Some animals apparently do recognize certain concretes as members of a particular group with particular characteristics; for instance, some monkeys have particular calls to warn their troops of a hawk, and other calls to warn of a snake. This, of course, requires that they—in some way—treat the concretes they encounter as units, “as members of a group of two or more similar members.”

I think Rand or Peikoff might answer that animals do this solely or primarily on the basis of instinct.

I’m not sure you can prove this, though, and I doubt that animals lack any form of unit perspective. For lots of concretes seeming to indicate that some animals do have some lesser form of unit perspective, see Are We Smart Enough to Know How Smart Animals Are? by Frans de Waal, which I reviewed here: https://theobjectivestandard.com/2018/09/are-we-smart-enough-to-know-how-smart-animals-are-by-frans-de-waal/

Personally, I think what makes man distinctive is that he can leverage the unit perspective to reason about things in their absence, bring things not directly open to his perception down to a level he can directly perceive, and abstract from abstractions.