Advertising coloration signals an animal's capability to other animals. These may be of the same species, as in sexual selection, or of different species, as in cleaning symbiosis. Signals, which often combine colour and movement, may be understood by many different species; for example, the cleaning stations of the banded coral shrimp Stenopus hispidus are visited by different species of fish, and even by reptiles such as hawksbill sea turtles. Sexual selection Cleaning symbiosis is a mutually beneficial association (symbiosis) between two species, where one (the cleaner) removes and eats parasites from the skin of the other (the client). Cleaning symbiosis is well-known among marine fish, where some small species of cleaner fish, notably wrasses but also species in other genera, are specialised to feed almost exclusively by cleaning larger fish and other marine animals. Other cleaning symbioses exist between birds and mammals, and in other groups. Cleaning behaviour was first described by the Greek historian Herodotus in about 420 BC, though his example (birds serving crocodiles) appears to occur only rarely. The role of cleaning symbioses has been debated by biologists for over thirty years. Some believe that cleaning represents selfless co-operation, essentially pure mutualism. Others such as Robert Trivers hold that it illustrates mutual selfishness, reciprocal altruism. Others again believe that cleaning behaviour is simply one-sided exploitation, a form of parasitism. Cheating, where either a cleaner sometimes harms its client, or a predatory species mimics a cleaner, also occurs. Predatory cheating is analogous to Batesian mimicry, as where a harmless hoverfly mimics a stinging wasp, though with the tables turned. Some genuine cleaner fish, such as gobies and wrasse, have the same colours and patterns, in an example of convergent evolution. Mutual resemblance among cleaner fish is analogous to Mullerian mimicry, as where stinging bees and wasps mimic each other. In his Histories (book II), the Ancient Greek historian Herodotus wrote: As [the crocodile] lives chiefly in the river, it has the inside of its mouth constantly covered with leeches; hence it happens that, while all the other birds and beasts avoid it, with the Trochilus it lives at peace, since it owes much to that bird: for the crocodile, when he leaves the water and comes out upon the land, is in the habit of lying with his mouth wide open, facing the western breeze: at such times the Trochilus goes into his mouth and devours the leeches. This benefits the crocodile, who is pleased, and takes care not to hurt the Trochilus. Herodotus thus claimed (about 420 BC) that Nile crocodiles had what would now be called a cleaning symbiosis with the bird he called the Trochilus, possibly a sandpiper; but while he was right about the existence of cleaning symbioses, there is little evidence for it in crocodiles. MacFarland and Reeder, reviewing the evidence, found that Extensive observations of Nile crocodiles in regular or occasional association with various species of potential cleaners (e.g. plovers, sandpipers, water dikkop) ... have resulted in only a few reports of sandpipers removing leeches from the mouth and gular scutes and snapping at insects along the reptile's body.