Why do some leaves taste bitter to animals?

Maybe you have crunched a green salad leaf and tasted something bitter on your tongue. Your nose scrunched up, and your mouth wanted to say yuck.

Out in the fields and woods, rabbits, deer and cows munch leaves all day long. Some of those leaves taste bitter to them too. So why do some leaves taste bitter to animals?

A plant stays in one spot its whole life. When a hungry deer comes along, the plant cannot run away or hide. Yet its leaves matter a lot, because the leaves catch sunlight to make food for the whole plant.

So the plant fills its leaves with a bitter juice. This juice is the plant's defense, which means its way of keeping itself safe. A plant cannot move, but its defense waits inside every leaf.

Now a rabbit hops up and nibbles one of those leaves. The bitter juice spreads over its tongue. A rabbit's tongue, just like yours, is covered in taste buds, which are tiny bumps that tell what a food tastes like.

The rabbit's taste buds notice the bitter juice right away. For an animal, a bitter taste is a warning that this food could give it a sore tummy. What do you think the rabbit does next?

The rabbit stops chewing, drops the leaf and hops off to munch some soft clover instead. The bitter defense has done its job. The plant keeps the rest of its leaves, still green and still catching sunlight.

Defense, taste buds. A plant cannot run away, so it fills its leaves with a bitter defense, and when an animal bites, its taste buds feel the bitter warning and it leaves the plant alone.

Sleep gently, and the next time you taste something bitter, you can smile and remember the little leaf keeping itself safe.