Oxpecker/mammal interactions are the subject of some debate and ongoing research. They were originally thought to be an example of mutualism, but recent evidence suggests that oxpeckers may be parasites instead. Oxpeckers do eat ticks, but often the ticks that have already fed on the ungulate host and there has been no proven statistically significant link between oxpecker presence and reduced ectoparasite load . However one study of impalas found that impalas which were used by oxpeckers spent less time grooming themselves suggesting they had fewer parasites. Oxpeckers have been seen opening new wounds and worsening existing ones in order to drink the blood of their perches . Oxpeckers also feed on the earwax and dandruffs of mammals, although less is known about the benefits of this to the mammal, it is suspected that this is also a parasitic behaviour. Some oxpecker hosts are intolerant of their presence. Elephants and some antelope will actively dislodge the oxpeckers when they land. Other species tolerate oxpeckers while they search for ticks on the face, what one author described as "appear[ing] ... to be an uncomfortable and invasive process."