Thermoregulation, energetics, and behavior.

http://dx.doi. org/10.1037/0000011-045 APA Handbook of Comparative Psychology: Vol. 1. Basic Concepts, Methods, Neural Substrate, and Behavior, J. Call (Editor-in-Chief) Copyright © 2017 by the American Psychological Association. All rights reserved. Temperature affects all aspects of biological function, including behavior (see Haynie, 2001). All behavior thus occurs within a thermodynamic context and is bounded by thermal and energetic constraints (see Careau, Killen, & Metcalfe, 2015; Mathot & Dingemanse, 2015). Some of these arise within biological systems, directly out of the mechanics of physics, biochemistry, and geometry. For example, chemical reaction rates are temperature dependent, as are many critical characteristics of enzymes and other proteins, including structure and function. The same is true for many properties of biological membranes (Hazel, 1995), including the permeability of chorionic and amniotic membranes (e.g., Bara & GuietBara, 1986) and the blood–brain barrier (Kiyatkin & Sharma, 2009). There are thus thermal optima for the myriad functions and complex biochemical reactions that drive living systems—optima that are carefully balanced and adjusted dynamically in the face of internal and external challenges.1 Neurons are among the most temperaturesensitive cells in the body (Kiyatkin, 2010; Rango, Arighi, & Bresolin, 2012). Many critical parameters of neuronal functioning (e.g., conduction velocity, refractory period) covary with temperature, implying coupling between brain temperature and neural processing (e.g., Blessing, Mohammed, & Ootsuka, 2012, 2013; Thiessen, 1983a) and optima for neuronal functioning (see Gisolfi & Mora, 2000; Kiyatkin, 2010). Indeed, the energetic efficiency of action potentials improves with increased temperature within the mammalian physiological range, with optimal efficiency at 37° C (Yu, Hill, & McCormick, 2012). Even small elevations (3° C–4° C) above this (e.g., because of fever, stroke, or ingestion of drugs such as MDMA [3,4-methylenedioxymethamphetamine]) can trigger neuronal dysfunction resulting in hallucinations, seizures, and damage (e.g., Brown & Kiyatkin, 2004). Likely because of these trade-offs, some of the most fascinating thermoregulatory adaptations have emerged in the context of regulating brain temperature (see Blumberg, 2002). Other constraints on temperature regulation are imposed from outside the animal. For example, by beginning life in the same locale in which their parents reproduced, most animals inherit basic ecological and climatic conditions and, within that context, select among or create microclimates. Add to this variation from daily and seasonal temperature rhythms, and we can recognize the bases of complex and varied adaptive strategies. In response to seasonal changes, some species migrate; others hibernate, estivate, or become hypometabolic (see Storey, 2015); many show season-specific changes in fur density, fat composition, and other metabolic and thermoregulatory phenotypes (Blumberg, 2002). With respect to daily changes, most species exhibit closely coordinated circadian rhythms of behavior and physiology, thereby optimizing the acquisition, utilization, and conservation (or loss) of thermal energy. For example, on a C H A P T E R 4 5