NARRATOR：

Listen to part of a lecture in a biology class.

FEMALE PROFESSOR：

OK, so we've been talking about what most animals do when they need food-

they simply go out and look for it.

But some animals do something entirely different when they need food.

Surprising as it may sound,

some animals actually spend a great deal of time taking care of their own food source....

they cultivate it-

sort of the way human farmers would-

so it will keep growing until it is ready for them to use.

Let's start with an animal that cultivates plants.

There's a certain fish-uh, it's called a damselfish-

that likes to eat a special kind of seaweed.

So wherever a patch of this seaweed grows,

there will usually be damselfish swimming above and around it.

Now the fish are there to provide protection from other plants so the seaweed can grow and then regrow as the damselfish eat it.

For example, if other plants start growing over the seaweed, blocking sunlight,

the damselfish remove those plants by biting off the parts that are getting too tall.

So, by protecting the seaweed from being over run and damaged by other plants,

the damselfish always have a supply of food ready to use.

Now some animals don't cultivate plants-

they take care of other animals as a source of food.

Take ants, for instance.

There are some species of black ants that care for tiny insects called aphids.

These aphids produce a sweet liquid that the ants like to eat,

so ants guard the aphids from being eaten by other animals

and help feed the aphids.

In fact, sometimes the ants even carry aphid eggs back to their own nests and raise the young aphids there.

Then the aphids grow and produce this sweet liquid that the ants eat.

So the ants make use of the aphids as a reliable source of food.