Parasite infection rather than tactile stimulation is the proximate cause of cleaning behaviour in reef fish.
Cleaning behaviour is a popular example of non-kin cooperation. However, quantitative support for this is generally sparse and the alternative, that cleaners are parasitic, has also been proposed. Although the behaviour involves some of the most complex and highly developed interspecific communication signals known, the proximate causal factors for why clients seek cleaners are controversial. However, this information is essential to understanding the evolution of cleaning. I tested whether clients (Hemigymnus melapterus) seek cleaners (Labroides dimidiatus) in response to parasite infection or whether clients seek cleaners for tactile stimulation regardless of parasite load. Cleaner fish and control fish were collected on 1 October 1998 from Lizard Island, Great Barrier Reef, Australia. Parasite (gnathiids) loads on client fish were manipulated and clients exposed to cleaner fish and control fish behind glass. I found that parasitized client fish spent more time than unparasitized fish next to a cleaner fish. In addition, parasitized clients spent more time next to cleaners than next to control fish, whereas unparasitized fish were not attracted to cleaners. This study shows, I believe for the first time, which is somewhat surprising, that parasite infection alone causes clients to seek cleaning by cleaners and provides insight into how this behaviour evolved.