selective pressures that shape the evolution of immune functioning. Integrating parasitic strategies of host exploitation can also contribute to understand the seemingly contradictory results that infection can enhance, but also protect from, autoimmune diseases. In the last decades, the incidence of autoimmune disorders has dramatically increased in wealthy countries of the northern hemisphere with a concomitant decrease of most parasitic infections. Experimental work on model organisms has shown that this pattern may be due to the protective role of certain parasites (i.e., helminths) that rely on the immunosuppression of hosts for their persistence. Interestingly, although parasite-induced immunosuppression can protect against autoimmunity, it can obviously favor the spread of other infections. Therefore, we need to think about the evolution of the immune system using a multidimensional trade-off involving immunoprotection, immunopathology and the parasitic strategies to escape the immune response.

Immune function is related to adult carotenoid and bile pigment levels, but not dietary carotenoid access during development, in female mallard ducks

Immune function can be modulated by multiple physiological factors, including nutrition and reproductive state. Because these factors can vary throughout an individual’s lifetime due to environmental conditions (e.g. nutrition) or life-history stage (e.g. adult reproduction), we must carefully examine the degree to which developmental versus adult conditions shape performance of the immune system. We investigated how variation in dietary access to carotenoid pigments – a class of molecules with immunostimulatory properties that females deposit into egg yolks – during three different developmental time points affected adult immunological and reproductive traits in female mallard ducks (Anas platyrhynchos). In males and females of other avian species, carotenoid access during development affects carotenoid assimilation ability, adult sexual ornamentation, and immune function, while carotenoid access at adulthood can increase immune response and reproductive investment (e.g. egg-laying capacity, biliverdin deposition in eggshells). We failed to find effects of developmental carotenoid supplementation on adult immune function (phytohemagglutinin-induced cutaneous immune response, antibody production in response to the novel antigen keyhole limpet hemocyanin [KLH], or oxidative burst, assessed by change in circulating nitric oxide levels), carotenoid-pigmented beak coloration, ovarian development, circulating carotenoid levels, or concentration of bile pigments in the gall bladder. However, we did uncover positive relationships between circulating carotenoid levels at adulthood and KLH-specific antibody production, and a negative relationship between biliverdin concentration in bile and KLH-specific antibody production. These results are consistent with the view that adult physiological parameters better predict current immune function than do developmental conditions and highlight a possible, previously unstudied relationship between biliverdin and immune system performance.

Attractive Skin Coloration: Harnessing Sexual Selection to Improve Diet and Health

In this paper we review the mechanisms through which carotenoid coloration could provide a sexually selected cue to condition in species with elaborate color vision. Skin carotenoid pigmentation induced by fruit and vegetable consumption may provide a similar cue to health in humans (particularly light-skinned Asians and Caucasians). Evidence demonstrates that carotenoid-based skin coloration enhances apparent health, and that dietary change can perceptibly impact skin color within weeks. We find that the skin coloration associated with increased fruit and vegetable consumption benefits apparent health to a greater extent than melanin pigmentation. We argue that the benefits to appearance may motivate individuals to improve their diet and that this line of appearance research reveals a potentially powerful strategy for motivating a healthy lifestyle.

Coloration of Anuran Tadpoles (Amphibia): Development, Dynamics, Function, and Hypotheses

Colorations of anuran tadpoles surely function in many of the same ways that have been ascribed to color and pattern in other animals, but the paucity of data forces one to look to other groups to generate hypotheses. Such an action often occurs because of the difficulty of defining specific fitness parameters to larval forms. The commonly muted colorations of tadpoles are typically considered to function only in some form of crypsis, but we discuss other functions in the particular context of behavioral ecology and changes induced by various kinds of coinhabitants. We review the development, terminology, diversity, and functions of coloration in tadpoles and then pose various questions for future research. We strongly support a broad-based perspective that calls for an integration of several fields of research.

What does carotenoid-dependent coloration tell? Plasma carotenoid level signals immunocompetence and oxidative stress state in birds-A meta-analysis

Mechanisms maintaining honesty of sexual signals are far from resolved, limiting our understanding of sexual selection and potential important parts of physiology. Carotenoid pigmented visual signals are among the most extensively studied sexual displays, but evidence regarding hypotheses on how carotenoids ensure signal honesty is mixed. Using a phylogenetically controlled meta-analysis of 357 effect sizes across 88 different species of birds, we tested two prominent hypotheses in the field: that carotenoid-dependent coloration signals i) immunocompetence and/or ii) oxidative stress state. Separate meta-analyses were performed for the relationships of trait coloration and circulating carotenoid level with different measures of immunocompetence and oxidative stress state. For immunocompetence we find that carotenoid levels (r = 0.20) and trait color intensity (r = 0.17) are significantly positively related to PHA response. Additionally we find that carotenoids are significantly positively related to antioxidant capacity (r = 0.10), but not significantly related to oxidative damage (r = -0.02). Thus our analyses provide support for both hypotheses, in that at least for some aspects of immunity and oxidative stress state the predicted correlations were found. Furthermore, we tested for differences in effect size between experimental and observational studies; a larger effect in observational studies would indicate that co-variation might not be causal. However, we detected no significant difference, suggesting that the relationships we found are causal. The overall effect sizes we report are modest and we discuss potential factors contributing to this, including differences between species. We suggest complementary mechanisms maintaining honesty rather than the involvement of carotenoids in immune function and oxidative stress and suggest experiments on how to test these.

Oxidative stress and condition-dependent sexual signals: more than just seeing red

The links between fitness, health, sexual signals and mate choice are complex and subject to ongoing study. In 1999, von Schantz et al. made the valuable suggestion that oxidative stress may be an important missing piece of this complex puzzle. Their suggestion has been enthusiastically tested, with over 300 studies citing their paper, but most effort has concerned carotenoid-based (and to a lesser extent melanin-based) visual signals, predominantly in birds and fishes. Today, we know a great deal more about oxidative stress and related physiology, in both a pathological and regulatory sense, than we did in 1999. We revisit von Schantz et al.'s predictions and, more importantly, highlight novel mechanisms that could link oxidative stress with a range of energetically demanding signals, greatly increasing the scope from visual signalling systems that are usually discussed and nearly always tested. In particular, we argue that differences between individuals in their ability to regulate physiology related to oxidative stress may be an important factor influencing the production of sexual signals and the costs that are incurred from investment.

The scent of senescence: sexual signalling and female preference in house mice

Sexual signals are expected to be costly to produce and maintain, thus ensuring that only males in good condition can sustain their expression at high levels. When males reach senescence they lose physiological function and condition, which could constrain their ability to invest in costly sexual signals, decreasing their attractiveness to mates. Furthermore, females may have evolved mating preferences that cause avoidance of senesced males to enhance fertilization success and viability of offspring. Among mammals, the size of antlers and other weapons can decrease with senescence, but changes in olfactory sexual signals have been largely unexplored. We examined changes in olfactory signals with senescence in house mice (Mus musculus domesticus), where males excrete volatile and involatile molecules in scent marks that elicit behavioural and priming responses in females. Compared to middle-aged males, the urine of senesced males contained a lower concentration of involatile signalling proteins (major urinary proteins or MUPs), and associated volatiles that bind to these proteins. The reduced intensity of male scent will affect the longevity of scent signals deposited in the environment and, accordingly, females were less attracted to urine from senesced males deposited 12 h previously. Females also discriminated against senesced males encountered behind a mesh barrier. These results reveal that investment in olfactory signalling is reduced during senescence and suggest that senesced males and their scent may be less attractive to females.

Immune activation reduces sperm quality in the great tit

Mounting an immune response against pathogens incurs costs to organisms by its effects on important life-history traits, such as reproductive investment and survival. As shown recently, immune activation produces large amounts of reactive species and is suggested to induce oxidative stress. Sperm are highly susceptible to oxidative stress, which can negatively impact sperm function and ultimately male fertilizing efficiency. Here we address the question as to whether mounting an immune response affects sperm quality through the damaging effects of oxidative stress. It has been demonstrated recently in birds that carotenoid-based ornaments can be reliable signals of a male's ability to protect sperm from oxidative damage. In a full-factorial design, we immune-challenged great tit males while simultaneously increasing their vitamin E availability, and assessed the effect on sperm quality and oxidative damage. We conducted this experiment in a natural population and tested the males' response to the experimental treatment in relation to their carotenoid-based breast coloration, a condition-dependent trait. Immune activation induced a steeper decline in sperm swimming velocity, thus highlighting the potential costs of an induced immune response on sperm competitive ability and fertilizing efficiency. We found sperm oxidative damage to be negatively correlated with sperm swimming velocity. However, blood resistance to a free-radical attack (a measure of somatic antioxidant capacity) as well as plasma and sperm levels of oxidative damage (lipid peroxidation) remained unaffected, thus suggesting that the observed effect did not arise through oxidative stress. Towards the end of their breeding cycle, swimming velocity of sperm of more intensely colored males was higher, which has important implications for the evolution of mate choice and multiple mating in females because females may accrue both direct and indirect benefits by mating with males having better quality sperm.

Effects of vitamin E and beta-carotene on sperm competitiveness

Sperm are particularly prone to oxidative damage because they generate reactive oxygen species (ROS), have a high polyunsaturated fat content and a reduced capacity to repair DNA damage. The dietary compounds vitamin E and beta-carotene are argued to have antioxidant properties that help to counter the damaging effects of excess ROS. Here in, we tested the post-copulatory consequences for male crickets (Teleogryllus oceanicus) of dietary intake of these two candidate antioxidants. During competitive fertilisation trials, vitamin E, but not beta-carotene, singularly enhanced sperm competitiveness. However, the diet combining a high vitamin E dose and beta-carotene produced males with the most competitive ejaculates, possibly due to the known ability of beta-carotene to recycle vitamin E. Our results provide support for the idea that these two common dietary compounds have interactive antioxidant properties in vivo, by affecting the outcomes of male reproductive success under competitive conditions.

Metal pollution indirectly increases oxidative stress in great tit (Parus major) nestlings

Metals can cause oxidative stress by increasing the formation of reactive oxygen species (ROS), when there are insufficient amount of antioxidants to defend against the growing amount of free radicals. We aimed to find out the most reliable biomarkers to detect pollution-related oxidative stress in wild birds by comparing oxidative stress status in great tit (Parus major) nestlings at populations in polluted and unpolluted areas. We also studied with experimental manipulations whether dietary carotenoid levels have any role in great tits' antioxidant defence and whether their carotenoid-based plumage colour was connected to an oxidative stress status. We used antioxidants (GSH, carotenoids) and several antioxidant enzymes (GP, GR, GST, SOD, and CAT) as indicators of the oxidative stress. We found no direct connections between dietary metal exposure and antioxidant or antioxidant enzyme levels. The activity of GP was, however, slightly higher in the polluted environment. This was due to poorer condition and subsequently higher level of oxidative stress in the nestlings in the polluted area. We also found a positive association between GP and an ambient temperature during the nestling period, which may be due to higher metabolic activity of partly poikilothermic nestlings in warm weather. The activity of GST was positively related to the number of nestlings at the sampling time. Fledging success was better in an unpolluted area, where also the nestling body mass was higher. Carotenoid treatment increased the plasma carotenoid concentrations 2.1 fold in carotenoid-supplemented birds, but was not associated with the oxidative stress biomarkers or metal levels. The yellow plumage colour was associated with dietary carotenoid levels in both study areas, but not with the metal exposure or the oxidative stress status. Our results suggest that at the exposure levels found in our study area, the enzyme activities do not indicate metal-related oxidative stress. Instead, GP can be used as an indicator of growth related oxidative stress, which is greater in the polluted area. The activity of this enzyme was, however, not directly related to metal exposure, but more likely to some secondary pollution-related change in the nestling condition.

Carotenoid supplementation positively affects the expression of a non-visual sexual signal

Carotenoids are a class of pigments which are widely used by animals for the expression of yellow-to-red colour signals, such as bill or plumage colour. Since they also have been shown to promote immunocompetence and to function as antioxidants, many studies have investigated a potential allocation trade-off with respect to carotenoid-based signals within the context of sexual selection. Although an effect of carotenoids on non-visual (e.g. acoustic) signals involved in sexual selection has been hypothesized, this has to date not been investigated. First, we examined a potential effect of dietary carotenoid supplementation on overall song rate during the non-breeding season in captive male European starlings (Sturnus vulgaris). After only 3–7 days, we found a significant (body-mass independent) positive effect of carotenoid availability on overall song rate. Secondly, as a number of studies suggest that carotenoids could affect the modulation of sexual signals by plasma levels of the steroid hormone testosterone (T), we used the same birds to subsequently investigate whether carotenoid availability affects the increase in (nestbox-oriented) song rate induced by experimentally elevated plasma T levels. Our results suggest that carotenoids may enhance the positive effect of elevated plasma T levels on nestbox-oriented song rate. Moreover, while non-supplemented starlings responded to T-implantation with an increase in both overall song rate and nestbox-oriented song, carotenoid-supplemented starlings instead shifted song production towards (reproductively relevant) nestbox-oriented song, without increasing overall song rate. Given that song rate is an acoustic signal rather than a visual signal, our findings therefore indicate that the role of carotenoids in (sexual) signalling need not be dependent on their function as pigments.

Carotenoid-based coloration, oxidative stress and corticosterone in common lizards

Environmental factors including stressors, health status and social context significantly affect carotenoid-based coloration. For instance, stressors may induce the diversion of carotenoids from pigmentation pathways, potentially explaining why stressed animals often exhibit reduced coloration. However, we recently showed that high blood corticosterone concentrations, which are part of the physiological stress response, are associated with increased redness of the belly in the common lizard (Lacerta vivipara). This result clearly contrasts with the findings of many studies of carotenoid-based coloration because corticosterone is believed to increase oxidative stress. Here, we examined whether these positive effects are influenced by differences in food availability. We tested the effect of high corticosterone levels on carotenoid-based coloration, antioxidant enzyme activity and oxidative damage in common lizards subject to low and high food availability. Food restriction abolished the carotenoid-based color enhancement when corticosterone concentrations in animals were high. We discuss how carotenoid-based color can honestly signal individual quality in this species and how the increased redness induced by corticosterone could be a terminal investment in an environment where long-term survival prospects are poor but not when immediate survival is endangered.

Effects of melatonin on in vitro development of mouse two-cell embryos cultured in HTF medium

Melatonin is capable of improving the developmental capacity of ovine, porcine and bovine embryos in vitro. However, whether melatonin possesses similar benefits to the in vitro mouse embryonic development has yet to be determined. In this study, we assessed the effects of various concentrations of melatonin (10-13 to 10-3 M) on the in-vitro development of mouse embryos cultured in HTF medium for 96 hr; embryos cultured without melatonin were used as control. The in vitro development of mouse two-cell embryos significantly benefited from treatment with melatonin in a concentration-dependent manner. The effects of melatonin on the rates of blastocyst formation, hatching/hatched blastocysts and cell number per blastocyst were bi-phasic; all significantly increased by melatonin at 10-13 to 10-5 M and decreased by melatonin at 10-3 M. Maximal benefit of melatonin on in vitro mouse 2-cell embryo development was achieved at a concentration of 10-9 M. In comparison to control, 10-9 M melatonin increased blastocyst formation rate from 48.08 +/- 5.25% to 82.08 +/- 2.34% (p < 0.05), hatched blastocyst rate from 25.65 +/- 11.79% to 66.47 +/- 4.94% (p < 0.05), and cell number per blastocyst 62.71 +/- 5.97 to 77.91 +/- 10.63 (p < 0.05). Thus, our datas demonstrated firstly that melatonin has beneficial effects on the in vitro development of 2-cell mouse embryos cultured in HTF medium.

The changing biological roles of melatonin during evolution: from an antioxidant to signals of darkness, sexual selection and fitness

Melatonin is a molecule present in a multitude of taxa and may be ubiquitous in organisms. It has been found in bacteria, unicellular eukaryotes, macroalgae, fungi, plants and animals. A primary biological function of melatonin in primitive unicellular organisms is in antioxidant defence to protect against toxic free radical damage. During evolution, melatonin has been adopted by multicellular organisms to perform many other biological functions. These functions likely include the chemical expression of darkness in vertebrates, environmental tolerance in fungi and plants, sexual signaling in birds and fish, seasonal reproductive regulation in photoperiodic mammals, and immunomodulation and anti-inflammatory activity in all vertebrates tested. Moreover, its waning production during aging may indicate senescence in terms of a bio-clock in many organisms. Conversely, high melatonin levels can serve as a signal of vitality and health. The multiple biological functions of melatonin can partially be attributed to its unconventional metabolism which is comprised of multi-enzymatic, pseudo-enzymatic and non-enzymatic pathways. As a result, several bioactive metabolites of melatonin are formed during its metabolism and some of the presumed biological functions of melatonin reported to date may, in fact, be mediated by these metabolites. The changing biological roles of melatonin seem to have evolved from its primary function as an antioxidant.

Reducing oxidative/nitrosative stress: a newly-discovered genre for melatonin

The discovery of melatonin and its derivatives as antioxidants has stimulated a very large number of studies which have, virtually uniformly, documented the ability of these molecules to detoxify harmful reactants and reduce molecular damage. These observations have clear clinical implications given that numerous age-related diseases in humans have an important free radical component. Moreover, a major theory to explain the processes of aging invokes radicals and their derivatives as causative agents. These conditions, coupled with the loss of melatonin as organisms age, suggest that some diseases and some aspects of aging may be aggravated by the diminished melatonin levels in advanced age. Another corollary of this is that the administration of melatonin, which has an uncommonly low toxicity profile, could theoretically defer the progression of some diseases and possibly forestall signs of aging. Certainly, research in the next decade will help to define the role of melatonin in age-related diseases and in determining successful aging. While increasing life span will not necessarily be a goal of these investigative efforts, improving health and the quality of life in the aged should be an aim of this research.