Repeatable individual differences in behaviour and physiology in juvenile horses from an early age

Abstract

Most people living or working in close association with animals are familiar with individual differences in their behaviour, and awareness is growing that understanding such differences can contribute to improved management, production and welfare. Nevertheless, due to the considerable logistic and financial difficulties of conducting longitudinal studies, particularly in the large, slow-growing mammalian species comprising an important part of the world’s domestic animals, there is still little information on how early such differences emerge and whether they persist across different developmental stages. In an extension of a previous study of the behavioural and physiological (heart rate variability, HRV) response of 30 Azteca-breed foals of both sexes to repeated brief maternal separation from birth to weaning, we tested the same animals post-weaning in two tests of brief separation from their social group when they were approximately nine months and one year old, and recording the same behavioural and physiological variables as previously. As in the earlier study, we found stable individual differences in several behavioural variables: vocalization rate (R = 0.54, P < 0.01), duration of locomotion (R = 0.46, P = 0.02), duration of high tail carriage (R = 0.31, P = 0.05), and head tosses (R = 0.49, P=0.02), also in three of the four HRV measures at the two ages; change in heart rate (R = 0.47, P < 0.001), in STDRR (R = 0.39, P = 0.02) and in RMSSD (R = 0.39, P = 0.02) from baseline to the separation. These corresponded to the individual differences we had found pre-weaning. In contrast to our previous study, however, we no longer found an association between the behavioural and physiological measurements at this later age. Including previous measurement from the same animals we conclude there is considerable stability in individual differences in behaviour and physiology in response to a brief and ecologically relevant stressor from as early as the first week of age, even though close correspondence between behavioural and physiological changes disappear through development. These differences might be reliable indicators of individuals’ suitability for later use in particular applied contexts.