Pigs can feel optimistic and pessimistic according to how they are being treated, scientists revealed today.
Experts from Newcastle University's School of Agriculture, Food and Rural Development found they were just as likely as humans to feel the glass was half empty or half full, depending on their living conditions, as hogs kept in piggy luxury were more likely to respond positively to a new experience than those in less stimulating pens.
The scientists hoped the research, which shows pigs are capable of feeling complex emotions, will have an impact on animal welfare.
Led by Dr Catherine Douglas, the team employed a technique to "ask" pigs if they are feeling optimistic or pessimistic about life as a result of the way in which they live.
In an experiment reminiscent of Pavlov's dogs, pigs were taught to associate a note on a glockenspiel with a treat - an apple - and a dog training "clicker" with something mildly unpleasant - in this case rustling a plastic bag.
Then they placed half the pigs in an enriched environment - with more space, freedom to roam in straw and play with toys - while the other half were placed in a smaller, boring environment with no straw and only one non-interactive toy.
The team then played an ambiguous noise - a squeak - and studied how the pigs responded.
Dr Douglas said the results were compelling.
"We found that almost without exception, the pigs in the enriched environment were optimistic about what this new noise could mean and approached expecting to get the treat," she said.
"In contrast, the pigs in the boring environment were pessimistic about this new strange noise and, fearing it might be the mildly unpleasant plastic bag, did not approach for a treat.
"It's a response we see all the time in humans where how we are feeling affects our judgment of ambiguous events.
"For example, if you're having a bad day - feeling stressed and low - and you're presented with an ambiguous cue such as your boss calling you into their office, the first thing that goes through your head is what have I done wrong?
"We call this a negative cognitive bias. But on a good day you greet the same ambiguous event far more positively, you might strut in expecting a slap on the back and a pay rise.
"This 'glass-half-empty versus glass-half-full' interpretation of life reflects our complex emotional states, and our study shows that we can get the same information from pigs.
"We can use this technique to finally answer important questions about animal welfare in relation to a range of farm environments, for pigs and potentially other farm animals."
Dr Douglas said that in their experiments they deliberately used only environments which could be easily mimicked on commercial farms.
"UK farmers have some very difficult decisions to make in order to compete in the global market and to suggest that we make all pig farms free range is simply not practical, possible or necessarily desirable," she explained.
"Unfortunately, consumers buying cheap imports have the biggest impact on animal welfare and this puts pressure on UK farmers who, as a rule, do their utmost to ensure good welfare of the UK pig herd.
"In our experiments the enriched environment was also a widespread commercial housing system, so a 'happy-pigs' system already exists on many farms across the UK."
Last year Dr Douglas published research that showed cows which were given a name and treated as an individual produced more milk than unnamed cattle.