Facial Indicators of Positive Emotions in Rats.

Kathryn Finlayson,Hanno Würbel,Sara Hintze,Jessica Frances Lampe,Luca Melotti,Edna Hillmann

doi:10.1371/journal.pone.0166446

Abstract

Until recently, research in animal welfare science has mainly focused on negative experiences like pain and suffering, often neglecting the importance of assessing and promoting positive experiences. In rodents, specific facial expressions have been found to occur in situations thought to induce negatively valenced emotional states (e.g., pain, aggression and fear), but none have yet been identified for positive states. Thus, this study aimed to investigate if facial expressions indicative of positive emotional state are exhibited in rats. Adolescent male Lister Hooded rats (Rattus norvegicus, N = 15) were individually subjected to a Positive and a mildly aversive Contrast Treatment over two consecutive days in order to induce contrasting emotional states and to detect differences in facial expression. The Positive Treatment consisted of playful manual tickling administered by the experimenter, while the Contrast Treatment consisted of exposure to a novel test room with intermittent bursts of white noise. The number of positive ultrasonic vocalisations was greater in the Positive Treatment compared to the Contrast Treatment, indicating the experience of differentially valenced states in the two treatments. The main findings were that Ear Colour became significantly pinker and Ear Angle was wider (ears more relaxed) in the Positive Treatment compared to the Contrast Treatment. All other quantitative and qualitative measures of facial expression, which included Eyeball height to width Ratio, Eyebrow height to width Ratio, Eyebrow Angle, visibility of the Nictitating Membrane, and the established Rat Grimace Scale, did not show differences between treatments. This study contributes to the exploration of positive emotional states, and thus good welfare, in rats as it identified the first facial indicators of positive emotions following a positive heterospecific play treatment. Furthermore, it provides improvements to the photography technique and image analysis for the detection of fine differences in facial expression, and also adds to the refinement of the tickling procedure.

Highlights

There is growing evidence that animals are capable of experiencing and expressing different emotions or affective states, including those of positive valence, such as happiness or pleasure [1,2]
Rats emitted a higher number of positive ultrasonic vocalisations (USVs) in the Positive Treatment (M = 115.87, Standard Error of the Mean (SEM) = 10.29, range: 44–188) compared to the Contrast Treatment (M = 1.07, SEM = 0.63, range: 0–8; t(14) = - 10.96, p < 0.001; Fig 6)
We found that Ear Colour was significantly pinker in the Positive compared to the Contrast Treatment, while the Rat Grimace Scale measures and the Nictitating Membrane visibility were not affected by treatments

Summary

Introduction

There is growing evidence that animals are capable of experiencing and expressing different emotions or affective states, including those of positive valence, such as happiness or pleasure [1,2]. Emotional states may be inferred from behaviour [3] including. Different emotional states can be expressed through changes in facial expression, as previously shown in human and non-human primate studies [10,11,12]. Facial expressions have been recognized in a wide variety of animal species with dissimilar facial musculature, including dogs [13], horses [14,15], rabbits [16], mice [17] and rats [18,19]

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