Hermes, my understanding of the literature is that it isn’t at all clear which species suffer pain and which ones don’t. And that estimate gets constantly revised in a more inclusive fashion. Ultimately, of course, it’s an empirical question.

But it doesn’t affect Bentham’s argument at all (and, by the way, the argument is not one from ignorance, which has a different structure). All he is saying is that whenever we discover that a species is capable of suffering pain we should elevate our ethical considerations about that species. Seems pretty good to me.