Ancient cat-sized horse

Global warming made an ancient type of horse shrink to the size of a cat, scientists have said.

Sifrhippus is the earliest known horse. It lived in the forests of North America more than 50 million years ago. It was small to start off with, about the size of a small dog.

Over the next 175,000 years the Earth's temperature rose by about 12C. During this time, Sifrhippus became even smaller until it was just the size of a cat!

Scientists from the USA looked at fossils of Sifrhippus's teeth and found that their change in size exactly matched the change in temperatures. They concluded that the animal had evolved to cope with the warmer atmosphere.

Fossilised teeth helped scientists to understand why ancient horses were smaller

Larger animals conserve more body heat than small ones. So it is better to be smaller in a warmer climate.

This explains why in two thirds of mammals, ones that live in hot countries are smaller than their cousins who live in colder places.

What will happen to animals in the future?

Now scientists are asking: how will animals and plants react to climate change in the future?

Experts have already started to notice that the size of some birds may be decreasing.

The ancient horse had 175,000 years to evolve. But global temperatures are predicted to rise by as much as 4C over the next 100 years. Will our creatures be able to adapt that quickly?