An essay in the New York Times offers a nice review of unlikely friendships
It's rare that researchers and others turn to popular press for an update about a particular topic. Nonetheless, an essay by Erica Goode in the New York Times called "Learning From Animal Friendships," provides a nice summary of what we know -- actually the little we know -- about cross-species friendships in nonhuman animals (animals).
There's no reason to put the word friends in quotes
For years some researchers actually debated whether or not animals "made friends," even among members of the same species. They would put the taboo "F" word, friends, in quotation marks, to reflect their uncertainty. Thus, the use of "friends" would mean two dogs or two cats who we would call buddies or BFF's were merely acting as if they were friends, but we really don't know if this is so. Surely, anyone who's lived with more than one dog or cat or rat, for example, or who's done long-term field work on social animals, know they form deep and meaningful friendships.
I have always found this uncertainty to be simply absurd, even when applying the most rigorous criteria to the concept of "friend." In the New York Times essay, Dr. Barbara King notes that a friendly relationship "must be sustained for some period of time; there must be mutuality, with both of the animals engaged in the interaction; and some sort of accommodation must take place in the service of the relationship, whether a modification in behavior or in communication."