A warthog puts up with getting its hair pulled by a mongoose in return for having parasites removed. A. Plumptre

Humans aside, mammals seldom form symbiotic relationships with other mammalian species, but one example has been confirmed in Uganda where banded mongooses (Mungos mungo) groom warthogs (Phacochoerus aethiopic). A deeper study of this cooperation may help us understand how humans came to domesticate animals, a major step in our rise to global domination.

Tourists visiting the Queen Elizabeth National Park have reported seeing warthogs lie down while mongooses climb over them. For the mongooses, it means a free high-protein meal of ticks, while the warthogs are freed from a painful and potentially disease-carrying parasite. The BBC filmed the behavior seven years ago, but the observation has only recently appeared in a scientific newsletter.

The interaction has been briefly described in Suiform Soundings, the newsletter of the International Union for the Conservation of Nature's (IUCN) specialist group on pigs, peccaries, and hippopotamuses. A thorough peer-reviewed study is yet to come.

The mongooses of the park have got so used to tourists that they allow humans to get unusually close, providing a boon to researchers, who have cataloged their remarkable sex lives in some detail, in addition to this grooming behavior.

"Such partnerships between different mammal species are rare, and this particular interaction illustrates a great deal of trust between participants," said Dr. Andy Plumptre of the World Conservation Society in a statement. "It makes you wonder what else may be happening between species that we don't see because, in order to see it, both species need to be unafraid of people."

The warthogs may appreciate a massage along with their delousing. A Plumptre

Plumptre notes that there have been rare accounts of different primate species grooming each other, but otherwise suitably flexible mammals seldom lend a paw to help out besieged members of other species.

The report brings into focus the question of why such mutually beneficial arrangements do not occur more often. Coral reef fish depend heavily on cleaner fish and cleaner shrimps, for example, to control their external parasite loads. Mutual grooming is common within many species, and in addition to removing biting insects and building social networks, it can provide an important part of an omnivorous species' diet. Extending the favor to neighbors who can't look after themselves looks like an obvious move.

Nevertheless, Plumptre notes that giant forest hogs live in the same location where the warthogs are being groomed, and appear not to have developed the same relationship with parasite-eating mongooses. It is also not known if the interaction is unique to the national park, or occurs more widely across Africa.

Plumptre noted that many other inhabitants of the African savannas, including rhinoceroses and zebras, get a similar parasite-removal service from oxpecker birds. Warthogs also benefit from the birds, as well as rubbing against trees and taking sand baths to get rid of ticks.

Although such mammal-mammal assistance is apparently rare, last year wolves and monkeys were found to have a symbiotic relationship in one area of Ethiopia. However, this only requires them to tolerate each other, and for the wolves not to eat juvenile monkeys, rather than the close contact that occurs between the warthogs and mongooses.