Vocalisations of the adult female domestic pig during a standard human approach test and their relationships with behavioural and heart rate measures.
Vocal communication in the domestic pig is generally not well documented. The aim of this experiment was to categorise and ascribe the function of the vocalisations of 67 Large WhitexLandrace gilts during a standard human approach test. At testing, each group of 3-5 gilts was moved to a handling area where each individual in turn was fitted with a heart rate monitor and introduced individually to a 2.4mx2.4m test arena. After 2min familiarisation, an unfamiliar human entered the pen and stood for 3min against one wall. Behaviour and sound were recorded continuously with sound recordings transferred onto computer for analysis. Three categories of calls were initially identified: single grunts, single squeals and rapidly repeated grunts. Sixty-six gilts performed single grunts, whereas only 28 and 16 gilts performed the other two categories, respectively. Single grunts could be sub-divided into two types based on sound amplitude profile. These types differed significantly in duration. Gilts performed more short and long grunts per minute during the 3min test period than during the familiarisation period. Most short grunts observed in a subset of 15 gilts were performed with the snout close to a pen surface or the human. The rate of short grunts during the test period was negatively correlated with the time taken to make contact with the human and positively correlated with the amount of locomotor behaviour carried out, the total number of interactions with the human and the total time spent within 0.5m of the human. Most long grunts observed in a subset of 15 gilts were performed with the snout away from any surface. The rate of long grunts during the test period positively correlated with amount of locomotor behaviour and heart rate, after the effect of activity had been removed. Squeals could similarly be sub-divided into long and short types on the basis of amplitude profile. Gilts that squealed carried out more locomotor behaviour, interacted with the human more, had higher mean heart rates and lower heart rate rise when touched by the human, suggesting a higher degree of arousal. Rapidly-repeated grunts were associated with close human interaction. The results indicate that the domestic pig performs a number of distinct vocalisations during isolation. Short single grunts appear to be associated with investigatory behaviour. Long single grunts may be a form of contact call, the rate of which is related to physiological and behavioural activity. Squeals may have similar function but result from a higher level of arousal. Short, rapidly-repeated grunts appear to have either a greeting or threat function. With further research, certain pig vocalisations may be identified as providing useful additional information about an individual's welfare.