Claudia Müller

Effects of a short period of elevated circulating corticosterone on postnatal growth in free-living Eurasian kestrels Falco tinnunculus

SUMMARY Environmental conditions affect growth and development and, through developmental plasticity, create phenotypic variation. In suboptimal conditions current survival is traded-off against development. Corticosterone, the main glucocorticoid in birds, may be involved in the reallocation of energy from growth to maintenance, but its effect on growth has rarely been investigated in altricial birds under natural conditions in the wild. In free-living Eurasian kestrel Falco tinnunculus nestlings, we artificially elevated corticosterone to stress-induced levels over 2–3 days in the middle of the nestling stage by implanting biodegradable implants, controlling the treatment with a placebo group. We measured the length of primary feather 8, hand length, tarsus length, body mass and subcutaneous fat stores from day 10 to 25. During corticosterone elevation, primary growth of cort-nestlings was significantly reduced to 71% of placebo-nestlings, hand and tarsus growth were significantly reduced to 14% and 26% of placebo-nestlings, respectively, and body mass increase stopped, while subcutaneous fat-store growth was not affected. Over the following 5 days, primary growth was still significantly suppressed to 84% of placebo-nestlings, while hand, tarsus and body mass growth were back to normal. During the subsequent 4 days, cort-nestlings partly compensated for the lag in body mass by significantly accelerating the body mass increase compared with placebo-nestlings. Before fledging, primary length was 10% shorter, hand and tarsus 5% and 4% shorter and body mass 8.5% lower in cort-nestlings than in placebo-nestlings, while fat score did not differ significantly between the two groups. Thus, we have shown that in free-living, altricial nestlings a few days of elevated plasma corticosterone levels alone, without food restriction, suppressed growth and this could only partly be compensated for afterwards. Feather, bone and body mass growth were reduced to different degrees, indicating that corticosterone had a differential effect on different structures. This demonstrates that corticosterone is probably involved in the control of developmental plasticity.

Effects of corticosterone pellets on baseline and stress-induced corticosterone and corticosteroid-binding-globulin

Exogenous administration of glucocorticoids is a widely used and efficient tool to investigate the effects of elevated concentrations of these hormones in field studies. Because the effects of corticosterone are dose and duration-dependent, the exact course of plasma corticosterone levels after exogenous administration needs to be known. We tested the performance of self-degradable corticosterone pellets (implanted under the skin) in elevating plasma corticosterone levels. We monitored baseline (sampled within 3min after capture) total corticosterone levels and investigated potential interactions with corticosteroid-binding-globulin (CBG) capacity and the endogenous corticosterone response to handling in Eurasian kestrel Falco tinnunculus and barn owl Tyto alba nestlings. Corticosterone pellets designed for a 7-day-release in rodents elevated circulating baseline total corticosterone during only 2-3 days compared to placebo-nestlings. Highest levels occurred 1-2days after implantation and levels decreased strongly thereafter. CBG capacity was also increased, resulting in a smaller, but still significant, increase in baseline free corticosterone levels. The release of endogenous corticosterone as a response to handling was strong in placebo-nestlings, but absent 2 and 8 days after corticosterone pellet implantation. This indicates a potential shut-down of the hypothalamo-pituitary-adrenal axis after the 2-3 days of elevated baseline corticosterone levels. 20 days after pellet implantation, the endogenous corticosterone response to handling of nestlings implanted with corticosterone pellets attained similar levels as in placebo-nestlings. Self-degradable pellets proved to be an efficient tool to artificially elevate circulating baseline corticosterone especially in field studies, requiring only one intervention. The resulting peak-like elevation of circulating corticosterone, the concomitant elevation of CBG capacity, and the absence of an endogenous corticosterone response to an acute stressor have to be taken into account.

Development of the adrenocortical response to stress in Eurasian kestrel nestlings: defence ability, age, brood hierarchy and condition.

The developmental hypothesis proposes that the adrenocortical response to stress during postnatal development in birds should not develop when the benefits of elevated corticosterone do not outweigh the deleterious effects on growth and development. We tested three predictions developed from this hypothesis in free-living, semi-altricial Eurasian kestrel Falco tinnunculus nestlings. We measured baseline and handling-induced corticosterone levels and the binding capacity of corticosteroid binding globulins CBG on day 10 and 21 of age and related these to age, the development of the defence behaviour, hatching asynchrony and fat stores (a measure of body condition). First, the adrenocortical response to handling (total plasma corticosterone) increased with age and thus during the time when nestlings developed the ability to defend themselves, but free corticosterone did not, because of a concomitant increase of CBG with age. Second, nestlings with adequate fat stores mounted a stronger adrenocortical stress response to an acute stressor, while nestlings with low fat stores avoided additional energy expenses. While baseline corticosterone levels were negatively related to fat stores, increase in corticosterone to handling was positively related. Third, both baseline corticosterone levels and the adrenocortical response to handling were not related to hatching order, but predominantly determined by body condition. The pattern of decreasing corticosterone levels with hatching order found in the lab seems to be neutralized by opposite effects of varying body condition on corticosterone levels in free-living birds. We argue that the postnatal adrenocortical response to stress is adaptively modulated by both variations in the release of corticosterone and in CBG, which is particularly important because elevated corticosterone may adversely affect the phenotype.

Systemische Risikofaktoren relativieren den alleinigen Einfluss von Ernährung und Bewegung bei der Entstehung von Übergewicht und Adipositas bei Kindern und Jugendlichen

Vor dem Hintergrund einer steigenden Pravalenz von Adipositas und Ubergewicht bei Kindern und Jugendlichen, die manchen Institutionen zufolge epidemische Ausmase anzunehmen scheint (WHO 1998), und dem Versagen der meisten bisherigen therapeutischen und praventiven Masnahmen (Stice et al. 2006) wurde in der vorliegenden Arbeit das komplexe Problem Adipositas auf physiologischer, soziologischer und psychologischer Ebene untersucht. Mittels standardisierter Datenerhebung wurde die Lebenssituation von 13-18jahrigen Jugendlichen und ihren Eltern in ihren verschiedenen Facetten erfasst und die Risikofaktoren fur Ubergewicht und Adipositas im Jugendalter ermittelt. Diese Altersgruppe ist von besonderer Relevanz, da ubergewichtige und adipose Jugendliche das hochste Risiko haben, diesen Gewichtsstatus im Erwachsenenalter beizubehalten (Wabitsch et al. 2002), und Praventionsprogramme im Vergleich zu anderen Altersgruppen (Kinder, Prapubertierende) den grosten Erfolg versprechen (Stice et al. 2006).