I do think certain aspects of an animals personality are there from birth, and some factors can exacerbate certain traits in an animals upbringing. I am thinking of a dog owned by people I know - the pup was "skittish" when they got him, never was comfortable with someone reaching over his head, and didn't know he was a dog - didn't know how to play with other dogs. A lot of that could have been corrected with early intervention, but instead, he was never socialized around other pups, and lived with an owner who tended to "bark too much" in human terms, so it fed into the pup's inborn anxieties. He became a sweet dog around people as long as they knew him, but also a barker, and never did learn to interact with canines.
Anyone who has ever spent time with more than one baby discovers that each infant has distinct personality traits, so it is natural that other creatures would be the same way!
I do think, for example, that it is probably easier to "ruin" a good dog or cat than "fix" one with problems, but isn't that almost a truism?
Bookmarks