Exogenous testosterone administration decreases rates of ovarian follicle growth in domestic chickens.

Avian egg yolks provide critical nutrients and energy reserves for a growing embryo. Once a follicle is selected into the ovulatory hierarchy, rings of yolk precursors are deposited daily until the yolk is large enough to ovulate. The composition of hen egg yolk and the mechanisms responsible for deposition in the ovarian follicle are well-studied. However, the factors that affect the rate of yolk deposition, and the number of days it takes for follicles to grow to the optimal size for ovulation, are less well-known. Social interactions appear to impact the rate of yolk deposition in many avian species and testosterone is a key hormone involved in regulating those interactions. Therefore, we hypothesized that testosterone would influence the rate of follicle growth. We tested the influences of exogenous testosterone treatment by rubbing either testosterone propionate cream (1.25 %) or a control cream to hen combs daily for 7 days to increase plasma testosterone concentrations. We then collected eggs and measured egg and yolk weight, yolk diameters, and stained and counted the number of concentric rings in each yolk. Testosterone-treated hens laid fewer eggs and grew their yolks more slowly than control hens. Testosterone also caused an increase in yolk weight but not yolk diameter. These results indicate that testosterone may play a role in the rate of yolk deposition in birds.

Prenatal testosterone triggers long-term behavioral changes in male zebra finches: unravelling the neurogenomic mechanisms.

BACKGROUND: Maternal hormones, like testosterone, can strongly influence developing offspring, even generating long-term organizational effects on adult behavior; yet, the mechanisms facilitating these effects are still unclear. Here, we experimentally elevated prenatal testosterone in the eggs of zebra finches (Taeniopygia guttata) and measured male aggression in adulthood along with patterns of neural gene expression (RNA-seq) and DNA methylation (MethylC-Seq) in two socially relevant brain regions (hypothalamus and nucleus taenia of the amygdala). We used enrichment analyses and protein-protein interaction networks to find candidate processes and hub genes potentially affected by the treatment. We additionally identified differentially expressed genes that contained differentially methylated regions. RESULTS: We found that males from testosterone-injected eggs displayed more aggressive behaviors compared to males from control eggs. Hundreds of genes were differentially expressed, particularly in the hypothalamus, including potential aggression-related hub genes (e.g., brain derived neurotrophic factor). There were also enriched processes with well-established links to aggressive phenotypes (e.g., somatostatin and glutamate signaling). Furthermore, several highly connected genes identified in protein-protein interaction networks also showed differential methylation, including adenylate cyclase 2 and proprotein convertase 2. CONCLUSIONS: These results highlight genes and processes that may play an important role in mediating the effects of prenatal testosterone on long-term phenotypic outcomes, thereby providing insights into the molecular mechanisms that facilitate hormone-mediated maternal effects.

Androgen and mineralocorticoid receptors are present on the germinal disc region in laying hens: Potential mediators of sex ratio adjustment in birds?

Female birds skew offspring sex ratios based on environmental and social stimuli; however, the mechanism mediating this phenomenon remains unknown. Growing evidence suggests that testosterone and corticosterone may influence meiosis, as they skew sex ratios when given immediately before chromosomal segregation. It is unclear if these hormones act on the germinal disc (GD) or through a downstream mediator. It is also unknown whether the GD contains receptors for these hormones. If testosterone and/or corticosterone act on the GD to skew sex ratios, then the GD should have receptors for them and that receptor levels should be higher in the GD regions compared to other follicular regions. Furthermore, fluctuations of receptor levels should occur near meiotic segregation. We collected ovarian follicles at 5 h pre-ovulation (just before meiotic segregation) and 20 h pre-ovulation (when sex chromosomes are arrested), and measured androgen receptor (AR) and mineralocorticoid receptor (MR) protein levels via Western blot. ARs and MRs were on the follicle in the GD and non-GD regions, and at 5 h and 20 h pre-ovulation. Both AR and MR protein levels were higher in the GD region than the non-GD region at both time points, but did not differ between time points. These results suggest that hen ovarian follicles have receptors for testosterone and corticosterone, and that the ability for testosterone to respond may be specifically higher in the GD-region, providing further support for the role of testosterone in the alteration of meiotic segregation.

Behavioral phenotype relates to physiological differences in immunological and stress responsiveness in reactive and proactive birds.

It has now been demonstrated in many species that individuals display substantial variation in coping styles, generally separating into two major behavioral phenotypes that appear to be linked to the degree of physiological stress responsiveness. Laying hens are perfect examples of these dichotomous phenotypes; white laying hens are reactive, flighty, and exhibit large hormonal and behavioral responses to both acute and chronic stress, while brown laying hens are proactive, exploratory, and exhibit low hormonal and behavioral responses to stress. Given the linkages between stress physiology and many other body systems, we hypothesized that behavioral phenotype would correspond to additional physiological responses beyond the stress response, in this case, immunological responses. Because corticosterone is widely known to be immunosuppressive, we predicted that the reactive white hens would show more dampened immune responses than the proactive brown hens due to their exposure to higher levels of corticosterone throughout life. To assess immune function in white and brown hens, we compared febrile responses, corticosterone elevations, feed consumption, and egg production that occurred in response an injection of lipopolysaccharide (LPS) or saline, inflammatory responses to phytohemagglutinin (PHA) injection in the toe web, innate phagocytic activity in whole blood, and antibody responses to an injection of Sheep Red Blood Cells (SRBCs). Contrary to our predictions, white hens had significantly greater swelling of the toe web in response to PHA and showed a greater inhibition of feeding and reproductive output in response to LPS. These results indicated that reactive individuals are more reactive in both stress and immunological responsiveness.

Behavioral phenotype predicts physiological responses to chronic stress in proactive and reactive birds.

Animal species display significant variation in personality traits among individuals, and two main coping styles have been identified and termed "proactive" and "reactive". Further, these coping styles appear to correlate directly with the strength of the physiological stress response exhibited by those individuals. In our study system, white laying hens are reactive, flighty, and exhibit large hormonal and behavioral responses to acute stress, while brown laying hens are proactive, exploratory, and exhibit low hormonal and behavioral responses to acute stress. The objective of the current study was to determine if personality type also corresponds to differences in multiple measures of stress when birds are subjected to a chronic stressor. We tested the responses of hens to chronic stress applied by providing feed according to an unpredictable schedule for 14 days, and measured corticosterone concentrations in circulation, expression of heat shock proteins (HSPs), molecules known to protect cells in response to stress, and the ratios of heterophils:lymphocytes in blood, two immune cells known to change in quantity in circulation during chronic stress. We predicted that white hens would show greater physiological responses to the chronic stress treatment. Plasma corticosterone levels significantly increased after 7 days of treatment and returned to baseline levels on day 14, but did not differ significantly between strains. H:L ratios, on the other hand, were significantly elevated by day 7 of treatment, and increased significantly more in brown hens than white. HSP70 and HSP90 expression levels were significantly higher after stress began in white hens than brown. Our results showed that brown hens were more reactive in one response (H:L ratios) while white hens were more reactive in another (HSP expression). These different reactions to the same stressor may represent different strategies for dealing with the same stressor.

Higher rates of internal ovulations occur in broiler breeder hens treated with testosterone.

Maximal profit in both the commercial egg and meat industries requires that the quantity of oviposited eggs closely matches the quantity of large yellow follicles maturing in the ovary. While laying hens are genetically selected for maximal egg production and strategies for management of broiler breeders have been constructed to achieve a similar outcome, a percentage of ovarian follicles that are selected into the ovulatory hierarchy in these hens still never make it to oviposition possibly due to atresia of large yellow follicles or internal ovulation of the oocyte into the peritoneal cavity rather than the oviduct. The causes and mechanisms responsible for these processes remain unclear, however, evidence in wild birds suggests that stressful and/or territorial challenges may stimulate oocyte losses. Since testosterone and corticosterone are central to the responses to territorial intrusions and stress, respectively, and since both large yellow follicles and the oviduct that will engulf them are sensitive to hormonal cues, one or both hormones may play a role in the loss of large yellow follicles via atresia and/or internal ovulation in laying hens. To test this, broiler breeder hens were treated with corticosterone or testosterone 5 h prior to ovulation and observed to see whether these treatments influenced the likelihood that a hen would lay an egg 24 h after the predicted ovulation time. A subset of hens that did not lay an egg were killed and dissected to look for evidence of follicle atresia and internal ovulation. Testosterone treatment resulted in significantly more oocyte losses, and 60% of these occurred due to internal ovulations, as was indicated by the presence of yolk in the peritoneal cavity. Corticosterone did not influence the rate of oocyte losses, follicle atresia, or internal ovulation. These results suggest that testosterone can cause disruptions that ultimately prevent the oviduct from capturing the oocyte after ovulation.

Phenotypic plasticity in response to breeding density in tree swallows: an adaptive maternal effect?

Territorial animals breeding in high-density environments are more likely to engage in aggressive competition with conspecifics for resources necessary for reproduction. In many avian species, increased competition among breeding females results in increased testosterone concentrations in egg yolks. Generally, elevated yolk testosterone increases nestling growth, competitive behaviors, and bold behavioral traits. However, few studies provide an environmental context with which to examine the potential adaptive benefits of these phenotypic changes. In this study, tree swallow (Tachycineta bicolor) breeding density was altered to modify levels of social competition and yolk testosterone. We measured nestling growth, competitive ability, and breathing rate in response to a stressor using a partial cross-foster design. Females breeding at high-density experienced more aggressive, competitive interactions and their eggs had higher testosterone concentrations. Nestlings that hatched in high-density environments grew faster and displayed more competitive behaviors and a higher breathing rate response to a stressor regardless of post-hatching density. Our study demonstrates that phenotypic plasticity occurs in response to yolk testosterone variation resulting from different breeding densities. These findings suggest that naturally-induced maternal effects prepare offspring for competitive environments, supporting the idea that maternal effects are adaptive.

Dietary yolk supplementation decreases rates of yolk deposition in Japanese quail.

Generation of egg yolk by birds requires the synthesis and deposition of large amounts of protein and lipid, and is often accompanied by the incorporation of additional physiological mediators. While there has been much work examining the relative quantities of yolk components, as well as potential adaptive patterns of their allocation, we still do not have a full understanding of what controls yolk formation and composition. Once ovarian follicles are recruited into the preovulatory hierarchy, the yolk is deposited in concentric rings, with one ring deposited per day. Previous studies have shown that there is substantial interspecific and intraspecific variation in the number of rings in yolks, and thus the number of days it took those yolks to grow. We hypothesized that the ability to grow follicles to maturity quickly is limited by the availability of materials to make yolk precursors in the female, either in body reserves or in dietary access. To test this, we supplemented the diets of Japanese quail with hard-boiled chicken yolk and examined the influences of treatment and female body condition on follicle growth rates. Contrary to predictions, females with higher body condition indices produced yolks that grew more slowly, and yolks from supplemented birds grew more slowly than controls. These results indicate that females can modulate the rate of yolk incorporation into developing follicles, and that an energy balance that is too high may not be optimal for the fast growth of developing ovarian follicles.

Dietary yolk supplements and rate of yolk deposition do not influence sex ratios in Japanese Quail.

Birds appear to have the ability to bias offspring sex ratios in response to environmental and social conditions. The mechanisms responsible remain unknown, however one previous study indicated a link between rates of ovarian follicle growth and the sexes of resulting eggs. This could indicate that either follicles destined to retain male or female follicles grow at different rates, or the rate of ovarian follicle growth determines the sex chromosome that will be retained to influence the sex of the resulting offspring. We tested for evidence of both possibilities by staining yolk rings that are indicative of daily growth. First, we tested for a correlation between the number of yolk rings and the sexes of the resulting germinal discs collected from each egg, and second, we tested whether experimentally decreasing follicle growth rates with a dietary yolk supplement impacts the sexes of the resulting germinal discs. There was no significant correlation between the number of yolk rings and the sexes of the resulting embryos, and decreasing follicle growth rates did not impact sexes of resulting germinal discs. These results indicate that offspring sex is not related to the rate of ovarian follicle growth in quail.

Maternal effects mediated by egg quality in the Yellow-legged Gull Larus michahellis in relation to laying order and embryo sex.

BACKGROUND: Maternal effects mediated by egg size and quality may profoundly affect offspring development and performance, and mothers may adjust egg traits according to environmental or social influences. In avian species, context-dependency of maternal effects may result in variation in egg composition, as well as in differential patterns of covariation among selected egg components, according to, for example, position in the laying sequence or offspring sex. We investigated variation in major classes of egg yolk components (carotenoids, vitamins and steroid hormones) in relation to egg size, position in the laying sequence and embryo sex in clutches of the Yellow-legged Gull (Larus michahellis). We also investigated their covariation, to highlight mutual adjustments, maternal constraints or trade-offs in egg allocation. RESULTS: Laying sequence-specific patterns of allocation emerged: concentration of carotenoids and vitamin E decreased, while concentrations of androgens increased. Vitamin A, estradiol and corticosterone did not show any change. There was no evidence of sex-specific allocation or covariation of yolk components. Concentrations of carotenoids and vitamins were positively correlated. Egg mass decreased along the laying sequence, and this decrease was negatively correlated with the mean concentrations of carotenoids in clutches, suggesting that nutritionally constrained females lay low quality clutches in terms of carotenoid content. Finally, clutches with smaller decline in antioxidants between first- and last-laid eggs had a larger increase in yolk corticosterone, suggesting that a smaller antioxidant depletion along the laying sequence may entail a cost for laying females in terms of increased stress levels. CONCLUSIONS: Since some of the analyzed yolk components (e.g. testosterone and lutein) are known to exert sex-specific phenotypic effects on the progeny in this species, the lack of sex-specific egg allocation by mothers may either result from trade-offs between contrasting effects of different egg components on male and female offspring, or indicate that sex-specific traits are controlled primarily by mechanisms of sexual differentiation, including endogenous hormone production or metabolism of exogenous antioxidants, during embryonic development.

Elevated testosterone during meiotic segregation stimulates laying hens to produce more sons than daughters.

Biases in avian sex ratios have been documented in relation to a variety of social and environmental conditions. Previous studies suggest that treatment with hormones can stimulate females to manipulate offspring sex, and that this effect occurs before ovulation. For example, acute and chronic treatments with testosterone stimulated significant skews towards male offspring. Hormones may act by influencing which sex chromosome is donated by the heterogametic female bird into the oocyte. However, it is difficult to pinpoint when effects of testosterone on offspring sex occurred in previous experiments because testosterone treatments were given either chronically over the entire period of follicular development or many hours before the critical period of chromosome segregation. We injected laying hens with testosterone injections 5 h prior to ovulation to target this critical period and quantified the sexes of the subsequently ovulated eggs. We hypothesized that an injection of testosterone coincident with segregation of sex chromosomes would stimulate hens to produce more male than female offspring. As hypothesized, hens injected with testosterone produced a significant bias towards male offspring compared to controls, nearly 70%. These results suggest that acute testosterone elevation during meiotic segregation may mediate skews in avian primary sex ratios.

Detrimental effects of carotenoid pigments: the dark side of bright coloration.

Carotenoid pigments produce yellow, orange, and red integumentary color displays that can serve as reliable signals of health and condition. In many birds and fish, individuals gain competitive or mating advantages by ingesting and utilizing large quantities of carotenoid pigments. Carotenoid pigments serve as antioxidants, performing important functions as free-radical scavengers. The beneficial effects of carotenoid pigments are well documented, but rarely have researchers considered potential detrimental effects of high-level accumulation of carotenoids. We maintained American goldfinches (Carduelis tristis) on high- or low-carotenoid diets through molt and tested for damage to the liver and skeletal muscle. High intake of carotenoids had no measurable effect on liver enzymes but caused an increase in creatine kinase, an indicator of skeletal muscle breakdown, and a reduction in vertical flight performance, a measure of skeletal muscle integrity. The detrimental effects of high-level carotenoid accumulation were approximately equivalent to the negative effects of removing carotenoids from the diet. The adverse effects observed in this study have important implications for theories of the function and evolution of colorful plumage.

Effects of an anthropogenic diet on indicators of physiological challenge and immunity of white ibis nestlings raised in captivity.

When wildlife forage and/or live in urban habitats, they often experience a shift in resource availability and dietary quality. Some species even use human handouts, such as bread, as well as human refuse, as a large part of their new diets; yet the influences of this nutritional shift on health and survival remain unclear. American white ibises are increasingly being seen in urban areas in Florida; they collect handouts, such as bread and other food items, from humans in parks, and are also found foraging on anthropogenic sources in trash heaps. We hypothesized that the consumption of these new anthropogenic food sources may trigger increases in indicators of physiological challenge and dampen immune responses. We tested this experimentally by raising 20 white ibis nestlings in captivity, and exposing 10 to a simulated anthropogenic diet (including the addition of white bread and a reduction in seafood content) while maintaining 10 on a diet similar to what ibises consume in more natural environments. We then tested two indicators of physiological challenge (corticosterone and heat shock protein 70), assessed innate immunity in these birds via bactericidal assays and an in vitro carbon clearance assay, and adaptive immunity using a phytohemagglutinin skin test. The anthropogenic diet depressed the development of the ability to kill Salmonella paratyphi in culture. Our results suggest that consuming an anthropogenic diet may be detrimental in terms of the ability to battle a pathogenic bacterial species, but there was little effect on indicators of physiological challenge and other immunological measures.

Corticosterone and testosterone treatment influence expression of gene pathways linked to meiotic segregation in preovulatory follicles of the domestic hen.

Decades of work indicate that female birds can control their offspring sex ratios in response to environmental and social cues. In laying hens, hormones administered immediately prior to sex chromosome segregation can exert sex ratio skews, indicating that these hormones may act directly on the germinal disc to influence which sex chromosome is retained in the oocyte and which is discarded into an unfertilizable polar body. We aimed to uncover the gene pathways involved in this process by testing whether treatments with testosterone or corticosterone that were previously shown to influence sex ratios elicit changes in the expression of genes and/or gene pathways involved in the process of meiotic segregation. We injected laying hens with testosterone, corticosterone, or control oil 5h prior to ovulation and collected germinal discs from the F1 preovulatory follicle in each hen 1.5h after injection. We used RNA-sequencing (RNA-seq) followed by DESeq2 and gene set enrichment analyses to identify genes and gene pathways that were differentially expressed between germinal discs of control and hormone-treated hens. Corticosterone treatment triggered downregulation of 13 individual genes, as well as enrichment of gene sets related to meiotic spindle organization and chromosome segregation, and additional gene sets that function in ion transport. Testosterone treatment triggered upregulation of one gene, and enrichment of one gene set that functions in nuclear chromosome segregation. This work indicates that corticosterone can be a potent regulator of meiotic processes and provides potential gene targets on which corticosterone and/or testosterone may act to influence offspring sex ratios in birds.

Validation of a noninvasive technique to quantify stress in northern bobwhite (Colinus virginianus).

Examination of the endocrine system through non-invasive fecal sampling may improve population management more than using demographic indicators alone. By addressing the physiological mechanisms that are influencing fitness, management actions can be proactively developed to alleviate stressors. Proactive determination of vulnerable populations is critical for species of concern, such as the Northern Bobwhite (Colinus virginianus), which have suffered decades of population decline. We validated an assay to noninvasively measure the adrenocortical response of captive reared bobwhite through fecal corticosterone metabolites (FCM). All individuals received three sequential 48-hour treatments in which samples were collected every 4 hours, including a reference period, an adrenocorticotropic hormone (ACTH) challenge and a biological stressor (exposure to a hunting dog). Reference FCM values had a mean concentration of 16.75 pg/mg (95% CrI: 13.68, 19.91) with adrenocortical activity increasing by 73% for the duration of the ACTH challenge (29.00 pg/mg; CrI: 25.01, 33.78). FCM concentrations remained similar to that of the reference levels during the biological stressor (16.56 pg/mg; CrI: 13.33, 19.92). Our study validates the use of feces to detect changes in FCM levels in our subject species but also demonstrates the complexity of FCM and the importance of both physiological and biological validation prior to field implementation.

Humans at tropical latitudes produce more females.

Skews in the human sex ratio at birth have captivated scientists for over a century. The accepted average human natal sex ratio is slightly male biased, at 106 males per 100 females or 51.5 per cent males. Studies conducted on a localized scale show that sex ratios deviate from this average in response to a staggering number of social, economical and physiological variables. However, these patterns often prove inconsistent when expanded to other human populations, perhaps because the nature of the influences themselves exhibit substantial cultural variation. Here, data collected from 202 countries over a decade show that latitude is a primary factor influencing the ratio of males and females produced at birth; countries at tropical latitudes produced significantly fewer boys (51.1% males) annually than those at temperate and subarctic latitudes (51.3%). This pattern remained strong despite enormous continental variation in lifestyle and socio-economic status, suggesting that latitudinal variables may act as overarching cues on which sex ratio variation in humans is based.

Frequent double ovipositions in two flocks of laying hens.

Laying hens have been selected over generations for maximal reproductive efficiency. As a result, the incidences of abnormal eggs and abnormal laying patterns are relatively low for these strains. While double ovulations, resulting in the production of double-yolked eggs, does occur on occasion in young birds just entering reproductive maturity, these double ovulations almost never result in 2 fully formed eggs, and are rare in hens that have reached peak reproductive productivity. However, there appears to be an increasing number of anecdotal accounts of backyard hens that lay more than 1 egg per day (hereafter called double oviposition). We hypothesized that double ovipositions in modern strains of laying hens are more common than previously thought. To test this, we observed a flock of Hy-Line W36 hens for 2 consecutive weeks, recording the time of oviposition for each hen on each day. We found that 13% of hens produced a double oviposition at least once during this 2-wk observation period, often producing a second egg within 2.5 h of the first. One hen produced double ovipositions on 3 d during this short period. We then monitored a second flock of hens for 6 wk, collecting eggs that were part of a double oviposition and, on one day, all eggs that were part of double ovipositions. Eggs that were part of double ovipositions showed no external shell abnormalities, and were similar in size, weight, and specific gravity to single ovipositions, suggesting that laying hens can, and do, produce 2 fully formed eggs within hours of one another. We suggest that this may represent selection for maximal reproductive performance in a modern strain of laying hens.

Yolk androgens as pleiotropic mediators of physiological processes: a mechanistic review.

Avian egg yolks are made up of complex mixtures of physiologically relevant substances. Androgens, in particular, accumulate in yolks in variable amounts based on social and environmental conditions experienced by laying females, and using experimental elevations of yolk androgen content, researchers have unveiled potent physiological and behavioral effects on offspring. These patterns and effects are exciting in an adaptive context, as the transfer of endogenously-produced substances such as androgens to egg yolks may allow fine manipulation of offspring phenotype to maximize reproductive success. However, to gain an in-depth understanding of how yolk androgens function in an adaptive context, we must first understand the complex entanglement of physiological and endocrine interrelationships that change after exposure to yolk androgens. Here, we take a comparative approach towards a discussion of how yolk androgens can simultaneously affect multiple body systems within developing birds, ultimately resulting in the large-scale phenotypic endpoints that may represent adaptive consequences of exposure to elevated levels of yolk androgens.

Livestock abundance predicts vampire bat demography, immune profiles and bacterial infection risk.

Human activities create novel food resources that can alter wildlife-pathogen interactions. If resources amplify or dampen, pathogen transmission probably depends on both host ecology and pathogen biology, but studies that measure responses to provisioning across both scales are rare. We tested these relationships with a 4-year study of 369 common vampire bats across 10 sites in Peru and Belize that differ in the abundance of livestock, an important anthropogenic food source. We quantified innate and adaptive immunity from bats and assessed infection with two common bacteria. We predicted that abundant livestock could reduce starvation and foraging effort, allowing for greater investments in immunity. Bats from high-livestock sites had higher microbicidal activity and proportions of neutrophils but lower immunoglobulin G and proportions of lymphocytes, suggesting more investment in innate relative to adaptive immunity and either greater chronic stress or pathogen exposure. This relationship was most pronounced in reproductive bats, which were also more common in high-livestock sites, suggesting feedbacks between demographic correlates of provisioning and immunity. Infection with both Bartonella and haemoplasmas were correlated with similar immune profiles, and both pathogens tended to be less prevalent in high-livestock sites, although effects were weaker for haemoplasmas. These differing responses to provisioning might therefore reflect distinct transmission processes. Predicting how provisioning alters host-pathogen interactions requires considering how both within-host processes and transmission modes respond to resource shifts.This article is part of the theme issue 'Anthropogenic resource subsidies and host-parasite dynamics in wildlife'.

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.