I think the question comes down to nomenclature. There are a variety of different forms of symbiosis, which can include parasitism (one party benefits at the cost of damage to the other), commensalism (one party benefits and the other is neither benefited, nor harmed) and mutualism (where both parties benefit). There's also amensalism, where one party impedes the success of another party, but derives no benefit from doing so. Each of these represent different forms of symbioses between different species.
I think it's true to say that there are a fairly broad range of mutualistic relationships out there, from fleeting transitory arrangements, to composite organisms. With regards the extreme scale of mutualism, the relationship typically described when many people used the word 'symbiosis', we think in terms of those such as that between Swollen Thorn Acacia trees and the ant species that live within them; or your choice of nectar producing plant + insect species used for pollen dispersal; or even more intertwined relationships in composite organisms such as corals (coral + algae) and lichen (fungi + algae).
At this level, representing a physical and physiological interaction, I am unaware of any such symbioses between mammals.
There are plenty of bird-mammal mutualisms however, one example being the relationship between the Honey Guide bird and the Honey badger (video) where the bird leads the badger to honey (benefit to badger) and the badger tears apart the hive (benefit to bird, which can now access honey).
Of course, what would happen if we removed one, or the other, of these organisms from the picture? In the strongest forms of mutualism (sometimes called obligate), we would perhaps expect one or both of the organisms to die. It's doubtful that there are any such mammal species with such a relationship, but I'm open to suggestions from other biologists here.
In answer, I think you've cited a fairly good example of a mutualism, but you need only look for close, mutually beneficial relationships between mammals to see other forms of mutualistic symbioses.
Last edited by Jim Caryl (24th Feb 2010 15:26:39)