INTRODUCTION
The ecological importance of phenotypic trait variation within species is gaining increasing attention (Forsman & Wennersten 2016; Rocheset al. 2018). Recent results demonstrate that intra-specific variation can play a considerable role on the biodiversity of an ecosystem, comparable or even stronger than variation across species (Roches et al. 2018). Individuality, which is also referred to as personality, is a statistical phenomenon characterized by individual variation being less than between individual variation and individuals differ along a behavioral dimension (Roche et al. 2016). By definition, this implies comparisons with other group members,i.e. an individual is bolder than another in multiple contexts and throughout time (Réale et al. 2010). Boldness, activity, aggressiveness and sociability/gregariousness are among the most studied personality traits (PTs). Covariation between personality traits defines a behavioral syndrome (Sih et al. 2004; Bell 2007). Recent work has integrated molecular and endocrinological mechanisms to further distinguish bold, aggressive and active individuals from their counterparts (Réale et al. 2010). The concept of syndromes relates to the older notion of coping styles (Koolhaas et al.1999), which highlights differences between reactive and proactive individuals in their capacities to respond to a challenge. Hence, reactive animals, those that are relatively shy, less aggressive and less active, are also characterized by high hypothalamic-pituitary-interrenal/adrenal (HPI/A) responses and low sympathetic activity compared to proactive ones in response to a stressor (Koolhaas et al. 2010). A very large body of literature has also demonstrated that individuals differ in a range of behavioral and physiological traits that are generally associated with being reactive or proactive, and many of these have ultimate consequences on traits associated with fitness in specific environments (see Table 1 for details).
A population is therefore behaviorally and physiologically structured along a continuum of reactive to proactive individuals. From an ecological point of view, this structure has major implications for the ecosystem’s dynamics since it drives intra-population competition as well as inter-species interactions (Bolnick et al. 2011; Sihet al. 2012). From an evolutionary point of view, variability in coping styles is also of primary importance since it determines a population’s capacity to adapt to environmental changes and therefore determines the response to natural selection (Dingemanse & Réale 2005). There is therefore an urgent need to acknowledge individual coping style as an important conservation actor in addition to the species diversity that is commonly considered by ecologists and managers (Brodie et al. 2018).
Through extensive harvesting, environmental pollution, habitat fragmentation, the introduction of exotic species, tourism, urbanization and climate change, humans modify the strength and direction of natural selection. This has profound consequences on the behavior and physiology of many species. Because of the relative explosion in the rate at which these anthropogenic disturbances have proliferated across the world, they are collectively referred to as human-induced rapid environmental changes or HIREC (Sih et al. 2011). Only few areas are still unaffected by HIREC, and therefore physiological responses of species to HIREC are of primary importance for conservation issues. While behavioral responses to HIREC have previously been reviewed (Sihet al. 2011; Tuomainen & Candolin 2011; Sih 2013; Wong & Candolin 2015), the associated underlying physiological mechanisms of the differences in coping style have been relatively ignored. Here, we focus on the multiple physiological and behavioral processes by which HIREC modifies the coping style structure of populations. A recent study demonstrates that one of the most important HIREC, climate change, facilitates the dominance of proactive species (i.e., fast-growing and opportunistic), and this has direct consequences on biodiversity (Brustolin et al. 2019). Based on recent literature, we propose that evolutionary responses to HIREC at the intraspecific level may generally favor reactive individuals in the wild. This evolutionary response is the consequence of three distinct time-related steps from habitat changes (migration or dispersal), to genetic evolution, detailed below.