A video of best buddies Suriya (orangutan) and Roscoe (pooch) is a YouTube sensation. An old tortoise and a young hippo have their own Web site and line of books detailing their friendship. You see stories of unlikely animal friends on the news: horses nuzzling cats, dogs caring for kittens and a pig nursing tiger cubs - and in May, Oprah even featured a dog and elephant who are best friends on her show.

These unusual relationships are celebrated and admired. But are they genuine - or do we just want them to be?

They're often actually the real deal, says John Wright, Ph.D., a certified applied animal behaviorist and professor of psychology at Mercer University in Macon, Ga. Given the right set of circumstances, and the right animals, they can form close and lasting relationships.

Heather MacLeod has seen it firsthand. She credits her white German shepherd, Faith, with helping save the life of her cat. Nearly nine years ago, MacLeod was driving home from work when a kitten tottered in front of her car. She swerved to avoid her, stopped the car, rescued the kitten and drove immediately to her veterinarian, who said she was so malnourished she probably wouldn't survive the night. But MacLeod named her Hope and took her home, where the dog and cat became so close that Hope even nursed from Faith, who had never been bred but nonetheless produced milk for the little kitten. MacLeod, who lives in New Brunswick, Canada, says the two are still the best of friends.