Oxidative stress and the effect of parasites on a carotenoid-based ornament

Oxidative stress, the physiological condition whereby the production of reactive oxygen and nitrogen species overwhelms the capacity of antioxidant defences, causes damage to key bio-molecules. It has been implicated in many diseases, and is proposed as a reliable currency in the trade-off between individual health and ornamentation. Whether oxidative stress mediates the expression of carotenoid-based signals, which are among the commonest signals of many birds, fish and reptiles, remains controversial. In the present study, we explored interactions between parasites, oxidative stress and the carotenoid-based ornamentation of red grouse Lagopus lagopus scoticus. We tested whether removing nematode parasites influenced both oxidative balance (levels of oxidative damage and circulating antioxidant defences) and carotenoid-based ornamentation. At the treatment group level, parasite purging enhanced the size and colouration of ornaments but did not significantly affect circulating carotenoids, antioxidant defences or oxidative damage. However, relative changes in these traits among individuals indicated that males with a greater number of parasites prior to treatment (parasite purging) showed a greater increase in the levels of circulating carotenoids and antioxidants, and a greater decrease in oxidative damage, than those with initially fewer parasites. At the individual level, a greater increase in carotenoid pigmentation was associated with a greater reduction in oxidative damage. Therefore, an individual's ability to express a carotenoid-based ornament appeared to be linked to its current oxidative balance and susceptibility to oxidative stress. Our experimental results suggest that oxidative stress can mediate the impact of parasites on carotenoid-based signals, and we discuss possible mechanisms linking carotenoid-based ornaments to oxidative stress.

Female ornamentation and egg carotenoids of six sympatric gobies

Belly colouration, gonad carotenoid concentration and skin transparency were quantified in gravid Gobiusculus flavescens, as well as in females of five sympatric gobies where belly ornamentation has not been described. Although G. flavescens females did, indeed, have far more colourful bellies than the other species, this could only in part be explained by a high concentration of total gonad carotenoids. Comparable, or occasionally higher, carotenoid levels were found in the gonads of other species. Instead, the unusual ornamentation of G. flavescens arises from a unique combination of carotenoid-rich gonads and a highly transparent abdominal skin.

Dietary carotenoid availability, sexual signalling and functional fertility in sticklebacks

In species where males express carotenoid-based sexual signals, more intensely coloured males may be signalling their enhanced ability to combat oxidative stress. This may include mitigating deleterious oxidative damage to their sperm, and so be directly related to their functional fertility. Using a split-clutch in vitro fertilization technique and dietary carotenoid manipulation, we demonstrate that in non-competitive fertilization assays, male three-spined sticklebacks (Gasterosteus aculeatus) that are fed higher (but biologically relevant) levels of carotenoids had a significantly increased fertilization success, irrespective of maternal carotenoid intake. Furthermore, within diet groups, a male's fertilization success was positively related to the expression of his carotenoid-based nuptial coloration, with more intensely coloured males having higher functional fertility. These data provide, to our knowledge, the first demonstration that dietary access to carotenoids influences fertilization success, and suggest that females could use a male's nuptial coloration as an indicator of his functional fertility.

Warning displays may function as honest signals of toxicity

Many prey species use colourful 'aposematic' signalling to advertise the fact that they are toxic. Some recent studies have shown that the brightness of aposematic displays correlates positively with the strength of toxicity, suggesting that aposematic displays are a form of handicap signal, the conspicuousness of which reliably indicates the level of toxicity. The theoretical consensus in the literature is, however, at odds with this finding. It is commonly assumed that the most toxic prey should have less bright advertisements because they have better chances of surviving attacks and can therefore reduce the costs incurred by signalling. Using a novel theoretical model, we show that aposematic signals can indeed function as handicaps. To generate this prediction, we make a key assumption that the expression of bright displays and the storage of anti-predator toxins compete for resources within prey individuals. One shared currency is energy. However, competition for antioxidant molecules, which serve dual roles as pigments and in protecting prey against oxidative stress when they accumulate toxins, provides a specific candidate resource that could explain signal honesty. Thus, contrary to the prevailing theoretical orthodoxy, warning displays may in fact be honest signals of the level of (rather than simply the existence of) toxicity.

Honest sexual signalling mediated by parasite and testosterone effects on oxidative balance

Extravagant ornaments evolved to advertise their bearers' quality, the honesty of the signal being ensured by the cost paid to produce or maintain it. The oxidation handicap hypothesis (OHH) proposes that a main cost of testosterone-dependent ornamentation is oxidative stress, a condition whereby the production of reactive oxygen and nitrogen species (ROS/RNS) overwhelms the capacity of antioxidant defences. ROS/RNS are unstable, very reactive by-products of normal metabolic processes that can cause extensive damage to key biomolecules (cellular proteins, lipids and DNA). Oxidative stress has been implicated in the aetiology of many diseases and could link ornamentation and genetic variation in fitness-related traits. We tested the OHH in a free-living bird, the red grouse. We show that elevated testosterone enhanced ornamentation and increased circulating antioxidant levels, but caused oxidative damage. Males with smaller ornaments suffered more oxidative damage than those with larger ornaments when forced to increase testosterone levels, consistent with a handicap mechanism. Parasites depleted antioxidant defences, caused oxidative damage and reduced ornament expression. Oxidative damage extent and the ability of males to increase antioxidant defences also explained the impacts of testosterone and parasites on ornamentation within treatment groups. Because oxidative stress is intimately linked to immune function, parasite resistance and fitness, it provides a reliable currency in the trade-off between individual health and ornamentation. The costs induced by oxidative stress can apply to a wide range of signals, which are testosterone-dependent or coloured by pigments with antioxidant properties.

Strength and cost of an induced immune response are associated with a heritable melanin-based colour trait in female tawny owls

1. Melanin pigments provide the most widespread source of coloration in vertebrates, but the adaptive function of such traits remains poorly known. 2. In a wild population of tawny owls (Strix aluco), we investigated the relationships between plumage coloration, which varies continuously from dark to pale reddish, and the strength and cost of an induced immune response. 3. The degree of reddishness in tawny owl feather colour was positively correlated with the concentration of phaeomelanin and eumelanin pigments, and plumage coloration was highly heritable (h(2) = 0.93). No carotenoids were detected in the feathers. 4. In mothers, the degree of melanin-based coloration was associated with antibody production against a vaccine, with dark reddish females maintaining a stronger level of antibody for a longer period of time compared to pale reddish females, but at a cost in terms of greater loss of body mass. 5. A cross-fostering experiment showed that, independent of maternal coloration, foster chicks reared by vaccinated mothers were lighter than those reared by nonvaccinated mothers. Hence, even though dark reddish mothers suffered a stronger immune cost than pale reddish mothers, this asymmetric cost was not translated to offspring growth. 6. Our study suggests that different heritable melanin-based colorations are associated with alternative strategies to resist parasite attacks, with dark reddish individuals investing more resources towards the humoral immune response than lightly reddish conspecifics.

Carotenoids, oxidative stress and female mating preference for longer lived males

Some of the most spectacular exaggerated sexual ornaments are carotenoid dependent. It has been suggested that such ornaments have evolved because carotenoid pigments are limiting for both signal expression and in their role as antioxidants and immunostimulants. An implicit assumption of this hypothesis is that males which can afford to produce more elaborate carotenoid-dependent displays are signalling their enhanced ability to resist parasites, disease or oxidative stress and hence would be predicted to live longer. Therefore, in species with carotenoid-dependent ornaments where a parent's future longevity is crucial for determining offspring survival, there should be a mating preference for partners that present the lowest risk of mortality during the breeding attempt, as signalled by the ability to allocate carotenoids to sexual displays. In an experimental study using three-spined sticklebacks (Gasterosteus aculeatus), we show that when dietary carotenoid intake is limited, males attempt to maintain their sexual ornament at the expense of body carotenoids and hence suffer from reduced reproductive investment and a shorter lifespan. These males also suffer from an increased susceptibility to oxidative stress, suggesting that this may constitute the mechanism underlying the increased rate of ageing. Furthermore, in pairwise mate-choice trials, females preferred males that had a greater access to carotenoids and chance of surviving the breeding season, suggesting that females can make adaptive mate choice decisions based on a male's carotenoid status and potential future longevity.

Availability of non-carotenoid antioxidants affects the expression of a carotenoid-based sexual ornament

Carotenoids are responsible for much of the yellow, orange and red pigmentation in the animal kingdom, and the importance of such coloration as an honest signal of individual quality has received widespread attention. In particular, owing to the multiple roles of carotenoids as pigments, antioxidants and immunostimulants, carotenoid-based coloration has been suggested to advertise an individual's antioxidant or immune defence capacity. However, it has recently been argued that carotenoid-based signals may in fact be advertising the availability of different antioxidants, many of which (including various vitamins, antioxidant enzymes and minerals) are colourless and so would be uninformative as components of a visual signal, yet often have greater biological activity than carotenoids. We tested this hypothesis by feeding male sticklebacks (Gasterosteus aculeatus) a diet containing a fixed level of carotenoids and either low or high, but biologically realistic levels of the colourless antioxidant vitamins C and E. High-antioxidant diet males produced significantly more intensely coloured (but not larger) carotenoid-based regions of nuptial coloration and were preferred over size-matched males of the opposite diet treatment in mate-choice trials. Furthermore, there were positive correlations between an individual's somatic antioxidant activity and signal intensity. Our data suggest that carotenoid-based ornaments may honestly signal an individual's availability of non-carotenoid antioxidants, allowing females to make adaptive mate-choice decisions.

Multiple pathways of maternal effects in black-headed gull eggs: constraint and adaptive compensatory adjustment

We investigated in the black-headed gull whether female deposition of antioxidants and immunoglobulins (enhancing early immune function), and testosterone (suppressing immune function and increasing early competitive skills) correlate suggesting that evolution has favoured the mutual adjustment of different pathways for maternal effects. We also took egg mass, the position of the egg in the laying sequence and offspring sex into account, as these affect offspring survival. Yolk antioxidant and immunoglobulin concentrations decreased across the laying order, while yolk testosterone concentrations increased. This may substantially handicap the immune defence of last-hatched chicks. The decrease in antioxidant levels was greater when mothers had a low body mass and when the increase in testosterone concentrations was relatively large. This suggests that female black-headed gulls are constrained in the deposition of antioxidants in last-laid eggs and compensate for this by enhanced testosterone deposition. The latter may be adaptive since it re-allocates the chick's investment from costly immune function to growth and competitive skills, necessary to overcome the consequences of hatching late from an egg of reduced quality.

Carotenoids and life-history evolution in animals

Animals must allocate finite resources amongst competing demands. A suite of such trade-offs is thought to occur in the deployment of carotenoids, being widely responsible for sexual coloration and also important in antioxidant and immune defences. Experimental manipulation of dietary carotenoid availability is a useful approach for elucidating the mechanistic bases of carotenoid allocation trade-offs. Recent work using birds has shown that both sexual display and immune defences can be limited by carotenoid availability, providing support for the hypothesis that males allocating greater amounts of carotenoids to sexual coloration are advertising their superior health. Carotenoid availability has also been shown to limit egg-laying capacity in birds, although it remains to be seen whether carotenoid display in females advertises reproductive potential. More experiments are required to ascertain the importance of direct (material) and indirect (genetic) benefits accruing through choosing to mate with individuals that have greater carotenoid display.

Egg-laying capacity is limited by carotenoid pigment availability in wild gulls Larus fuscus

In birds, experimentally increased egg production can reduce maternal condition, parenting ability and survival, and the quality of the eggs themselves. Such costs probably reflect resource limitation, but the identity of the resource(s) in question remains unclear. Carotenoids are antioxidants and immunomodulants that birds can only obtain in their diet. Trade-offs in the allocation of limiting carotenoids between somatic maintenance and egg production could therefore be an important factor underlying reproductive costs. We show that in wild lesser black-backed gulls, Larus fuscus, dietary carotenoid availability (i) constrained the capacity to re-lay following clutch removal; and (ii) affected the relationship between yolk mass and egg mass. However, whether carotenoids are limiting for egg production directly, by stimulating the synthesis or antioxidant protection of yolk precursors, or indirectly via effects on maternal health, requires further study.

Neonatal nutrition, adult antioxidant defences and sexual attractiveness in the zebra finch

Early nutrition has recently been shown to have pervasive, downstream effects on adult life-history parameters including lifespan, but the underlying mechanisms remain poorly understood. Damage to biomolecules caused by oxidants, such as free radicals generated during metabolic processes, is widely recognized as a key contributor to somatic degeneration and the rate of ageing. Lipophilic antioxidants (carotenoids, vitamins A and E) are an important component of vertebrate defences against such damage. By using an avian model, we show here that independent of later nutrition, individuals experiencing a short period of low-quality nutrition during the nestling period had a twofold reduction in plasma levels of these antioxidants at adulthood. We found no effects on adult external morphology or sexual attractiveness: in mate-choice trials females did not discriminate between adult males that had received standard- or lower-quality diet as neonates. Our results suggest low-quality neonatal nutrition resulted in a long-term impairment in the capacity to assimilate dietary antioxidants, thereby setting up a need to trade off the requirement for antioxidant activity against the need to maintain morphological development and sexual attractiveness. Such state-dependent trade-offs could underpin the link between early nutrition and senescence.

Carotenoid modulation of immune function and sexual attractiveness in zebra finches

One hypothesis for why females in many animal species frequently prefer to mate with the most elaborately ornamented males predicts that availability of carotenoid pigments is a potentially limiting factor for both ornament expression and immune function. An implicit assumption of this hypothesis is that males that can afford to produce more elaborate carotenoid-dependent displays must be healthier individuals with superior immunocompetence. However, whether variation in circulating carotenoid levels causes variation in both immune function and sexual attractiveness has not been determined in any species. In this study, we show that manipulation of dietary carotenoid supply invokes parallel changes in cell-mediated immune function and sexual attractiveness in male zebra finches (Taeniopygia guttata).

Carotenoids and egg quality in the lesser blackbacked gull Larus fuscus: a supplemental feeding study of maternal effects

Egg quality is a phenotype of, and can profoundly influence fitness in, both mother and offspring. However, the physiological mechanisms that underlie this maternal effect are poorly understood. Carotenoids are hypothesized to enhance antioxidant activity and immune function, and are responsible for the pigmentation of egg yolk. The proximate basis and consequences of this maternal investment, however, have not previously been studied in wild birds. In this supplemental feeding study of lesser black-backed gulls, Larus fuscus, carotenoid-fed females are shown to have increased integument pigmentation, higher plasma concentrations of carotenoids and antioxidant activity, and lower plasma concentrations of immunoglobulins (Igs) in comparison with controls. In turn, carotenoid-fed females produced eggs containing high carotenoid but low Ig concentrations (i.e. passive immunity), whereas control females produced eggs containing low carotenoid but high Ig concentrations. Within-clutch patterns of these resources varied over the laying sequence in a similar manner in both carotenoid-fed and control nests. Our results suggest that carotenoids could be one resource responsible for egg quality maternal effects in birds. We discuss the possible implications of carotenoid-mediated effects on phenotype for fitness in mothers and their offspring.

Carotenoid discrimination by the avian embryo: a lesson from wild birds

The concentrations (microg/g wet yolk) of total carotenoids in eggs of the common moorhen (Gallinula chloropus), American coot (Fulica americana) and lesser black-backed gull (Larus fuscus), collected in the wild, were 47.5, 131.0 and 71.6, respectively. In contrast to data for eggs of the domestic chicken, beta-carotene was a significant component in the yolks of these three wild species, forming 25-29% by wt. of the total carotenoids present. The concentration of total carotenoids in the livers of the newly-hatched chicks was 5-10 times higher than in the other tissues and beta-carotene was again a major component, forming 37-58% of the hepatic carotenoids. In the newly-hatched gull, the proportions of both lutein and zeaxanthin were very low in the liver but high in the heart and muscle when compared with the yolk. By contrast canthaxanthin, echinenone and beta-carotene were very minor constituents of heart and muscle when compared with their proportions in the yolk of the gull. The proportions of lutein and zeaxanthin in the liver of the newly-hatched coot and moorhen were also far lower than in the yolk whereas the liver was relatively enriched with beta-cryptoxanthin, beta-carotene and (in the moorhen) echinenone. The results indicate that avian embryos discriminate between different carotenoids during their distribution from the yolk to the various tissues.