It is not just people who kiss and make up, according to researchers who have shown that certain fish will stroke the fins of others to apologise for their bad behaviour.
The scientists believe it is the first documented example of one species reconciling with another outside of the mammal world.
鈥淚t鈥檚 a fascinating find,鈥� says Lee Dugatkin, a behavioural ecologist from the University of Louisville, Kentucky, who has written several books on animal co-operation.
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Redouan Bshary from the University of Cambridge and his colleague Manuela Wurth from the Technical University of Munich, Germany, studied the cleaner fish Labroides dimidiatus.
These fish survive by eating the parasites, scales and mucous of 鈥渃lient鈥� reef fish. The nibbling keeps the reef fish clean, and they seem to enjoy the treatment. 鈥淭hey鈥檙e kind of out of control,鈥� says Bshary about the enraptured client fish. 鈥淭hey鈥檒l actually drift and bump into coral reefs.鈥�
Bshary and Wurth knew that cleaner fish sometimes stroke their client fish without picking off parasites. They do this with their back fins, facing away from the client, so it is impossible for them to be feeding at the same time.
To find out why they do this, the biologists spent 112 hours underwater in the Ras Mohammed National Park in Sinai, Egypt, watching 12 individual cleaner fish.
They found that the cleaners were more likely to caress their clients鈥� fins if they had accidentally bitten them too hard during their last meal, forcing the client to flee or to chase the offender away. 鈥淚t鈥檚 a very gentle stroking,鈥� says Bshary, who has felt the fish tickle his hands. 鈥淚t鈥檚 quite pleasant.鈥�
Cleaners were also more likely to stroke fish who sometimes ate cleaners, as if to prove they were more valuable alive. And they used their seductive stroking to convince moving clients to settle down, making it easier to feed.
Bshary says this kind of behaviour has only been seen in mammals before 鈥� including primates, sheep, goats, dolphins and hyenas. He suspects that other animals exhibit apologetic behaviour too. 鈥淚 would expect that as soon as you have co-operation with some possibility for conflict, you should have reconciliation,鈥� he says.
Dugatkin says 鈥渢he cleaner-client scenario is a text-book case of reciprocity. It鈥檚 nice to see that people are doing better studies on them.鈥�
He adds that most mathematical models which describe inter-species interactions work best when the animals have a way to correct their accidental mistakes.
Journal reference: Proceedings of the Royal Society London B (vol 268, p 1495)