Interbreed

They can interbreed.

As a human cannot really howl like a wolf or chirp like a bird, if there were aliens, it would be unlikely that a human could speak the alien language, as the organ for speech or communication would be different, as a bird's beak is to a human mouth. There are science fiction shows that depict different alien humanoid races from different worlds, and they could interbreed, because maybe these shows assume that there was a common ancient humanoid colonization that resulted in different evolutions on different worlds. Possibly then, in such a scenario, different humanoids could speak one another's language.

The rypes and our red grouse are identically the same bird, though disease has never been known in the former, and even interbreeding with the red grouse, to the limited extent they are ever likely to do, should do more good than harm.

The city's two-inch Environmental Review Technical Manual instructs regulators to consider adverse environmental impacts on "any subspecies" or "any distinct population segment . . . which interbreeds when mature."

The rock dove has so interbred with the feral pigeons from doocots and racing pigeon lofts that, as a pure species, it may possibly now be extinct in Britain.

In the wild, deer mice and oldfield mice never interbreed.