A stallion may not be able to breed naturally, or old age may decrease his performance.

Mr. Harju said he had hoped the ferrets would breed naturally this year.

Some species do not breed naturally with others, and medical intervention is needed to produce a child.

Our kind will never breed naturally on Luna.

If they can find a way to let us breed naturally, we'll have a new species: Homo astrum, who can live between the planets.

They live and breed naturally there now.

I mean that horses and asses naturally breed from one another.

So I see a slow decline in men's fertility until, eventually, men can no longer breed naturally.

There's also a huge bird population, and the area is one of the few places in India where flamingos are known to breed naturally.

The only level within the strata allowed to breed naturally was command.