Vocal repertoire of adult domestic pigs in a laboratory environment

Abstract

The use of domestic pigs in clinical training and biomedical research is expanding rapidly, increasing the need for reliable, noninvasive indicators of health and welfare. Vocal analysis offers a non-invasive promising tool, yet the acoustic repertoire of adult domestic pigs remains poorly defined. However, the vocalization repertoire of adult domestic pigs has yet to be characterized. This study characterizes the vocal repertoire of adult pigs housed in a biomedical research laboratory. Twelve mixed-breed pigs (2-3 months old; 5 males, 7 females) were recorded during routine husbandry and experimental procedures. Vocal classification was conducted using perceptual and objective clustering techniques. First, aural- visual (AV) inspection of spectrograms was used to construct a hierarchical repertoire. Second, a two-step cluster analysis based on six acoustic parameters (5% frequency, first quartile frequency, center frequency, 90% bandwidth, interquartile range bandwidth, and 90% duration) provided an objective classification. Agreement between methods was evaluated using Cramers V. A total of 1,136 vocalizations from 69 recordings were analyzed. AV classification revealed five major vocal classes-- grunt, squeal, complex, scream, and bark--subdividing into 16 distinct call types. Standardized definitions integrating descriptive and quantitative criteria are provided. The two-step cluster analysis identified two clusters as the optimal statistical solution, with moderate agreement between methods (Cramers V = 0.67, p < 0.0001). Most AV-defined call types aligned with previously reported repertoires, although whines, yelps, and stable screams were unique to this study. While two-cluster solutions are commonly reported, our findings indicate that richer acoustic structure exists and that high gradation among pig calls may limit the resolution of statistical clustering. These results establish a detailed acoustic framework for adult pig vocalizations and provide essential groundwork for developing predictive models to enhance welfare assessment and support comparative research in laboratory-housed pigs.