Yolk Androgen Levels Research Articles

Prenatal androgen exposure modulates cellular and humoral immune function of black-headed gull chicks

Avian eggs contain considerable amounts of maternal yolk androgens, which have been shown to beneficially influence the physiology and behaviour of the chick. As androgens may suppress immune functions, they may also entail costs for the chick. This is particularly relevant for colonial species, such as the black-headed gull (Larus ridibundus), in which the aggregation of large numbers of birds during the breeding season enhances the risk of infectious diseases for the hatching chick. To test the effect of maternal yolk androgens on the chick's immune function, we experimentally manipulated, in a field study, yolk androgen levels within the physiological range by in ovo injection of either androgens (testosterone and androstenedione) or sesame oil (control) into freshly laid eggs. We determined cell-mediated immunity (CMI) and humoral immunity of the chicks at the beginning of the nestling period to evaluate early modulatory effects of yolk androgens on immune function. Embryonic exposure to elevated levels of androgens negatively affected both CMI and humoral immunity in nestling gull chicks. Consequently, maternal yolk androgens not only entail benefits of enhanced competitiveness and growth as previously shown, but also costs in terms of immunosuppression. The outcome of embryonic yolk androgen exposure thus likely depends on the post-hatching circumstances for the developing offspring such as parasite exposure and degree of sibling competition.

Egg yolk androgen levels increase with breeding density in the European Starling,Sturnus vulgaris

SummaryHigh breeding density can cause elevated plasma androgen levels in adult birds. Since maternal androgens are deposited into egg yolk, high breeding density may result in elevated yolk androgen levels as well.The relationship between breeding density and yolk androgen levels was examined in the European Starling,Sturnus vulgaris. The concentration and total content of yolk androstenedione and yolk testosterone were measured in eggs from 24 clutches distributed across nine different colonies of nestboxes.Yolk androstenedione and testosterone levels were significantly higher in colonies where a greater proportion of nestboxes had active nests.Yolk testosterone levels were significantly higher, and yolk androstenedione levels were marginally higher, in colonies with a greater absolute number of active nests.Yolk androgen levels were not related to the number of active nests in adjacent nestboxes.We conclude that female starlings nesting in colonies with higher breeding densities transfer more androgen to their eggs.This relationship may be mediated by increased interfemale aggression, particularly towards floater females searching for mates or nests to brood parasitize, in high‐density colonies. Such a relationship between maternal environment and maternal yolk androgens may represent adaptive maternal modification of offspring phenotype or a non‐adaptive physiological constraint which females cannot avoid.

Determinants of within‐ and among‐clutch variation in levels of maternal hormones in Black‐Headed Gull eggs

Summary1. Females of egg‐laying vertebrates may adjust the development of their offspring to prevailing environmental conditions by regulating the deposition of hormones into their eggs. Within‐ and amng‐clutch variation in levels of steroid hormones were studied in the egg yolks of the Black‐Headed Gull (Larus ridibundus, Linnaeus) in relation to environmental conditions at the nest site. This species breeds in colonies of different densities and in different habitats, and the chicks hatch asynchronously.2. Egg yolks contained very high levels of androstenedione, substantial levels of testosterone and moderate levels of 5α‐dihydrotestosterone. Oestrogen (17β‐oestradiol) was not detected.3. Androgen levels increased strongly with laying order, irrespective of egg or yolk mass. This may compensate for the disadvantages of the later hatching chicks. These results have implications for adaptive hypotheses that were proposed for asynchronous incubation.4. Eggs of lighter clutches contained more androgens, perhaps to compensate for a lower nutritional quality of these eggs.5. Birds breeding in the periphery of a colony, being relatively more aggressive and having relatively large territories, laid eggs that contained more androgens than those of birds breeding in the centre. These high yolk androgen levels may facilitate growth and motor development of the chicks, which may be especially important for chicks developing at the periphery of a colony. Reduced levels may be adaptive for birds breeding in the centre, where risk of infectious diseases is high, since steroids may be immunosuppressive.6. Corrected for nest distance, clutches of birds in high vegetation, where predation risk is less severe and therefore competition for nest sites perhaps high, contained relatively high levels of androgens. It is suggested that the level of yolk androgens reflects the hormonal condition of the female, that in turn is influenced by her characteristics such as her age and aggressiveness, and the level of social stimulation.