Vocalisation of domestic pigs (Sus scrofa domestica) as an indicator for their adaptation towards ambient temperatures

It was tested whether the vocalisation of pigs can be used to assess their adaptability to ambient temperatures. It was hypothesised that the proportion of high-frequency vocalisation, indicating discomfort, would increase with behavioural reactions to both low and high ambient temperatures and that this relationship is modified by the weight of pigs. In addition, the saliva concentration of cortisol was measured to test for a relationship between the rate of high-frequency vocalisation and the (hypothalamic-pituitary-adrenocortical)-HPA-axis activity. Subjects were 12 groups of fattening pigs (nine pigs per group, observed at weights of 25-35, 50-70 and > 80 kg). Eight groups were tested in summer and four groups in winter and there were three to six observational nights per group. Mean temperatures in the pens during the observational nights ranged from 4 to 26degreesC. Vocalisation and behaviour were recorded from 23:00 to 5:00 h and vocalisations were analysed, using an artificial neural network (STREMODO(R)). The saliva concentration of cortisol at 6:00 h was analysed during winter experiments. Data were analysed, using linear mixed-effects models that took the hierarchical experimental design into account. With decreasing temperatures, huddling increased (P < 0.001). This increase was weight-dependent with pigs huddling less with increasing weight (P < 0.001). The increase in huddling was also accompanied by an increase in high-frequency vocalisation (P < 0.01). We found a weight-dependent covariation between huddling and high-frequency vocalisation. Pigs produced more high-frequent vocalisation and huddled less with increasing weight (P < 0.01). There was no statistically significant relationship between cortisol and high-frequency vocalisation. Lying in the dung area occurred only at temperatures above 21degreesC and was not related to high-frequency vocalisation. Our findings suggest that huddling results in a state of discomfort in at least medium-weight and heavy pigs and that this is reflected by changes in the vocal behaviour. Smaller pigs seem to be less affected by huddling than heavy pigs. The use of high-frequency vocalisation was a powerful indicator of behavioural thermoregulation in pigs of medium and heavy weight at low temperatures. (C) 2004 Elsevier B.V. All rights reserved.