Cleaner fish live in groups led by one dominant male with a harem of up to 16 females. All cleaner fish are born female and turn into males when they become the biggest fish in their group. A male cleaner fish usually partners with the biggest female fish in the harem for cleaning duties.

"Our research shows that male cleaner fish are sensitive to their female partner's size. One reason for keeping a cheating female in check may be to stop her eating too much and then challenging his position as the dominant male on the reef," says Dr Nichola Raihani, lead author from ZSL.

The research also shows that the male cleaner fish distinguish between high and low value meals and will punish the female more severely if she drives off a high-value client.