An ethogram of piglet vocalizations during suckling

There is obvious confusion in the literature on pig vocalizations. It is not clear whether pig calls are discrete or continuous in form and function, and whether they are specific signals. The aim with this study was to try to classify piglet vocalizations uttered during nursing, and the hypothesis was that the sounds are, in fact, structurally discrete. All vocalizations uttered during 33 nursings in 3 litters during 3 days after farrowing were recorded, and 106 sounds were processed in a spectrograph. The spectrograms were analysed by the following method. Firstly, they were classified in groups on the basis of a number of physical parameters which were quantified for each spectrogram. Secondly, the inter-class dissimilarity was tested with single-link cluster-analysis. Thirdly, the correlation of the physical parameters was tested within classes with correlation and regression calculations. The analysis supported the existence of 5 discrete classes of piglet vocalizations: croaking; deep grunt; high grunt; scream; and squeak. In croaking, scream and squeak there was a correlation between basic frequency and time, indicating a continuous variation of intensity within the discrete call classes. It was further concluded that any viewpoint on the function of the sounds must be based on studies of statistical links between specific signals and certain behaviour in the receiver.