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We have investigated the vocalisation structure (frequency, duration and energy repartition) and its relationship with emotional states in pigs, especially through the human-animal relationship. These studies were part of a European Eranet project called Soundwel and have given encouraging results, as they may allow the development of automated software to monitor animal welfare using real time recording of vocalisations in farms.

Pigs are social animals and communicate using a large variety of call types (Tallet et al, 2020). Since 2016, Peagase UMR have coordinated the European project called Soundwel, aiming at better understanding of the links between vocal expression and emotional states. The acoustic structure of vocalisations depends on the perception of the environment by the animal, and especially their perception of humans. One part of the project was to use vocalisation structure to better understand the perception that piglets had of a human.

Vocal expression and emotional state

In the first study, we compared vocal responses to conspecifics (pen mates) or humans (one familiar human, providing additional positive contacts to the piglets), after piglets were trained to anticipate both types of partner. Piglets produced grunts at a higher rate when anticipating the arrival of their pen mates than when anticipating ‘their human’. Grunts were noisier (spectral noise) when piglets expected to meet with their conspecifics, whereas they were longer and higher pitched when they expected to meet with the human. These characteristics indicate a positive perception of the anticipation of conspecifics and a state close to frustration before meeting with the human. This study demonstrates for the first time that vocal expression of anticipation in piglets is specific to the type of expected event. In addition, tamed piglets have a different vocal expression from non-tamed piglets, suggesting a long term effect of the human-animal relationship on vocal communication.

Figure 1 : Vocal expression of anticipation of conspecifics or a familiar human. A: Experimental set up and room. B: Procedure of the conditioning. Piglets learnt to anticipate the arrival of two conspecifics or a familiar human, during twelve repeats per (pseudo)social partner. An audio visual stimulus announced the entrance of the partner. The duration of the visual stimulus was progressively increased to generate an anticipation phase. For the last trial, the entrance of the partner was delayed to trigger frustration. After acoustic analysis of grunts, we can see the effect of the anticipation and frustration on (C) the duration of grunts and (D) their spectral composition.

Vocalisation structure and perception of enrichments and humans

In a second study, after a two week familiarization of piglets with both a human, providing additional positive contacts, and an object, known as enrichment, we tested the impact of a reunion with each of the stimuli after a social isolation (known to be stressful).
Vocal expression showed that the human triggered a positive state (shorter grunts than during isolation), indicating reassurance for piglets, whereas the object did not.
Piglets’ emotional state and their proximity to conspecifics and humans is reflected in the structure of their vocalisations, especially their grunts (duration, spectral noise and frequency band), revealing the mental representation that piglets build toward humans.

Figure 2 : Effect of a post isolation reunion with an object, a familiar human or nothing on (A) behaviour of piglets, (C) duration of grunts produced during the test and (B) their spectral composition.
These results provide a useful acoustic basis for the development of automated software to classify and detect emotions in pigs’ vocalisations.

Further reading

• Céline Tallet, Lisette M C Leliveld, E.F. Briefer. Vocalizations. Pigs welfare in practice, 5M Publishing, pp.56, 2020, Animal Welfare in Practice, 978-1-7891-8-1050. ⟨https://hal.inrae.fr/hal-02503320⟩
• Avelyne Villain, Azélie Hazard, Margot Danglot, Carole Guérin, Alain Boissy, et al.. Piglets vocally express the anticipation of pseudo-social contexts in their grunts. Scientific Reports, Nature Publishing Group, 2020, 10 (1), pp.18496. ⟨10.1038/s41598-020-75378-x⟩. ⟨https://hal.inrae.fr/hal-03025185⟩
• Avelyne Villain, Mathilde Lanthony, Carole Guérin, Céline Tallet. Manipulable object and human contact: preference and modulation of emotional states in weaned pigs. Frontiers in Veterinary Science, Frontiers Media, 2020, 7, ⟨10.3389/fvets.2020.577433⟩. ⟨https://hal.inrae.fr/hal-03030949⟩
• Data available on Data Inrae : https://data.inrae.fr/dataverse/SOUNDWEL