Testing personality-pace-of-life associations via artificial selection: insights from selected lines of rainbow trout on the context-dependence of correlations

More than a decade of study since the personality pace-of-life syndrome (POLS) hypotheses were first proposed, there is little support for it within species. Lack of experimental control, insufficient sampling in the face of highly labile behavioural and metabolic traits, and context dependency of trait correlations are suggested as reasons. Here, I argue that artificial selection and/or use of existing selected lines represents a powerful but under-used approach to furthering our understanding of the POLS. To illustrate this potential, I conducted a focussed review of studies that compared the behaviour, metabolism, growth and survival of an artificially selected fast-growing rainbow trout relative to wild unselected strains, under varying food and risk conditions in the laboratory and field. Resting metabolic rate, food intake, and behaviours that enhance feeding but increase energy expenditure (activity, aggression, boldness), were all higher in the fast strain in paired contrasts, under all food and risk conditions, both in the laboratory and the field. Fast-strain fish grew faster in almost every food and risk situation except where food was highly limited (or absent), had higher survival under low or zero predation risk, but had lower survival under high risk. Several other traits rarely considered in POLS studies were also higher in the fast strain, including maximum swimming speed, and hormones (growth hormone (GH), thyroid hormone (T3) and insulin-like growth factor (IGF-1)). I conclude: (i) assumptions and predictions of the POLS hypothesis are well supported, and (ii) context-dependency was largely absent, but when present revealed trade-offs between food acquisition and predation risk. This focused review highlights the potential of artificial selection in testing POLS ideas, and will hopefully motivate further studies using other animals.

Energy allocation is revealed while behavioural performance persists after fire disturbance

Metabolic physiology and animal behaviour are often considered to be linked, positively or negatively, according to either the performance or allocation models. Performance seems to predominate over allocation in natural systems, but the constraining environmental context may reveal allocation limitations to energetically expensive behaviours. Habitat disturbance, such as the large-scale fire that burnt wetlands of Biebrza National Park (NE Poland), degrades natural ecosystems. It arguably reduces food and shelter availability, modifies predator-prey interactions, and poses a direct threat for animal survival, such as that of the wetland specialist root vole Microtus oeconomus. We hypothesized that fire disturbance induces physiology-behaviour co-expression, as a consequence of changed environmental context. We repeatedly measured maintenance and exercise metabolism, and behavioural responses to the open field, in a root voles from post-fire and unburnt locations. Highly repeatable maintenance metabolism and distance moved during behavioural tests correlated positively, but relatively labile exercise metabolism did not covary with behaviour. At the same time, voles from a post-fire habitat had higher maintenance metabolism and moved shorter distances than voles from unburnt areas. We conclude there is a prevalence of the performance mechanism, but simultaneous manifestation of context-dependent allocation constraints of the physiology-behaviour covariation after disturbance. The last occurs at the within-individual level, indicating the significance of behavioural plasticity in the context of environmental disturbance.

Individual variation in heat substitution: is activity in the cold energetically cheaper for some individuals than others?

In many endotherms, a potentially important yet often overlooked mechanism to save energy is the use of the heat generated by active skeletal muscles to replace heat that would have been generated by thermogenesis (i.e., "activity-thermoregulatory heat substitution"). While substitution has been documented numerous times, the extent of individual variation in substitution has never been quantified. Here, we used a home-cage respirometry system to repeatedly measure substitution through the concomitant monitoring of metabolic rate (MR) and locomotor activity in 46 female white-footed mice (Peromyscus leucopus). A total of 117 measures of substitution were taken by quantifying the difference in the slope of the relationship between MR and locomotor activity speed at two different ambient temperatures. Consistency repeatability (±se) of substitution was 0.313±0.131 - hence, about a third of the variation in substitution occurs at the among-individual level. Body length and heart mass were positively correlated with substitution whereas surface area was negatively correlated with substitution. These two sub-organismal traits accounted for the majority of the among-individual variation (i.e., individual differences in substitution were not significant after accounting for these traits). Overall, our results imply that the energetic cost of activity below the thermoneutral zone is consistently cheaper from some individuals than others, and that the energy saved from substitution might be available to invest in fitness-enhancing activities.