Stress responses in an outbred, wild-derived hamster, Phodopus campbelli: Individuality in the cortisol response to acute restraint stress and the absence of strong association with behavioural responses to acute restraint
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Glucocorticoid reactivity is often used as a biomarker of emotional stress status in animals and man. Likewise, behavioural responses of individuals to standardized stresses are also widely used to assess the magnitude of perceived stress. Unfortunately, there is little evidence that widely used physiological measures and widely used behavioural measures yield similar, or even correlated, results. The current study provides an explicit test of the relationship between the two approaches. Dwarf hamsters (Phodopus campbelli) were subjected to a standardized, on-back restraint in a socially subordinate position and had plasma samples drawn under brief, home-cage, isoflurane anesthesia to quantify cortisol before and after the acute stress. Individual responses were transformed into parameters describing the absolute change in cortisol, the proportional changes in cortisol, and the overall exposure to cortisol across stress and recovery. There was exceptional individual variability in cortisol responses to the standardized stressor, including monotonic declines, and cluster analysis failed to effectively partition between-individual variation by sex, age, or housing conditions, for any parameter. Next, physiological measures, and a battery of behavioural tests routinely used to assess, anxiety, stress, and emotionality differences, as well as resistance to capture and intruder aggression, were combined in an expanded population including individuals from genetic lines selected to increase or decrease cortisol concentration. In general, dwarf hamsters showed less neophobia than is typical of laboratory mice and rats (e.g. animals quickly dropped from the elevated plus maze). In addition, results were combined to create behavioural indices by orienting each test score relative to an emotionality continuum and combining tests based on, a priori, expectations of similarity. Multivariate regression failed to detect covariation between physiological and behavioural measures, despite the large number of parameters describing those responses. Although the experimental design did not include replication, or comparison of physiological responses to different stressors, it was clear that behavioural and physiological responses were neither interchangeable nor statistically associated. Thus, results challenge widespread assumptions about the interaction between the glucocorticoid response to stress, and the manifestation of that stress through alterations in behaviour. Additional research exploring physiology-behaviour associations in genetically diverse populations is warranted.