We recently published a paper looking at chipmunk stress reactivity, and its links with life-time exploration patterns and reproductive output in eastern chipmunks. It investigates how individual cortisol stress response is associated to its lifetime exploration pattern and its reproductive output. It also presents a way to study the stress reactivity of individuals in the wild, over many months.
My thesis tested the idea that behaviour and life history can co-evolve whenever behaviour mediate how individuals negotiate life history trade-offs. On important such trade-off is the one between what an individual invests in its current reproduction (for example how many offspring to produce this year) and what it can invest in its future reproduction, or how likely it is to survive to see other reproductive seasons and how many offspring it will be able to produce. Some individuals will favor current reproduction, at the expense of their future reproduction, while some others will limit their current reproduction in order to have a higher survival and potentially more breeding attempts over their life. Whenever individuals are on different life history trajectories, we can expect them to differ in their propensity to express behavioural traits that are good for their current reproduction (for example being aggressive, faster explorer or bold), or their future reproduction (for example being shy, risk averse and exploring the environment slowly). I showed that, in eastern chipmunks, individuals with a faster exploration pattern will typically reproduce earlier and attain their lifetime maximum fecundity earlier in their life, compared to slower exploring individuals. You can read more about this here and here.
The main control mechanism regulating both the behaviour and life history involves a group of hormones called gluco-corticoids, like cortisol and corticosterone. Cortisol (the main gluco-corticoid in chipmunks) helps regulating behaviour and life history traits during the stress response, that is observed in most vertebrate animals whenever the environment is perturbed and threatens the individual’s survival and future reproduction. During the stress response, GC will shut-down functions associated with current reproduction and shunt the organism’s resources to functions that promote survival (and consequently future reproduction). In nesting birds, the stress response will inhibit parental and foraging behaviour, and can favor individual survival at the expense of their current reproduction. An interesting aspect is that animals facing a very short or demanding reproductive season may even down-regulate their ability to produce cortisol under stress, to prevent energy from being diverted from reproduction.
Based on these hypotheses, fast exploring chipmunks should have a higher stress reactivity when exposed to the various perturbation of their natural environment. At the same time, chipmunks that invest more in their reproduction in a given year (by producing a larger litter) should down-regulate their stress reactivity. We found that, indeed, individuals showed consistent differences in the variability of their cortisol level, that covaried with their exploration level. This result is really interesting because it suggest that the links between individual behaviour and stress reactivity we observe in laboratory settings, over short period of time in many species are probably still relevant in natural conditions, over significant portions of the animals’ lives. We also found that individuals with slower exploration patterns exhibited cortisol levels that were more variable (suggesting they have a higher stress reactivity). Another key finding, is that even after correcting for individual differences in exploration, we also observed that females raising a larger litter exhibited even less variable cortisol levels. This suggests that females could actually down-regulate cortisol stress reactivity according to their reproductive effort a given year.