Prenatal androgen exposure modulates cellular and humoral immune function of black-headed gull chicks

Avian eggs contain considerable amounts of maternal yolk androgens, which have been shown to beneficially influence the physiology and behaviour of the chick. As androgens may suppress immune functions, they may also entail costs for the chick. This is particularly relevant for colonial species, such as the black-headed gull (Larus ridibundus), in which the aggregation of large numbers of birds during the breeding season enhances the risk of infectious diseases for the hatching chick.To test the effect of maternal yolk androgens on the chick's immune function, we experimentally manipulated, in a field study, yolk androgen levels within the physiological range by in ovo injection of either androgens (testosterone and androstenedione) or sesame oil (control) into freshly laid eggs. We determined cell-mediated immunity (CMI) and humoral immunity of the chicks at the beginning of the nestling period to evaluate early modulatory effects of yolk androgens on immune function.Embryonic exposure to elevated levels of androgens negatively affected both CMI and humoral immunity in nestling gull chicks. Consequently, maternal yolk androgens not only entail benefits of enhanced competitiveness and growth as previously shown, but also costs in terms of immunosuppression. The outcome of embryonic yolk androgen exposure thus likely depends on the post-hatching circumstances for the developing offspring such as parasite exposure and degree of sibling competition.     

Differential maternal testosterone allocation among siblings benefits both mother and offspring in the zebra finch Taeniopygia guttata

Parents are selected to preferentially invest in the offspring with highest reproductive value. One mechanism for achieving this is the modification of competitive asymmetries between siblings by maternal hormones. In many organisms, offspring value varies according to birth position in the brood, which determines survival chances and competitive advantage over access to resources. In birds, variation in yolk androgen allocation over the laying sequence is thought to modulate dominance of senior chicks over junior brood mates. We tested this hypothesis in zebra finches, which show a naturally decreasing pattern of within-clutch testosterone allocation. We abolished these within-clutch differences by experimentally elevating yolk testosterone levels in eggs 2-6 to the level of egg 1, and we assessed fitness measures for junior offspring (eggs 2-6), senior offspring (egg 1), and their mothers. Testosterone-injected eggs hatched later than control eggs. Junior, but not senior, chicks in testosterone-treated broods attained poorer phenotypic quality compared to control broods, which was not compensated for by positive effects on seniors. Mothers were generally unaffected by clutch treatment. Thus, naturally decreasing within-clutch yolk testosterone allocation appears to benefit all family members and does not generally enhance brood reduction by favoring senior chicks, in contrast to the widely held assumption.     

Maternal androgens in black-headed gull (Larus ridibundus) eggs: consequences for chick development

We tested the hypothesis that mother birds counterbalance the negative effects of hatching asynchrony for later-hatched chicks by increasing the yolk androgen concentrations in consecutive eggs of their clutch. In doing so, they may adaptively tune each offspring's competitive ability and, thus, growth and survival. However, evidence in support of this hypothesis is contradictory. The yolk concentrations of maternal androgens in the eggs of black-headed gulls increase significantly with the laying order of the eggs in a clutch. We experimentally tested the functional consequences of this increase on chick development under natural conditions by injecting eggs with either an oil or androgen solution. We created experimental clutches in which androgen levels either stayed constant or increased with laying order while controlling for differences in egg quality by using only first-laid eggs. We then compared development, growth and survival between these broods. Androgen treatment enhanced embryonic development because androgen-treated eggs hatched half a day earlier than controls, while their size at hatching was similar to oil-treated controls. Androgen treatment did not increase chick survival, but it enhanced growth. Androgen-treated, third-hatched chicks had a higher body mass and longer legs than third-hatched chicks that hatched from oil-treated eggs. At the same time, growth of first chicks (which were all oil treated) was reduced by the presence of two androgen-treated siblings, suggesting that yolk androgens enhance the competitive ability of later-hatched chicks. Our results support the hypothesis that transfer of different amounts of androgens to the eggs of a clutch is a mechanism by which mothers maximize their reproductive output.     

Penguin Chicks Benefit from Elevated Yolk Androgen Levels under Sibling Competition

Crested penguins (genus Eudyptes) have a peculiar hatching pattern, with the first-laid egg (A-egg) hatching after the second-laid egg (B-egg) and chicks from A-eggs typically having a much lower survival probability. Maternal yolk androgens have been suggested to contribute to the competitive superiority of the B-chick in southern rockhopper penguins Eudyptes chrysocome, given their important role in mediating sibling competition in other species. We therefore increased the yolk androgen levels in freshly-laid eggs and examined the consequences for sibling competition - via effects on embryonic developmental times, chick growth and early survival. We placed one androgen-treated egg and one control egg into each foster nest, matching them for mass, laying date and laying order. The androgen treatment did not significantly affect embryonic developmental times or chick measurements at hatching. However, elevated yolk androgen levels benefitted chick growth in interaction with the number of siblings in a brood. Chicks from androgen-treated eggs had faster growth in the presence of a sibling than chicks from control eggs. Under these circumstances they also had a higher survival probability. Thus maternal androgens appear to reinforce the observed hatching pattern, facilitating brood reduction. This contrasts to most previous studies in other species where yolk androgens have been shown to compensate for the negative consequences of delayed hatching within the brood hierarchy.     

Sex-specific effects of yolk testosterone on survival, begging and growth of zebra finches

Yolk androgens affect offspring hatching, begging, growth and survival in many bird species. If these effects are sex-specific, yolk androgen deposition may constitute a mechanism for differential investment in male and female offspring. We tested this hypothesis in zebra finches. In this species, females increase yolk-testosterone levels and produce male-biased sex ratios when paired to more attractive males. We therefore predicted that especially sons benefit from elevated yolk androgens. Eggs were injected with testosterone or sesame oil (controls) after 2 days of incubation. Testosterone had no clear effect on sex-specific embryonic mortality and changed the pattern of early nestling mortality independent of offspring sex. Testosterone-treated eggs took longer to hatch than control eggs. Control males begged significantly longer than females during the first days after hatching and grew significantly faster. These sex differences were reduced in offspring from testosterone-treated eggs due to prolonged begging durations of daughters, enhanced growth of daughters and reduced growth of sons. The results show that variation in maternal testosterone can play an important role in avian sex allocation due to its sex-specific effects on offspring begging and growth.     

Do group size and laying order influence maternal deposition of testosterone in smooth-billed ani eggs?

The avian egg contains maternal hormones that affect behavior, growth, morphology, and offspring survival. Evidence to date suggests that patterns of yolk androgen deposition could provide females with a means to manipulate sibling competition and, thereby, increase their fitness. We examined yolk testosterone (T) concentrations in eggs of the smooth-billed ani (Crotophaga ani) to understand patterns of androgen deposition in eggs of this plural-breeding joint-nesting cooperatively breeding species. We tested the hatching asynchrony adjustment hypothesis, which states that increases in yolk androgen levels over the laying sequence function to mitigate the disadvantage of being a later-hatched chick in species without adaptive brood reduction. We also investigated the effect of group size on yolk T deposition to test the hypothesis that females in multi-female groups could give a competitive edge to their own chicks by depositing higher T levels in their eggs. Predictions of the hatching asynchrony adjustment hypothesis were supported in both single- and multi-female groups as yolk testosterone levels increased from early- to late-laid eggs. This suggests that ani females can influence nestling competition and chick survival by within-clutch differential T allocation. Unexpectedly, we did not observe an effect of group size on yolk T deposition. Yolk testosterone concentrations may not be a mere reflection of a female's hormonal status as female plasma circulating levels of T did not vary in the same direction as yolk T levels. Results of this study therefore support the idea that females may adaptively manipulate chick behavior through hormonal deposition in eggs.     

Yolk androgens and embryo sex: maternal effects or confounding factors?

Maternal effects occur when offspring phenotype is affected by environmental factors experienced by the mother and, in egg-laying species, are often mediated via egg resources. There is currently great interest among behavioural ecologists in maternally allocated yolk androgens, especially their relationship with offspring sex and development. Such studies need embryonic tissue for sexing, however, requiring eggs to be incubated (usually for 3 days). Therefore, there are concerns about whether the androgen concentrations assayed reflect those allocated by the mother. In addition, studies showing sex biases in maternal allocation of androgens could be confounded if male and female embryos uptake or metabolise androgens at different rates. We ran a series of experiments using zebra finch (Taeniopygia guttata) eggs to address these potential confounding factors. First we showed, using eggs naturally incubated for up to 5 days, that eggs containing embryos had lower yolk androgen concentrations than eggs that had failed to form embryos. We then tested various hypotheses for this difference using controlled incubation treatments. Our results suggested that (a) embryo development causes the yolk to become progressively more diluted with albumin; and (b) between 3 and 5 days of incubation embryos start uptaking or metabolising androgens. Crucially, we found no decline in yolk androgen concentration at 3 days incubation, and no evidence for sex-specific rates of uptake or metabolism of androgens. This strongly suggests that yolk androgen levels up to 3 days incubation do reflect those allocated by the mother, and that studies of sex biased maternal allocation of yolk androgens are not confounded by sex differences in embryo development.     

Functional significance of variation in egg-yolk androgens in the American coot

Maternally derived hormones in cleidoic eggs have been implicated in mediating growth, behavior, and social interactions among offspring. Given these widespread and significant effects, hormonal investments have the potential to greatly influence fitness of offspring. Intraspecific variation can exist at three levels (within individual eggs, among eggs within clutches, and among eggs from different females), each of which has different implications for offspring. We characterized all three levels of variation in maternally derived androgens (testosterone and androstenedione) present in yolks of American coot eggs. We found no variation in testosterone levels within eggs which suggests that embryos are exposed to constant androgen levels during development, and that field-based yolk biopsies are an appropriate way to sample eggs for this species. Within clutches, early-laid eggs had higher androgen levels than late-laid eggs, and this pattern may exacerbate negative effects of hatching asynchrony on chicks from late-hatching eggs if androgens provide chicks with a behavioral or growth advantage over chicks from eggs with lower androgen levels. American coots lay large clutches, and unequal resource allocation among offspring may be the optimal strategy for females with access to limited resources. Most of the variation in androgen levels occurred among eggs from different females. Females nesting on ponds with two other pairs laid eggs with significantly higher androgen levels than females living on ponds with fewer pairs. This suggests that increased territory defense behaviors influence the levels of androgens allocated to eggs and may be one mechanism underlying density-dependent effects on reproduction.     

Maternal yolk androgens stimulate territorial behaviour in black-headed gull chicks

Avian eggs contain substantial amounts of maternal androgens. The concentrations of these yolk androgens are affected by the maternal environment, such as the level of social competition, parasite exposure or food conditions. Since yolk androgens have been shown to affect a wide array of offspring traits, they may adjust the chicks to the expected post-hatching environment, but experimental evidence is still scarce. We investigate in colonial breeding black-headed gulls whether high concentrations of yolk androgens, such as those found in environments with high numbers of social interactions, facilitate aggressiveness and territorial behaviour of the chicks. Black-headed gulls are highly suitable for this, as the semi-precocial chicks defend the natal territory and food against intruders. We manipulated yolk androgen concentrations and investigated their role in both within-nest and between-nest aggression. We found that chicks hatching from androgen-treated eggs defended the natal territory more often than their nest mates from control eggs, without increasing sibling aggression. This suggests that variation in yolk androgen concentrations in relation to the social environment of the mother may indeed allow adjustment of the offspring''s behaviour to the expected frequency of territorial interactions with conspecifics post-hatching.     

Brood reduction in the Red-necked Grebe Podiceps grisegena

Brood reduction in Red-necked Grebes Podiceps grisegena breeding on fish ponds in south-eastern Poland occurred either through the desertion of the last-laid eggs after partial hatching of the clutch and/or the selective starvation of the smallest chicks. Abandonment of unhatched eggs was not influenced by the number of young already hatched or by the breeding date, but it was more likely in larger clutches and in families suffering chick starvation. Chicks from the largest broods had a higher probability of survival until fledging than those from single-chick broods. Larger chicks obtained food more successfully through better positioning during food delivery. In families that did not suffer brood reduction, chicks were better provisioned with food than in reduced broods. Although allocation of food among chicks in reduced broods was more skewed to the disadvantage of the younger siblings, dominant chicks obtained less food prior to brood reduction than dominant siblings in unreduced broods. Sibling aggression did not differ between unreduced and reduced broods before death of the weakest chicks. Post-laying adjustment of the number of offspring to prevailing feeding conditions occurred at two stages: by parental manipulation of the number of hatched eggs at the time when parents and chicks leave the nest and by competition between chicks. It is suggested that late egg desertion may be an adaptive mechanism of brood-size adjustment, when elimination of the weakest chicks through sibling competition is not very efficient.     

Hatching asynchrony and brood reduction influence immune response in Common Kestrel Falco tinnunculus nestlings

The onset of incubation before the end of laying imposes asynchrony at hatching and, therefore, a size hierarchy in the brood. It has been argued that hatching asynchrony might be a strategy to improve reproductive output in terms of quality or quantity of offspring. However, little is known about the mediating effect of hatching asynchrony on offspring quality when brood reduction occurs. Here, we investigate the relationship between phenotypic quality and hatching asynchrony in Common Kestrel Falco tinnunculus nestlings in Spain. Hatching asynchrony did not increase breeding success or nestling quality. Furthermore, hatching asynchrony and brood reduction had different effects on nestlings’ phytohaematogglutinin (PHA)‐mediated immune response and nestling growth. In asynchronous and reduced broods (in which at least one nestling died), nestlings showed a stronger PHA‐mediated immune response and tended to have a smaller body size compared with nestlings raised in synchronous and reduced broods. When brood reduction occurred in broods hatched synchronously, there was no effect on nestling size, but nestlings had a relatively poor PHA‐mediated immune response compared with nestlings raised in asynchronous and reduced broods. We suggest that resources for growth can be directed to immune function only in asynchronously hatched broods, resulting in improved nestling quality, as suggested by their immune response. We also found that males produced a greater PHA‐mediated immune response than females only in brood‐reduced nests without any effect on nestling size or condition, suggesting that females may trade off immune activities and body condition, size or weight. Overall, our results suggest that hatching pattern and brood reduction may mediate resource allocation to different fitness traits. They also highlight that the resolution of immune‐related trade‐offs when brood reduction occurs may differ between male and female nestlings.     

Interfemale variation in egg yolk androgen allocation in the European starling: do high-quality females invest more?

Androgenic hormones occur naturally in bird egg yolk and are known to enhance growth in canary and gull chicks. Gil et al. (1999, Science, 286, 126-128) have recently proposed that female allocation of androgens to eggs represents a form of costly maternal investment. This hypothesis predicts that females of high quality or high reproductive potential should invest more yolk androgens in their eggs than females of low quality or low reproductive potential. We tested these predictions by examining interfemale variation in allocation of the androgenic steroids testosterone and androstenedione to eggs in a wild population of European starlings, Sturnus vulgaris. We collected 30 full clutches of eggs and captured 22 of the females that laid these clutches for phenotypic measurements. In agreement with the hypothesis, we found that there was significant interfemale variation in yolk androgen concentrations. Furthermore, older females deposited more androstenedione and testosterone in eggs than 1-year-old females, and females laying early or large clutches deposited more testosterone in eggs than females laying late or small clutches. However, females in good body condition did not deposit more androgens in eggs than females in poor body condition. Large-yolked eggs had a higher total yolk androgen content than small-yolked eggs, but larger eggs did not have higher concentrations of yolk androgens. The results in general support the investment hypothesis of yolk androgen allocation. However, further data are needed regarding the costs of yolk androgen allocation and the effects of yolk androgens on offspring to substantiate the assumptions of the hypothesis. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Yolk testosterone stimulates growth and immunity in house finch chicks

Female birds deposit variable amounts of androgens, such as testosterone, into the yolks of their eggs. Evidence suggests that yolk androgens play an important role in the determination of offspring phenotype. While androgens are generally regarded as anabolic and immunosuppressive, studies of the behavioral and physiological effects of yolk androgens on offspring of several avian species have been conflicting, leaving the adaptive significance associated with deposition patterns of yolk androgens unclear. We injected either a physiological dose of testosterone or a control vehicle into house finch (Carpodacus mexicanus) eggs and examined the effects of these injections on offspring growth and immunity. Two days after hatching, nestlings from eggs treated with testosterone were significantly larger than nestlings from eggs treated with a control injection, suggesting a stimulatory effect of yolk androgens in early development. By 8 d after hatching, however, this effect disappeared, and chicks from the two treatment groups were similar in size. Nestlings in the testosterone treatment group showed a significantly larger swelling response to phytohemagglutinin than control nestlings 15 d after hatching, which is close to fledging. Overall, our observations show that when food resources are abundant, testosterone stimulates both early growth and immunity in developing house finches.     

Yolk androgens in the barn swallow (Hirundo rustica): a test of some adaptive hypotheses

Maternal effects such as androgen in avian eggs can mediate evolutionary responses to selection, allowing manipulation of offspring phenotype and promoting trans-generational adaptive effects. We tested the predictions of two adaptive hypotheses that have been proposed to explain female variation in yolk androgen allocation in birds, using the barn swallow Hirundo rustica as a model. We found no support for the first hypothesis proposing that yolk androgen varies as a function of breeding density in order to prepare offspring for different breeding densities. However, we found experimental support for the hypothesis that female yolk androgen allocation depends on mate attractiveness and that it constitutes an example of differential allocation. Females increased the concentration of androgens in their eggs when mated to males with experimentally elongated tails. Female phenotypic quality as measured by arrival date and clutch size was positively related to egg androgen concentration, consistent with the hypothesis that this is a costly investment, constrained by female condition. We found correlative evidence of a direct relationship between egg androgen concentration and performance of offspring as measured by mass increase.     

Yolk androgens reduce offspring survival

Females may favour some offspring over others by differential deposition of yolk hormones. In American kestrels (Falco sparverius), we found that yolks of eggs laid late in the sequence of a clutch had more testosterone (T) and androstenedione (A4) than yolks of first-laid eggs. To investigate the effects of these yolk androgens on nestling 'fitness', we injected both T and A4 into the yolks of first-laid eggs and compared their hatching time, nestling growth and nestling survival with those of first-laid eggs in which we injected vehicle as a control. Compared to controls, injection of T and A4 at a dose intended to increase their levels to those of later-laid eggs delayed hatching and reduced nestling growth and survival rates. Yolk androgen treatment of egg 1 had no effect on survival of siblings hatching from subsequently laid eggs. The adverse actions of yolk androgen treatment in the kestrel are in contrast to the favourable actions of yolk T treatment found previously in canaries (Serinus canaria). Additional studies are necessary in order to determine whether the deposition of yolk androgens is an adaptive form of parental favouritism or an adverse by-product of endocrine processes during egg formation. Despite its adaptive significance, such 'transgenerational' effects of steroid hormones may have helped to evolutionarily shape the hormonal mechanisms regulating reproduction.     

Do mothers regulate facultative and obligate siblicide by differentially provisioning eggs with hormones?

We examined androgens in clutches of two booby species that differ in their sibling conflict. Blue-footed booby Sula nebouxii chicks show an aggression-submission relationship, aggression is normally moderate and siblicide is facultative. Brown booby Sula leucogaster chicks show an aggression-aggression relationship, aggression of both chicks can be relentless and siblicide is obligate. The parental favoritism hypothesis predicts that egg mass, yolk mass and yolk androgens should decline with laying order less in the blue-footed booby than in the brown booby, to promote the survival of the former's junior chick. The eggs of the blue-footed booby had higher yolk concentrations of 5α-dihydrotestosterone (DHT) and lower concentrations of testosterone (T); both species had similarly high yolk concentrations of androstenedione (A). Intra-clutch variation in yolk DHT, T and A failed to support our predictions. In both species, first and second eggs showed similar concentrations of all three hormones and were of similar size, although in the blue-footed booby (only) yolk masses declined with laying order. Brown booby mothers make junior chicks vulnerable to siblicide by hatching them 5 d after their broodmates, but not by differential allocation of egg androgens or nutrients. Blue-footed booby mothers appear to prepare junior chicks for thriving in a subordinate non-provocative role by hatching them 4 d after their broodmates and providing them with 10% less yolk. To orchestrate agonism within the brood, these boobies may rely more on hatch intervals and yolk provision than on androgens.     

Egg‐yolk androgen and carotenoid deposition as a function of maternal social environment in barn swallows Hirundo rustica

Evidence is mounting that female animals use egg‐yolk compounds (e.g. steroids, antioxidants) to adaptively engineer the quality of their offspring as a function of several maternal and environmental factors. Though adjustments to yolk allocation have been well‐characterized as a function of parental phenotypes, we know very little about how an individual's social environment influences yolk composition. Here, we consider how two types of yolk compounds, androgens and carotenoids, relate to the maternal social environment during the egg‐laying period, controlling statistically for known correlations between various aspects of parental quality and egg yolk compounds. Barn swallows Hirundo rustica erythrogaster breed in groups of highly variable size and spacing, allowing us to test whether or not the social environment is correlated with these maternal effects. We found no relationship between carotenoid levels in eggs as a function of colony size, colony density, or nearest‐neighbor distance. However, eggs from females in larger groups had lower concentrations and total amounts of yolk androgens than those from females in smaller, less dense social settings. Our results counter previous predictions and literature, showing that females breeding in large groups deposit more androgen in eggs, mechanistically, because they compete more with conspecifics and have higher circulating androgen levels themselves and, functionally, because it could be advantageous for their offspring to show high androgen‐mediated competitive abilities early in life. Instead, because group size in this species is governed largely by site fidelity and the availability of old nests for re‐use, and because reproductive output does not differ as a function of group size, it may be that competition is greater for limited nests in small groups, thus elevating androgen levels. Further, yolk androgens were previously shown to be affected by male quality, and the greater concentrations and amounts of yolk androgens in smaller sites may reflect differential allocation to darker males found at these sites.     

EVOLUTION OF OBLIGATE SIBLICIDE IN BOOBIES. 2: FOOD LIMITATION AND PARENT–OFFSPRING CONFLICT

Proximate limitation on parental food delivery has long been invoked to explain the evolution of single-chick broods of pelagic seabirds such as masked boobies (Sula dactylatra). A second possible proximate limit on brood size is siblicide driven by genetic parent–offspring conflict (POC) over brood size, if siblicidal offspring can reduce brood size to one even if the parents' optimal brood size is greater than one. I tested these two hypotheses by experimentally suppressing obligate siblicide in masked booby broods and comparing breeding parameters of these broods with unmanipulated single-chick control broods. Per capita mortality rate of experimental nestlings was higher than that of controls, but this deficit was more than made up by larger brood size. Parents of experimental broods brought more food to offspring, had higher fledging success, and apparently incurred no additional major short-term cost of reproduction, relative to parents of control broods, thus refuting the food limitation hypothesis. Estimates of inclusive fitness of chicks in experimental broods were higher than were those of control nestlings, a result inconsistent with the POC hypothesis that the siblicidal offspring's optimal brood size is one while the parents' optimum is greater than one. This discrepency between natural brood size and apparent brood size optima might be resolved in several ways: experimental artifacts may give misleading estimates of optimal brood size; experimental and control offspring may have different reproductive values at the time of fledging; nestling masked boobies may face a special frequency-dependent case of POC in which the high risk of sharing a nest with a siblicidal sibling makes invasion of other behavioral genotypes difficult even when offspring and parent inclusive fitnesses are higher from a nonsiblicidal brood of two than from a brood of one.     

Elevated yolk androgen levels benefit offspring development in a between-clutch context

The field of androgen deposition in avian eggs and its consequencesfor offspring development has received a lot of attention inrecent research. However, although variation within clutchesin yolk androgens is relatively well understood, the adaptivesignificance of patterns of variation between clutches remainsrather unclear. Furthermore, it has been hypothesized that yolkandrogens act as a means of an adaptive maternal effect to adjustoffspring to a given posthatching environment. Thus, the consequencesof maternal yolk androgens for offspring development are likelyto depend on the specific environment of a given brood. We experimentallymanipulated yolk androgen concentrations in spotless starlingeggs, using a between-brood design in which full broods weremanipulated applying either an androgen or sham treatment, inorder to test the effects of between-brood variation in yolkandrogen levels. We also included in the analysis several femalecharacters that have been shown to affect androgen deposition.Androgen-chicks tended to gain more mass, a similar effect tothat shown in previous studies where direct competition betweenchicks belonging to different treatments was allowed, but didnot gain a survival benefit. Androgen-chicks had wider beakflanges, an effect that has not been described previously andcould play an important role in food acquisition. In addition,androgen-chicks had higher endogenous plasma levels of androgens,which could induce higher begging intensity. We hypothesizethat these effects are an important mechanistic link for ourunderstanding of how yolk androgens exert their effects on offspringdevelopment after hatching. Contrary to our predictions, wefound no evidence that the observed effects depended on theenvironment under which it was investigated. Because high yolkandrogen levels seem to be beneficial for nestlings in thisspecies, we hypothesize that yolk hormone deposition could becostly for females or alternatively that potential negativeeffects later in adulthood may constrain maternal hormone allocation.     

Pre-breeding diet affects the allocation of yolk hormones in zebra finches Taeniopygia guttata

The ability of mothers to modify offspring phenotype to match prevailing environmental conditions is an important component of reproductive success, especially in variable environments. Pre-breeding conditions, such as food abundance, may have significant consequences for both the number and quality of offspring a female produces as well as her ability to rear the offspring. In an experiment where pre-breeding diet was manipulated, we investigated if allocation of yolk androgens (testosterone and 5α-dihydrotestosterone) was related to the quality of diet experienced prior to breeding. Female zebra finches Taeniopygia guttata produced larger clutches on high quality diet than on low quality diet but with no differences in egg mass. Yolk androgen levels were repeatable within subsequent clutches of the same female and females did not change mean androgen content in eggs in relation to diet quality. However, within-in clutch pattern of yolk testosterone and DHT changed with diet treatment. Testosterone and DHT decreased with laying order on the low quality diet but remained constant on the high quality diet. Differential yolk androgen allocation within the clutch may alter the competitive differences between chicks and provide females the possibility to adjust reproductive investment and offspring phenotype already at the egg stage.