In my case, it wasn’t until last year that the theory that a pig has the IQ of a three-year-old child suddenly hit home. My younger self, who saw a breakfast not featuring a bacon sandwich as an opportunity lost, had found the idea pretty easy to ignore. Only three? Then crisp me up some more, streaky if you have it.

When you have a three-year-old human actually living in the same house as you, the idea carries a little extra weight. The kid could tell meandering anecdotes aged two, knows more about the construction industry than some junior site managers, and can carry off a passable Italian accent whenever his pop-up plywood pizzeria is open for business.

What, then, was going on inside a pig’s mind as it shuffled around its patch of mud or, more likely, industrial grating? I read, I researched. At one sitting, I even went as deep as the second page of search results.

Though, for the record, their lack of opposable thumbs makes them terrible at FIFA

It didn’t take long to find out that the domestic pig — or swine to give them their other, equally unflattering name — might even be smarter than your average preschooler. Up there, in fact, with dolphins and chimpanzees at the very front row of the animal classroom. They have distinct personalities, excellent memories, form complex social networks and have a sense of the future; not the greatest attribute to have when that future almost certainly involves an ineffective stun gun and the inside of a cured meat selection.

They also love creative play and demonstrate an inner life or sense of “I”. One research project put them to work playing video games in which they outperformed dogs and even chimps. Though, for the record, their lack of opposable thumbs makes them terrible at FIFA.

Even their filthy reputation is in dispute. In a Swiss study, a group of wild boars were given apple slices coated in sand. Instead of eating them straight away, they first took them to a stream to rinse them off. Perhaps later stopping for a shaded picnic. Frequently, people who’ve had pigs as pets, report them as more emotionally intelligent than dogs; the same dogs we like to dress in miniature gilets and buy chewy treat stockings for at Christmas.

From childhood, we see them as greedy and ridiculous, but is this just part of a long-term PR job to make it much easier to overlook their mass slaughter?

True, pigs are much less comfortable to have on your lap. Especially those sows who are kept almost permanently pregnant in gestation crates. And let’s be honest, their looks haven’t helped them much. From childhood, we see them as greedy and ridiculous, but is this just part of a long-term PR job to make it much easier to overlook their mass slaughter?

What isn’t in doubt is that humans have won. And as the argument goes, this is just the natural order of things. Except, here is the rub, preferably paprika-based. When I first bought into the old “way of the jungle” argument three decades ago, I’m pretty sure I’d been sold on the principle that they were dumb.

In the years since, plenty of other compelling arguments for giving up meat have piled up —habitat loss, climate change, waste pollution, personal health and, of course, cruelty. Writer and historian Yuval Noah Harari suggests “the treatment of domesticated animals in industrial farms is perhaps the worst crime in history”. A pretty big statement in itself; somehow made bigger when you also know a pig can use a joystick.

I went to see a pig in the flesh at a city farm. And it was the flesh that was most troubling. The soft nose and ears, overriding pinkness, darting eyes and sporadically hairy skin; skin that burnt from the sun and flesh that humans notoriously found the hardest meat to digest. There were no obvious signs of genius on show, but it wasn’t so hard to see this pig as a distant human relative who took a turning off the evolutionary crossroads in the murky, soupy past. A catastrophically bad turning as it turned out. More like going the wrong way down a motorway slip road.

Back in search mode, I found one evolutionary biologist with a convoluted theory that homo sapiens are actually descendants of an ancient cross between pig and chimpanzee. Perhaps this explained why Fijian cannibals would use the phrase “puaka balava” meaning “long pig” to describe human meat, so distinguishing it from “puaka dina”, or “real pig”. Was it a coincidence that modern medicine found pig organs the most successful for human transplants?

For me at least, there was a lot on the line. Could I get through Christmas without a ham?

My initially idle research had somewhat backfired. The celebratory hog roast in our local market on full display of passing children had taken on the feel of an arthouse horror film. I could have joined the new wave of “conscious” omnivores who like to declare that they now only eat pork as a novelty and only from farms that offered space, fresh air and ideally stories before lights out. Infinitely preferable, but now I knew too much. I made a pact with the pigs.

For me at least, there was a lot on the line. Could I get through Christmas without a ham? Would I ever be allowed back to Spain? If they say the smell of bacon can still tempt vegetarians, I knew I was in the clear when the aromas from a traditional London “caff” had precisely the opposite effect. Initially, it made me feel noble and interesting to declare my new dietary requirements, hinting at a cultural or spiritual heritage more exotic than third generation East Surrey.

“Why pork?” people would occasionally ask. And I would explain; the IQ, the sense of self, the human trapped in a pig’s body thing. If they were still listening, or at least within earshot, I might even end with a line like: “Perhaps our greatest arrogance is to associate intelligence with speech.”

“So, presumably you don’t eat octopus, either?” asked one friend. Octopus? Who didn’t love those salads with the potatoes? Preferably on a terrace in the South of France. My grasp of their cognitive capabilities was sketchy, however, so I delved a little deeper, coming across Peter Godfrey Smith’s work on the subject. By the end of it I questioned why there wasn’t a cephalopod heat on University Challenge.

These animals can “solve complex puzzles requiring pushing or pulling actions,” reads their Wikipedia entry, “and can also unscrew the lids of containers and open the latches on acrylic boxes.” On second thoughts, maybe The Crystal Maze is more appropriate, but these guys are undeniably impressive.

Anyway, abstaining from octopus was hardly going to send shockwaves through my weekly meal plan. I added it to the contraband list. Except it did raise the question of where this moral experiment was taking me. It started to feel as if I needed to determine whether my dinner could take on an entry-level Sudoku before ordering.

It started to feel as if I needed to determine whether my dinner could take on an entry-level Sudoku before ordering

I looked to the philosophers. Aristotle said man’s superiority to beasts gave him the right to use them as he saw fit, but he was born in the fourth century BC and hadn’t seen the emerging research on juvenile bird intelligence from the University of Konstanz.

Every day was triggering a new trail of academic research papers. Chicken can add and subtract, show empathy and self-awareness and are workable problem solvers. Sheep have shown they could navigate out of a complex maze, and can recognise 50 different faces for up to two years. Just last week I’d failed to recognise the guy from the local dry cleaners.

When I searched for “cow intelligence” I could hardly look. For all these years, their apparently blank faces belied the fact they were emotional, affectionate, could be pessimists or optimists and even had occasional “Eureka” moments.

At least there were still fish. Pending the latest reports on overfishing that is, but at least I still had fish. And a pescatarian future wasn’t so bad at all. Yet only the briefest scan revealed that with seafood — or marine life as it’s also known — we are perhaps most guilty of the idea that anything remotely different from us was floating protein.

Surely I had mussels? Couldn’t my future at least include the occasional moules-frites?

Astonishingly, fish appear to show primate levels of intelligence, can recognise themselves in mirrors, are capable of planning ahead, frequently display emotion, have long memories and show better self awareness than humans. Prawns, although “no creative geniuses”, show innovation. Even tiny crayfish display anxiety that can be relieved by Valium.

Surely I had mussels? Couldn’t my future at least include the occasional moules-frites? I soon realised I was in the bargaining stage of grief. Sure enough, I came across a team of prominent neuroscientists who in 2012 signed The Cambridge Declaration on Consciousness stating “that the physical processes associated with consciousness in humans could be found in many other creatures, including insects and molluscs”.

Fucking molluscs. In just a few months, I had very gradually and reluctantly pulled my fingers from my ears and look where it had got me. What was the stage after bargaining? Depression. It was looking like tomato soup for dinner.