When water interacts with temperature: Ecological and evolutionary implications of thermo‐hydroregulation in terrestrial ectotherms

Abstract

The regulation of body temperature (thermoregulation) and of water balance (defined here as hydroregulation) are key processes underlying ecological and evolutionary responses to climate fluctuations in wild animal populations. In terrestrial (or semiterrestrial) ectotherms, thermoregulation and hydroregulation closely interact and combined temperature and water constraints should directly influence individual performances. Although comparative physiologists traditionally investigate jointly water and temperature regulation, the ecological and evolutionary implications of these coupled processes have so far mostly been studied independently. Here, we revisit the concept of thermo‐hydroregulation to address the functional integration of body temperature and water balance regulation in terrestrial ectotherms. We demonstrate how thermo‐hydroregulation provides a framework to investigate functional adaptations to joint environmental variation in temperature and water availability, and potential physiological and/or behavioral conflicts between thermoregulation and hydroregulation. We extend the classical cost–benefit model of thermoregulation in ectotherms to highlight the adaptive evolution of optimal thermo‐hydroregulation strategies. Critical gaps in the parameterization of this conceptual optimality model and guidelines for future empirical research are discussed. We show that studies of thermo‐hydroregulation refine our mechanistic understanding of physiological and behavioral plasticity, and of the fundamental niche of the species. This is illustrated with relevant and recent examples of space use and dispersal, resource‐based trade‐offs, and life‐history tactics in insects, amphibians, and nonavian reptiles.

Short‐term change in water availability influences thermoregulation behaviours in a dry‐skinned ectotherm

“…These results call for a better consideration of joint water and thermal availability changes in future climate change studies of ectotherms (Rozen-Rechels et al, 2019). Models ignoring this dual effect might overestimate or underestimate activity restriction amplitude in terrestrial ectotherms depending on future climate trends and interactive effects of water and heat on behavioural thermoregulation.…”

“…Yet, the thermal dependence of activity patterns of these species is more complex than previously thought and our understanding of it requires more quantitative data (Gunderson & Leal, 2015Vickers, Manicom, & Schwarzkopf, 2011). In addition, current mechanistic models neglect the water dependence of behavioural thermoregulation Rozen-Rechels et al, 2019;Ryan et al, 2016;Sannolo & Carretero, 2019), which becomes critical given predicted changes in drought propensity as the climate is warming worldwide (Field, Barros, Stocker, & Dahe, 2012). In fact, we still lack empirical evidence to understand and parameterize the joint effects of water and heat stress on ectotherms activity for future mechanistic modelling.…”

“…shade selection; Blouin-Demers & Weatherhead, 2002;Muri et al, 2015) changes in activity patterns. In addition, behavioural thermoregulation also depends on water availability, and future changes in water availability may cause behavioural and ecological changes in terrestrial ectotherms independently from heat constraints (Cahill et al, 2012;Rozen-Rechels et al, 2019). Understanding behavioural consequences of both heat and water restriction is thus essential to predict activity and growth patterns (Kearney, Munns, Moore, Malishev, & Bull, 2018;Pirtle, Tracy, & Kearney, 2019).…”

“…When trophic interactions are controlled for, behavioural responses of organisms to water availability could thus be driven by direct physiological effects or by indirect microclimatic changes (Rozen-Rechels et al, 2019).…”

“…Thermoregulation and hydroregulation behaviours are intimately related (Rozen-Rechels et al, 2019), as heat and water stress conditions are often correlated, making it hard to disentangle one from the other. Yet, mechanistic models of ectotherms populations typically describe behavioural thermoregulation with simple thermal functions that fail to predict how these behaviours should respond to joint changes in temperature and water availability (Gunderson & Leal, 2016;.…”