According to a Time magazine cover story about animal friendships, animals such as chimpanzees, dolphins and even horses form close friendships but dogs not so much. Before you start shouting, consider that the definition of friendship isn't just best buddies who seek out each other at the dog park. Animal friends, like close human friends, work at the relationship, which involves sharing over time, sacrificing and grieving. There are plenty of anecdotes about animal buddies, especially on YouTube, but not all the animals shown are friends and anecdotes don't make science.
In the article James Serpell, author and director of the Center for the Interaction of Animals and Society, says that dogs may form "friendship-type relationships" with other dogs in the same household (not Miro and Alanis, though); but they don't form strong friendships any more than members of wolf packs do. Did your dog willingly share his dinner with you or another dog today? Or does he basically think you should share with him? And isn't that the way we like it, not just because we don't want to eat kibble or raw chicken necks?
The article's author Carl Zimmer writes, "Thanks to domestication, dogs have become capable of being sweet and loyal to humans, but it's likely that they treat us more as guardians than friends. Dogs are neither our best friends nor one another's--which is not to say they're not warm and wonderful company all the same."
For the purpose of scientific study, the definition of friendship has to be strict. The advantage is that these studies of friendship will underline what we animal bloggers already know about animal thinking and feeling. The fact that dogs don't form the same kinds of inter-and intra-species friendships as other animals also confirms what we know: dogs are in a class by themselves.