Androgens In Eggs Research Articles

Prenatal manipulation of yolk androgen levels affects egg size but not egg colour in a songbird

Maternal allocation of androgens to the egg yolk allows mothers to adaptively manipulate offspring phenotype. Increases of egg androgen levels have often been shown to induce sex-specific effects. Some previous studies suggest that females specifically may suffer a fitness reduction after early exposure to high androgen levels. In this study, we explored whether female birds that developed exposed to high yolk androgen concentrations would reduce their maternal investment later in life. We explored two different aspects of female reproduction in spotless starlings (Sturnus unicolor): egg coloration (a post mating sexual signal in this species) and egg size. We manipulated androgen yolk levels in a large sample of clutches of this species. We monitored the reproduction of the females that hatched from these eggs, measuring the size and colour of the eggs they laid throughout their life (from 1 to 7 years of age). We found no overall difference in egg colour in relation to treatment, clutch number or age. However, females exposed to high egg androgen levels showed a steeper decrease in egg size along the laying order than controls. This pattern likely results in a more unbalanced distribution of resources within the clutch, possibly favouring brood size reductions in experimental females. In addition, control and experimental females differed in how egg volume changed with age. These results show that some egg characteristics may be affected by the female’s exposure to yolk androgens during her own embryonic development. Our research calls for further long-term research on the influence of prenatal androgens on the fitness mechanisms regulating reproductive investment and its potential signalling role in a perspective of sexual selection. In birds, mothers prepare the offspring to the environment they will encounter at hatching by depositing a mixture of compounds into the eggs. Androgens specifically exert a variety of effects during embryo development and chick growth, but some of these may be negative in adult life. In particular, early androgen may be detrimental for females in the long term, decreasing maternal investment and fitness. Using the spotless starling as study species, we tested whether an experimental increase in yolk androgens negatively affects breeding investment, measured by egg colour (a sexual signal) and egg size. We found no effects of the treatment on egg colour. However, females exposed to high androgen levels during their development reduced egg size along the laying order more strongly than controls. This may affect nestling survival, and indicates that prenatal yolk androgens may influence some egg characteristics with known relevance for offspring viability.

Egg Testosterone Differentially Affects Telomere Length in Somatic Tissues of Yellow-Legged Gull Embryos.

Maternal decisions on egg composition have major consequences for offspring. Maternal egg androgens have diverse, often contrasting, effects depending on offspring trait and life stage, suggesting that mothers face trade-offs in egg hormone transfer. However, the effect of egg androgens on embryonic telomere length, which is a major trait potentially affecting performance, has been never investigated. We administered a physiological dose of testosterone (T) to yellow-legged gull (Larus michahellis) eggs and found that, compared to controls, telomere length shortly before hatching was reduced in the liver but unaffected in the brain, heart, and pectoralis muscle. Telomere length varied across somatic tissues, and, independent of egg treatment, it was not correlated between them, suggesting independent telomere dynamics. Thus, we showed for the first time that increased egg T can increase telomere shortening in the embryo and that maternal T allocation strategies may evolve also in response to such effect. Moreover, contrary to observations in adult birds, at the embryonic stage telomere length in one somatic tissue may not reflect telomere length in other body districts.

Avian yolk androgens are metabolized rather than taken up by the embryo during the first days of incubation.

Several studies show effects of yolk androgens in avian eggs on the phenotype of the offspring. Yolk hormone concentrations decline strongly within the first few days of incubation. Although early embryonic uptake of yolk androgens is suggested by the presence of radioactivity in the embryo when eggs are injected with radiolabelled androgens, these studies do not verify the chemical identity of radioactive compound(s), although it is known that these androgens can be metabolized substantially. By using stable isotope-labelled testosterone and androstenedione in combination with mass spectrometry, enabling verification of the exact molecular identity of labelled compounds in the embryo, we found that after 5 days of incubation the androgens were not taken up by the embryo. However, their concentrations in the entire yolk albumen homogenates declined strongly, even when corrected for dilution by albumen and water. Our results indicate metabolism of maternal androgens, very likely to 5β-androstane-3α,17β-diol, etiocholanolone and their conjugated forms. The results imply that the effects of increased exposure of the embryo to maternal androgens take place either before this early conversion or are mediated by these metabolites with an as yet unknown function, opening new avenues for understanding hormone-mediated maternal effects in vertebrates.

Maternal egg hormones in the mating context: The effect of pair personality

AbstractAnimal personality traits emerge developmentally from the interaction of genetic and early environmental factors. Maternal hormones, such as androgens (testosterone, T and androstenedione, A4), transferred to embryos and egg yolks may simultaneously organize multiple behavioural and physiological traits. Although previous studies demonstrated an association between the mother's personality and yolk androgen levels, the independent effects of the male partner's personality and pair combination remains unknown.We test this association using an ecological model species for personality research, the great tit (Parus major) using multiple approaches: (1) a wild population, (2) a randomly mated captive population and (3) an experimental study with (dis)assortatively mated pairs from lines selected for fast exploration/boldness or slow exploration/shyness.Egg androgen concentrations were associated with variation in female personality traits, and the experimental data suggested that this is independent of male personality: Experimental females from the slow‐shy line tended to have higher egg T concentrations than females from the fast‐bold line, with no effect of male personality. Shy females from the wild population had higher egg A4 concentration than bold females. However, in the correlative data yolk hormones were linked with male personality, as well as the interaction between female and male traits: Male handling responsiveness correlated negatively with egg A4 concentration in wild birds. In randomly mated birds, pairs that were mated assortatively for personality had lower egg T concentrations than disassortatively mated pairs.Given that egg androgens are known mediators of avian personality, our results suggest that maternal hormones might contribute to the heritability of personality, may be sensitive to the social context of mating, and act as key drivers of individual differences.Aplain language summaryis available for this article.

Maternal androgens in avian brood parasites and their hosts: Responses to parasitism and competition?

In the coevolutionary dynamic of avian brood parasites and their hosts, maternal (or transgenerational) effects have rarely been investigated. We examined the potential role of elevated yolk testosterone in eggs of the principal brood parasite in North America, the brown-headed cowbird, and three of its frequent host species. Elevated maternal androgens in eggs are a common maternal effect observed in many avian species when breeding conditions are unfavorable. These steroids accelerate embryo development, shorten incubation period, increase nestling growth rate, and enhance begging vigor, all traits that can increase the survival of offspring. We hypothesized that elevated maternal androgens in host eggs are a defense against brood parasitism. Our second hypothesis was that elevated maternal androgens in cowbird eggs are a defense against intra-specific competition. For host species, we found that elevated yolk testosterone was correlated with parasitized nests of small species, those whose nest success is most reduced by cowbird parasitism. For cowbirds, we found that elevated yolk testosterone was correlated with eggs in multiply-parasitized nests, which indicate intra-specific competition for nests due to high cowbird density. We propose experimental work to further examine the use of maternal effects by cowbirds and their hosts.

Maternal transfer of androgens in eggs is affected by food supplementation but not by predation risk

Mothers may affect the future success of their offspring by varying allocation to eggs and embryos. Allocation may be adaptive based on the environmental conditions perceived during early breeding. We investigated the effects of food supplementation and predation risk on yolk hormone transfer in the pied flycatcher Ficedula hypoleuca. In a food supplementation experiment, females were food‐supplemented prior to and during egg‐laying and androgen concentrations were measured throughout the laying order. Predation risk was investigated in three different studies combining both correlative data, where flycatchers bred in close proximity to two different predator species that prey upon adult flycatchers (either Tengmalm's owl Aegolius funereus or pygmy owl Glaucidium passerinum), and an experimental manipulation, where flycatchers were exposed to cues of a nest predator (least weasel Mustela nivalis). Females receiving food supplementation laid eggs with lower concentrations of androstenedione (A4) than females not receiving food supplements. Yolk testosterone (T) concentration showed the same pattern but the difference was not statically significant. Testosterone (but not A4) concentration increased within clutches, from the first to the last egg, independently of the food supplementation. Females breeding under high predation risk did not differ from control females in their yolk androgen levels (A4, T or progesterone). However, concentrations of A4 tended to be lower in the proximity of pygmy owls, which could indirectly increase offspring survival after fledging. Food supplementation during egg‐laying seems to have a stronger impact on maternal transfer of androgens than predation risk. Food availability and predation risk could differentially affect the trade‐offs of androgen allocation for the offspring when raised in good vs. dangerous environments.

Individual and population-level sex-dependent lateralization in yellow-legged gull (Larus michahellis) chicks

Behavioral lateralization at the population level is widespread across vertebrates, with considerable variation among species. However, evidence for individual-level and sex-dependent lateralization is sparse and inconsistent in fish, reptiles and birds. In addition, covariation of lateralization with position in the laying sequence, which is expected because the concentration of maternal egg hormones varies with laying order, has never been investigated. We analyzed lateralization of yellow-legged gull (Larus michahellis) chicks in reverting from supine to prone posture (‘RTP’ response) and in pecking at a dummy parental bill to solicit food provisioning (‘begging’ response). Chicks were lateralized both at the population and at the individual level in the RTP response and at the individual level in begging. Lateralization in the RTP was sex-dependent, as females showed a leftward preference. Lateralization in either motor task was not correlated within individuals. Lateralization did not differ among families, suggesting little additive genetic variation. Lateral preference in begging response varied according to laying order and matched variation in egg androgens concentration. Our study confirms previous findings on population-level lateralization and adds to the scant information on individual-level and heritable variation in lateralization in birds. Moreover, it hints at epigenetic components in lateralization depending on maternal effects.

Maternal dietary carotenoids mitigate detrimental effects of maternal GnRH on offspring immune function in Japanese quail Coturnix japonica

Maternal resources deposited in eggs can aff ect the development of several off spring phenotypic traits and result in tradeoff s among them. For example, maternal androgens in eggs may be benefi cial to off spring growth and competitive ability, but detrimental to immunocompetence and oxidative stress. In contrast, maternal antioxidants in eggs may be benefi cial if they mitigate oxidative stress and immunosuppressive eff ects of androgens. We investigated possible interactive eff ects of maternal steroids and carotenoids on aspects of off spring physiology and phenotype, by simultaneously manipulating levels of androgens (via gonadotropin-releasing hormone, GnRH-challenges) and carotenoids (via diet supplementation) in captive female Japanese quail Coturnix japonica during egg laying. Carotenoid supplementation of hens, which elevates yolk concentrations of carotenoid and vitamins A and E, enhanced egg hatching success, off spring survival to age 15 d, and size of the bursa of Fabricius in off spring. In contrast, repeated maternal GnRH challenges, which elevated yolk testosterone concentrations, enhanced off spring neonatal size, but negatively aff ected bursa size. However, interaction among the treatments suggests that the positive eff ect of maternal carotenoid supplementation on plasma bactericidal capacity was mediated by maternal GnRH challenges. Chicks originating from carotenoid-supplemented hens were less immunosuppressed than those originating from carotenoid-supplemented GnRH-challenged hens, which were less immunosuppressed than chicks from GnRH-challenged females not supplemented with carotenoids. Females availability of carotenoid enriched diets allows them to enhance the development of off spring immune system via carotenoids and vitamins deposited in egg yolks and off set detrimental eff ects of androgens deposited by GnRHchallenged females.

Prenatal exposure to testosterone impairs oxidative damage repair efficiency in the domestic chicken ( Gallus gallus )

Elevated levels of maternal androgens in avian eggs affect numerous traits, including oxidative stress. However, current studies disagree as to whether prenatal androgen exposure enhances or ameliorates oxidative stress. Here, we tested how prenatal testosterone exposure affects oxidative stress in female domestic chickens (Gallus gallus) during the known oxidative challenge of an acute stressor. Prior to incubation, eggs were either injected with an oil vehicle or 5 ng testosterone. At either 17 or 18 days post-hatch, several oxidative stress markers were assessed from blood taken before and after a 20 min acute stressor, as well as following a 25 min recovery from the stressor. We found that, regardless of yolk treatment, during both stress and recovery all individuals were in a state of oxidative stress, with elevated levels of oxidative damage markers accompanied by a reduced total antioxidant capacity. In addition, testosterone-exposed individuals exhibited poorer DNA damage repair efficiencies in comparison with control individuals. Our work suggests that while yolk androgens do not alter oxidative stress directly, they may impair mechanisms of oxidative damage repair.

Differential effects of yolk testosterone and androstenedione in embryo development and nestling growth in the spotless starling (Sturnus unicolor)

Yolk androgens in avian eggs play a significant role in embryo and nestling development. However, few studies have examined the differential effect of two of the main yolk androgens, testosterone (T) and androstenedione (A4). Here, we injected eggs of spotless starlings with physiological levels of either T, A4, the combination T+A4 or vehicle substance (control), to examine the differential ability of these steroids to influence nestling development. We found that the duration of the embryonic period was increased by T, and less so by A4, but not by the combination T+A4. Body condition was reduced in all experimental treatments where A4 was present, particularly so in the combination T+A4. Tarsus length was increased in males by A4, and in a lower degree by T, whereas the combination T+A4 inhibited growth. However, these differences in tarsus length between groups disappeared at the end of the nestling period. Cell-mediated immune responsiveness was marginally affected by the interaction between treatment and sex. These patterns suggest that in this species, T has a stronger influence during embryo development than A4, whereas during nestling development the capacities of both androgens to influence growth are similar. The combination T+A4 showed non-additive effects, suggesting either some kind of inhibition between the two androgens, or else an excessive effect due to a bell-shaped pattern of response. Our results suggest a complex picture of sex and age-dependent effects of T and A4, and underline the necessity of further research in the metabolism and action of egg androgens.

Sex‐specific effects of yolk androgens on begging behavior and digestion in pied flycatchers

The hormonal environment during early development, such as maternally derived androgens in bird eggs, has been shown to shape offspring in ways that may have important short‐ and long‐term consequences for phenotype and behavior. However, studies on the effects of androgens on begging behavior and growth in birds have provided variable results and sex‐specific effects are not well understood. We experimentally elevated yolk androgen levels in whole clutches in a passerine bird, the pied flycatcher Ficedula hypoleuca and studied effects on male and female begging behavior, ingestion and digestion. Yolk androgen elevation led to shorter latencies to beg in both male and female nestlings but to greater begging rates only in males. Both responses could be used as cues to solicit more food from parents. Yolk androgens had no effect on food intake, fecal wet mass or fecal water content. However, the proportion of organic matter in feces was lower in androgen‐treated than control males, potentially suggesting that the androgen‐treated individuals had more efficient digestion. We discuss the alternative pathways of effects of yolk androgens on begging, growth and digestion. If the effects of yolk androgens on phenotype and behavior are causally linked, there is less scope for the mother to adjust specific offspring traits independently from other traits. The sex‐specific effects of yolk androgens should be studied more, also in within‐clutch context, as the costs and benefits for each sex may further differ in relation to hatching order.

Long-lasting effects of yolk androgens on phenotype in the pied flycatcher (Ficedula hypoleuca)

The hormonal environment during early development, such as maternally derived androgens in bird eggs, shapes the development of the offspring in ways that may have important long-term consequences for phenotype and behavior and, ultimately, fitness. We studied the long-term effects of yolk androgens on several phenotypic and physiological traits in male and female pied flycatchers (Ficedula hypoleuca) by experimentally elevating yolk androgen levels and rearing birds in common-garden environment in captivity. We found that high yolk androgen levels increased the basal metabolic rates in both females and males in adulthood. High yolk androgen levels did not affect male melanin coloration or plumage ornaments, or timing or speed of moult in either sex. No effect of androgen treatment on cell-mediated or humoral immune response was found in either sex. Covariation among the measured phenotypic traits was further not altered by androgen treatment. Our results suggest that exposure to high androgen levels can have long-lasting effects on some offspring traits, but do not seem to lead to different phenotypes. Furthermore, the role of yolk androgens affecting sexually selected male traits in our study species seems to be minor. The fitness consequences of yolk androgen-induced higher metabolic rates remain to be studied.

Long-term effects of yolk androgens on phenotype and parental feeding behavior in a wild passerine

Early growth conditions, such as exposure to maternally derived androgens in bird eggs, have been shown to shape offspring in ways that may have important long-term consequences for phenotype and behavior. Using an experimental approach, we studied the long-term effects of yolk androgens on several phenotypic traits and parental behavior in adult and female collared flycatchers (Ficedula albicollis). We elevated yolk androgen levels and monitored the experimental recruits the following breeding seasons. Androgen treatment had a sex-dependent effect on adult body condition, yolk androgen-treated males being heavier than control males when controlling for size, a result which may be caused potentially by selective mortality, physiological differences, or different life-history strategies. Androgen treatment did not however affect the expression of sexually selected plumage ornaments (forehead and wing patch size), UV coloration, or parental feeding rate in either sex. Our results suggest that yolk androgens are unlikely to affect sexual selection via plumage characteristics or contribute to breeding success via altered parental care. Yolk androgens do not seem to act as a means for female collared flycatchers to enhance the attractiveness of their sons. The lower return rate previously observed for androgen-treated male offspring compared to controls may therefore not be due to lower mating or breeding success, but may rather reflect lower survival or higher dispersal propensity of yolk androgen-treated males.

Relationships between yolk androgens and nest density, laying date, and laying order in Western Burrowing Owls (Athene cunicularia hypugaea)

Increases in yolk androgens within and among avian clutches have been correlated with decreased incubation time, increased aggression within a nest, increased begging behaviour, decreased immune response, and decreased life span. Although the mechanisms that lead to variability in yolk androgens within and between clutches are not completely known, yolk androgens can be a function of both social and environmental conditions. We were interested in if and how nesting density, laying date, and laying order influenced yolk androgens in Western Burrowing Owls ( Athene cunicularia hypugaea (Bonaparte, 1825)) in which nest density varies considerably. In 2006 and 2007, we used radioimmunoassay to quantify the concentrations of testosterone, 5α-dihydrotestosterone, and androstenedione in the egg yolks from one early and one late-laid egg in 47 nests of Burrowing Owls located in the Morley Nelson Snake River Birds of Prey National Conservation Area in southern Idaho. Nesting density had no detectable effect on yolk androgens. Yolk androgens varied temporally and peaked in the middle of the laying season while being low before and after this time period. Within nests, late-laid eggs had higher testosterone and dihydrotestosterone than early-laid eggs; adrostendione exhibited a similar pattern in one but not both years of our study. It is possible that the seasonal pattern in yolk androgens that we observed is related to aspects of mate quality for females or declining chances of fledging success for later nesting females, whereas rises in egg androgens between early and late eggs within clutches could reflect a mechanism to assist nestlings from late-laid eggs that hatch one to several days after their siblings to better compete for resources within the nest or promote survival in the presence of larger siblings.

Yolk androgen deposition in rockhopper penguins, a species with reversed hatching asynchrony

To maximize fitness, females should invest optimally in the siblings within a litter or brood and adapt this investment to environmental conditions. Chick mass and yolk androgens have been shown to influence the outcome of sibling competition. In birds, asynchronous hatching plays a major role in this process and often leads to brood reduction. We studied maternal deposition of yolk androgens in eggs of southern rockhopper penguins (Eudyptes chrysocome chrysocome). Contrary to other avian models, laying and hatching sequences do not coincide in this species, which exhibits reversed hatching asynchrony. This provides a unique model to test whether the first egg to hatch (B-egg), which is the most likely to survive, differs in composition from the second egg to hatch (A-egg). We found that B-eggs had higher egg masses, yolk masses, yolk androgen concentrations and total yolk androgen amounts than A-eggs. This was observed consistently for the three androgens analyzed (testosterone, androstenedione and 5α-dihydrotestosterone). Laying date affected androgen deposition into A- and B-eggs differently. Interestingly, late clutches had proportionally higher androgen levels in the B-egg compared to the A-egg than early clutches. We discuss these results in relation to the chronology of egg formation and the potential effect of the observed differences on embryo development and brood reduction.

Yolk hormones and sexual conflict over parental investment in the pied flycatcher

Female birds might be able to manipulate the parental effort of their male partner through elevated transfer of hormones to the eggs, since these hormones affect many chick traits that males might use as cues for adjusting the level of their investment. We experimentally studied whether female pied flycatchers Ficedula hypoleuca could manipulate male investment via yolk androgens. There is much more variation in yolk androgen levels between females than within clutches, and in order to change the androgen levels of the eggs, we swapped whole clutches between nests. To estimate the androgen levels of the clutch, we measured the androgen content of a single egg per clutch. Females did not succeed in manipulating male effort using yolk androgens, since there was no relationship between the division of parental care within a pair and either original or foster egg androgen levels. One of these relationships should have occurred if females were manipulating males. The proportion of feeding visits by the male was higher when the male was old (55%) than when he was young (45%) and females laid eggs with higher androgen levels when mated with a young male. Young males did not exhibit any responses to yolk androgen levels either, which indicates that females cannot exploit their effort more than that of old males. We suggest that females may allocate yolk androgens to adjust the growth trajectories of the chicks to poor growing conditions when mated with young males that are poor providers or occupying a poor territory.

Maternal yolk testosterone in canary eggs: toward a better understanding of mechanisms and function

Maternal yolk androgens in avian eggs have been shown to affect numerous offspring traits. These changes in offspring phenotype represent examples of maternal effects and are thought to adjust offspring development to the posthatching environment. When studying the functional consequences of yolk hormones it is, therefore, crucial to manipulate the yolk androgen concentrations as well as the environmental conditions under which the study is performed. However, so far context-dependent effects of maternal yolk hormones have not sufficiently been taken into account, which might contribute to the current level of inconsistency in yolk androgen-mediated effects. We experimentally elevated the yolk testosterone concentrations and manipulated the sibling size hierarchy. We focused on the effects of yolk testosterone on growth and monitored begging behavior and parental feeding preferences in search of the underlying mechanisms of changes in growth. Experimental changes in the yolk testosterone concentrations significantly affected offspring phenotype. However, elevated yolk testosterone concentrations only improved the growth of chicks that were at a competitive disadvantage, whereas it benefited all chicks when placed in staged competition for food as juvenile. This emphasizes the compensatory role of yolk androgens in the context of hatching asynchrony and its context dependency. Enhanced growth did not coincide with intensified begging. Neither males nor females preferentially fed chicks hatching from eggs with elevated yolk testosterone concentrations. Enhanced growth rather resulted from yolk testosterone induced changes in physiology, of which the detailed mechanisms are yet unknown.

Yolk hormones have sex-specific long-term effects on behavior in the pied flycatcher ( Ficedula hypoleuca)

The hormonal environment during early development, such as maternally derived androgens in bird eggs, shapes the development and phenotype of the offspring in ways that may have important long-term consequences for behaviour. We studied the effects of yolk androgens on multiple behavioural traits in female and male pied flycatchers ( Ficedula hypoleuca) by experimentally elevating androgen levels (testosterone and androstenedione) in the eggs. The birds were housed in a common-garden environment in captivity until full independence, after which their behaviour was tested. We found that androgen-treated males were more likely than control males to explore a novel environment and showed higher activity in the presence of a novel object. In response to a simulated predator attack, androgen-treated males mainly showed freezing behaviour, while control males showed escape behaviour. Females from the androgen treatment and control group showed no differences in these behaviours. Androgen treatment did not affect neophobia (latency to approach the novel object) or dominance behaviour in either sex. Behaviour in the novel environment and towards a novel object was repeatable, but behaviours in the different experiments were mostly not inter-correlated. These results indicate that yolk androgens have various long-lasting effects on behaviour, especially in males, but that they do not induce a distinct behavioural syndrome. As behaviour is strongly linked with fitness, our results suggest that yolk androgens may play a role in determining fitness, and thus play a potentially adaptive role.

Long-lasting consequences of elevated yolk testosterone levels on female reproduction

Maternal yolk androgens in bird eggs represent an important pathway along which offspring phenotype is shaped. Most of the hormone-mediated maternal effects are highly important in the context of sibling competition. However, there is also increasing evidence for long-lasting effects far beyond the nestling period, and these effects may have important consequences on the reproductive success of the offspring. Here, we investigated the effects of experimentally elevated yolk testosterone concentrations on growth and reproduction in female canaries. Elevated yolk testosterone concentrations enhanced the post-natal growth rate, but not the asymptotic mass, and reduced the survival probability. The latter may be a consequence of the higher growth rate, which may have rendered females hatching from testosterone-treated eggs (T-females) more vulnerable to harsh environmental conditions. Adult T-females made a larger investment in their clutch by laying more but not heavier eggs than females hatching from control-treated eggs. Our results suggest that the observed long-lasting effect on clutch size relates to changes in the growth trajectory rather than being a direct consequence of testosterone, since studies manipulating early growth conditions obtained similar results. Clearly, further studies are now required in order to investigate the intriguing relationship between yolk testosterone, elevated growth rates, and clutch size.

Sex-specific effects of prenatal testosterone on nestling plasma antioxidant capacity in the zebra finch

Trans-generational transfer of non-genetic, maternal resources such as hormones can have a substantial influence on offspring phenotype in many vertebrate species. In birds, maternal androgens enhance both growth and competitive behaviour, but also suppress the immune system. It has been hypothesised that high levels of egg androgens could also influence the prooxidant-antioxidant balance through their positive effect on growth and metabolism. We tested this hypothesis in the zebra finch (Taeniopygia guttata). Eggs were injected with testosterone dissolved in sesame oil or sesame oil only (control). We subsequently assessed the effect of the egg hormone manipulation on nestling growth and nestling plasma total antioxidant capacity (TAC). Growth rates of zebra finch nestlings were not significantly affected by egg hormone treatment. However, male offspring hatched from eggs with experimentally elevated testosterone had reduced plasma TAC at 10 days of age compared with male offspring hatching from control eggs. At the age of 34 days, males had similar plasma TAC irrespective of egg treatment. No effects of egg testosterone manipulation on nestling plasma TAC were found in females. Our results demonstrate that embryonic exposure to elevated levels of testosterone modulates chick antioxidant status, but this seems to be independent of chick growth. Sex-specific effects of prenatal testosterone on plasma TAC of zebra finch nestlings may have important consequences for sex allocation.