One of the defining features of mammals is that they have belly buttons. And except for platypuses that lay eggs and marsupials whose umbilical cords fall off in their mother’s pouch so a scar never forms it’s true that the majority of mammals have belly buttons.

But what about other members of the animal kingdom? Do they also have belly buttons?

Well, it may surprise you to learn that birds also have navels – that’s the proper name for a belly button. And here’s why. Inside a bird’s egg is a tiny cord that connects the embryo to the yolk sac. When a chick hatches it does have a scar where the cord was attached but it is so small that it is hard to see except in newborn chicks.

The scar quickly fades and by the time the chick fledges it has all but disappeared. So although birds do have belly buttons they disappear so quickly to all intents and purposes they don’t in the way we would usually define a belly button.