Quantifying the autonomic response to stressors – one way to expand the definition of "stress" in animals

Abstract

Quantifying the impact of changes or stimuli in the external and internal environment that are challenging (“stressors”) to whole organisms is difficult. To date, physiological ecologists and ecological physiologists have mostly used measures of glucocorticoids (GCs) to assess the impact of stressors on animals. This is of course too simplistic as Hans Seyle himself characterized the response of organisms to “noxious stimuli” using multiple physiological responses. Possible solutions include increasing the number of biomarkers to more accurately characterize the “stress state” of animal or just measuring different biomarkers to more accurately characterize the degree of acute or chronic stressors an animal is experiencing. We focus on the latter and discuss how heart rate (HR) heart rate variability (HRV) may be better predictors of the degree of activation of the sympathetic-adrenal-medullary system and complement or even replace measures of GCs as indicators of animal health, welfare, fitness, or their level of exposure to stressors. The miniaturization of biological sensor technology (“bio-sensors” or “bio-loggers”) presents an opportunity to reassess measures of stress and develop new approaches. We describe some modern approaches to gathering these HR and HRV data in free-living animals with the aim that heart dynamics will be more integrated with measures of GCs as bio-markers of stress and predictors of fitness in free-living animals.

Heart rate as a measure of emotional arousal in evolutionary biology

“…Heart rate provides an accurate and comprehensive measure of emotional arousal in non-human animals [9]. Here, I propose that research on heart rate modulation in a wide variety of species and different contexts could be of interdisciplinary interest and would provide important insights for basic research, animal welfare and conservation.…”

“…variation in the time interval between heartbeats, have been suggested to reflect the balance between the regulation of the sympathetic and parasympathetic branches of the automatic nervous system [6,7]. As these responses are plastic and context-dependent [8], heart rate measures can provide important insight into how animals perceive and respond to their environment [9]. Further, heart rate is positively correlated to metabolic rate and thus can be used as a reliable proxy of energy expenditure [10].…”