HIREC-driven superiority of plastic individuals
Natural selection will favor well-adapted heritable phenotypes in a new environment and lead to genotypic change over time. However, most environmental changes associated with HIRECs seem to occur rapidly and plasticity plays an essential role in determining the phenotype that will survive (Diamond & Martin 2016). Phenotypic changes for individuals exposed to HIREC are largely documented in response to exposition to contaminants, temperature, acidification, environmental noise, etc. Individuals with the best capacities to respond rapidly via changes in behavior (van Baaren & Candolin 2018) or physiology (Taff & Vitousek 2016) are therefore the most likely to survive HIREC, as seen with climate change (Beever et al. 2017). In a number of taxa, proactive and reactive phenotypes diverge in their capacities to respond to environmental changes (Table 1). Case studies highlighted the ability of reactive individuals to be more behaviorally plastic in a wide range of circumstances. In rats selected for their aggressive behavior, proactive individuals developed routines, a superior strategy when in predictable environments (or situations) but not when the environment frequently changes (Benus et al. 1991).
Plasticity has nevertheless an energetic cost (Moran 1992; Murrenet al. 2015). Consequently, in a stable environment, proactive individuals, which are less plastic than reactive individuals, are expected to better perform because less energy is invested in coping abilities, memory and learning capacities (Table 1). The costs of plasticity have been demonstrated in great tits (Parus major ), where fast exploring/proactive individuals performed better in stable environments, while the slow exploring/reactive individuals performed better in a variable and fluctuating environment (Dingemanse & de Goede 2004). And, after transport from UK to Norway, reactive rainbow trout (Oncorhynchus mykiss ), had greater feeding motivation and started to win dyadic fights against proactive opponents which was not the case prior transportation (Ruiz-Gomez et al. 2008). Moreover, reactive rainbow trout are also more efficient at finding food than proactive ones in response to environmental changes (Ruiz-Gomez et al.2011). In addition, reactive fish performed better in cognitively complex foraging tasks, even after modifications to their environment (White et al. 2017). Similar conclusions were drawn in birds (Verbeek et al. 1994) and pigs (Bolhuis et al. 2004) where proactive individuals were less successful in reversal learning than reactive pigs, suggesting that proactive individuals display a less plastic behavior. In a monogamous species, the red point cichlid (Amatitlania siquia ), reactive individuals were better at varying their behavioral profile within pair (Laubu et al. 2016) which is important to enhance their reproductive success (Gabriel & Black 2012; Harris & Siefferman 2014). Finally, fish with different coping styles also diverge in their sensitivity to environmental cues. Indeed, reactive individuals have been shown to be more responsive to negative, aversive, stimuli while proactive individuals are more sensitive to positive stimuli (Millot et al. 2014).
Altogether, reactive individuals have increased behavioral capacities to cope with rapid environmental changes. This is also seen at the physiological level, with reactive individuals showing higher capacities to mount physiological responses required to cope with environmental challenges (Fig. 1.2) due to greater HPI/A axis activation when facing a stress.