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Using reaction norms to identify glucocorticoid phenotypes and their relationship to fitness in individuals of a wild vertebrate.

Periodic Reporting for period 1 - CortFit (Using reaction norms to identify glucocorticoid phenotypes and their relationship to fitness in individuals of a wild vertebrate.)

Reporting period: 2015-04-01 to 2017-03-31

Hormones are a key physiological mechanism modulating the response to environmental change, yet understanding about individual variation in hormonal traits is lacking. Recently a novel approach was proposed to study changes in a trait across an environmental gradient, known as a reaction norm (RN). A key characteristic of this approach is the specific focus on plasticity as an important characteristic of an individual’s phenotype, which is critical when studying inherently plastic traits such as hormones. Understanding how hormones modulate the response to environmental change is critical in light of the unprecedented rate at which global change is occurring. Glucocorticoid (GC) hormones are important in modulating the response to energetic challenges at baseline, and in modulating the response to stressors as part of the stress response. Another critical component of the hypothalamic-pituitary-adrenal (HPA) axis, the main physiological system involved in the release of GCs, is a negative feedback loop. Strong negative feedback is deemed important to prevent negative effects of prolonged exposure to elevated GC levels, such as reduced reproductive success and survival. One way to assess HPA axis functioning is the HPA function test, measuring baseline GC, the GC stress response, and negative feedback efficacy through a dexamethasone challenge. Combining the RN approach with the HPA function test provides a comprehensive assessment of the GC flexibility of an individual, which can then be correlated to fitness measures such as reproductive success. The aim of this work was to identify individual GC characteristics that are repeatable and related to reproductive success. The research proposed consists of an interdisciplinary approach using techniques from physiological, behavioral, and evolutionary ecology, as well as from the field of endocrinology. The research undertaken under this project aimed to address two specific objectives:

Objective 1: Determine glucocorticoid flexibility of individuals under standardized conditions

Objective 2: Determine relationships between glucocorticoid flexibility, reproductive performance and fitness in free-living great tits
Study 1: Glucocorticoid response of captive great tits to mild, short term, changes in ambient temperature. This project addressed objective 1 and aimed to establish a RN in response to a change in temperature. We predicted that lower temperatures lead to increased baseline GC associated with increased metabolic rate in order to maintain body temperature. This project is one of the first attempts at applying a RN approach to GCs. Fifteen great tits (Parus major) were captured from a local forest and acclimatized to an indoor aviary. Next the birds were exposed to three different temperature regimes for three days each, e.g. cold, neutral, and warm. During the neutral treatment the normal temperature of the aviaries was used, e.g. 19C. The cold and warm treatment consisted of an average temperature of 14C and 24C, respectively. Birds were sampled once at prior to treatment, and again after 3 days of exposure to each treatment. Aside from plasma GCs, descriptive measures, such as weight, and behavior behavioral observations, were collected. This project is currently undergoing final data analysis and manuscript writing. Preliminary data was presented at the 2015 ICCPB conference in Krakau, Poland.

Study 2: Does individual GC flexibility predict reproductive success in free-roaming great tits? This project addressed objective 2, and aimed to assess individual GC responsiveness and HPA axis functioning in great tits in winter. We expected plasticity in baseline GC, e.g. a stronger response to temperature manipulation, to be positively related to reproductive success. Furthermore, we expected a more responsive HPA axis, e.g. strong stress response and robust negative feedback, to be associated with higher reproductive success. Prior to the breeding season, 24 animals were exposed to a temperature change and an HPA-function test. Great tits roost overnight in nest boxes in winter, providing an opportunity for relatively easy capture. Additionally, temperature inside a nest box can be manipulated relatively easily. Birds were caught at night, and assigned to either a control (10 animals), or a temperature manipulation group (14 animals). Upon capture, each bird was sampled to obtain baseline GCs. Temperature manipulation consisted of placing a heating pad in the bottom of the nest box, which was run off of a car battery. Birds were then returned to the box and left alone for three hours, after which each bird was recaptured and sampled again. Control birds were treated identical, except the heating pad was never turned on. After the second sample, birds were exposed to standardized stressor, containment in a cloth bag. After 30 minutes, a stress-induced sample was taken, immediately followed by a dexamethasone injection to induce negative feedback. A final sample was taken 90 minutes post injection. During the breeding season, 18 birds were found again actively building nests, and could thus be tracked to obtain measures of reproductive performance and success, including laying date, number of eggs, hatching success, feeding rate, and fledging success. This allowed us to investigate how winter GC flexibility is associated with reproductive success. This project is currently undergoing final data analysis and manuscript writing. Preliminary data was presented at the 2016 SICB conference in Portland, Oregon, USA.

Public outreach was an important aspect of this project. During the spring of 2015, a local high school biology class was guided around my experimental forest. This offered a great opportunity to interact with young students about research and to provide hands on experience on what it is like to be a field biologist. Additionally, some of the ideas and work described in this project were presented during seminars at Carleton University in Canada (February 2015), and Linkoping University in Sweden (October of 2015).
To conclude, this project adds significant progress to the state of the art of the field of ecological endocrinology. As both studies involved repeated sampling of the same individuals, this work provides further data on repeatability of GC characteristics. Combining the HPA function test and the GC response to temperature changes provides critical data on how individual GC characteristics, and individual GC flexibility, relate to reproductive success. This provides invaluable insight into the role of GCs in modulating the response to environmental change, as well as the role GCs play in survival and reproductive success. The research performed under this project is an important contribution to the advancement of basic science regarding the role of GCs. Furthermore, as the HPA axis and GCs are highly conserved among vertebrates, including humans, and because of the interactions between GCs and the immune system, disease, metabolism, and reproduction, furthering the understanding of the GC system will have far-reaching implications for a range of scientific disciplines. For example, dysregulation of GCs and the HPA-axis has been associated with a large number of afflictions and diseases in humans. Understanding individual plasticity in GC flexibility and functioning will thus have a significant and broad impact not just on science, but also on society at large.