title: Long-term physiological trends and their drivers: linking hair hormone concentrations with telemetry data in GPS-collared Serengeti wildebeest
Creators: name: Buchanan, Callum
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: Serengeti, wildebeest, migration, GPS, telemetry, physiology, hair, hormone, cortisol, progesterone, aldosterone, stress, pregnancy, sodium.

description: The environmental context in which animals live, such as their exposure to predation or availability of food, can strongly shape their physiological state and subsequent behaviour. Additionally, changes in reproductive state also affect animal behaviour. Temporal physiological data is therefore of great value in the study of free-living animals and how they interact with their environment. However, longitudinal measures of physiological state within individual animals are rarely studied, particularly in wild populations, because they are difficult to collect. Some substances which circulate in the blood, including drugs and endogenous steroid hormones, are incorporated into hair as it grows, in concentrations that reflect their circulating concentrations at that time. Since most regions of hair grow throughout the life of an individual, by sectioning hair samples along their growth axis we can create sequential, distinct samples. Measuring hormone concentrations in each of these sections therefore allows for a temporal record of chronic hormone concentrations, covering the time the sample was grown. This thesis is composed of a general introduction (Chapter 1) and discussion (Chapter 6), as well as four standalone data chapters (Chapters 2-5). Chapter 2 used lengths of regrown hair collected from 6 wildebeest to calculate hair growth rate, showing no significant difference between individuals. This hair growth rate was used to calculate dates of growth for all sections of hair. Cortisol, the primary hormone responsible for the stress response, was measured in wildebeest tail hair, showing variation along the sample, and was stable in wildebeest hair when measured in pg cortisol/cm hair. Cortisol was also measured and varied along the length of a hair sample collected from a ~100-year-old taxidermied oryx, highlighting that cortisol appears to remain stable over longer periods of time, showing promise for the collection of physiological data from museum specimens. Chapter 3 implemented this method to produce cortisol profiles for hair samples (sectioned longitudinally into 8mm segments) collected from GPS-collared wildebeest (Connochaetes taurinus) in the Serengeti-Mara ecosystem. Cortisol data was then paired with a variety of environmental and anthropogenic factors extracted from telemetry data to determine drivers of stress. Hair cortisol concentrations were found to be negatively related with NDVI (a remotely sensed metric of vegetative productivity) and proximity to villages. Cortisol was generally higher in samples taken from animals found dead than those that were live captured, suggesting chronic stress impacts negatively on survival. Interestingly, the effect of NDVI on cortisol differed between animals found dead and those captured. These results suggest that chronic stress increases when wildebeest are in locations where food availability is low and where villages are nearby and that chronically stressed animals (carcass samples) perceived stressors differently to the individuals who were alive at sampling. Chapter 4 measured progesterone, one of the primary hormones responsible for the regulation of pregnancy, in wildebeest hair samples, showing variation between and within samples. It was not stable along the growth axis of the hair, but it was possible to correct for this post hoc, using relative metrics compared against a long-term mean. Corrected progesterone concentrations were significantly higher during the time between the rut and calving period. This corresponds to the time when the vast majority of migratory wildebeest are pregnant, showing promise for the use of this technique to determine reproductive trends in wild animals. Chapter 5 focused on aldosterone, the primary hormone responsible for the regulation of sodium and water balance within the body. Sodium is an essential element which wildebeest obtain through forage; however, the sodium content of forage varies across the Serengeti-Mara ecosystem. Aldosterone was measured in tail hair taken from wildebeest resident to a high-sodium region of the ecosystem, and migrants, to determine whether spatial variation in sodium within the landscape could be detected in the physiological response of the animals. Aldosterone concentrations showed variation within and between hair samples, as well as a general increase along the growth axis of the hair; however, it was possible to detrend the data using the correction method developed in Chapter 2. Overall, aldosterone was higher, and showed greater seasonal variation, in migratory wildebeest than in wildebeest resident to sodium-rich areas of the ecosystem, which suggests that migrants experience greater sodium stress. Aldosterone levels also correlated with season (i.e. proximity to time of maximum cumulative rainfall) and NDVI (i.e. vegetation greenness), which suggests that wildebeest are most sodium-deprived at the end of the dry season, when they occupy the most sodium-poor area of the ecosystem. This thesis validates a relatively straight-forward and robust method for the collection of chronic physiological time series data relating to stress, reproduction, and sodium homeostasis. Additionally, samples collected from geo-tagged animals can be used to study the relationship between an animal’s physiology and their environment. The ability to link spatial information with an individual animal’s physiology over long time periods opens exciting new avenues for research relevant to movement ecology, conservation management and animal welfare across a wide range of taxa and settings.