How come loads of people say they hate cats and hardly anyone says they hate dogs? Well, Dr Jonica Newby has a theory about that, and it's all to do with dogs' eyebrows.

Dr Jonica Newby

I have always loved cats. So how come so many people say they hate cats? Whereas hardly anyone in our culture says they hate dogs? Well, I have a theory about that. And it's all to do with eyebrows.

NARRATION

All animals have brow ridges because they protect the eyes. But there's something peculiar about what we call the eyebrow. It seethes with a life of its own, and very few animals can do this. Dogs have proper eyebrows. Cats don't. Why? I asked around, but you'll be shocked to discover there's hardly any research on this. But a few top veterinary anatomists got on the case, and this is what we've all come up with.

Dr Jonica Newby

The key seems to be this little muscle here. The levator anguli oculi medialis. And according to this bible of dog anatomy, it's a small, strong muscle that passes through the portions of the upper lid that bear the hairs designated as the eyebrow. Now it's unusual to call a little muscle like this 'strong', so that immediately catches the attention ...hello sweetie ... and its job is to lift the upper lid and erect the hairs of the eyebrow.

NARRATION

Now if you look at the equivalent muscle in the cat, it's not strong, and attaches all the way across the eye ridge. Which means the cat can't do much interesting with that muscle. Whereas the dog's levator anguli oculi medialis is perfectly placed to raise just one edge of its eyebrow. But the deeper why question is, why do dogs have this special eyebrow muscle, and most animals don't? Well the best theory concerns the evolution of social living. In general, the most social animals have the most expressive faces.

Dr Jonica Newby

Ooh!

NARRATION

One study showed foxes who hunt alone had about half the facial expressions of wolves who work in packs. In fact, in wolves and dingoes, the eyebrows are often even a different colour, exaggerating the movement.

Dr Jonica Newby

And the most intelligent social species, with no fewer than two muscles dedicated to eyebrow movement is us. And we like eyebrows. So here's my theory.

NARRATION

Because dogs have such expressive faces, and especially eyebrows, we humans feel intuitively we can read them. Whereas cats, who evolved to hunt alone, can barely even move their face above the eyes. We have to learn to read them.

Dr Jonica Newby

But people, it's not their fault. Cats are loving and lovable - they just didn't evolve proper eyebrows. Did you, sweetie? Mm. I love cat fishing.