© Peter Buchner
What are the effects of moderate physiological stress on reproduction and growth of birds?
Animals have to cope with predictable characteristics of their environment such as seasonal changes in resource availability and climate, but also with a wide variety of unpredictable environmental events. These perturbation factors have the potential to cause physiological stress. Survival and reproductive success of animals and thus their fitness depend on both the adaptations to the general qualities of their habitat and on efficient strategies to respond to unpredictable, unfavourable events. In response to such unpredictable events, animals may resort to behavioural and physiological patterns putting them into an emergency life-history stage. However, the fitness-relevance of such a response as a life history trait has been rarely investigated.
Hormones are a crucial link between the genome and its phenotypical expression. The endocrine system adjusts the physiology, behaviour and development to the prevailing environmental conditions, including unpredictable, unfavourable events. Thus, physiological control mechanisms translate cues from the environment and from the condition of the bird into the phenotype. If this affects development and growth in early life, such effects are particularly long-lasting. Therefore, stressful events may not only have an immediate effect, but may also shape the phenotype on a longer term scale or even permanently.
Our general aim is to determine the costs and benefits of a moderate physiological stress response in two particularly sensitive life-cycle stages, reproduction and postnatal development, under natural environmental conditions. Hence, the study contributes to understand how animals cope with unpredictable and suboptimal environmental conditions.
The study is on free-living European kestrels (Falco tinnunculus) and barn Owls (Tyto alba). For both reproduction and postnatal development, the effects of stress are first tested experimentally (by an artificially induced physiological stress response) and, secondly, validated for their ecological relevance by analysing the effects of naturally occurring variation in glucocorticoid levels.
Concerning reproduction, we investigate how a moderate physiological stress response (intermediate levels of glucocorticoids) affects the reproductive investment of females (e.g. timing of reproduction, clutch size) and males (food provision to nestlings). The hypothesis to be tested is that glucocorticoids contribute to regulate the trade-off between self-maintenance and reproductive investment. Attention is also given to the fitness costs of such a stress response, i.e. survival of parents as well as survival and quality of their offspring.
Concerning postnatal development, we investigate the costs and benefits of a moderate physiological stress response (intermediate levels of glucocorticoids) in parent-dependent offspring. Short-term effects (during the nestling phase) on growth and development are measured. Particular emphasis is on long-term effects of postnatal stress, i.e. on the learning abilities to catch prey, time to independence from the parents, regulation of dispersal, and survival. Therefore, birds are followed by radio-tracking for several months after fledging. The aim here is to test the hypothesis that stress in early life has long-lasting effects on learning and behaviour and, hence, that postnatal stress conditions influence post-fledging mortality.
In both species a moderately increased corticosterone level affected the development of the nestlings: growth was temporarily slowed down and the nestling time prolonged. The ability of the kestrel nestlings to react to stress increased during growth with increasing age and body mass. Independently of hatching order, nestlings with high body mass showed a stronger stress response than those with low body mass. Apparently the nestlings reduced the stress response during the first two weeks to avoid the negative effect of reduced growth. Kestrel nestlings showed a change in the heterophil/ lymphocyte- ratio (H/L-ratio) in response to stress, but to other stressors than those inducing a corticosterone response. Therefore the H/L ratio offers an additional tool to measure stress. In the barn owl nestlings increased corticosterone concentrations suppress part of the immune system. To conclude, mildly elevated corticosterone affects the development of nestlings. Whether it also influences the ability to learn hunting and whether survival between fledging and independency is increased will be analysed on the basis of available telemetry data.
The extent to react to stress in barn owl nestlings was heritable and linked to the eumelanic coloration. Nestlings with big black spots released less corticosterone during stressful events and had a higher clearance rate than nestlings with small spots. This means that if stress sensitivity is signalled by eumelanic coloration, this signal might be subjected to sexual selection.
The effect of slightly elevated corticosterone concentrations in adult barn owl males showed that these individuals temporarily provided the nestlings with less food than the control males did. The reduced food supply had the effect that the nestlings grew slower and reacted with an increased response to a stressor. Moreover, barn owls reacted differently to stress depending on their melanin-based colour morph. Barn owl males with large eumelanic spots were less sensitive to exogenous corticosterone and reduced their nestling provisioning rate to a smaller extent than small-spotted males. Growth of the large-spotted nestlings was less affected by increased corticosterone than the growth of the small-spotted ones. Again, stress sensitivity may be signalled through the melanin-based black spots, and this trait may hence be under sexual selection.
Prof Alexandre Roulin, Université de Lausanne