By Jody Bourton
Earth News reporter
You scratch my back I'll scratch yours?
A warthog has been pictured being groomed by a huge bird known as a ground hornbill.
The warthog approached the southern ground hornbill seeking the favour, and the bird obliged by removing parasites from the warthog's body.
Similar interactions occur between warthogs and other animals such as banded mongooses.
But hornbills are not known to groom in this way, say scientists who photographed the incident.
Details of the behaviour are reported in the African Journal of Ecology.
"The warthogs approached the hornbills and then lay down on their sides to be cleaned," explains Mr Hendri Coetzee of North West University, Potchefstroom, South Africa.
"The warthogs were very nervous, because this behaviour most probably makes them more vulnerable to predation."
Mr Coetzee says he repeatedly observed similar interactions between common warthogs (Phacochoerus africanus) and southern ground hornbills (Bucorvus leadbeateri) in the Mabula Game Reserve in South Africa's Limpopo Province.
The arrangement is mutually beneficial: the warthog gets a cleaning service and the ground hornbill a nutritious and easily obtainable food source.
Other animals will also groom warthogs, picking off parasites from the wild pig's body.
For example, banded mongooses remove ticks from warthogs, in what is believed by scientists to be the only symbiotic relationship between two mammal species.
Oxpecker birds also regularly clean the skins of a number of African mammals, including zebra and hippos.
But the interactions with the hornbills stood out because the warthogs usually initiated the grooming.
Southern ground hornbills are large black plumaged birds, with powerful beaks that can kill tortoises and large snakes.
"What surprised me was how delicately they were removing parasites from the warthogs," says Mr Coetzee.
"Adult warthogs even tolerated the ground hornbills probing their ears and around their more delicate areas under their tails."
Mr Coetzee speculates that the animals might be behaving this way in part because they are living on a game reserve, where they might be less threatened and more relaxed.
"It is most likely the result of learned behaviour and regular contact between the same individuals living under somewhat artificial circumstances, where the risk of predation is reduced," he explains.