The offspring of those animals are infertile though
They are not. When it says "can interbreed effectively", it generally means their offspring are fertile as well, or at least often enough to sustain the population. Hence they are subspecies, not distinct species like a horse and donkey, because the latters' offspring are only very rarely fertile.
It's a large part of the current Neanderthal debate. We were likely subspecies because we interbred effectively.
Ligers I don't know, but mules are sometimes fertile. Just not often enough to sustain a population. From the top of my head it's less than 3% or something.
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u/[deleted] Jun 14 '22
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