Resource allocation under stress: mechanisms and strategies in a long-lived bird
MetadataVis full innførsel
- Institutt for biologi 
The marine environment is currently undergoing critical modification in the wake of global climate change. Marine animals are, consequently, expected to experience increasing periods of nutritionally stressful conditions. Pronounced changes occur in the distribution, timing, quantity, and quality of food available to marine predators, increasing the likelihood of food limitations. At the same time, the variability in behavioral and physiological responses of marine predators to nutritional stress is still poorly understood. Even more so, fitness consequences of variable responses remain largely unknown and therefore unpredictable. In this thesis, I am exploring responses to stress and consequences thereof in an arctic-breeding, long-lived seabird species (the black-legged kittiwake, Rissa tridactyla). Between 2010 and 2013, I measured and experimentally elevated nutritional stress in individuals at two focal kittiwake colonies (an Atlantic and a Pacific colony), and collected data on behavioral, physiological, and fitness effects of this stress. Behavioral effects of stress were measured through observations of parental care during the breeding season and by recording migration schedules during over-winter treks. Physiological effects were explored by measuring changes in energy stores, energy expenditure, and glucocorticoid hormones. Effects on fitness were assessed by measuring changes in telomere length (a physiological proxy for life expectancy), effects on offspring and reproductive success, and the apparent survival of adult kittiwakes. Responses to stress that were measured at both focal colonies were evaluated against the background of population-specific life history strategies in North Atlantic and North Pacific kittiwakes, addressing the question of whether intra-specific variation in responses to stress can be predicted based on life history theory. An examination of the relationship between varying nutritional stress levels and endogenous energy stores revealed a consistently nonlinear relationship in kittiwakes at two life history stages, during incubation and during chick-rearing. While birds remained relatively lean under benign conditions, moderate increases in stress led to an accumulation of energy stores. Loss of endogenous energy stores was again associated with high stress levels. Next, intra-specific behavioral and physiological responses to stress were shown to be predictable based on life history theory. Considering that Atlantic kittiwakes are supposedly fast-lived while Pacific kittiwakes are relatively slow-lived, I was able to predict contrasting responses in parental care and consequences for the physiological state, growth and survival of offspring. Furthermore, contrasting responses to stress between populations were also reflected in allocation of energy and the consequences of this variation in allocation. Specifically, while stress resulted in higher energy expenditure at both colonies, increasing energy expenditure was associated with lower survival and faster telomere loss at the Atlantic colony. In contrast, increasing energy expenditure was associated with higher survival and better telomere maintenance at the North Pacific colony. Aside from these intriguing findings on intra-specific differences in responses and associated contrasting fitness consequences, corticosterone was revealed as a mediator of carry-over effect from stress experienced during reproduction on migration in kittiwakes. Particularly, individuals that were experimentally stressed during breeding migrated earlier and stayed longer at the wintering grounds, but this applied only to female kittiwakes. Finally, migration and stress during reproduction were demonstrated to interactively determine telomere dynamics. This finding establishes the importance of migration on individual fitness, in particular the delaying effect of time spent at the wintering grounds on the loss of life expectancy. In summary, this thesis identifies important connections between physiological mechanisms involved in trade-offs, specifically the effects of stress hormones, energy expenditure, and parental behavior on reproduction, migratory behavior, self-maintenance and longevity. Findings from this thesis may help to understand and predict the effects of a changing, increasingly stressful environment on seabirds, and might serve as an important stepping stone towards a mechanistic understanding of impacts of stress on animals more generally.