Somehow September has passed by without a single post. During that time, the Mule has travelled to the other side of the world and back (primarily for a one day workshop in Switzerland). Also, James Glover (regular contributor to the blog) and I have been exploring the statistical significance of global temperatures. That will, eventually, crystallise into a future post but in the meantime James has been driven to reflect on cats rather than climate.

There are, apparently, two kinds of people. Those who like cats and those who don’t have personalities. I am of the former and am onto my 5th and 6th cats (a mother/daughter pair of rescue cats). I’ve been reading (another) book on cat behaviour which traces the domestication of the cat from solitary hunters to domestic pets (John Bradshaw’s Cat Sense: The Feline Enigma Revealed). Most domesticated animals are herd beasts whose natural behaviours lend them to domestication. A really great read on this is Jared Diamond’s Guns, Germs and Steel. Cats, however, are naturally solitary creatures whose real benefit to humans became obvious when agrarian societies stored grains which attracted rodents, the cat’s natural food source. It’s hard to imagine now, when we get our daily bread from Woolies, but think back to the day when farmers were (literally) plagued by mice and rats, and cats served to control them.

As a kid growing up in suburban Townsville we had an un-neutered tom cat called Whiskey. We weren’t allowed to play with Whiskey, and I have vague memories of him bringing home litters which lived briefly under the house and my mother throwing him the occasional piece of liver on the back steps. He wasn’t what you would call a friendly cat. As an 8 year old we moved and I recall driving with my father to take Whiskey to a “cat home”. I still have an image of dozens of cats climbing up the side of a large wire cage. I am guessing Whiskey didn’t last there for long, and, of course was happily re-homed with another loving family. Yes, that’s what happened.

Almost every website on cats says not to feed them cow’s milk because adult mammals don’t produce lactase, the enzyme required to break down lactose in milk, into sugar. Mammals stop producing lactase once they are weaned because their mothers no longer provide them with milk and they instead produce enzymes which turn proteins, in animal and vegetable matter, into sugars. Producing lactase would be pointless and require resources better devoted to other enzymes and hence has been adapted against. The idea is that if cats can’t digest lactose, it stays in their gut and bacteria feeding on it leads to an upset stomach and diarrhoea. But I see several problems with this view.

1. Humans can produce lactase as adults*, due to a variety of different genetic mutations which stop the shutdown of lactase production in adults. So the genetic mutation doesn’t have to suddenly find a way to produce lactase, just a way to stop stopping it. Basically this is because of the nutritional benefits of cow’s milk to dairy farmers which started about 10,000 years ago. Comparisons of 10,000 year old human DNA to modern descendants of dairy farmers show this is a widespread adaptation due to its obvious nutritional benefits. Indigenous Australians and Inuit don’t have this mutation because they have no dairy farmer ancestors. This is still an open question however as curdled milk and cheese doesn’t have much lactose so do not require lactase to digest them. Personally I suspect that hunters which killed a lactating cow were able to drink the milk immediately and benefited. Other theories say cow’s milk, as an alternative to water, may have saved them from diseases. Not all humans can do this. My own father, for example, can’t drink milk.
2. Cats are quickly put off foods that make them feel sick and my cats love milk. It’s possible there is something in milk which they love (like cat nip) even if it makes them sick, but they are quick learners and I doubt it.
3. There is a lack of eye witness evidence from vets and catteries back in the day when cats were fed milk that they suffered diarrhoea when they drink milk. But none of the evidence against cats drinking cow’s milk seems to be based on this. I’ve not found a single account of someone whose cats were fed cow’s milk and suffered.
4. Cats have adapted to human living rapidly in the last 2-3 thousand years. This is equivalent to 4-5 times the length of time for humans due to their shorter lifespans, about the same time humans have adapted to drinking milk as adults.
5. It makes sense that cats which were given milk by humans, and could process it, would have a better chance of reproducing. It would have a nutritional advantage over cats which couldn’t, the same evolutionary pressure on humans should operate on cats and they should (most of them anyway) have adapted to being able to drink milk as adults.
6. I can’t find a single study which shows cats can’t produce lactase as adults, it just seems to be assumed because they are non-human mammals.

My guess is that cats descended from European cats can (most of them anyway) drink cow’s milk safely. If they drink it and come back for more it probably doesn’t upset them. My own cats, when they drink milk, run around like kids on sugary drinks, displaying very kittenish behaviour. That makes me think they are turning lactose into sugar, which means they are still producing lactase as adults.

I still find it quite amazing how memes like “cats shouldn’t drink milk” propagate across the internet without any back up evidence–like an actual study which shows it. Like climate skeptics, cat people latch onto “evidence” which supports their point of view. In any event if anyone has firm evidence that adult cats don’t produce lactase I would be happy to hear about it.

Two cats both called Minoo because cats don’t actually know their names

* Editor’s note: a recent episode of Science Friday touched on this and other evolutionary changes in the human diet. The theme of the podcast is that humans are still evolving, faster than ever. So, perhaps cats are too, as James suggests.

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