| 1. |
- Sandell, Maria, et al.
(author)
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Yolk androgens and the development of avian immunity: an experiment in jackdaws (Corvus monedula).
- 2009
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In: Journal of Experimental Biology. - : The Company of Biologists. - 1477-9145 .- 0022-0949. ; 212:Pt 6, s. 815-822
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Journal article (peer-reviewed)abstract
- 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.
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| 2. |
- Tobler, Michael, et al.
(author)
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Costly steroids: egg testosterone modulates nestling metabolic rate in the zebra finch
- 2007
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In: Biology letters. - : The Royal Society. - 1744-9561 .- 1744-957X. ; 3:4, s. 408-410
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Journal article (peer-reviewed)abstract
- The transfer of non-genetic resources from mother to the offspring often has considerable consequences for offspring performance. In birds, maternally derived hormones are known to influence a variety of morphological, physiological and behavioural traits in the chick. So far, the range of these hormonal effects involves benefits in terms of enhanced growth and competitive ability as well as costs in terms of immunosuppression. However, since yolk hormones can enhance growth and begging activity, high levels of these hormones may also involve energetic costs. Here, we show experimentally that elevated levels of prenatal testosterone increase resting metabolic rate in nestling zebra finches (Taeniopygia guttata). Surprisingly, however, elevation of prenatal testosterone did not result in higher growth rates and, thus, differences in resting metabolism do not seem to be linked to nestling growth. We conclude that apart from immunosuppressive effects, high levels of egg steroids may also entail costs in terms of increased energy expenditure.
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| 3. |
- Tobler, Michael, et al.
(author)
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Maternal androgens in the pied flycatcher; timing of breeding and within-female consistency
- 2007
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In: Oecologia. - : Springer Science and Business Media LLC. - 1432-1939 .- 0029-8549. ; 151:4, s. 731-740
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Journal article (peer-reviewed)abstract
- 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
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| 4. |
- Tobler, Michael
(author)
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Maternal programming: costs, benefits and constraints of maternal hormone transfer
- 2007
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Doctoral thesis (other academic/artistic)abstract
- Avian mothers convey significant quantities of androgens (i.e. ?male? steroids) to their eggs, which are known to positively influence embryonic development, post-natal growth and competitive behaviour of the offspring. However, there is large inter-female variation in the amount of androgens transferred to the eggs. Recent studies suggest that this variation may be maintained because the positive effects on growth and behaviour are counterbalanced by negative effects on the immune function of the offspring. This thesis deals with the causal and functional aspects of maternal androgen transfer in birds. In a study on a wild pied flycatcher (Ficedula hypoleuca) population, I investigated female plasticity in androgen transfer with respect to the timing of breeding. Eggs of replacement clutches produced late in the season contained on average lower levels of androgens compared to first clutches produced early in the season. 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. Differential androgen transfer with respect to timing of breeding is discussed as an adaptive female tactic to optimize reproductive output. To examine the effects of egg androgens on offspring phenotype, I compared offspring hatched from eggs with experimentally elevated androgen levels with offspring hatched from eggs with sham-manipulated androgen levels (controls). I found that exposure to elevated egg levels of testosterone (T) increased metabolism in zebra finch (Taeniopygia guttata) chicks. Extra egg T also enhanced growth in male zebra finch chicks, but at the same time reduced their ability to resist oxidative stress during the early growth phase. These results are the first to demonstrate that high levels of egg androgens can entail costs in terms of increased energy expenditure and increased susceptibility to oxidative stress. To evaluate whether there are any long-term effects of egg T on offspring physiology and behaviour, I followed a cohort of zebra finches hatched from manipulated eggs. Although no effects of egg T on adult immunity were found, there appeared to be a positive effect of high levels of egg T on oxidative stress resistance in adult males when they were subjected to an immunization with a pathogen. Furthermore, egg T influenced adult behaviour in males and females. High levels of egg T facilitated the ability of adult zebra finches to habituate to a novel situation. The effects of egg androgens on growth and immunity were also investigated in a study on wild jackdaws (Corvus monedula). In this study, there was a clear negative effect of high egg androgen levels on nestling immunity. The study shows that the effect is present during different developmental periods of the nestlings, and most interestingly, still present just before fledging. The results from these experimental studies add to our understanding of how transgenerational transmission of hormones affect the formation of the offspring phenotype.
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| 5. |
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| 6. |
- Tobler, Michael, et al.
(author)
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Sex-specific effects of prenatal testosterone on nestling plasma antioxidant capacity in the zebra finch.
- 2009
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In: Journal of Experimental Biology. - : The Company of Biologists. - 1477-9145 .- 0022-0949. ; 212:1, s. 89-94
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Journal article (peer-reviewed)abstract
- 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.
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| 7. |
- Tobler, Michael, et al.
(author)
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Yolk testosterone modulates persistence of neophobic responses in adult zebra finches, Taeniopygia guttata
- 2007
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In: Hormones and Behavior. - : Elsevier BV. - 1095-6867 .- 0018-506X. ; 52:5, s. 640-645
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Journal article (peer-reviewed)abstract
- 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. (C) 2007 Elsevier Inc. All rights reserved.
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