Table 2 Description of the behaviours observed in the video analysis of the encounters by two independent observers

Walking (duration in s)

Moving its legs in order to advance in any direction of the pen

Immobile and/or ground sniffing (duration in s)

Pig is upright on all four legs, neither moving forwards nor backwards [63], either exploring with the snout and mouth or not

Reciprocal look (duration in s)

Both pigs look at each other at the same moment. The head and the eyes were directed to the other pig. May be a sort of synchronised behaviour, which may indicate a positive welfare state [64, 65]

Agonistic behaviour (binary)

Actively displacing another pig, pushing it, biting it, or actively pursuing it [5, 6]

Agonistic signal–threat (frequency)

Movement of the head or body oriented to the receiver, without contact [66]

Fighting (binary)

Attacks consisting of head-to-head blows, head-to-body blows, shoving, biting and/or physical displacements [8]

Fighting starter (binary)

Pig that starts the fighting by attacking the other pig (head-to-head blows, head-to-body blows, shoving, biting and/or physical displacements) [8]

Encounter duration (in s)

Total duration of the encounter

Need to stop the encounter (binary)

The encounter was stopped when pigs started fighting or when one pig performed three bites on the other. If none of this occurs, the encounter ended at 5 min

Total social nosing (binary)

The pig touches the nose, head or other part of the body of another pig [22]

Positive social nosing (binary)

Social nosing that does not prime aggression after five seconds

Negative social nosing (binary)

Social nosing that primes aggression after five seconds

Proximity (duration in s)

Spatial proximity, being close to each other during specific activities or resting periods. Depending on the context, it could be considered a type of affiliative behaviour [18]. Nine squares divided the test area (3 × 3): when most of the body of both pigs (or all of the body) were in two consecutive squares, thus not separated by another square, we considered that they were in proximity

Tail movement (duration in s)

Lateral tail wagging, mostly from side to side, linked to positive emotions [67]. Each time that the movement stops for at least two seconds, we consider another movement [67]

Latency of first contact without aggression (in s)

Seconds that the pig lasts to start the first direct contact with the other pig, which did not follow aggression until five seconds later

Latency of first contact with aggression (in s)

Seconds that the pig lasts to start the first direct contact with the other pig, which followed aggression until five seconds later