Jaanis Lodjak will defend his doctoral thesis titled "Association of insulin-like growth factor I and corticosterone with nestling growth and fledging success in wild passerines" on 22 April at 10.15 at Vanemuise 46, room 301.
Supervisor: teadur Marko Mägi, PhD (TÜ ÖMI)
Opponent: professor Simon Verhulst (Groningeni Ülikool, Holland)
Summary: The central goal of evolutionary ecology is to explain how certain traits, like growth rate, body weight and survival, have evolved to their current values. Postnatal growth rate is an important life-history trait that is directly associated with survival during the transition to independence and in the overwinter period. Since natural conditions constantly change, animals have to be physiologically flexible and forever adapt to the environment to keep the postnatal growth rate in the highest levels possible given the surrounding conditions. Hormones such as insulin-like growth factor 1 (IGF-1) and glucocorticoids (stress hormones) are potentially important mediators of developmental plasticity, although their function is quite poorly understood in free-living animals to date. In this thesis, brood size manipulation and a hormone-injection experiment were conducted in two free-living passerines, the great tit (Parus major) and pied flycatcher (Ficedula hypoleuca), with the main aim to explore the developmental plasticity of IGF-1 and corticosterone (main glucocorticoid in birds) functions in relation to environmental challenges (e.g. changing nutritional conditions). More specifically, the aim was to see how these two hormones are connected with the variability in growth rates, physiological conditions and survival rates of nestlings during the nest-bound period in experimentally manipulated growing conditions. Results showed that nestlings in better nutritional conditions (decreased broods) had increased postnatal growth rate, better body condition and also higher IGF-1 levels prior to fledging than those in worse nutritional conditions (control and enlarged broods). Plasma IGF-1 levels were higher in the middle of the nestling period, when growth rate was most rapid, than in the pre-fledging stage. Interestingly, at the pre-fledging stage, IGF-1 levels were negatively associated with the growth rate of those nestlings that grew in nutritionally better conditions. Compared to less mature nestlings, this indicates an IGF-1 down-regulation in more mature nestlings, that reached optimal fledging mass quicker, and that probably needed to invest less into somatic growth and structural size at the end of the fledging period. Furthermore, it was shown that feather corticosterone (the measure incorporates an integral hormonal profile over a relatively long time period, during which the feather was growing) and its association with growth rate and fledging success were significantly affected by the treatment only in coniferous forests, where growth conditions had a tendency to be poorer than in deciduous forests. More specifically, it was found that feather corticosterone was negatively related to fledging success and unimodally associated with the growth rate in the coniferous habitat. In the latter case, the positive association between growth rate and corticosterone levels became negative as corticosterone levels increased. Lastly, it was shown that the association between levels of IGF-1 prior to fledging and feather corticosterone depended on the physiological condition of nestlings. Namely, there was a positive association between the hormone levels in nestlings from decreased broods, which where in better condition, and a negative association in nestlings from enlarged broods, which were in worse condition. Interestingly, the interaction between levels of IGF-1 and corticosterone was also related to the survival of the nestlings. The results of the current thesis suggest that nestling plasma IGF-1 and integral feather corticosterone levels vary depending on resource (e.g. food) availability. These hormones potentially play an important role in regulating nestling fitness in altricial birds, by mediating physiological trade-offs in the regulation of their postnatal growth rate, physiological condition and fledging success in the stochastic environment.