The hyaenas were a given. And solitary: http://en.wikipedia.org/wiki/Solitary_animal. Tigers, bears and wolverines, oh my. Solitary being merely another point on the distribution of social structure. One wonders whether other categories in this this semi-ordinal distribution - solo, pack, herd - can be readily identified for mammals. Cetacean pods probably won't do - roughly ten individuals which is as good as pack, really. A preference for duos would certainly be different. This implies long-term monogamy and being on the K side of r/K selection but actually I don't think it really requires it. Then again, you could argue that some of the solitary Carnivora do this, serially. Also prairie voles. Does a transient association for mating count? Seems like cheating. Gibbons? No, looks like family groups, so... pack, essentially. Some big birds, anyway. Please Register or Log in to view the hidden image! No, not this big bird.