Another issue is that evaluating memory disorders like AD is really hard if your subjects don't have language and can't discuss their subjective experience. A mouse that can still find its way around a maze that it learned is hard to compare to a person that can still find their way to their office but can't remember colleagues' names or whether they are married. Apes wouldn't help with the problem that AD symptoms are tied up with relationships and experiences unique to the only species that has language. (Side note: as someone approaching retirement from a family with a strong history of AD, I'm not entirely sure that the unique faculty of language is worth its associated cost: that we're the only species whose members know they will senesce and die,) Edit: shit, that's maudlin. Need a drink. Ignore me.