The “not so innocent” mutualism between oxpeckers and herbivores
Mutualistic relationships are something really handsome and interesting. Mutualism is a relationship between two species which benefit to one another. There are obligate mutualisms where one or both species is completly dependent on the other, for example figs and fig wasps. And then there are non-obligate mutualisms where the species are not totally dependent and would still survive without the other. Mutualism seems something very good and usuful, but as we learn more about it we discover that sometimes this is not always as innocent as it seems.
Red billed oxpecker (Buphagus erythrorynchus) on zebra
Red billed oxpecker (Buphagus erythrorynchus) on an impala
“Oxpeckers love eating ticks (an adult eats ~100 adult ticks or >1000 nymphs per day ).”
Yellow billed oxpecker (Buphagus africanus) on a giraffe
Let’s take the oxpeckers for example. There are two species of oxpeckers: the red billed and the yellow billed. Oxpeckers get nearly all of their food by eating parasites off of large mammals. They live so close to these animals that even their body has been adapted to them: their beaks are perfect for combing through the fur and their toe structure is different from other birds and allows them to get a grip on animal’s fur at any angle.
Ticks suck animal’s blood causing anemia and spreading diseases like Lyme disease, so our romantic view of the oxpecker-mammal relationship was that the birds were helping the large mammals by taking off ticks, and getting food in return. In fact many researchers have studied the stomach contents of oxpeckers and estimated that an individual oxpecker can eat about 100 adult ticks a day (and about 1000 nymphs a day).
“.. ticks are really bad for mammals, spreading disease and slowing calf growth (by <44kg/yr in impala).”
Other studies have shown that the blood-feeding of oxpeckers is dependent on the quantity of ticks. When ticks are many, the birds tend to eat them but when the tick number reduces, they start feeding on wounds. These birds actually eat blood, not meat, and when they eat ticks it’s just because they contain blood. Probably they prefer to eat ticks, when they are many, because it is easier than digging into the wounds.. but this is our humble opinion.
Oxpeckers also warn their hosts of danger. However, just like the story with humans and honeyguides, all is not simple!
While the benefit of tick removal is measured (impala with oxpeckers have fewer ticks and spend less time grooming), the impact of wound-opening is hard to quantify. If all was rosy, we’d expect mammals to welcome oxpeckers, but they don’t (cit. Colin Beale)
Field observations of this special relationship between red‐billed oxpeckers and ungulates in Nakuru National Park, Kenya, revealed that specific hosts frequently tried to manipulate oxpecker feeding. This involved a repertoire of behaviour called resistance behaviour, and often resulted in the oxpeckers either changing their position on the host’s body or going away. Cape buffalo, the most frequently used host, exhibited little resistance behaviour. Waterbuck were also popular oxpecker hosts, but frequently performed vigorous resistance behaviour. Impala, the third most largely utilized host species, also used resistance behaviours, but allowed a higher proportion of oxpeckers to feed without trouble.