Which comes first in human temperature regulation: the physiological or the behavioural response?

The ability of humans to maintain thermal homeostasis requires engagement of both physiological and behavioural responses. We are all acutely aware of two major physiological thermoregulatory responses, sweating in the heat and shivering in the cold, yet we may not be as cognizant of our behavioural responses. If we think about a common behaviour used in both the heat and the cold, seeking shelter, it becomes apparent that our behaviour is aimed at maintaining thermal homeostasis, and we may be unconsciously attempting to minimize our reliance on physiology (Schlader et al. 2013). The literature is replete with studies aimed at understanding either the physiological response(s) or the behavioural response(s); however, very few studies are aimed at understanding the intersection of physiology and behaviour in human thermoregulation. Schlader and colleagues have been at the forefront of addressing this important gap in the literature and, in the current issue of Experimental Physiology, expand on their previous work by addressing the question of whether changes in metabolic heat production and sweating precede thermal behaviour (Schlader et al. 2009, 2013, 2016a,b). Using a novel experimental design, in which participants were free to move between cool and warm rooms, Schlader et al. (2016a) found that initiation of thermal behaviour precedes any changes in metabolic heat production when going from a cool to a warm room as well as changes in sweating when going from a warm to a cool room. Conversely, initiation of thermal behaviour appeared to coincide with changes in cutaneous vasomotor tone such that there was substantial cutaneous vasoconstriction when moving from a cool to a warm room and maximal withdrawal of vasoconstrictor tone when moving from warm to cool (and perhaps engagement of the active vasodilator system; Schlader et al. 2016a). Inasmuch as changes in skin blood flow can affect skin temperature, the findings of Schlader et al. (2016a) appear to support the notion that changes in skin temperature are the primary determinants of thermal behaviour (Cabanac et al. 1972; Schlader et al. 2013). If, as proposed above, we accept the concept that thermal behaviour is a mechanism to maintain thermal homeostasis and, perhaps, to minimize our reliance on physiology, these findings make sense. Changes in skin temperature can have a large influence on thermal comfort and, as such, changes in skin blood flow and skin temperature appear to drive changes in our thermal behaviour. The present study by Schlader et al. (2016a) raises some important and interesting questions. What happens in more extreme temperatures? The temperature of the cool room ( 18°C) in the present study represents a rather mild autumn/early winter day in many parts of the USA (Schlader et al. 2016a). Would changes in metabolic heat production, skin temperature and cutaneous vasomotor tone be antecedent to initiation of thermal behaviour in more extreme environments? Are these thermal behaviours modified by disease, body fat and/or fitness? What happens during exercise? How does humidity in a hot environment affect behaviour? While asking whether changes in physiology or behaviour come first may appear to be a ‘chicken or the egg’ question, the data from the present study have helped to clarify the question and push us a little more toward an answer of ‘behaviour’. Although there is clearly more work to do in this area, it is encouraging to see studies focused on the intersection of physiology and our behaviour. As highlighted both in the present study and in our own everyday experiences, our behaviour affects our physiology and vice versa.