It’s your hormones, deer. Individual variation in hormone levels within a wild population of red deer: causes and consequences.
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Date
26/11/2015Author
Pavitt, Alyson
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Abstract
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.