Faster exploring chipmunks are better at reproducing early in life and produce bigger litters earlier than slower exploring ones. This variation is maintained by fluctuations in food abundance, favouring faster explorers during years of mast, and slower explorer during years of low food abundance.
One of the most interesting questions in evolutionary ecology is how consistent behavioural differences (I’ll call them personality) are maintained in animal populations. This is indeed a bit puzzling because personality implies that animals cannot adjust perfectly their behaviour to any situation. and so in a changing world, personality may actually be bad an animal’s fitness. Of course a lot of work now shows that personality is often (but not always) adaptive. That is, personality may result from past selective pressures and arises because the optimal behaviour in a given situation varies from an individual to the next. A particularly important idea is that personality is maintained because it involves behaviours associated with evolutionary trade-offs and life history differences. In a nutshell, individuals who have a higher expected reproductive success in the future, should express less risky behaviours, such as exploration, aggressiveness, or boldness, just because they have more to loose.
This idea predicts that we should observe personality differences when individuals differ in their future fitness expectations (how high is your future reproductive success supposed to be). Testing this is pretty difficult to achieve as one needs to manipulate the fitness expectations of individual and assess the effect it has on personality. The only experiment conducted so far in the laboratory shows that manipulating the fitness expectations of great tits does indeed changes their behaviour. Needless to say, this would be pretty difficult to conduct in free ranging animals. However, we may also use to our advantage species or populations where individual life history is constrained by features of the environment and vary predictably.
tested whether exploration (a personality trait, describing the tendency to investigate novel environments) is associated with how individuals maximize their reproduction over their life (I’ll call that their reproductive life history) in a eastern chipmunks population located near the Mounts Sutton in southern Québec. Chipmunks are pretty good models to investigate how personality differences are maintained because they show consistent differences in how they explore their environment. They also occupy the same burrow throughout their life, and so they are easy to follow from one year to the next (a good feature if you want to follow their behaviour and reproduction during their whole life). Most people also think they are pretty cute.
I followed chipmunks from 4 birth cohorts and measured their exploration using open-field tests. During these tests, we place individuals in a novel arena and record how much distance they cover as a meaure of their exploration. The interesting thing is that, throughout their entire life, some individuals are consistently faster explorers: they will move around the arena intensely at first but decrease almost completely their exploration during the following minute. At the other extreme, some other individuals will be slower explorers and have a moderate but constant exploration level throughout the test. We captured all the juveniles produced in the population at their maternal burrow, and determined their father using micro-satellite analyses.
We found that faster exploring chipmunks attain their maximal reproductive success early in their life, where slower explorers seem to do better toward the end of their reproductive life span. Which is in line with the prediction of the pace of life hypothesis because individuals who have higher fitness expectations (slower explorers) exhibit a risk-averse behaviour. On the contrary, individuals who accomplished most of their reproductive success early in life are expressing a riskier, faster exploration pattern.
An interesting feature of this species is that individuals are able to reproduce for the first time at varying ages depending on their birth cohorts. This happens because chipmunks synchronize their mating seasons with the occurences of masts events, when trees like beeches or oaks released a ton of food. Mast event occur on a 2 or 3 year cycle. This provided us with a nice ‘natural’ experiment, because chipmunks have different life histories, just depending on their birth cohorts. Comparing the average exploration of individuals among the birth cohorts, we also found that cohorts with the opportunity to reproduce early in their life were faster explorers. Hence it suggest that the relationship between exploration and life history may arise because mast events favor individuals with different exploration and life history in this population.
You can find more details by reading the full paper in Journal of Animal Ecology.