A Quantitative Analysis of Facial Expression in the Plains Zebra

Abstract

Abstract and SummaryThis study on the facial expressions of the plains zebra is concerned with those facial expressions about which the literature is not consistent as far as the description and functional naming are concerned. In order to establish which facial expressions occur a vector analysis approach was adopted in which a quantitative inventory was made of combinations of the main facial expression components. 20 combinations (facial expressions) were found to occur frequently. We then grouped these facial expressions on the basis of similarities in the contexts in which they occur, using calculations of (dis)similarity measures, followed by several types of cluster analysis. In this way we found 7 groups of facial expressions which, with the help of the contextual parameters used in the analysis, could be further reduced to 6 groups and could be interpreted and labeled as: submission (bared teeth with ears forward, sideward or backward, chewing movements and occasionally head lowering); play face (biting intentions with ears forward or sideward); threat (ears backward, often together with the head pointing towards the opponent); herding threat (a head‐low posture with ears backward); greeting (ears forward, repeated mouth openings without showing teeth); and another submissive expression (head low, ears backward). Methods, results and interpretations are discussed in the light of the existing literature and hopefully some of the confusion about zebra facial expression is removed. However, some reserve remains justified with respect to the extent to which the proposed model can be generalized, because it is founded on data from only one zebra herd with a particular composition.