Effects of prenatal testosterone exposure on antioxidant status and bill color in adult zebra finches.

Permanent offspring modification through maternal hormone transfer is thought to be a tool for mothers to influence life-history trajectories of individual offspring. In birds, yolk hormones influence numerous aspects of the offspring's physiology, including antioxidant status, an important physiological measure that is linked to growth, reproductive effort, and survival. While it is evident that yolk hormones can affect antioxidant status of nestlings, it is not known whether their effect extends beyond the nestling stage. In this study, we use the zebra finch (Taeniopygia guttata) to test experimentally whether exposure to elevated yolk testosterone (T) levels can result in long-term effects on antioxidant status and traits likely to be associated with this measure. Our data show a significant but sex-specific effect with respect to a period from fledging to the age of 7 mo; T males had higher antioxidant status than control males, whereas antioxidant levels did not differ among females and were intermediate compared with the two male groups. Bill color, a trait associated with carotenoids (a specific group of antioxidants) and known to be under the control of circulating levels of T, was not affected by our yolk T manipulation. Bill color (alone or in covariation with egg treatment or sex) did not predict immune responsiveness or antioxidant status. Moreover, there was only weak evidence that antioxidant status predicted the strength of different immune responses. Antioxidant status (in covariation with egg treatment and sex) predicted levels of circulating total antibody levels but did not predict the strength of cell-mediated and humoral immune responses. Our results suggest that yolk T affects antioxidant status independently of these other traits.

Long-lasting consequences of elevated yolk testosterone for metabolism in the zebra finch.

Resting metabolic rate is a common way of quantifying the cost of living in endothermic animals. The trait often makes up a substantial part of an animal's energy budget and can also be related to sustainable peak work rate as well as to daily energy expenditure. Studies have shown that metabolic rates are often heritable, but much of the variation seems to be caused by other factors (e.g., environmental and maternal effects). In a previous study, in ovo exposure to increased levels of testosterone induced metabolic costs early in life. It is, however, unknown whether in ovo androgens also have long-term effects on individual metabolic rates. In this study, we show that experimentally increased levels of in ovo testosterone in zebra finches (Taeniopygia guttata) result in a 7% higher resting metabolic rate when they are adults. This shows that maternally transferred hormones can induce long-term effects on metabolic demands and potentially influence variation in life-history strategies among offspring. Variation in maternal hormone transfer may also explain some of the large interindividual variation observed in metabolic rates.

Stress hormone dynamics: an adaptation to migration?

The hormone corticosterone (CORT) is an important component of a bird's response to environmental stress, but it can also have negative effects. Therefore, birds on migration are hypothesized to have repressed stress responses (migration-modulation hypothesis). In contrast to earlier studies on long-distance migrants, we evaluate this hypothesis in a population containing both migratory and resident individuals. We use a population of partially migratory blue tits (Cyanistes caeruleus) in southern Sweden as a model species. Migrants had higher CORT levels at the time of capture than residents, indicating migratory preparations, adaptation to stressors, higher allostatic load or possibly low social status. Migrants and residents had the same stress response, thus contradicting the migration-modulation hypothesis. We suggest that migrants travelling short distances are more benefited than harmed by retaining the ability to respond to stress.

Patterns and dynamics of rest-phase hypothermia in wild and captive blue tits during winter.

We evaluated biotic and abiotic predictors of rest-phase hypothermia in wintering blue tits (Cyanistes caeruleus) and also assessed how food availability influences nightly thermoregulation. On any given night, captive blue tits (with unrestricted access to food) remained largely homeothermic, whereas free-ranging birds decreased their body temperature (T(b)) by about 5 degrees C. This was not an effect of increased stress in the aviary as we found no difference in circulating corticosterone between groups. Nocturnal T(b) in free-ranging birds varied with ambient temperature, date and time. Conversely, T(b) in captive birds could not be explained by climatic or temporal factors, but differed slightly between the sexes. We argue that the degree of hypothermia is controlled predominantly by birds' ability to obtain sufficient energy reserves during the day. However, environmental factors became increasingly important for thermoregulation when resources were limited. Moreover, as birds did not enter hypothermia in captivity when food was abundant, we suggest that this strategy has associated costs and hence is avoided whenever resource levels permit.

Yolk androgens and the development of avian immunity: an experiment in jackdaws (Corvus monedula).

Maternally derived androgens have been shown to influence offspring phenotype in various ways. In birds, the benefits of prenatal androgen exposure, such as increased competitiveness and accelerated growth in nestlings, have been suggested to be balanced by costs, such as reduced immune function. In this study, we used an integrative approach to examine the influence of yolk androgens on the development of immune function in jackdaw (Corvus monedula) nestlings. Specifically, we tested whether the effects of yolk androgens on offspring immunity may extend over the first few days of life and be detectable even after several weeks. We manipulated yolk androgen concentrations in jackdaw eggs and estimated immune responsiveness by challenging the young with different pathogens at different stages of the nestling period. Six-day-old chicks hatched from eggs with elevated yolk androgen levels had lower pre-challenge antibody titres against lipopolysaccharide (LPS) than control chicks. However, antibody titres against LPS did not differ between treatment groups eight days after a challenge with LPS. During the late nestling phase, both humoral (towards diphtheria/tetanus antigens) and cell-mediated (towards phytohaemagglutinin) immune responsiveness were lower in chicks from yolk androgen-treated eggs compared with control chicks. Our experimental study on jackdaw chicks shows that elevated yolk androgen levels result in a general immunosuppression in offspring; this conclusion was based on results for several immunological tests of both humoral and cell-mediated immunity conducted at 1-2 and 3-4 weeks of age.

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

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

Yolk testosterone modulates persistence of neophobic responses in adult zebra finches, Taeniopygia guttata.

Individual differences in animal behavior can be attributed to genetic as well as non-genetic influences. One mechanism by which the behavioral phenotype of an individual can be shaped is via transmission of maternal sex steroids. In this study, we examined the role of yolk testosterone (T) in controlling neophobia in 9-month-old, sexually mature zebra finches (Taeniopygia guttata). Offspring hatched from either T-treated or control eggs were subjected to a sequential series of behavioral tests in which we measured the neophobic response and its persistence towards two unfamiliar stimuli. Birds from T-treated and control eggs did not differ in their latencies to approach and eat a novel food source during their first encounter. However, egg treatment affected subsequent habituation. Latencies decreased in both groups over a habituation period of 5 days, but considerably more so in T-offspring. Although males appeared to approach novel food faster than females, there was no overall sex effect during the habituation period. When a novel object was added in combination with the previously learned food stimulus, this caused an behavioral shift in approach latencies. In males, control offspring had significantly shorter latencies than T-offspring, whereas there was no difference among females. The latency to eat in the same test was not significantly affected by sex or egg treatment. Our results demonstrate long-term effects of prenatal T on neophobic responses in adult zebra finches. We hypothesize that prenatal T may be one underlying mechanism for individual differences routine formation.

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

Maternal hormones can have substantial phenotypic effects in the progeny of many vertebrates. It has been proposed that mothers adaptively adjust hormone levels experienced by particular young to optimize their reproductive output. In birds, systematic variation in egg hormone levels has been related to different female reproductive strategies. Because in many bird species prospects of the offspring change seasonally and with brood number, strategic adjustment of yolk androgen levels would be expected. To test this idea, we induced pied flycatcher (Ficedula hypoleuca) females to nest twice during the same season by removing their first clutches shortly after clutch completion. We collected eggs of first and replacement clutches to measure yolk concentrations of androstenedione (A4) and testosterone (T) and captured the females that laid these clutches for phenotypic measurements. Although average egg androgen levels were remarkably consistent within females, hormone patterns differed considerably between first and replacement clutches. Eggs of replacement clutches were heavier with larger yolks compared to first clutches, but they contained on average lower levels of androgens. Within clutches, androgen concentration increased over the laying sequence in the first clutch, but decreased or remained more constant over the laying sequence in the replacement clutch. Mean yolk T, but not A4 levels, were negatively associated with laying date for both breeding attempts. Moreover, females in good body condition produced eggs containing lower levels of androgens than females in poor condition. Our results are consistent with the idea that differences in yolk androgen levels may be one mechanism underlying seasonal variation in reproductive success and it is possible that changes in egg androgen patterns may reflect a change in female reproductive strategy. High within-female consistency also highlights the possibility that there may be some underlying genetic variation in yolk androgen levels.

Transgenerational priming of immunity: maternal exposure to a bacterial antigen enhances offspring humoral immunity.

Young vertebrates have limited capacity to synthesize antibodies and are dependent on the protection of maternally transmitted antibodies for humoral disease resistance early in life. However, mothers may enhance fitness by priming their offspring's immune systems to elevate disease resistance. Transgenerational induced defences have been documented in plants and invertebrates, but maternal priming of offspring immunity in vertebrates has been essentially neglected. To test the ability of mothers to stimulate the immune systems of offspring, we manipulated maternal and offspring antigen exposure in a wild population of birds, pied flycatchers (Ficedula hypoleuca). We show that immunization of the mother before egg laying apparently stimulates a transgenerational defence against pathogens by elevating endogenous offspring antibody production. If the disease environments encountered by mothers and offspring are similar, this transgenerational immune priming may allow young to better cope with the local pathogen fauna.