Current Advances in Assessment of Dog's Emotions, Facial Expressions, and Their Use for Clinical Recognition of Pain

Abstract

Animals' facial expressions are involuntary responses that serve to communicate the emotions that individuals feel. Due to their close co-existence with humans, broad attention has been given to identifying these expressions in certain species, especially dogs. This review aims to analyze and discuss the advances in identifying the facial expressions of domestic dogs and their clinical utility in recognizing pain as a method to improve daily practice and, in an accessible and effective way, assess the health outcome of dogs. This study focuses on aspects related to the anatomy and physiology of facial expressions in dogs, their emotions, and evaluations of their eyebrows, eyes, lips, and ear positions as changes that reflect pain or nociception. In this regard, research has found that dogs have anatomical configurations that allow them to generate changes in their expressions that similar canids-wolves, for example-cannot produce. Additionally, dogs can perceive emotions similar to those of their human tutors due to close human-animal interaction. This phenomenon-called "emotional contagion"-is triggered precisely by the dog's capacity to identify their owners' gestures and then react by emitting responses with either similar or opposed expressions that correspond to positive or negative stimuli, respectively. In conclusion, facial expressions are essential to maintaining social interaction between dogs and other species, as in their bond with humans. Moreover, this provides valuable information on emotions and the perception of pain, so in dogs, they can serve as valuable elements for recognizing and evaluating pain in clinical settings.

Keywords: animal welfare; emotions; human-dog interaction; pain; positive and negative stimuli.

Figure 1

Review methodology.

Figure 2

Characteristics of an infant-like, or paedomorphic face. A 3-month-old Golden Retriever puppy with gastrointestinal disease. The image shows the broad forehead and large eyes suggestive of sadness, two traits that may also be associated with chronic pain [39].

Figure 3

Neurobiology of facial expression in dogs. During a threat exposure, such as another dog, unfamiliar person, or a dispute over territory, the neural mechanism in the amygdala reacts to catecholamine secretion (A and NA). The catecholamines stimulate the motor cortex and its efferent fibers to modify a facial expression. Movements such as flattening the ears to the side (EAD105) and lifting the upper eyelids to have a wider field of vision (AU101) are characteristic of an expression of fear. OXT: oxytocin; NA: noradrenaline; A: adrenaline; DA: dopamine; SE: serotonin.

Figure 4

Endocrine and motor control of facial expression during pleasant emotions in canines. When the amygdala associates the presence of humans or conspecifics to a positive emotion, the levels of OXT, DA, and SE increase in response to the stimulus; this activates the facial units that enlarge the eyes and retract the muscles of the mouth to produce a simulated “smile”. OX: oxytocin; NA: noradrenaline; A: adrenaline; DA: dopamine; SE: serotonin.

Figure 5

Facial expressions in dogs and their links to humans. Image of a one-year-old female Chihuahua showing marked changes in facial expressions upon recognizing similar emotions in her owner’s gestures. (A) Neutral. Relaxed expression; (B) Sadness. Broad forehead, large eyes, ears erect with caudal inclination, closed snout; (C) Pleasure. Tension in the labial commissure, eyes semi-closed with facial mimicking that simulates the form of a human smile; (D) Surprise. Eyebrows raised to show apparently larger, more expressive eyes, with ears raised and labial tension.

Figure 6

Changes in facial expression during exposure to various stimuli. (A). Neutral. No relation or expression; (B). Anger. Observations include a fixed gaze with ears at medium height or pulled back with flared nostrils and tensed cheeks; (C). Fear. The gaze has deviated with the ears pulled back, exposing the frontal region as a signal of submission; (D). Surprise. The eyebrows are inexpressive, perhaps with a slight raising of the medial section to give the appearance of large eyes, with the ears at medium height; (E). Pleasure. Eyebrows and ears are clearly raised with the mouth open and tension in the labial commissure.

Figure 7

Facial expressions represent aggression. The perception of an adverse external stimulus (e.g., a person or another animal) triggers a neuroendocrine response with an elevation in catecholamine levels, together with a decrease in circulating levels of OXT, DA, and SE. When this negative emotion is recognized in the amygdala, its connections to the motor cortex and facial muscles (trough the facial nerve) cause the flattening of the ears (EAD105), lifting of the upper lip to bare their teeth (AU116), nostril opening (AD38), more visible white sclera (AU101), and vocalizations (AU34) as a sign of threat. OXT: oxytocin; NA: noradrenaline; A: adrenaline; DA: dopamine; SE: serotonin.

Figure 8

Facial expressions related to fear can provoke aggression. (A) Nose, sclera, and snout. Increased exposure of the sclera is caused by raised eyebrows and tension in the nasal region with flared nostrils, accompanied by an open mouth, raised upper lip, and exposed fangs. Additionally, dogs may emit vocalizations. (B) Ears and gaze. The positioning of the ears is commonly seen to be at a medium height concerning the frontal region; the animal’s gaze is fixed in the direction of the threatening stimulus.

Figure 9

Facial expressions related to emotions [21,25,28,102,106,123,133].

Figure 10

Expressions related to pain. (A). Mild pain. Expressions are characterized by a sustained raising of the eyebrows with displays of the frontal region of the head and lateral abduction of the ears, generally associated with a sad face, as represented in this image of a one-year-old female Pitbull, which shows a slight anaphylactic reaction. (B). Severe pain. Expressions are characterized by semi-closed eyelids, cheek tension, and vocalizations, as can be seen in the image of this two-year-old female Dalmatian with acute polyradiculoneuritis.