Androgen Deposition Research Articles

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.

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

The field of androgen deposition in avian eggs and its consequences for offspring development has received a lot of attention in recent research. However, although variation within clutches in yolk androgens is relatively well understood, the adaptive significance of patterns of variation between clutches remains rather unclear. Furthermore, it has been hypothesized that yolk androgens act as a means of an adaptive maternal effect to adjust offspring to a given posthatching environment. Thus, the consequences of maternal yolk androgens for offspring development are likely to depend on the specific environment of a given brood. We experimentally manipulated yolk androgen concentrations in spotless starling eggs, using a between-brood design in which full broods were manipulated applying either an androgen or sham treatment, in order to test the effects of between-brood variation in yolk androgen levels. We also included in the analysis several female characters that have been shown to affect androgen deposition. Androgen-chicks tended to gain more mass, a similar effect to that shown in previous studies where direct competition between chicks belonging to different treatments was allowed, but did not gain a survival benefit. Androgen-chicks had wider beak flanges, an effect that has not been described previously and could 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 hypothesize that these effects are an important mechanistic link for our understanding of how yolk androgens exert their effects on offspring development after hatching. Contrary to our predictions, we found no evidence that the observed effects depended on the environment under which it was investigated. Because high yolk androgen levels seem to be beneficial for nestlings in this species, we hypothesize that yolk hormone deposition could be costly for females or alternatively that potential negative effects later in adulthood may constrain maternal hormone allocation.

Food availability affects the maternal transfer of androgens and antibodies into eggs of a colonial seabird

Mothers can improve the quality of their offspring by increasing the level of certain components in their eggs. To examine whether or not mothers increase deposition of such components in eggs as a function of food availability, we food-supplemented black-legged kittiwake females (Rissa tridactyla) before and during egg laying and compared deposition of androgens and antibodies into eggs of first and experimentally induced replacement clutches. Food-supplemented females transferred lower amounts of androgens and antibodies into eggs of induced replacement clutches than did non-food-supplemented mothers, whereas first clutches presented no differences between treatments. Our results suggest that when females are in lower condition, they transfer more androgens and antibodies into eggs to facilitate chick development despite potential long-term costs for juveniles. Females in prime condition may avoid these potential long-term costs because they can provide their chicks with more and higher quality resources.

Yolk androgen deposition as a female tactic to manipulate paternal contribution

Yolk androgen deposition as a female tactic to manipulate paternal contribution

Orchestration of avian reproductive effort: an integration of the ultimate and proximate bases for flexibility in clutch size, incubation behaviour, and yolk androgen deposition

How much effort to expend in any one bout of reproduction is among the most important decisions made by an individual that breeds more than once. According to life-history theory, reproduction is costly, and individuals that invest too much in a given reproductive bout pay with reduced reproductive output in the future. Likewise, investing too little does not maximize reproductive potential. Because reproductive effort relative to output can vary with predictable and unpredictable challenges and opportunities, no single level of reproductive effort maximizes fitness. This leads to the prediction that individuals possessing behavioural mechanisms to buffer challenges and take advantage of opportunities would incur fitness benefits. Here, we review evidence in birds, primarily of altricial species, for the presence of at least two such mechanisms and evidence for and against the seasonal coordination of these mechanisms through seasonal changes in plasma concentrations of the pituitary hormone prolactin. First, the seasonal decline in clutch size of most bird species may partially offset a predictable seasonal decline in the reproductive value of offspring. Second, establishing a developmental sibling-hierarchy among offspring may hedge against unpredictable changes in resource availability and offspring viability or quality, and minimize energy expenditure in raising a brood. The hierarchy may be a product, in part, of the timing of incubation onset relative to clutch completion and the rate of yolk androgen deposition during the laying cycle. Because clutch size should influence the effects of both these traits on the developmental hierarchy, we predicted and describe evidence in some species that females adjust the timing of incubation onset and rate of yolk androgen deposition to match clutch size. Studies on domesticated precocial species reveal an inhibitory effect of the pituitary hormone prolactin on egg laying, suggesting a possible hormonal basis for the regulation of clutch size. Studies on the American kestrel (Falco sparverius) and other species suggest that the seasonal increase in plasma concentrations of prolactin may regulate both a seasonal advance in the timing of incubation onset and a seasonal increase in the rate of yolk androgen deposition. These observations, together with strong conceptual arguments published previously, raise the possibility that a single hormone, prolactin, functions as the basis of a common mechanism for the seasonal adjustment of reproductive effort. However, a role for prolactin in regulating clutch size in any species is not firmly established, and evidence from some species indicates that clutch size may not be coupled to the timing of incubation onset and rate of yolk androgen deposition. A dissociation between the regulation of clutch size and the regulation of incubation onset and yolk androgen deposition may enable an independent response to the predictable and unpredictable challenges and opportunities faced during reproduction.

Orchestration of avian reproductive effort: an integration of the ultimate and proximate bases for flexibility in clutch size, incubation behaviour, and yolk androgen deposition

How much effort to expend in any one bout of reproduction is among the most important decisions made by an individual that breeds more than once. According to life-history theory, reproduction is costly, and individuals that invest too much in a given reproductive bout pay with reduced reproductive output in the future. Likewise, investing too little does not maximize reproductive potential. Because reproductive effort relative to output can vary with predictable and unpredictable challenges and opportunities, no single level of reproductive effort maximizes fitness. This leads to the prediction that individuals possessing behavioural mechanisms to buffer challenges and take advantage of opportunities would incur fitness benefits. Here, we review evidence in birds, primarily of altricial species, for the presence of at least two such mechanisms and evidence for and against the seasonal coordination of these mechanisms through seasonal changes in plasma concentrations of the pituitary hormone prolactin. First, the seasonal decline in clutch size of most bird species may partially offset a predictable seasonal decline in the reproductive value of offspring. Second, establishing a developmental sibling-hierarchy among offspring may hedge against unpredictable changes in resource availability and offspring viability or quality, and minimize energy expenditure in raising a brood. The hierarchy may be a product, in part, of the timing of incubation onset relative to clutch completion and the rate of yolk androgen deposition during the laying cycle. Because clutch size should influence the effects of both these traits on the developmental hierarchy, we predicted and describe evidence in some species that females adjust the timing of incubation onset and rate of yolk androgen deposition to match clutch size. Studies on domesticated precocial species reveal an inhibitory effect of the pituitary hormone prolactin on egg laying, suggesting a possible hormonal basis for the regulation of clutch size. Studies on the American kestrel (Falco sparverius) and other species suggest that the seasonal increase in plasma concentrations of prolactin may regulate both a seasonal advance in the timing of incubation onset and a seasonal increase in the rate of yolk androgen deposition. These observations, together with strong conceptual arguments published previously, raise the possibility that a single hormone, prolactin, functions as the basis of a common mechanism for the seasonal adjustment of reproductive effort. However, a role for prolactin in regulating clutch size in any species is not firmly established, and evidence from some species indicates that clutch size may not be coupled to the timing of incubation onset and rate of yolk androgen deposition. A dissociation between the regulation of clutch size and the regulation of incubation onset and yolk androgen deposition may enable an independent response to the predictable and unpredictable challenges and opportunities faced during reproduction.

Yolk androgens vary inversely to maternal androgens in Eastern Bluebirds: an experimental study

1 Female birds deposit variable amounts of androgens in the yolks of their eggs, and it has been suggested that yolk androgen deposition is an adaptive mechanism preparing offspring for a competitive environment. Breeding pairs of Eastern Bluebirds (Sialia sialis) Linnaeus were stimulated with an intruder presentation while ovarian follicles were developing. Yolk steroid concentrations in eggs laid by stimulated females were then compared with yolk steroid concentrations in eggs laid by control females. Additionally, blood samples taken from a subset of control and stimulated females were analysed for plasma steroid hormone concentrations. We predicted that female bluebirds experiencing a simulated intrusion would experience increased levels of circulating plasma androgens that would be reflected by larger amounts of androgens deposited in their eggs compared with control females. 2 Patterns of steroid concentrations differed between egg yolks and female plasma. In egg yolks, androstenedione was the predominant hormone, followed by testosterone. Yolks contained minimal amounts of both corticosterone and oestradiol. In female plasma, however, corticosterone was the predominant hormone, while sex steroids were found at low levels. 3 Yolk steroid concentrations did not vary with laying order in either the control or the stimulated group, a result expected due to the relatively synchronous nature of incubation behaviour exhibited by Eastern Bluebird females. 4 Yolk androgen concentrations in eggs laid by stimulated females were significantly higher than in those laid by control females, suggesting that females increase yolk androgen deposition in response to aggressive encounters. 5 Females exposed to an intruder presentation contained significantly lower levels of plasma androgens than control females. We suggest that the deposition of androgens in the eggs serves as an adaptive method of regulating circulating androgen levels in the female, preventing potentially disruptive elevations in circulating androgen concentrations during a particularly sensitive period in the reproductive cycle.

Female house martins (Delichon urbica) reduce egg androgen deposition in response to a challenge of their immune system

Female birds deposit in the yolks of eggs substantial amounts of androgens, such as testosterone and androstenedione. These androgens have been shown to speed up nestling development, induce a fast development of ornaments and increase dominance in adults. Experiments in several species have reported that females invest greater amounts of androgens in the eggs fathered by attractive males, suggesting that yolk androgen is a costly investment for either the offspring or the mother. There is some evidence that nestling immunocompetence may be partially suppressed by high levels of yolk androgens, but it is not known whether this is also the case for females. We tested this hypothesis in the house martin by inducing an immune challenge through an injection of sheep red blood cells, a standard challenge of the humoral immune system. Experimental birds laid eggs with lower amounts of yolk androstenedione than controls, and there was a similar non-significant trend for testosterone. Furthermore, the probability of laying a replacement clutch was higher for birds that had laid a first clutch with relatively high levels of yolk testosterone. These results suggest that yolk androgen deposition is limited by immune costs in the female, and that only females in good condition may afford to invest high levels of androgen in eggs in this species.

Yolk androgen deposition as a compensatory strategy

The deposition of androgens into the yolks of eggs can have long-lasting effects on the growth and development of young birds. It has been proposed that female birds and reptiles deposit yolk androgens according to the differential allocation hypothesis (DAH), which posits the allocation of more resources to offspring sired by more attractive, higher-quality males. We examined deposition patterns of yolk androgens in relation to mate attractiveness in the house finch Carpodacus mexicanus. Contrary to the predictions of the DAH, female house finches deposited significantly more androgens into eggs sired by less attractive males. We propose that, rather than serving as resources, androgens are used as mediators in a compensatory distribution strategy, enabling females to improve the quality of young produced with less attractive males.

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.

Negative effects of elevated testosterone on female fecundity in zebra finches

Although factors influencing androgen deposition in the avian egg and its effects on nestling fitness are recently receiving considerable attention, little is known about the potential costs of high testosterone levels in the females. Our study aimed at determining the effect of injections of testosterone (T) in female zebra finches ( Taeniopygia guttata), on clutch size, egg mass, yolk mass, and yolk androgen content. Females were given a single bolus injection of T in a range of doses after laying the first egg. Results show that administration of T negatively affected clutch size; the strength of this effect increased with increasing doses of T. Females injected with the highest testosterone dose showed suppressed oviposition of the third and the fourth eggs. Interestingly, testosterone administration made females produce eggs with relatively large yolks, suggesting that T may mediate the trade-off between number and size of eggs. Testosterone injection resulted in elevated levels of androgen in the eggs, in contrast to control clutches, which showed a decreasing pattern of androgen concentration along the laying sequence. We conclude that high androgen investment in eggs may be limited by physiological requirements of the ovulatory process.

Within-clutch patterns of yolk testosterone vary with the onset of incubation in black-headed gulls

Hatching asynchrony in birds produces an age and size hierarchy among siblings. Later-hatching chicks have a competitive disadvantage, and brood reduction may occur when food availability is insufficient to raise all chicks. When early-hatched chicks fail to survive or if the circumstances allow raising all chicks, mothers should reverse the disadvantage to later-hatched chicks. Increasing deposition of maternal androgens with the laying sequence has been suggested to compensate for detrimental effects of hatching asynchrony, allowing a more precise adjustment of the survival probabilities of each chick. Here, we show for black-headed gulls that the increase in yolk testosterone with each successive egg is greater when the mother incubates longer before clutch completion, which is the major determinant of the degree of hatching asynchrony. This finding supports the idea that yolk testosterone has a compensatory function in the context of hatching asynchrony. Our data further show that if the time needed to complete a clutch is lengthened, the developmental differences due to incubation between the first- and the last-laid eggs increase. In addition, the onset of incubation before clutch completion occurs sooner as the breeding season progresses. Both long inter-egg intervals and the seasonal shift in incubation behavior enhance the necessity of compensation for later-hatching chicks. Indeed, yolk levels of testosterone increased more steeply over the laying order, if the duration of the egg-laying period was extended and in later-laid clutches. We suggest that prolactin plays a key role in the adjustment of testosterone allocation to the incubation pattern.

Sex-specific patterns of yolk androgen allocation depend on maternal diet in the zebra finch

Females are predicted to adjust their reproductive investment in relation to resource quality. In zebra finches (Taeniopygia guttata), diet quality has been found to influence egg mass both between and within clutches. We tested the prediction that diet quality also affects the quantity of maternally allocated yolk testosterone and 5a-dihydrotestosterone (DHT) between and within clutches. We also investigated whether this pattern differed between male and female eggs. Females laid eggs on a high-quality (HQ) or a low-quality (LQ) diet. Eggs were removed at laying and artificially incubated for 72 h, after which time embryos were sexed and yolk androgens assayed. Diet treatments were then swapped and the experiment repeated. Because there was evidence of a carry-over effect between breeding rounds, we based our conclusions mainly on the results from the first breeding round. On the HQ diet, but not on the LQ diet, infertile eggs contained more testosterone than did fertile eggs in round one. Although there were no overall differences in within-clutch patterns of androgen deposition between the diets, this changed when embryo sex was taken into account. On the HQ diet, testosterone decreased with laying sequence for male eggs but increased with laying sequence for female eggs. On the LQ diet, mothers’ male eggs contained more testosterone and DHT than did female eggs regardless of position in the laying sequence. Our data suggest that there are complex, context-dependent mechanisms of sexspecific androgen allocation in this species. Key words: diet quality, sex allocation, Taeniopygia guttata, yolk androgens, zebra finch. [Behav Ecol]

Sexually attractive phrases increase yolk androgens deposition in Canaries ( Serinus canaria)

The androgen concentration in birds’ eggs varies with laying order, breeding conditions, and mate attractiveness. In passerine birds, mate attractiveness depends upon song quality. The aim of our study was to evaluate the effect of one criterion used by females to assess male song quality that is to say the presence of sexually attractive phrases on yolk androgen deposition. Twenty-five female Canaries were assigned to three experimental groups; in the first group, the females were allowed to hear songs made up with attractive phrases; in the second group, they were allowed to hear songs made up with non-attractive phrases; and in the control group, the females could not hear any song. Our results show that females allowed to hear songs with attractive phrases deposit significantly higher amounts of androgens (mostly testosterone) in their eggs than females without acoustical stimulation. The females exposed to songs with non-attractive phrases had androgen amounts halfway between the two other groups. This suggests that when females are paired with mates able to sing attractive phrases they can allocate more androgens in their eggs during the pre-laying period.

Sex differences in yolk hormones depend on maternal social status in Leghorn chickens (Gallus gallus domesticus).

Maternal hormones are known to be present in avian eggs and can have beneficial effects on chick development. Recently, differences in avian yolk steroid concentrations between the sexes have been demonstrated, and in this context steroids have been proposed to be part of the avian sex-determining mechanism. In our study, we show that it is very unlikely that androgen concentrations alone are the decisive part of the sex-determining mechanism. We found that sex-specific differences in the yolk hormones strongly depend on the social rank of the mother. First, dominant females, but not subdominant females, allocated significantly more testosterone to male eggs than to female eggs. Second, subordinate females increased the testosterone concentrations of female eggs. This pattern of yolk hormone deposition can be functionally explained. In polygynous species such as the chicken, reproductive success is more variable in males than in females. Parental investment in sons or daughters is therefore expected to occur in direct relation to parental rearing capacities. We found that the social status of a hen was indeed negatively correlated with her maternal capacities (for example, body mass, egg mass). Differential androgen deposition might thus provide a mechanism for adaptive maternal investment depending on both the sex of the egg and the social status of the mother.

Plasma Corticosterone in Nestling American Kestrels: Effects of Age, Handling Stress, Yolk Androgens, and Body Condition

The effects of age, handling-induced stress, yolk androgens, and body condition on plasma corticosterone levels were investigated in free-living nestling American kestrels, Falco sparverius, a semialtricial falcon species. In an observational study, corticosterone levels varied with age and handling time. Specifically, corticosterone was low until age 15 days and then rose from age 20 through 25 days. Nestlings as young as age 10 days showed a handling-induced rise in corticosterone. Neither sex nor hatching order of the nestling affected corticosterone levels. Concentrations of maternally derived yolk androgens have previously been shown to be lower in first-laid than in later-laid eggs in the clutch. In an experimental study, androgens were injected into the yolk of the first-laid egg to elevate its levels to those of later-laid eggs, a treatment that substantially reduces nestling body condition compared with that of controls. Yolk androgen treatment elevated posthatching corticosterone levels compared with those of controls, and corticosterone levels were negatively correlated with body condition. These findings indicate that even very young, developing birds can show stress-induced increases in corticosterone and that age-related changes in corticosterone secretion may be modified by body condition and maternal effects such as yolk androgen deposition. The short- and long-term consequences of high glucocorticosteroid levels in young, developing vertebrates are largely unknown.

Hatching asynchrony and maternal androgens in egg yolks of House Wrens

Synchronously and asynchronously hatching clutches of House Wrens Troglodytesaedon usually do not differ in reproductive success. Thus late‐hatching nestlings in asynchronously hatching clutches somehow overcome any age‐ and size‐related disadvantages of hatching after their nest‐mates. One possible way for them to do this is for female House Wrens to add maternal androgens to the yolk of late‐hatching eggs. We tested this hypothesis in a wild population of House Wrens that produces both asynchronously and synchronously hatching clutches. Yolks of eggs from both types of clutches were biopsied and the eggs returned to their nests to hatch. Radioimmunoassays revealed that total androgen levels in the yolk varied within and among clutches. However, total androgen levels in yolks did not vary predictably with egg position in either synchronously or asynchronously hatching clutches. Thus, deposition of androgens in yolk did not follow the expected pattern based on the potential for sibling competition in House Wrens.

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.

The price of eggs: increased investment in egg production reduces the offspring rearing capacity of parents

Understanding the selective pressures shaping the number of offspring per breeding event is a key area in the study of life-history strategies. However, in species with parental care, costs incurred in offspring production, rather than rearing, have been largely ignored in both theoretical and empirical studies until relatively recently. Furthermore, the few experimental studies that have manipulated production costs have not yet teased apart effects that operate via the parental phenotype from effects on the quality of the resulting young. To examine whether increased egg production influences parental brood rearing capacity independently of effects operating via egg quality, we experimentally increased egg production in gulls and then examined their capacity to rear a control clutch. We found that the capacity of parents to rear the control brood was substantially reduced solely as a consequence of having themselves produced one extra egg. The paradox that, in many species, parents apparently aim for fewer young per breeding event than the experimentally and theoretically demonstrated optimum, has partly arisen from the failure to take into account the constraints imposed by production costs.