Abstract
During their domestication via artificial selection, humans have substantially modified the morphology and thus visual appearance of non-human animals. While research highlights the negative impact of these modifications on physical functioning, little is known about their impact on behavior and signaling, either toward humans or conspecifics. Changes in the appearance of the face, such as those associated with, but not limited to, facial expressions, form an important part of non-verbal communication. In companion animals, the face is one of their most visually diverse features (due to human-driven selection), which may impact the visual clarity of expressions and other forms of signaling. Using the domestic cat as our model, we applied a new analytical technique in order to understand the impact of breed variation on relative positioning of facial landmarks, chosen specifically for their association with the production of various facial movements, and the expression of affect. We then assessed the extent to which facial appearances known to be associated with a specific underlying state (i.e., pain, assessed via a validated, facial pain score), could be reliably detected in a morphologically diverse population. Substantial baseline variation in landmarks was identified at both the cephalic (e.g., brachycephalic, dolichocephalic, mesocephalic) as well as breed levels. While differences in facial pain scores could successfully differentiate between “pain” and “no pain” in the facial appearance of domestic shorthaired cats (DSH), these differences were no longer detectable when assessed within a larger more morphologically diverse population, after corrections for multiple testing were applied. There was also considerable overlap between pain scores in the DSH “pain” population and the neutral faces of other breeds. Additionally, for several paedomorphic breeds, their neutral face shapes produced scores indicative of greater pain, compared to most other breeds, including the DSH cats actually in pain. Our findings highlight the degree to which anthropocentric selection might disrupt the communicative content of animals' faces, in this case the domestic cat. These results also suggest a potential human preference for features extending beyond the infantile, to include negatively-valenced facial forms such as pain.
Highlights
The domestication syndrome highlights the rapid, cross-species convergence of key phenotypic changes not usually seen in wild populations
Fifty facial images were obtained from a population of mixed breed (i.e., Domestic short-haired type) cats (2.8 ± 0.5 kg; 14.1 ± 5.2 months of age) undergoing ovariohysterectomy as described in Brondani et al [46]. These data were originally generated during the development of a previous postoperative composite pain detection scale [UNESP-Botucatu MCPS; Brondani et al [46] and were subsequently used to develop a geometric morphometric method to assess facial expressions in DSH cats [45]]
The facial landmarks used in this study were, where possible, located in relation to muscle insertion points, and in all cases positioned relative to areas of the face that are expected to vary as a result of changes in facial expression [see [41, 45]]
Summary
The domestication syndrome highlights the rapid, cross-species convergence of key phenotypic changes not usually seen in wild populations. Variation in humandriven selection for specific features has led to much greater levels of intra-species behavioral and morphological diversity than may typically be present in their wild progenitors [4,5,6]. In companion animals, such anthropogenic selection has contributed to the creation of breeds with exaggerated characteristics or various morphological “extremes,” affecting a range of features [e.g., the size and shape of their face and ears, their limbs, tails, and general body size and shape; [7,8,9]]. The value of effective communication in these contexts may be high, yet it is possible that such animals might be at a disadvantage, given the radical morphological differences they display at a species level [e.g., [7, 9, 15]]
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