It’s your hormones, deer. Individual variation in hormone levels within a wild population of red deer: causes and consequences.
Whilst individual differences in circulating hormone levels can influence life history traits throughout an animal’s lifetime, this remains a poorly understood area of research, particularly for wild systems where sufficient sets of individual-based data are rare. This thesis aimed to address this dearth of information by identifying key drivers of hormone variation, as well as exploring potential fitness consequences within a single system of wild red deer (Cervus elaphus) on the Isle of Rum National Nature Reserve in Scotland. It focussed on both androgen (e.g. testosterone) and glucocorticoid (e.g. cortisol) levels, and examined among-individual variation in these two hormone groups from samples collected using both traditional (blood: chapters 3 & 4) and non-invasive (faecal: chapters 5 & 6) methods. Results showed both intrinsic and extrinsic factors to influence an individual’s hormone levels. In general, current or recent environment explained the greatest variation, with both hormone groups exhibiting strong temporal trends at multiple scales. Concentrations changed substantially across an individual’s lifetime as they aged (chapters 5 & 6), and calves born in different years differed in their neonatal testosterone levels (chapter 3). Hormone levels also varied across the year, showing clear seasonal cycles which peaked during key reproductive events: the calving season in females (chapter 6) and the rut in males (chapter 5). An individual’s current life history state was also important, particularly a female’s reproductive state (chapter 6). Whilst there was some evidence of maternal effects on neonatal hormone levels (chapter 3) these were not extensive, and maternal hormone concentrations did not appear to influence those in their new-born calves (chapter 6). There was, however, evidence of neonatal circulating testosterone levels being heritable, and despite overall differences between the sexes the underlying genetic architecture of this trait did not differ between male and female calves (chapter 4). Associations were also found between an individual’s hormone levels and their fitness, although these consequences were only apparent in short-term fitness measures or proxies such as reproductive behaviour (e.g. male reproductive effort in chapter 5). Effects were also not ubiquitous within the population. Whilst a calf’s circulating testosterone levels indicated their probability of surviving their first year of life, these effects were only apparent in firstborn males, a group which is particularly vulnerable to mortality (chapter 3). In general, this thesis suggests that the fitness consequences identified by broad-scale hormone manipulation studies can still be found when looking at subtle individual-level differences. The limited evidence of persistent hormone phenotypes (indicated by the lack of among individual variance for most measures, chapter 5 & 6) does, however, emphasise the importance of repeatedly sampling individuals before drawing extensive conclusions about fitness consequences.