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Classic evolutionary theory predicts that predation will shift trait means and erode variance within prey species; however, several studies indicate higher behavioral trait variance and trait integration in high-predation populations. These results come predominately from field-sampled animals comparing low- and high-predation sites and thus cannot isolate the role of predation from other ecological factors, including density effects arising from higher predation. Here, we study the role of predation on behavioral trait (co)variation in experimental populations of guppies (Poecilia reticulata) living with and without a benthic ambush predator (Jaguar cichlid) to better evaluate the role of predation and where density was equalized among replicates twice per year. At 2.5 years after introduction of the predators (∼10 overlapping generations), 40 males were sampled from each of the six replicate populations and extensively assayed for activity rates, water column use, and latency to feed following disturbance. Individual variation was pronounced in both treatments, with substantial individual variation in means, temporal plasticity, and predictability (inverse residual variance). Predators had little effect on mean behavior, although there was some evidence for greater use of the upper water column in predator-exposed fish. There was greater variance among individuals in water column use in predator-exposed fish, and they habituated more quickly over time; individuals higher in the water column fed slower and had a reduced positive correlation with activity, although again this effect was time specific. Predators also affected the integration of personality and plasticity—among-individual variances in water column use increased, and those in activity decreased, through time—which was absent in controls. Our results contrast with the extensive guppy literature showing rapid evolution in trait means, demonstrating either increases or maintenance of behavioral variance u

Behavioural studies have shown that even after accounting for individual differences in contextual and temporal plasticity, considerable unexplained (residual) variation remains. Recent studies show that individuals differ in the magnitude of residual variation (= their predictability), but hardly any studies exist that assess whether this individual attribute itself is repeatable and potentially subject to selection, and whether predictability is related to aspects of different underlying state variables. Using data on the latency to emerge after disturbance of 100 pill bugs, Armadillidium vulgare, measured 24 times each over time and across contexts, we found substantial among-individual variation in mean boldness (latency) and in their behavioural predictability. Individual mean boldness across weeks was highly consistent over time, as was individual predictability; by contrast, unadjusted repeatability of boldness scores (the familiar ‘intraclass correlation’) and repeatability adjusted for time-related behavioural changes were low to moderate, indicating substantial residual variation. Individual mean boldness was not related to individual predictability, indicating that while selection can potentially act on individual means and individual variances, correlated selection is unlikely, for the traits assessed. Neither boldness nor predictability in boldness was related to mass or to sex, nor did they vary over time concurrent with gains in mass under ad libitum food conditions and in experience with the behavioural assay, indicating they were not clearly related to these potentially important life history and state variables.

Predation risk during early ontogeny can impact developmental trajectories and permanently alter adult phenotypes. Such phenotypic plasticity often leads to adaptive changes in traits involved in anti-predator responses. While plastic changes in cognition may increase survival, it remains unclear whether early predation experience shapes cognitive investment and drives developmental plasticity in cognitive abilities. Here, we show that predation risk during early ontogeny induces developmental plasticity in two cognitive domains. We reared female guppies Poecilia reticulata with and without predator cues and tested their adult cognitive abilities. We found that females reared under simulated predation took longer to learn a simple association task, yet outperformed animals reared without predation threat in a reversal learning task testing cognitive flexibility. These results show that predation pressure during ontogeny shapes adult cognitive abilities, which we argue is likely to be adaptive. Our study highlights the important role of predator-mediated developmental plasticity on cognitive investment in natural populations and the general role of plasticity in cognitive performance.

Behavioral and physiological ecologists have long been interested in explaining the causes and consequences of trait variation, with a focus on individual differences in mean values. However, the majority of phenotypic variation typically occurs within individuals, rather than among individuals (as indicated by average repeatability being less than 0.5). Recent studies have further shown that individuals can also differ in the magnitude of variation that is unexplained by individual variation or environmental factors (i.e., residual variation). The significance of residual variation, or why individuals differ, is largely unexplained, but is important from evolutionary, methodological, and statistical perspectives. Here, we broadly reviewed literature on individual variation in behavior and physiology, and located 39 datasets with sufficient repeated measures to evaluate individual differences in residual variance. We then analyzed these datasets using methods that permit direct comparisons of parameters across studies. This revealed substantial and widespread individual differences in residual variance. The magnitude of individual variation appeared larger in behavioral traits than in physiological traits, and heterogeneity was greater in more controlled situations. We discuss potential ecological and evolutionary implications of individual differences in residual variance and suggest productive future research directions.

The study of behavioral syndromes aims to understand among-individual correlations of behavior, yielding insights into the ecological factors and proximate constraints that shape behavior. In parallel, interest has been growing in behavioral plasticity, with results commonly showing that animals vary in their behavioral response to environmental change. These two phenomena are inextricably linked-behavioral syndromes describe cross-trait or cross-context correlations, while variation in behavioral plasticity describes variation in response to changing context. However, they are often discussed separately, with plasticity analyses typically considering a single trait (univariate) across environments, while behavioral trait correlations are studied as multiple traits (multivariate) under one environmental context. Here, we argue that such separation represents a missed opportunity to integrate these concepts. Through observations of multiple traits while manipulating environmental conditions, we can quantify how the environment shapes behavioral correlations, thus quantifying how phenotypes are differentially constrained or integrated under different environmental conditions. Two analytical options exist which enable us to evaluate the context dependence of behavioral syndromes-multivariate reaction norms and character state models. These models are largely two sides of the same coin, but through careful interpretation we can use either to shift our focus to test how the contextual environment shapes trait covariances.

Predation risk during early ontogeny can impact developmental trajectories and permanently alter adult phenotypes. Such phenotypic plasticity often leads to adaptive changes in traits involved in anti-predator responses. While plastic changes in cognition may increase survival, it remains unclear whether early predation experience shapes cognitive investment and drives developmental plasticity in cognitive abilities. Here, we show that predation risk during early ontogeny induces developmental plasticity in two cognitive domains. We reared female guppies Poecilia reticulata with and without predator cues and tested their adult cognitive abilities. We found that females reared under simulated predation took longer to learn a simple association task, yet outperformed animals reared without predation threat in a reversal learning task testing cognitive flexibility. These results show that predation pressure during ontogeny shapes adult cognitive abilities, which we argue is likely to be adaptive. Our study highlights the important role of predator-mediated developmental plasticity on cognitive investment in natural populations and the general role of plasticity in cognitive performance

Quantifying individual variation in labile physiological or behavioral traits often involves repeated measures through time, so as to test for consistency of individual differences (often using repeatability, R) and/or individual differences in trendlines over time. Another form of temporal change in behavior is temporal autocorrelation, which predicts observations taken closely together in time to be correlated, leading to nonrandom residuals about individual temporal trendlines. Temporal autocorrelation may result from slowly changing internal states (e.g., hormone or energy levels), leading to slowly changing behavior. Autocorrelation is a well-known phenomenon, but has been largely neglected by those studying individual variation in behavior. Here, we provide two worked examples which show substantial temporal autocorrelation (r > 0.4) is present in spontaneous activity rates of guppies (Poecilia reticulata) and house mice (Mus domesticus) in stable laboratory conditions, even after accounting for temporal plasticity of individuals. Second, we show that ignoring autocorrelation does bias estimates of R and temporal reaction norm variances upwards, both in our worked examples and in separate simulations. This bias occurs due to the misestimation of individual-specific means and slopes. Given the increasing use of technologies that generate behavioral and physiological data at high sampling rates, we can now study among- and within-individual changes in behavior in more detailed ways, including autocorrelation, which we discuss from biological and methodological perspectives and provide recommendations and annotated R code to help researchers implement these models on their data.

Predation is a near ubiquitous factor of nature and a powerful selective force on prey. Moreover, it has recently emerged as an important driver in the evolution of brain anatomy, though population comparisons show ambiguous results with considerable unexplained variation. Here, we test the reproducibility of reduced predation on evolutionary trajectories of brain evolution. We make use of an introduction experiment, whereby guppies (Poecilia reticulata) from a single high predation stream were introduced to four low predation streams. After 8-9 years of natural selection in the wild and two generations of common garden conditions in the laboratory, we quantified brain anatomy. Relative brain region sizes did not differ between populations. However, we found a general increase and striking variation in relative brain size of introduced populations, which varied from no change to a 12.5% increase in relative brain weight, relative to the ancestral high predation population. We interpret this as evidence for non-parallel evolution, which implies a weak or inconsistent association of relative brain size with fitness in low predation sites. The evolution of brain anatomy appears sensitive to unknown environmental factors, or contingent on either chance events or historical legacies of environmental change.