Evidence that stress-induced changes in surface temperature serve a thermoregulatory function

Abstract

The fact that body temperature can rise or fall following exposure to stressors has been known for nearly two millennia; however, the functional value of this phenomenon remains poorly understood. We tested two competing hypotheses to explain stress-induced changes in temperature, with respect to surface tissues. Under the first hypothesis, changes in surface temperature are a consequence of vasoconstriction that occur to attenuate blood loss in the event of injury and serve no functional purpose per se; defined as the 'haemoprotective hypothesis'. Under the second hypothesis, changes in surface temperature reduce thermoregulatory burdens experienced during activation of a stress response, and thus hold a direct functional value: the 'thermoprotective hypothesis'. To understand whether stress-induced changes in surface temperature have functional consequences, we tested predictions of these two hypotheses by exposing black-capped chickadees (n=20) to rotating stressors across an ecologically relevant ambient temperature gradient, while non-invasively monitoring surface temperature (eye region temperature) using infrared thermography. Our results show that individuals exposed to rotating stressors reduce surface temperature and dry heat loss at low ambient temperature and increase surface temperature and dry heat loss at high ambient temperature, when compared with controls. These results support the thermoprotective hypothesis and suggest that changes in surface temperature following stress exposure have functional consequences and are consistent with an adaptation. Such findings emphasize the importance of the thermal environment in shaping physiological responses to stressors in vertebrates, and in doing so, raise questions about their suitability within the context of a changing climate.

Heat dissipation capacity influences reproductive performance in an aerial insectivore

“…The estimated percentage total surface area trimmed was ∼7% (see Appendix for details). We are confident that this amount of trimming is sufficient to induce heat loss, as seen in similar treatments by Nord and Nilsson (2019) and implied in equations from Robertson et al (2020) (see Appendix for details). Constriction of the vasculature in the exposed area around the brood patch would probably be insufficient to counter the energy loss induced by trimming.…”

Body surface temperature responses to food restriction in wild and captive great tits

“…If so, we would have expected to see a considerably faster return to before-food restriction values than in this study, based on the timeline of the thermal response to an acute stressor in periorbital skin in the closely related blue tit (Cyanistes caeruleus) (Jerem et al, 2019). This provides evidence for selective vasoconstriction of the bill as opposed to a global drop in peripheral temperature as is expected in response to an acute stressor (e.g., Herborn et al, 2015;Nord and Folkow, 2019;Robertson et al, 2020).…”

Thermal flexibility is a repeatable mechanism to cope with environmental stressors in a passerine bird

“…Blair et al 1959;Yokoi 1966;Nord & Folkow 2019;Winder et al 2020). In some species, these changes in T s appear to endow individuals with greater heat conservation in the cold, and greater heat dissipation in the warmth, thus reducing their demands for costly thermogenesis or evaporative cooling respectively (Jerem et al 2018;Robertson et al 2020a;Winder et al 2020). In this way, total energetic expenditure may be balanced in challenging environments by allocating energy toward more immediate and higher-cost threats (e.g.…”

“…the perceived stressors) and away from less immediate and lower-cost threats (e.g. thermal challenges; Jerem et al 2018; Robertson et al 2020a). In urban environments, where individuals regularly contend with both physical and thermal challenges, such flexibility of T s and peripheral heat loss (here, non-evaporative heat-loss; henceforth, "q Tot ") could be particularly advantageous, with those capable of enhanced flexibility (particularly during stress exposures) being better able to balance energy expenditure and, therefore, being favoured by selection (see Parsons 2005).…”

“…Hill et al 1980;Powers et al 2015) and both temperature of, and heat loss from this region have previously been shown to respond to stress exposure (e.g. Jerem et al 2015;Ikkatai & Watanabe 2015;Herborn et al 2018;Robertson et al 2020a). We, therefore, chose to use temperature of, and heat loss from, the eye region as our indicators of T s and q Tot in this study.…”