As extensively reviewed by Endler and Parker (1990), Lazarus (1993a), and Suls and David (1996), in humans, interest in coping concepts became more prominent during the 1960s and 70s as interest in the study of stress increased. In the early years, coping was conceptualized as a psychological style or inner (primarily unconscious) psychodynamic response in defense of personal (“ego”) integrity and threat management. For example, Sigmund Freud described 10 defense mechanisms: regression, repression, reaction formation, isolation, undoing, projection, introjection, turning against the self, reversal, and sublimation (in Freud, 1992). Some defense styles were considered healthier or less regressed than others (Lazarus, 1993a). Other researchers formulated the ego defenses as trait-like coping styles. For example, Byrne proposed a continuum between repression, i.e., behavior mechanisms of a predominantly avoiding (denying, repressing) type, and sensitization, i.e., behavior mechanisms of predominantly approaching (intellectualizing, obsessional) behaviors (Byrne, 1961; Suls and David, 1996). Some 13 additional formulations on the approach-avoidance scale are reviewed by Roth and Cohen (1986).

In contrast, the coping process approach developed by Lazarus and colleagues in the 1970s and 80s regarded coping strategies to be a function of the situational context in which it occurs, as influenced by external environmental forces (Lazarus, 1993a). According to this approach, coping processes change over time and are not based on preconceived notions of inherent health and adaptiveness or pathology and maladaptiveness. This approach emphasizes two major functions of coping: “problem” focused coping, i.e., to actively attempt to change the environment by removing the source of stress or remove oneself from the source of stress, versus “emotion focused” coping, i.e., to reduce the emotional impact or the negative appraisal of stress (Folkman and Lazarus, 1980). In particular, emotion focused coping may predominate in situations that are perceived to be “refractory to change,” whereas problem focused coping may predominate in situations that are perceived as “controllable by action” (Lazarus, 1993a). However, it became empirically clear that the situational context does not account for all of variation in coping behaviors (Suls and David, 1996). In the 1980s and 1990s, research revealed that coping strategies may be influenced by personality variables, such as the tendency to be optimistic vs. pessimistic (Scheier et al., 1986). In fact, it was reported that some 30% of the variation in single-act behaviors can be explained by personality traits and a considerably larger portion may be explained when behavioral cross-situational aggregates rather than situation-specific single-act behaviors are used (Kenrick and Funder, 1988). Indeed, Kato and Pedersen (2005) observed that genetic influences on coping scales were partly attributable to genetic factors that are associated with personality traits.

Therefore, to some extent, coping strategies were shown to be consistent, showing a stable pattern over time and across stressful encounters and could be viewed as a stable coping disposition or style. A well-described coping style is called the Type A pattern, which Friedman and Rosenman (1974) defined as “an action–emotion complex that can be observed in any person who is aggressively involved in a chronic, incessant struggle to achieve more and more in less and less time, and if required to do so, against the opposing efforts of other things or other persons” (in Matthews, 1982). As reviewed by Matthews (1982), this behavioral style can be regarded as a continuum of behaviors ranging from the extreme Type A to the opposite extreme Type B. Type A personalities are characterized as having increased levels of competitiveness, aggressiveness, achievement striving, ambition, impatience, and hostility (Heilbrun and Renert, 1986). In response to stress, Type A personalities show an active response and a greater tendency to rely on achievement related, solution-oriented, problem-focused coping, directed at bolstering the perception of controllability (Evans and Fearn, 1985; Hart, 1988). In the 1980s, personality dimensions were extended to broader sets of personality dimensions, such as “the Big Five”: Neuroticism (N) vs. Emotional Stability; Extraversion (E) or Surgency; Openness to Experience (O) or Intellect; Agreeableness (A) vs. Antagonism; and Conscientiousness (C) or Will to Achieve (Costa and McCrea, 1992), which are also associated with stress-related coping processes (Penley and Tomaka, 2002).

Speisman et al. (1964) observed that “control of the environment” was an important modulating factor that determines stress response characteristics. Indeed, the interaction between behavioral type (A or B) with “locus of control” significantly influenced manager work satisfaction and health outcomes (Kirkcaldy et al., 2002). Managers with external locus (believing that events in their lives are a function of luck, chance, fate, God(s), or powerful others) showed significant lower job satisfaction levels and more negative health consequences than managers with internal locus (believing that events are a function of their own behavior and/or ability, personality, or effort), especially when this characteristic was combined with a Type A personality. Speisman et al. (1964) identified that controllability was related to separate catecholamine (adrenaline and noradrenaline) and cortisol factors. The catecholamine factor is predominantly related to coping “effort,” whereas the cortisol factor is predominantly related to coping “distress,” i.e., cortisol tends to increase in novel and unfamiliar situations that evoke feelings of uncertainty and anxiety. Comparing response to a low-control and a high-control task, they observed that during the high-control task adrenaline, but not cortisol, increased (effort without distress), whereas during the low-control task both adrenaline and cortisol increased (effort with distress). In addition, Speisman et al. (1964) linked personality, perception of control, and situational context, in that together they form the threat meaning or “harmful significance” of an event. Indeed, “Stressors, like beauty, lie in the eye of the beholder,” i.e., it is only when a situation is perceived as a potential threat that a stress response is initiated (Everly and Lating, 2013). For example, an event may be perceived as an unpleasant threat but also as an exhilarating challenge, and resulting from the motivation toward the event, both the appraisal of the event and the subsequent coping response can be described as a function of stress emotional states, including anger, anxiety, guilt, shame, sadness, envy, jealousy, and disgust, but also happiness, pride, relief, love, hope, compassion, and gratitude (Lazarus, 1993b). Frankenhaeuser and Lundberg (1985) confirmed that catecholamine output under different psychosocial conditions was linearly related to the intensity of the emotional experience (and consequently the degree of subjective stress), whether pleasant or unpleasant.

Methods used for coping assessment in animals are fundamentally different from those used in human research. As reviewed by Parker and Endler (1992), coping research in humans is primarily based on various self-report coping scales and measures in response to (the thought of) a stressful event, such as the Ways of Coping Questionnaire (Folkman and Lazarus, 1988). Because animals cannot directly communicate their cognitive emotional appraisal, motivation, or behavioral approach toward a stressful event these questionnaires have no use in animal research. Instead, the behavioral and/or physiological response to social and non-social situations is evaluated. However, although coping assessment methods are inherently different in animals and humans, research in coping strategies in humans and animals has, with few (notable) exceptions (Zozulya et al., 2008; Cavigelli et al., 2013), surprisingly little overlap in its discussion and implications of the results. Whereas human research is based on (unconscious or conscious) psychological coping mechanisms that are thought to be influenced by personality traits, since assessment methods in animals are unable to evaluate the latter, coping in animal research is described by behavioral and physiological responses that are themselves equated to a personality concept. Based on earlier work studying individual differences between animals during defensive behavior, it was hypothesized that two fundamentally distinct coping styles exist: animals that respond to social interaction with an active fight–flight response that is characterized by a behavioral and neuroendocrine response highly suited to either attack or flight, versus animals that respond with a passive conservation-withdrawal response that is characterized by a neuroendocrine response resulting in behavioral inhibition (Benus et al., 1992). Blanchard et al. (2011) discussed fight–flight vs. conservation-withdrawal strategies in a social confrontation from a process approach as a function of the situational context in which it occurs, as influenced by external environmental forces: situational risk assessment (i.e., appraisal through auditory, visual, and olfactory detection) shapes the defensive response choice on a likelihood of success prediction compared with taking a different approach. For example, an animal may flight if an escape route is available, hide if there is a place of concealment, freeze if there is neither an escape route nor a hiding place, show defensive threats as the threat stimulus approaches, and defensive attack when contact with the threat stimulus is imminent. Indeed, the distance between the subject and the threat was found to be a rather precise determinant of freezing (longer distances) vs. fight behavior (shorter distances) (Blanchard et al., 2011). In contrast, similar to humans, a coping style is defined as a coping strategy that is consistent when measured repeatedly in the same situation, as well as in response to different situations, i.e., (at least to some degree) independent of the situational context (Koolhaas et al., 1999). In a social confrontation, fight–flight-type animals are more aggressive, attack, or actively try to escape when defeated, and are characterized by a predominantly sympathetic adrenal-medullary response pattern, whereas conservation-withdrawal-type animals are less aggressive, more immobile, hardly respond to attacks, and respond with a parasympathetically dominated response and greater adrenocortical activity. Indeed, Koolhaas et al. (1999) showed that mouse lines divergently selected for aggressive social behavior (attack latency) were representative of two distinct coping styles: proactive, aggressive (fight–flight type) animals and reactive, non-aggressive (conservation-withdrawal type) animals (Benus et al., 1991). In addition to differences in social behavior, the lines also differed with respect to other behavioral components. As opposed to reactive animals, proactive animals are fast exploring, impulsive, actively manipulate events, score high in frustration tests, and are risk takers and novelty seekers (Benus, 1988; Coppens et al., 2010). Overall, aggressive mice were better shock avoiders (Benus et al., 1989) and had greater mobility in a novel object test (Caramaschi et al., 2009). They tended to react in a rather routine way to changes in the environment (i.e., intrinsic behavioral control), whereas non-aggressive individuals seemed to be more attentive to their environment (i.e., extrinsic behavioral control) (Benus et al., 1987). Further research showed that also the social response of aggressive mice toward a conspecific was more routine-like and inflexible than that of non-aggressive mice (Benus et al., 1990).

Coping styles in rodent studies are extensively reviewed by Cavigelli et al. (2013). After the rodent studies, coping styles were reported in other species as well, in particular in animals managed by humans, such as pigs (Bolhuis et al., 2005), chickens (Jensen et al., 2005), cattle (Hopster, 1998), fish (Øverli et al., 2004), horses (Budzyńska, 2014), dogs (Horváth et al., 2007), and cats (Natoli et al., 2005), but also in natural species such as birds (Carere et al., 2001), marmots (Costantini et al., 2012), wild rabbits (Rödel and Monclús, 2011), chipmunks (Montiglio et al., 2012), salamanders (Crane et al., 2012), crickets (Kortet and Hedrick, 2007), and spiders (Johnson and Sih, 2005). It is now clear that variation in behavioral traits has a clear genetic basis and that behavioral traits do not inherit independently of each other (Van Oers et al., 2005). Because individual behavioral traits within a style are genetically correlated, (divergently) selecting for extremes of one behavioral style trait, such as aggression in the studies of Koolhaas et al. (1999), will result in a correlated response in other style traits. For example, Sluyter et al. (1995) showed that mice selected for short attack latency showed more nest-building behavior than those selected for high attach latency, whereas mice in a reciprocal selection experiment selected for high nest-building activity were more aggressive than those selected for low nest-building activity. Annen and Fujita (1985) showed that rats selected for “low emotional reactivity” (i.e., high ambulation and low defecation scores in a brightly lit runway) were also more aggressive than those selected for “high emotional reactivity or timidity,” and mink selected for confident reaction to humans showed more exploratory behavior across several social and non-social situations than mink selected for fearful reaction toward humans (Malmkvist and Hansen, 2002). Mice selected for high wheel-running behavior (voluntary physical activity) were more explorative in the open-field test, and showed more risk-taking behavior in approaching a novel object relative to a control line (Jónás et al., 2010). Great tits selected for slow exploratory performance took longer to attack than those selected for fast exploratory performance (Carere et al., 2005).

Livestock animals require to cope with the demands of a semi-natural production environment that includes few features that the individual can manipulate or change (Wechsler, 1995). Several studies have indicated that the behavioral organization of livestock has not been changed to a great extent by domestication; therefore, intensive housing conditions are likely to trigger behavioral and physiological coping efforts in response to aspects including limited space allowance, absence of key stimuli, and semi-natural feeding regimes and social group compositions (Duncan, 1998; Bracke and Hopster, 2006). Because unsuccessful coping is closely related to the development of abnormal (stereotypic) behavior, coping ability in livestock animals directly relates to the individual’s ability to adapt to specific management conditions and therefore animal wellbeing (Wechsler, 1995). In this context, coping research in livestock species is mostly directed to evaluation of the response to novel environmental cues, and (anti)social behavior.

Hessing et al. (1993) classified piglets in response to a restraint backtest as resistant, doubtful, or non-resistant. A high consistency in their behavioral response was observed in successive restraint tests, in addition, individuals that resisted in the backtest were also classified as “aggressive” individuals immediately after relocation and mixing, whereas individuals that did not resist were classified as “non-aggressive” individuals. The authors indicated that the results may be particularly important in realizing the optimum group composition based on individual characteristics. Indeed, Bolhuis et al. (2005) showed that resistant pigs initiated more fights, started fighting earlier, and spent more time fighting after regrouping. In addition, resistant pigs appeared to have a higher propensity to develop inflexible behavioral routines (Bolhuis et al., 2004). D’Eath and Burn (2002) and Cassady (2007), however, did not find any relationship between resistance in the backtest and aggressive behavior in a resident–intruder test in piglets. Neither did Janczak et al. (2003), who observed repeatability in response to specific or closely related stimuli, but did not find any correlation between resistance to the backtest, and a resident-intruder, human in the home cage, or novel object test.

The conclusion was shared by Herskin et al. (2004) in dairy cows, who observed stimulus specificity in the reactions of dairy cows toward different novel object tests. Also Gibbons et al. (2009) observed consistency over time in flight response scores and consistency between different human approach situations, but no consistency was found between human approach situations and novel object test scores. However, Müller and Von Keyserlingk (2006) observed that beef cattle having higher temperament as measured by repeated measurements of flight speed from a squeeze chute were more agitated during a social separation test as measured by locomotion and behavioral states. In rangeland-raised beef cattle, Wesley et al. (2012) observed consistent behavioral styles in foraging behaviors across contexts (confinement vs. rangeland pasture), and also MacKay et al. (2013) observed a correlation between observations in a short-term temperament test and aggression at feeders.

Korte et al. (1998) observed that chicks from a low feather pecking line of laying hens had a higher parasympathetic activity in response to manual restraint than chicks from a high feather pecking line, and concluded that individuals of both lines were passive and active, respectively, in terms of coping style. This was supported by Jensen et al. (2005), who observed that feather pecking behavior in chickens was positively correlated with activity in an open-field test, novel food/novel object test, and in a restraint test, which suggests that feather pecking might be genetically linked to a proactive coping style. However, Jones et al. (1995) observed that individuals from a chicken line selected for high feather pecking showed more freezing, and less vocalization and activity in an open-field, which is more indicative of a reactive coping style.

It has been proposed that lack of repeatability of intra-test results may result when tests are more situation-specific and therefore less indicative of true differences in individual stress-coping behavior measurable throughout an individual’s life (Cassady, 2007). Lack of correlations between both intra- and inter-test results have been proposed to result from differences in stability of underlying emotions, or the perception of or the biological motivation toward certain tests (Ruis et al., 2000). In addition, Ruis et al. (2000) indicated that the (dynamics of) group compositions may affect the individual coping response of animals and as a result the correlations between results and the interpretation of data.

A renewed interest in coping styles has emerged in the fields of ecology, evolutionary, and natural biology, with a focus on the structure of coping styles, its proximate and ultimate causes, its relation to life-history strategies, and consequently its ecological and evolutionary significance (Wolf and Weissing, 2012; MacKay and Haskell, 2015). Thus far, research into coping behavior in the context of natural evolution had followed the coping process approach, under the understanding that behavioral strategies were potentially infinitely plastic and, therefore, natural selection favored different optimal strategies in different situational contexts (Sih et al., 2004; Bell, 2007). In the ecological literature, coping style, or within-individual consistency in behavior, is also referred to as “behavioral type”; proactive, aggressive copers are referred to as “bold types” and reactive, non-aggressive copers as “shy types.” In addition, a new dimension has been added, the behavioral syndrome, which refers to between-individual consistency in behavior across multiple situations, or “the correlation of rank-order differences between individuals over time and/or across situations” (Bell, 2007). Behavioral syndromes are thus a property not only of the individual, but also of the population, since they describe the distribution of behavioral types (or coping strategies) within a population (MacKay and Haskell, 2015). As reviewed by DeWitt et al. (1998), adaptive phenotypic plasticity infers costs such as those related to the production and maintenance of sensory and regulatory mechanisms of plasticity, and information acquisition. However, the cost and benefits of limited plasticity, as implied by the existence of behavioral syndromes, may trade-off with the ability to adapt to a variety of environments. Therefore, the existence of coping styles implies that individuals may be adapted to a certain environment but display behaviors that are suboptimal in a different one. For example, proactive copers rely mainly on internally organized predictions, which may be fast but also inaccurate in new situations, whereas passive copers are more guided by external stimuli than active copers and hence their behavior is more flexible and adaptable to variable and unpredictable environmental conditions (Benus, 1988; Coppens et al., 2010). Similarly, too high levels of aggression may be unsuitable in contexts where caution and care may be more appropriate, but unaggressive individuals may do poorly in competitive situations (Sih et al., 2004). However, although the flexibility or variety of behavioral strategies employed by an individual in different situations is, to some extent, restricted (behavioral types), the magnitude of the response in different situations may be flexible (behavioral syndrome), while consistently differing from that of other individuals in the population (Biro and Stamps, 2008).

Coping styles are closely related to individual fitness and are part of life-history strategies, since they are related to risk-taking behavior and form general adaptive response patterns in reaction to everyday challenges and stress (Coppens et al., 2010). In addition, activity, exploration, boldness, and aggression are energetically costly. Life history is commonly defined as a set of evolved behavioral and physiological strategies that more or less influence longevity and reproduction and may include fitness traits such as reproductive success, survival, viability, fecundity, mating success, and age at maturity (Schluter et al., 1991). A fundamental assumption of life-history theory is that resources are limited and need to be distributed among growth, reproduction, and maintenance, or stored for future use. Since resources used for one purpose are no longer available for other purposes, trade-offs are inevitable. Therefore, coping styles can be expected to trade off against other life-history traits (Wolf et al., 2007; Wolf and Weissing, 2012). Natural selection results in the optimal allocation of resources across important life-history functions (Brommer, 2000; Roff, 2007). Since nature has not favored any coping strategy in particular, both strategies may have benefits and should be considered as alternative strategies to cope with environmental demands (Coppens et al., 2010). More recently, behavioral syndrome theory has been integrated in the life-history pace-of-life syndrome hypothesis, thus mapping behavioral styles on the “slow” (low metabolic rate, slow development, late maturation, long life span, high investment in few offspring) to “fast” (high metabolic rate, fast development, fast maturation, low survival, low investment in many offspring) pace-of-life scale, and connecting them with a series of metabolic, hormonal, and immunity traits that underlie this syndrome (Réale et al., 2010). Indeed, several empirical studies provide evidence for links between behavioral types and food intake, growth, and reproductive traits (Biro and Stamps, 2008). Bold individuals with high levels of aggression may have better access to resources necessary for fast development and early reproduction, and have more reproductive success, but this may also be associated with a higher risk of mortality. Indeed, Smith and Blumstein (2008) showed in a meta-analysis that bolder individuals have greater reproductive success but this incurs a survival cost. Thus, shy individuals that have reduced short-term reproductive success but live longer may have the same overall fitness as bold individuals.

The Type A behavior pattern in humans arose from the observation by Friedman and Rosenman that patients with cardiac disorders seemed to have different behavioral characteristics than noncardiac patients. Meanwhile, Type A as an independent risk factor for coronary heart disease (CHD) has been firmly established (Matthews, 1982). However, the nature of the link with Type A behavior was not immediately recognized, partly because the definition of Type A behavior is broad and complex and has been described (and sometimes misclassified) in a wide variety of ways (Friedman and Booth-Kewley, 1987). As proposed by Lazarus (1993b), it appears that it is the stress emotional state that determines whether individuals classified as Type A are prone to CHD while those classified as Type B are not (Friedman and Booth-Kewley, 1987). For example, it appears that emotional reactions of impatience, hostility or repressed hostility, anger, aggression, and tenseness are characteristic of the CHD-prone person. Therefore, hostile, competitive, aggressive striven Type A individuals may be prone to CHD but not active, hard-working, confident, charismatic, socially skilled, dominant, vigorous, ambitious individuals. Likewise, relaxed, easygoing, reserved Type B individuals may not be prone to CHD, but individuals that experience strong emotions but have some difficulty in expressing them openly may be prone to CHD. In addition, depression and anxiety appear to be underlying risk factors for CHD that are not always clearly expressed in the expected behavioral type (Diamond, 1982; Friedman and Booth-Kewley, 1987). Folkman et al. (1986) showed that individuals who display more confrontive coping (e.g., “stood my ground and fought for what I wanted”), distancing (e.g., “went on as if nothing had happened”), self-controlling (e.g., “I tried to keep my feelings to myself”), accepting responsibility (e.g., “criticized or lectured myself”), and escape avoidance behaviors (e.g., “wished that the situation would go away or somehow be over with”) had a lower somatic health status, whereas individuals that felt more “mastery” (i.e., that regard one’s life chances as being under one’s control in contrast to being fatalistically determined) were healthier. However, Folkman (1984) indicates that in humans, being in control may not always be stress reducing and result in a positive situational appraisal.

Also in other animals, proactive and reactive individuals are found to differ in the physiological and neuroendocrinological response to stress, which may have implications for their health status. Several studies, as reviewed by Koolhaas (2008), demonstrate that the nature of the neuroendocrine stress response may modulate the immune response and individual vulnerability to disease. The response of proactive animals is often found to be dominated by an enhanced sympathetic and (nor-)adrenergic response, resulting in high plasma levels of adrenalin and noradrenalin and a high heart rate and blood pressure, whereas the response of reactive animals is dominated by enhanced parasympathetic activation and a high hypothalamic–pituitary–adrenal (HPA) response, resulting in a bradycardia response in reaction to a sudden unpredicted stressor and increased plasma levels of corticosteroids. However, when defeated in a social confrontation, proactive animals appear to respond with a corticosterone response that is higher than that of defeated reactive mice (Carere et al., 2001; Koolhaas, 2008). Morrow-Tesch et al. (1994) showed that both socially dominant and submissive pigs were immune compromised (elevated numbers of neutrophils, decreased antibody production) compared with socially intermediate pigs and concluded that although dominance may afford the animal greater priority to resources (like food and mates), it may have some immunosuppressing effects as well.

The reactivity of the stress response is shaped both by genetics as well as by gene × environment interactions resulting in epigenetic modifications (Rauw and Gomez Raya, 2017). Kadarmideen and Janss (2007) estimated that cortisol in a pig selection experiment is highly genetically determined with heritabilities of 0.40–0.70. They observed that cortisol levels are determined by a mixture of genes with large and small effects, but also detected a major gene with an additive effect of 86 ng/ml. Sautron et al. (2015) identified 65 genes in pigs as biological markers of HPA axis activation at the gene expression level. In chickens, Fallahsharoudi et al. (2017) detected one genome-wide significant QTL on chromosome 5 and two suggestive QTL which together explained 20% of the variance in corticosterone response.

Although energetics is a fundamental assumption of life-history theory, surprisingly little is published to date on theory or observation linking behavioral styles and syndromes to energy budgets and resource allocation. However, because exploration, boldness, and aggression are energetically costly, from a resource allocation point of view it can be expected that different coping strategies may constitute different metabolic costs and present different trade-offs with growth, reproduction, and immune function. For example, depending on body weight, a minimum cost can be defined for such activities as walking, running, flying, and swimming (Tucker, 1970; Videlex and Nolet, 1990). Fighting depletes energy reserves and (anticipated) metabolic consequences may determine strategic decision making during social interaction (Neat et al., 1998). In fish, Neat et al. (1998) showed that escalated fighting is costly for both winners and losers, but especially for losers. Indeed, aggressive disposition may be beneficial if it allows for greater food intake and faster growth. Similarly, for bold individuals, the benefits in terms of dispersal potential and gaining access to new resources may be offset by increased energetic costs from higher metabolic rates when at rest (Myles-Gonzales et al., 2015). Indeed, when food cannot be monopolized, the greater costs of behaviors associated with an increased standard metabolic rate may significantly reduce growth (Vøllestad and Quinn, 2003; Georgiev et al., 2013). Careau et al. (2010) found that aggressive dog breeds have higher energy needs than unaggressive ones and, in addition, docile dogs live longer than bold ones. Biro and Stamps (2010) reviewed literature that reported significant positive relationships of several types of behavior (aggressive behavior, success in competitive interactions, activity rates, boldness, scrounging, courtship) with resting metabolic rate and therefore maintenance cost in a diverse array of taxa.

Because behaviors underlying fight–flight behavior are fueled by a greater metabolic rate, it may appear that fight–flight coping styles are costlier than conservation–withdrawal styles. However, the multi-axial physiological stress response may also demand a significant amount of resources. Indeed, the concept of allostasis, i.e., energy demanded by physiological mediators or “homeostats” of the physiological stress response is deeply integrated into the concept of life history theory and trade-offs through resource allocation (Goldstein, 2003; Rauw and Gomez Raya, 2017). One of the best known physiological mediators is the HPA axis, which functions as a primary mediator of energy balance homeostasis, as a master organizer of life-history transition, and as an integral responder to stressors (Crespi et al., 2013). Glucocorticoid-induced physiological changes include gluconeogenesis for providing energy to fuel the greater metabolic demands, and the stimulation of feeding behavior to replenish depleted energy stores following a stress response (Matteri et al., 2000). Resources used by this and other physiological mediators determine the total energy demanded or “allostatic load,” which is a function of the response to metabolic demand of daily and seasonal routines and of unpredictable stressors (Romero et al., 2009). When stimulation of the mediators is prolonged or severe, such that costs are larger than those available in the reserve, resources must be reallocated away from other biological functions, which then become impaired (Moberg, 2000). For example, increased costs associated with the stimulation of the HPA axis result in reduced growth rates in all livestock species (Rauw and Gomez Raya, 2015). In addition, glucocorticoids are largely immunosuppressive, which could be an adaptive mechanism to reallocate resources from activation of the immune defense system (Råberg et al., 1998). Therefore, the reduced cost of the coping response of reactive individuals that result from reduced levels of activity and aggression may (partly) be offset by increased energetic costs associated with a higher HPA response.