Alison M. Bell

There is a growing interest among behavioral ecologists in behavioral syndromes and animal personality. Studies of behavioral syndromes repeatedly measure the same individuals to quantify within-individual consistency and between-individual variation in behavior. Often these studies measure behavior in different contexts or in different behavioral assays to determine whether individual differences in behavior in one context are related to behavior in other contexts, that is, a behavioral syndrome. For studies of behavioral syndromes, there is not universal agreement about whether it is preferable to randomize the order of different assays or to administer them in a fixed order. Here, I articulate the advantages and disadvantages of testing in a randomized or fixed order and offer some recommendations according to the goals and power of the experiment. In general, studies using within-subjects designs that are primarily interested in mean-level differences between treatments should randomize the order that individuals experience different treatments. Under certain conditions, studies of behavioral syndromes should also administer the assays in a randomized order, but only if the study is sufficiently powerful to statistically account for carryover and period effects. If the experimenter is interested in behavioral syndromes that are caused by carryovers, it is often preferable to test in a fixed order. If the experimenter wants to guard against carryovers, but the experiment is not sufficiently powerful to account for carryover and period effects, then a compromise is to test in a fixed order, but to test individuals in the context that is most likely to affect subsequent behavior last.

Predation has an important influence on life history traits in many organisms, especially when they are young. When cues of trout were present, juvenile sticklebacks grew faster. The increase in body size as a result of exposure to cues of predators was adaptive because larger individuals were more likely to survive predation. However, sticklebacks that had been exposed to cues of predators were smaller at adulthood. This result is consistent with some life history theory. However, these results prompt an alternative hypothesis, which is that the decreased size at adulthood reflects a deferred cost of early rapid growth. Compared to males, females were more likely to survive predation, but female size at adulthood was more affected by cues of predators than male size at adulthood, suggesting that size at adulthood might be more important to male fitness than to female fitness.

Consistent individual differences in behaviour, aka personality, pose several evolutionary questions. For example, it is difficult to explain within-individual consistency in behaviour because behavioural plasticity is often advantageous. In addition, selection erodes heritable behavioural variation that is related to fitness, therefore we wish to know the mechanisms that can maintain between-individual variation in behaviour. In this paper, we argue that whole genome expression data can reveal new insights into the proximate mechanisms underlying personality, as well as its evolutionary consequences. After introducing the basics of whole genome expression analysis, we show how whole genome expression data can be used to understand whether behaviours in different contexts are affected by the same molecular mechanisms. We suggest strategies for using the power of genomics to understand what maintains behavioural variation, to study the evolution of behavioural correlations and to compare personality traits across diverse organisms.

Individual animals of the same species inhabiting environments which differ in the frequency and magnitude of stressors often exhibit different physiological and behavioral responses to stressors. Here, we compare the respiratory response to confinement stress, and behavioral responses to ecologically relevant challenges among sticklebacks from 11 different populations varying in predation pressure. We found that sticklebacks from high predation populations breathed faster in response to confinement stress and were, on an average, more behaviorally responsive to a pike behind glass compared with sticklebacks from low predation populations. These patterns differ from the results of studies on other species, highlighting the need for a conceptual framework to understand the proximate and ultimate factors shaping variable responses to stressors over developmental and evolutionary time. Moreover, physiological and behavioral responses were integrated with each other, both at the individual and population levels. In general, fish that were more aggressive and bold in the presence of a predator breathed faster, independent of body size. These results are consistent with the growing body of evidence that individuals differ in a suite of physiological and behavioral mechanisms for coping with challenges in the environment.

A perplexing new question that has emerged from the recent surge of interest in behavioural syndromes or animal personalities is – why do individual animals behave consistently when behavioural flexibility is advantageous? If individuals have a tendency to be generally aggressive, then a relatively aggressive individual might be overly aggressive towards offspring, mates or even predators. Despite these costs, studies in several taxa have shown that individuals that are more aggressive are also relatively bold. However, the behavioural correlation is not universal; even within a species, population comparisons have shown that boldness and aggressiveness are correlated in populations of sticklebacks that are under strong predation pressure, but not in low predation populations. Here, we provide the first demonstration that an environmental factor can induce a correlation between boldness and aggressiveness. Boldness under predation risk and aggressiveness towards a conspecific were measured before and after sticklebacks were exposed to predation by trout, which predated half the sticklebacks. Exposure to predation generated the boldness–aggressiveness behavioural correlation. The behavioural correlation was produced by both selection by predators and behavioural plasticity. These results support the hypothesis that certain correlations between behaviours might be adaptive in some environments.