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.