Why do dogs wag when they're happy?

Same tail: dogs wag nonstop, wolves rarely do

When you come home, your dog's tail just won't stop (how happy it really is in that moment is another episode).

Its cousin the wolf is far less of a wagger. Scientists who compared them found that, of all the dog family, dogs wag the most often and in the most situations.

Same tail. So how did it become
"so glad you're here"
on a dog?

A wolf's tail says "you're the boss"

Wolves do wag, but it means something else. When a low-ranking wolf meets a higher-ranking one, it drops its tail and wags it fast: you're the boss, don't hurt me.

A hungry wolf pup licks an adult's mouth, wagging as it goes. The adult then brings up a bit of the meat it ate to feed it.

For a wolf, a wag says:
I mean no harm,
I want to get along with you.

Puppies and wolf pups, raised the same way

Scientists in Hungary had people hand-raise both newborn puppies and wolf pups.

At four or five weeks old, the puppies wagged as soon as they saw their carer. The wolf pups almost never did.

Same upbringing, and still that big a difference. So it isn't all learned: part of it is already built in.

The fearless foxes of Siberia

Why did dogs become such waggers? Scientists have two ideas.

Idea one: it came along for free. Starting in 1959, Soviet scientists in Siberia bred foxes. In every generation, only the least fearful foxes got to have pups.

They never once picked foxes for wagging. Yet dozens of generations later, the foxes wagged at people, and their tails had started to curl, just like a dog's.

Pick for calm, friendly animals,
and the wagging tags along.

People just love a steady rhythm

Idea two: people chose it. Human brains love rhythm. Anything that ticks along evenly just feels good to watch, the way a drumbeat makes you want to nod along.

Dogs that wagged more won people over, so they were more likely to be kept, fed and to have puppies. Generation after generation, dogs wagged more and more.

Neither idea
has been settled yet.
It may be a bit of both.

Leaning right, or leaning left

Scientists in Italy filmed thirty dogs and measured the angle of every swing to find this lean.

Another study even found that more aggressive dogs wagged their tails more.

So a wag says:
I'm worked up,
and I want to connect with you.

Happy is just the most common way of being worked up. To tell if a dog is really happy, look at the whole dog: ears, mouth, and whether its body is loose and wiggly.

A cat says hello with its tail up

Other animals have their own way of saying it. A cat coming over to say hello and rub against your legs doesn't wag. It holds its tail straight up.

Same body part, different language. Don't read a cat's tail the way you'd read a dog's.