To look at this, the researchers housed pigs in 16 groups of six. They then took two of the pigs from each of these groups and either trained them to anticipate that something good would happen, or that something bad would happen. They did this by playing the pigs some music and then either giving them a good experience (food) or a stressful experience (social isolation and handling). This meant that the pigs either learned that the music predicted food or stress.
The researches then took two of the pig’s penmates (‘naïve’ pigs) and put them with the pig that had either been trained to one of these two things. All the pigs were then played the music that held meaning to the trained pigs (which, incidentally, was Bach or a military march). The pigs trained that the music meant food showed ‘happy’ behaviours (play behaviour, wagging their tail and barking) while the pigs trained that the music predicted something stressful behaved stressed (standing ‘alert’, put their ears back, urinated and defecated).
Now, the researchers wanted to see whether the naïve pigs that had never experienced either the food or the stressful situations themselves would react to the behaviour of the other pig when the music was played. This ‘emotional contagion’ (sharing the emotional response someone else is having) is one key aspect of the ability to empathize. They found that the pigs did indeed react to the behaviour of the other pig: when a naïve pig was near a trained pig that was acting stressed, the naïve pig also became more alert and also put their ears back.