Maternal androgens in the pied flycatcher: timing of breeding and within-female consistency

Effects of prenatal testosterone on cumulative markers of oxidative damage to organs of young adult zebra finches (Taeniopygia guttata)

We tested the hypothesis that exposure of avian embryos to androgens in ovo entails long-term costs in the form of oxidative damage to vital cells and organs in adulthood. We injected zebra finch eggs with testosterone (T), monitored postnatal growth, and analyzed markers of oxidative damage in heart and liver in mature birds. We measured 8-oxo-2'-deoxyguanosine and isoprostanes, markers of oxidative damage to DNA and membrane lipids, respectively. T treatment (1) reduced growth rates of female but not male nestlings vs. controls; (2) resulted in less accumulation of 8-oxo-dG, but not IsoPs, in liver tissue of 60-day-old females, but not males; and (3) a trend toward elevated 8-oxo-dG levels in heart tissue of males and females at 60 and 180 days old combined. These results generally support the testosterone oxidative damage hypothesis, in that embryonic exposure to higher T resulted in damage to DNA of heart tissue in both sexes. They also suggest that sex-specific effects of androgens on early growth rates may carry over as differences in some forms of oxidative damage in adults. This supports a basic tenet of evolutionary aging theory that developmental influences early in life can be linked to costs later on.

Seasonal changes in yolk hormone concentrations carry-over to offspring traits

Yolk hormones are substances which transmit non-genetic factors from the mother to the next generation. The systematic changes of yolk hormone concentrations within asynchronously hatching clutches have been interpreted as a means to adaptively shape the offspring's phenotype. However, in synchronously hatching clutches the role of yolk hormones is less understood. We investigated whether seasonal changes between eggs in the yolk hormones testosterone (Testo), progesterone (Prog) and corticosterone (Cort) also occur in the grey partridge, a synchronously hatching precocial species without direct food competition between siblings. Specifically we asked whether yolk hormone concentrations systematically vary with season and whether they affect the offspring's hatching mass, mass gain, circulating baseline and stress-induced Cort. Additionally, we investigated the effect of genetic background and food availability on yolk hormone concentrations by subjecting grey partridge hens of two strains (wild and domesticated) to two different feeding regimes (predictable vs. unpredictable feeding) during egg laying. We hypothesized that egg hormone concentrations change over the season, but breeding in captivity over many generations and ad libitum food access could have resulted in domestication effects which abolished potential seasonal effects. Results showed that progressing season had a strong positive effect on yolk Prog and yolk Testo, but not on yolk Cort. Feeding regimes and strain had no effect on yolk hormones. Offspring mass and mass gain increased and baseline Cort decreased with progressing season. In addition, yolk Testo correlated positively with offspring mass gain and negatively with baseline Cort, while yolk Prog had a positive correlation with baseline Cort. Strain and feeding regimes of the mother had no effect on offspring traits. In conclusion, grey partridge chicks hatching late in the season might benefit from the increased concentrations of the growth-stimulating yolk Testo and by this catch-up in development. Hence, yolk hormone concentration could adaptively shape the offspring phenotype in a precocial species.

Between- and Within-Individual Variation of Maternal Thyroid Hormone Deposition in Wild Great Tits (Parus major)

Maternal hormones are often considered a mediator of anticipatory maternal effects; namely, mothers adjust maternal hormone transfer to prepare the offspring for the anticipated environment. The flexibility for mothers to adjust hormone transfer is therefore a prerequisite for such anticipatory maternal effects. Nevertheless, previous studies have focused only on the average differences of maternal hormone transfer between groups and neglected the substantial individual variation, despite the fact that individual plasticity in maternal hormone transfer is actually the central assumption. In this study, we studied the between- and within-individual variation of maternal thyroid hormones (THs) in egg yolk of wild great tits (Parus major) and estimated the individual plasticity of maternal yolk THs across environmental temperature, clutch initiation dates, and egg laying order using linear mixed effects models. Interestingly, our models provide statistical evidence that the two main THs-the main biologically active hormone T3 and T4, which is mostly considered a prohormone-exhibited different variation patterns. Yolk T3 showed significant between-individual variation on the average levels, in line with its previously reported moderate heritability. Yolk T4, however, showed significant between-clutch variation in the pattern over the laying sequence, suggesting a great within-individual plasticity. Our findings suggest that the role and function of the hormone within the endocrine axis likely influences its flexibility to respond to environmental change. Whether the flexibility of T4 deposition brings a fitness advantage should be examined along with its potential effects on offspring, which remain to be further investigated.

Temperature-induced variation in yolk androgen and thyroid hormone levels in avian eggs

Global warming has substantially changed the environment, but the mechanisms to cope with these changes in animals, including the role of maternal effects, are poorly understood. Maternal effects via hormones deposited in eggs, have important environment-dependent effects on offspring development and fitness: thus females are expected to adjust these hormones to the environment, such as the ambient temperature. Longer-term temperature variation could function as a cue, predicting chick rearing conditions to which yolk hormone levels are adjusted, while short-term temperature variation during egg formation may causally affect hormone transfer to eggs. We studied the effects of ambient temperature on yolk androgens (testosterone and androstenedione) and thyroid hormones (thyroxine and triiodothyronine) in great tits (Parus major) using data from unmanipulated clutches from a wild population and from aviary birds (ad libitum food) exposed to different experimental temperature treatments during five years. Both in the wild and in captivity, longer-term pre-laying ambient temperature was not associated with clutch mean yolk hormone levels, while the way androstenedione and thyroxine levels varied across the laying sequence did associate with pre-laying temperature in the wild. Yolk testosterone levels were positively correlated with short-term temperature (during yolk formation) changes within clutches in both wild and captivity. We also report, for the first time in a wild bird, that yolk thyroxine levels correlated with a key environmental factor: thyroxine levels were negatively correlated with ambient temperature during egg formation. Thus, yolk hormone levels, especially testosterone, seem to be causally affected by ambient temperature. These short-term effects might reflect physiological changes in females with changes in ambient temperature. The adaptive value of the variation with ambient temperatures pre-laying or during egg formation should be studied with hormone manipulations in different thermal environments.

Timing of Breeding Determines Growth and Development in a Long-Distance Migratory Bird

The timing of breeding can have significant consequences for adult and offspring fitness, yet our current hypotheses of seasonal timing focus on the parent perspective. When offspring survival is affected by timing of breeding, we expect to see offspring mechanisms to detect and respond to cues of seasonal timing. Avian embryos respond to photoperiod and seasonal cues during development and in this study we evaluate the influence of photoperiod and season on posthatching growth and development in Franklin's gull (Leucophaeus pipixcan). Early- and late-season chicks exposed to short and long photoperiods during development were reared under common garden conditions. Photoperiod had no effect on posthatching growth and development, but seasonal effects present in the egg (maternal egg effects or genetic) did influence posthatching growth and development. Late-season chicks grew body mass and primary flight feathers at faster rates than early-season chicks, which we hypothesize facilitates independence and readiness for migration. Growth rates are complex phenotypes and we propose a general growth model that incorporates delays in negative feedback systems regulating growth. We show that the timing of breeding programs intrinsic growth rates in offspring, which suggests that many of the metrics used to describe seasonal patterns of reproductive success may be biased.

Yolk steroids in great tit Parus major eggs: variation and covariation between hormones and with environmental and parental factors

Abstract

Avian mothers can potentially alter the phenotypes of their offspring by varying the concentration of steroid hormones in their eggs. We explored variation in androstenedione (A4), testosterone (T), 5α-dihydrotestosterone (DHT), 17β-estradiol (E2), and corticosterone (CORT) in the yolks of 12 free-living great tit Parus major clutches. We analyzed variation and covariation in greater detail than previous studies, using models for variation with laying sequence that take into account variable clutch size and comparing correlations between pairs of hormones at the within- and between-clutch levels. We also investigated relationships between hormone levels and various environmental, life history, and parental traits. For three of the five steroids, we found no significant correlates, but based on individual statistical tests (a) DHT varied between clutches with male age (1 year old vs older); (b) DHT and CORT were negatively correlated within clutches with the average temperature on the day (DHT and CORT) or 3 days (DHT only) preceding laying; and (c) DHT in the last egg of the clutch relative to the clutch mean was positively correlated with the interval between clutch completion and the onset of incubation (incubation delay). Relationships with ambient temperature and incubation delay have not previously been reported for any yolk hormone in birds. Intriguingly, the three relationships for DHT are consistent with more DHT being transferred to eggs in situations that could be more energetically challenging for the female. More research is needed to determine the generality of the patterns we found and to understand their functional significance.

Significance statement

The yolks of birds’ eggs contain steroid hormones produced by the mother which can affect the development and behavior of the resultant chicks. We analyzed five steroid hormones in the yolks of wild great tits and show for the first time that yolk hormone levels are related to ambient temperature in the day(s) just before laying and, in the last-laid egg, with the day it is laid relative to the onset of incubation, and that the concentrations of pairs of yolk hormones can vary with each other in a different way between and within clutches. These results contribute insights into the ways in which yolk hormones may adaptively modify the chicks or may reflect physiological processes occurring in the mother.

Electronic supplementary material

The online version of this article (doi:10.1007/s00265-016-2107-1) contains supplementary material, which is available to authorized users.

Hormonally-mediated maternal effects in birds: Lessons from the flycatcher model system

Maternal effects are a crucial mechanism in many taxa in generating phenotypic variation, affecting offspring development and fitness and thereby potentially adapting them to their expected environments. Androgen hormones in bird eggs have attracted considerable interest in past years, and it is frequently assumed that their concentrations in eggs are shaped by Darwinian selection. Currently, however, the data is scattered over species with very different life-history strategies, environments and selection pressures, making it difficult to draw any firm conclusions as to their functional significance for a given system. I review the evidence available as to the function, variation and potential adaptive value of yolk androgens (testosterone, T and androstenedione, A4) using one well-studied wild bird model system, the European flycatchers Ficedula hypoleuca and Ficedula albicollis. These species both show genetic and environmental variation in yolk androgen levels, along with fitness correlations for the female, suggesting the potential for selection. However, variation in yolk T and A4 seem to be differentially affected, suggesting that maternal constraints/costs shape the transfer of the yolk steroids differently. Most of the environmental variation is consistent with the idea of high yolk androgen levels under poor rearing conditions, although the effect sizes in relation to environmental variation are rather small in relation to genetic among-female variation. Importantly, within-clutch patterns too vary in relation to environmental conditions. Yolk androgens seem to have multiple short- and long-term effects on phenotype and behavior; importantly, they are also correlated with the fitness of offspring and mothers. However, the effects are often sex-dependent, and not universally beneficial for the offspring. Unfortunately, conclusive data as to the adaptive benefits of clutch mean androgen levels or within clutch-patterns in different environmental conditions is still lacking.

Context-dependent effects of yolk androgens on nestling growth and immune function in a multibrooded passerine

Female birds may adjust their offspring phenotype to the specific requirements of the environment by differential allocation of physiologically active substances into yolks, such as androgens. Yolk androgens have been shown to accelerate embryonic development, growth rate and competitive ability of nestlings, but they can also entail immunological costs. The balance between costs and benefits of androgen allocation is expected to depend on nestling environment. We tested this hypothesis in a multibrooded passerine, the spotless starling, Sturnus unicolor. We experimentally manipulated yolk androgen levels using a between-brood design and evaluated its effects on nestling development, survival and immune function. Both in first and replacement broods, the embryonic development period was shorter for androgen-treated chicks than controls, but there were no differences in second broods. In replacement broods, androgen-treated chicks were heavier and larger than those hatched from control eggs, but this effect was not observed in the other breeding attempts. Androgen exposure reduced survival with respect to controls only in second broods. Regarding immune function, we detected nonsignificant trends for androgen treatment to activate two important components of innate and adaptive immunity (IL-6 and Ig-A levels, respectively). Similarly, androgen-treated chicks showed greater lymphocyte proliferation than controls in the first brood and an opposite trend in the second brood. Our results indicate that yolk androgen effects on nestling development and immunity depend on the environmental conditions of each breeding attempt. Variation in maternal androgen allocation to eggs could be explained as the result of context-dependent optimal strategies to maximize offspring fitness.

Optimizing the trade-off between offspring number and quality in unpredictable environments: testing the role of differential androgen transfer to collared flycatcher eggs

According to the brood reduction hypothesis, parents adjust their brood size in response to current environmental conditions. When resources are abundant, parents can successfully raise all hatched offspring, but when resources are scarce, brood reduction, i.e., the sacrifice of some siblings to secure the quality of a subset of offspring, may maximize fitness. Differential transfer of maternal androgens is one potential proximate mechanism through which female birds may facilitate brood reduction because it may alter the relative competitive ability of sibling nestlings. We tested the hypothesis that female collared flycatchers (Ficedula albicollis) manipulate sibling competition by transferring less androgens to eggs late in the laying sequence. We experimentally elevated androgen levels in i) whole clutches and ii) only the two last laid eggs, and compared growth and begging behavior of offspring from these treatments with a control treatment. By using three treatments and video assessment of begging, we examined the effects of within-clutch patterns of yolk androgen transfer on levels of sibling competition in situ. When androgens were elevated in only the two last laid eggs, begging was more even among siblings compared to control nests. We also found that female nestlings receiving additional yolk androgens showed higher mass gain later in the breeding season, while their male counterparts did not. Our results suggest that females may improve reproductive success in unpredictable environments by altering within-clutch patterns of yolk androgen transfer. We discuss the possibility that life-history divergence between the co-occurring collared and pied flycatcher (Ficedula hypoleuca) is amplified by patterns of yolk androgen transfer.

Between-female variation in house sparrow yolk testosterone concentration is negatively associated with CYP19A1 (aromatase) mRNA expression in ovarian follicles

Maternally-derived yolk androgens influence the development and long-term phenotype of offspring in oviparous species. Between-female variation in the amounts of these yolk androgens has been associated with a number of social and environmental factors, suggesting that the variation is adaptive, but the mechanisms behind it are unknown. Using two different approaches, we tested the hypothesis that variation in yolk androgen levels across individuals is associated with variation in their capacity to synthesize androgens. First, we injected female house sparrows with exogenous gonadotropin-releasing hormone (GnRH) to maximally stimulate ovarian steroidogenesis. Second, we collected pre-ovulatory follicle tissue and quantified the mRNA expression of four key enzymes of the steroid synthesis pathway: steroidogenic acute regulatory protein (StAR), cytochrome P450-side chain cleavage enzyme (CYP11A1), 17β-hydroxysteroid dehydrogenase (HSD17B1), and aromatase (CYP19A1). Thirty minutes after GnRH injection, androgen concentrations in both the plasma and in the yolks of pre-ovulatory follicles were significantly elevated compared to controls. However, this measure of steroidogenic capacity did not explain variation in yolk testosterone levels, although physiological differences between house sparrows and more widely studied poultry models were revealed by this approach. Steroidogenic enzyme mRNA levels were detectable in all samples and were significantly lower in the most mature pre-ovulatory follicles. Of the four measured genes, CYP19A1 expression exhibited a significant negative relationship with yolk testosterone concentrations in laid eggs, revealing a key mechanism for between-female variation in yolk testosterone. Furthermore, this suggests that any factors which alter the expression of CYP19A1 within an individual female could have dramatic effects on offspring phenotype.

Genetic differences in yolk testosterone levels influence maternal hormone deposition in the second laying cycle in Japanese quails

Maternally-derived yolk androgens exhibit distinct among- and within-female variations but limited data refer to inter-seasonal changes of maternal hormones in the yolk. We investigated the deposition of yolk testosterone (T) across two laying cycles in Japanese quail. To test how genetically-determined differences influence between cycle variations in yolk androgens we compared females from low (LET) and high (HET) egg T lines at the end of the first and at the beginning of the second laying cycle after an induced moult. Line differences in yolk T levels exhibited high consistency exceeding two reproductive cycles. Yolk T concentrations increased in the second laying cycle in HET but not in LET females. Plasma T levels did not differ between cycles in both lines and no line differences were found either before or after the moult indicating the presence of mechanisms limiting the increase of T concentrations in the circulation. Differences in the yolk T levels were not accompanied by changes in the egg and yolk mass. The HET quail laid eggs with heavier eggshell than the LET quail. Our results demonstrate different abilities of mothers to deposit T in their eggs over two reproductive seasons with expected consequences on the development of their progeny.

Sources of variation in yolk hormone deposition: consistency, inheritance and developmental effects

Maternal effects occur when the phenotype of the mother affects the phenotype of their offspring. They are thought to have evolved to translate the environmental conditions experienced by the mother into adaptive phenotypic variation of the offspring. However, the integration of environmental cues allowing adaptive responses requires some form of plasticity that depends on the interaction of the maternal phenotype and her environment. In birds, maternal yolk hormones represent such a pathway for maternal effects, and their adaptive significance depends thus on the plasticity in maternal yolk hormone deposition. We studied sources of variation in yolk testosterone deposition, focusing on the often neglected contribution of the (partly heritable) maternal phenotype. We investigated consistency and heritability of yolk testosterone deposition in captive canaries of which the F(1) generation was raised in foster nests and analyzed the potential effects of the early developmental conditions. We found significant female consistency across years in egg mass, yolk mass and total amount of yolk testosterone but not in yolk testosterone concentrations. Females varied the yolk testosterone concentrations of their eggs across years mainly via changes in yolk mass. The heritable variation in egg mass, yolk mass and amount of yolk testosterone but not yolk testosterone concentrations was within the range of previous studies, but not significantly different from zero. Finally, the growth of the daughters as nestling had a significant effect on their yolk testosterone deposition at adulthood indicating the transgenerational potential for environmental effects - via the effects of yolk hormones on offspring development.

Geographical variation in egg mass and egg content in a passerine bird

Reproductive, phenotypic and life-history traits in many animal and plant taxa show geographic variation, indicating spatial variation in selection regimes. Maternal deposition to avian eggs, such as hormones, antibodies and antioxidants, critically affect development of the offspring, with long-lasting effects on the phenotype and fitness. Little is however known about large-scale geographical patterns of variation in maternal deposition to eggs. We studied geographical variation in egg components of a passerine bird, the pied flycatcher (Ficedula hypoleuca), by collecting samples from 16 populations and measuring egg and yolk mass, albumen lysozyme activity, yolk immunoglobulins, yolk androgens and yolk total carotenoids. We found significant variation among populations in most egg components, but ca. 90% of the variation was among individuals within populations. Population however explained 40% of the variation in carotenoid levels. In contrast to our hypothesis, we found geographical trends only in carotenoids, but not in any of the other egg components. Our results thus suggest high within-population variation and leave little scope for local adaptation and genetic differentiation in deposition of different egg components. The role of these maternally-derived resources in evolutionary change should be further investigated.

Experimental evidence for genetic heritability of maternal hormone transfer to offspring

In many animal species, embryos are exposed to maternal hormones that affect their development. Maternal hormone transfer varies with environmental conditions of the mother and is often interpreted as being shaped by natural selection to adjust the offspring to prevailing environmental conditions. Such hormone transfer requires genetic variability, which has not yet been experimentally demonstrated. Our study reports direct evidence for additive genetic variance of maternal androgens through a bidirectional selection on yolk testosterone (T) levels in Japanese quail. Lines selected for high egg T (HET) and low egg T (LET) concentration differed in yolk levels of this androgen, resulting in high realized heritability (h² = 0.42)Correlated responses to selection on other gonadal hormones indicated that selection specifically targeted biologically active androgens. Eggs of HET quail contained higher androstenedione and lower estradiol concentrations than did those of LET quail, with no line differences in yolk progesterone concentration. Plasma T concentrations in adult females were not affected by selection, seriously challenging the hypothesis that transfer of maternal hormones to offspring is constrained by hormone levels in a mother's circulation. Our results suggest that transfer of maternal T represents an indirect genetic effect that has important consequences for the evolution of traits in offspring.

Digit ratios have poor indicator value in a wild bird population

Early androgen exposure is known to have long-lasting effects on phenotype, behaviour and even fitness, but difficulties in measuring the exposure hinders the study of its importance in evolutionary context. Digit ratios have been highlighted as a potential easy-to-measure indicator of early steroid exposure, as they have been suggested to reflect steroid, mainly testosterone levels during prenatal development. However, evidence for digit ratios reflecting early steroid levels is weak, as experimental studies, especially in wild populations, are scarce. We studied the association between maternally derived yolk androgens and digit ratios (2D:4D, 2D:3D and 3D:4D) using both correlative data and a rather high level of experimental elevation of yolk androgens in a passerine bird, the pied flycatcher (Ficedula hypoleuca). We also examined whether digit ratios have indicator value in an evolutionary context by studying correlations between digit ratios and reproductive traits, secondary sexual traits and exploratory behaviour. We did not find any association between digit ratios and yolk androgen level either in correlative or experimental data. Digit ratios were neither related to any of the reproductive and secondary sexual traits or exploratory behaviour measured. There was, however, a sex difference in 2D:3D and 3D:4D of adult birds (due to second and fourth digits being shorter in females), which was not apparent in fledglings or captivity-raised juveniles. This suggests that either the sex difference may develop as late as during the sexual maturation for breeding. These results indicate that, in this species, digit ratios are not reliable markers of maternally derived yolk androgen exposure and that they bear little relevance as correlates of the adaptive traits we measured.

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.

Heritable variation in maternal yolk hormone transfer in a wild bird population

Differential reproductive investment by the mother can critically influence offspring development and phenotype, and strong selection is therefore expected to act on such maternal effects. Although a genetic basis is a prerequisite for phenotypic traits to respond to selection and thus to evolve, we still know very little about the extent of heritable variation in maternal effects in natural populations. Here, we present the first estimates of intrafemale repeatability across breeding seasons and estimates of heritability of hormone-mediated maternal effects in a wild population of collared flycatchers (Ficedula albicollis). We found that maternal yolk testosterone (T) concentrations, yolk mass, and egg mass were moderately to highly repeatable within females across years, whereas intrafemale consistency of maternal yolk androstenedione (A4) deposition was low yet statistically significant. Furthermore, maternal yolk T transfer, yolk mass, and egg mass were significantly heritable, whereas yolk A4 transfer was not. These results strongly suggest that two major maternal yolk androgens are differentially regulated by genes and the environment. Selection on heritable variation in maternal yolk T deposition has the potential to shape the rate and direction of phenotypic change in offspring traits and can thereby accelerate or impede the response to selection in natural populations.

Variability of yolk testosterone concentrations during the reproductive cycle of Japanese quail

Deposition of yolk androgens can vary among females as well as within eggs of one female. Numerous external and internal factors can contribute to this variability. In our study, we investigated the systemic variation of yolk testosterone concentrations during the laying cycle of Japanese quail reared in stable environmental and social conditions. Testosterone was analysed in three eggs collected per female at the beginning, top and the end of a reproductive period and the extent of inter- and intra-female differences in yolk deposition of this androgen was quantified. Yolk testosterone concentrations and the yolk testosterone content decreased from the early to the latest stage of reproductive period. Testosterone concentrations in the egg yolk as well as the age-dependent pattern significantly differed among individual females. We found high repeatability of yolk testosterone among 3 eggs of individual females together with high repeatability between 3 stages of the reproductive cycle. Testosterone in the egg yolk correlated positively with eggshell weight. Our results suggest that precocial birds with long laying sequences display higher inter-female differences in yolk testosterone concentrations compared with intra-individual variability. The decreased testosterone deposition with age may influence the development and behaviour of the young hatched at different stages of the female's reproductive cycle.

Yolk androgens do not appear to mediate sexual conflict over parental investment in the collared flycatcher Ficedula albicollis

Males and females are in conflict over parental care, as it would be favourable for one parent to shift labour to the other. Yolk hormones may offer a mechanism through which female birds could influence offspring traits in ways that increase the relative investment by the male. We studied the role of yolk androgens in mediating sexual conflict over parental care in the collared flycatcher (Ficedula albicollis). In a cross-fostering experiment, the male's proportion of total feeding visits increased with increasing androgen levels in the foster eggs. This could suggest that sexual conflict over parental care may be influenced by the female's differential allocation of yolk androgens or a maternal effect associated with yolk androgens. However, when we experimentally elevated yolk androgen levels, male feeding rates did not differ between control and androgen-manipulated nests. This suggests that other egg components correlated with yolk androgen levels, rather than yolk androgen levels per se, may influence male parental effort. In conclusion, yolk androgens per se do not appear to mediate sexual conflict over parental investment in the collared flycatcher.

Selection on personality in a songbird affects maternal hormone levels tuned to its effect on timing of reproduction

The increase or decrease in yolk androgens over the laying sequence of a clutch in birds may mitigate or enhance, respectively, the disadvantage of the last-hatched chicks, providing a potentially adaptive tool to adjust brood size to food conditions. This variation may involve a genetic component on which Darwinian selection can act. We found that two lines of a wild bird species selected for bold and shy personalities show, respectively, increased and decreased androgen concentrations over the laying sequence. The line showing the increase laid earlier in the season, when food conditions are normally sufficient to raise the whole brood. The line showing the decrease laid later, when food is normally scarce, which may facilitate brood reduction. The results indicate a correlated response in maternal hormone transfer to genetic selection on personality, which relates to ecological conditions.

Are yolk androgens adjusted to environmental conditions? A test in two seabirds that lay single-egg clutches

It is widely believed that female birds strategically allocate androgens to yolk in the manner that best equips offspring for feeding conditions during their development. Because most avian studies have focused on multi-egg clutch species, and interpreted results within the framework of sibling competition, we still know little about how yolk androgens might be allocated in direct response to environmental conditions. Most oceanic birds are long-lived and lay single-egg clutches, and their breeding success is tightly linked to highly variable marine production. That combination: a variable breeding environment, long lives, and single-egg clutches, makes oceanic birds good subjects to test hypotheses about yolk androgen allocation strategies. We measured concentrations of two yolk androgens, androstenedione (A4) and testosterone (T), in the single-egg clutches laid by early-laying Cassin's (Ptychoramphus aleuticus) and rhinoceros (Cerorhinca monocerata) auklets at Triangle Island, British Columbia, Canada, in 2002-2004. Environmental conditions including sea-surface temperatures and the timing and intensity of marine primary production varied over the 3 years, and in response, both the timing and success of seabird breeding varied. As in other avian species, concentrations of A4 and T varied markedly among individual eggs in both species (by factors of 3-8), yet contrary to expectation, little of the variation could be attributed to year effects. The high interindividual variation and the lack of interannual variation suggest a non-adaptive explanation for yolk androgen deposition relative to environmental conditions in these species.

Consistent variation in yolk androgens in the Australian Brush-turkey, a species without sibling competition or parental care

Maternal hormones are an excellent pathway for the mother to influence offspring development, and birds provide exceptional opportunities to study these hormone-mediated maternal effects. Two dominant hypotheses about the function of yolk androgens in avian eggs concern maternal manipulation of sibling competition and post hatching paternal care. In megapodes, however, neither sibling competition nor post hatching parental care exists. Eggs are incubated by external heat sources, and chicks dig themselves out of their underground nest and live independently of their parents and their siblings. In this first study on egg androgens of such a megapode, the Australian Brush-turkey Alectura lathami, we found nevertheless substantial amounts of maternal androgens. Since size of the incubation mound, incubation temperature, egg size and laying date greatly vary in this species, we analysed variation in testosterone (T), androstenedione (A4) and dihydrotestosterone (DHT) in relation to these factors. T concentrations were significantly higher in eggs from bigger mounds and laid at greater depth, which may compensate via anabolic effects for the longer duration and higher energetic requirements of chicks when digging themselves out. T concentrations were higher in smaller eggs, and both yolk A4 and T concentrations increased with laying date, perhaps as a compensatory measure, while DHT concentrations only varied across different mounds. These results indicate that maternal androgens may influence offspring development outside the contexts of sibling competition or parental care.