The term "mental retardation" may not be the most appropriate, but I know no other way to put it; for now.
I am thinking of the kind of mental retardation that can be observed in the behaviour of an animal -- in comparison to other individuals of his kind and age.
For example, I have seen quite a number of cats, and a few stand out. (I know, this is just a speculation based on street observation -- but I hope someone can tell me more.)
One of the cats is an orphan and has been abandoned by his owners at the age of about 5 months; he has been adopted by a family in my street, and is now about 2 years old. One immediately notices that the cat is somehow different -- the look in his eyes is stupid, yes, I have no other way to say it. Normally, cats look into one's eyes, but he doesn't. He is uncharacteristically shy (there's plenty of undomesticated cats here which you cannot catch, but they aren't nearly as shy as he). When he came into the neighbourhood, we had him for a while, and he wasn't one bit playful, and he was very clumsy, nothing seemed to interest him. He is now very thin, considering the food the cats get at that house, he is also malnourished -- but the other three cats living there are all fine.
I wonder why he is this way. The early trauma that he must have experienced as a kitten certainly left traces on his psyche -- but what kind of damage has been done?
Is his odd behaviour of a psychological nature (and could technically be cured with the right nurture) -- or has that early trauma (esp. malnutrition) left damage to his brain, which then results in odd behaviour -- or was he "born with it"?