Let me introduce you to ‘pig 3’. Around the turn of the Millennium, she performed a mental feat that, if she were an ape, would probably have been accepted as evidence for a 'theory of mind' – the ability to attribute knowledge, beliefs or desires to others. The fact that few people have heard of this Einstein among swine neatly illustrates a bias that has blighted studies of animal minds.

In experiments run at the University of Bristol, pig 3 was one of ten juveniles tested in a maze in which a researcher had placed food down one of four corridors, out of her sight. In each trial, another pig had a similarly blocked view, but a third could see what was going on. When the animals were released, pig 3 alone consistently followed the knowledgeable companion to the food.

It was an astonishing result, given that previous attempts to get chimpanzees to understand human experimenters’ knowledge about hidden food had required extensive training. At about the same time, the penny finally dropped among scientists studying chimps. A team led by Brian Hare, now at Duke University in Durham, North Carolina, showed that the apes pass the test if you replace the human experimenters with knowledgeable chimps.

It was basically the same finding as for pig 3, but the work was written up rather differently. While Hare and his colleagues wrote about what chimpanzees “know”, the paper describing pig 3’s performance stressed more mundane possibilities – in particular that she had earlier learned to follow animals that are in a position to see where food was placed, without any deeper understanding of why that’s a good strategy.

“We wrote it up in a very modest way,” says Richard Byrne of the University of St Andrews, a member of the team – in part, he admits, because of concerns about reactions from the meat industry. Byrne told me that he thinks it more plausible that pigs are actually capable of understanding what their companions know.

Suzanne Held of the University of Bristol – who actually ran the experiments – thinks the cautious approach was correct, but she argues that researchers working with apes should exercise similar restraint. “Neither of us could rule out that the animals had made some association beforehand,” she says.

Let’s be clear: I’m not suggesting that pigs are as bright as chimps, for which there is other evidence for something approaching a theory of mind. My point is simply that our double standards can make the differences between species seem greater than they really are.

For pig 3, the gulf between human attitudes towards apes and swine couldn’t have been more significant. When Byrne learned of the results, he asked Held about running further studies with this remarkable animal. It was too late. All of the pigs that participated in the experiment had been tested in a short interlude before their final journey to the slaughterhouse: pig 3 was already bacon.