EquiFACS: The Equine Facial Action Coding System

Abstract

Although previous studies of horses have investigated their facial expressions in specific contexts, e.g. pain, until now there has been no methodology available that documents all the possible facial movements of the horse and provides a way to record all potential facial configurations. This is essential for an objective description of horse facial expressions across a range of contexts that reflect different emotional states. Facial Action Coding Systems (FACS) provide a systematic methodology of identifying and coding facial expressions on the basis of underlying facial musculature and muscle movement. FACS are anatomically based and document all possible facial movements rather than a configuration of movements associated with a particular situation. Consequently, FACS can be applied as a tool for a wide range of research questions. We developed FACS for the domestic horse (Equus caballus) through anatomical investigation of the underlying musculature and subsequent analysis of naturally occurring behaviour captured on high quality video. Discrete facial movements were identified and described in terms of the underlying muscle contractions, in correspondence with previous FACS systems. The reliability of others to be able to learn this system (EquiFACS) and consistently code behavioural sequences was high--and this included people with no previous experience of horses. A wide range of facial movements were identified, including many that are also seen in primates and other domestic animals (dogs and cats). EquiFACS provides a method that can now be used to document the facial movements associated with different social contexts and thus to address questions relevant to understanding social cognition and comparative psychology, as well as informing current veterinary and animal welfare practices.

Fig 1. The facial muscles of the horse.

NB. Levator labii superioris a.n. represents the levator labii superioris alaeque nasi, which is often also called the levator nasolabialis. Synonyms for the levator annuli oris fascialis muscle include the dilator nares muscle and the caninus muscle. The frontoscutularis has a frontal and a temporal arm.

Fig 2. The facial landmarks of the horse.

Fig 3. A guide to anatomical direction.

Fig 4. The area and direction of movement in AU101, inner brow raiser.

The picture shows the left eye of the horse.

Fig 5. AU10, upper lip raiser (+ 16 + 17 + 25 + 27 + AD1 [AD1 seen in part B only]).

Part A shows appearance changes 1, 2, 3, 4, and 7; part B shows appearance changes 2, 4, 5, 6, and 7.

Fig 6. The location and direction of movement in AU113, sharp lip puller (indicated by the arrow).

The lips are in the neutral position. AU113 is very difficult to code from photographs unless there is a very strong action, or it is combined with AU12, lip corner puller.

Fig 7. AU12, lip corner puller, without AU113, sharp lip puller, (A) and AU12 with AU113 (B).

Note the difference in the shape at the corner of the mouth. AU12 produces a curvature at the mouth corner; however, when AU113 is applied with AU12 the mouth has an angular appearance at the top corner, with corresponding wrinkles in the skin surrounding the mouth. See S15 Video for a demonstration of these actions acting simultaneously.

Fig 8. Direction and area of AUH13, nostril lift.

The picture on the left shows the neutral face, the picture on the right shows the face with a mild action of AUH13 applied. The arrow illustrates the location and direction of movement.

Fig 9. AD160, lower lip relax, seen from a distance and a close up view.

Note that from a distance, although subtle, AD160 can be identified by the visible pink flesh from the inside of the lower lip.

Fig 10. Profile view of AD160, lower lip relax.

Panel A demonstrates the outline of the lower lip when AD160 is present, and panel B when AD160 is released.

Fig 11. The lower lip when AU16, lower lip depressor, is acting in isolation (A) and in combination with AU17, chin raiser (B).

Note the square shape of the lip and definition in the skin covering the mental region in combination AU16+17.

Fig 12. An example of the wrinkles seen with AU122, upper lip curl.

These wrinkles are also characteristic of AU10, upper lip raiser, however in AU10 there is no puckering or eversion of the lip.

Fig 13. The general neutral ear position of a horse, although there is some individual variation.

Fig 14. The starting ear position (A) and then with EAD103, ear flattener, applied (B).

Note how the angle of the ears to the front of the face alters.

Fig 15. The starting ear position (A) and then with EAD103, ear flattener, and EAD104, ear rotator, applied (B).

An example of how the ears can almost disappear in a frontal view of a strong action.