Not many would recommend that a guy tell his gal pal to slim down, but for some fish in tropical coral reefs it's a necessary way of life. New research reveals that male cleaner fish aggressively nudge their mates to watch their diet, as a way to prevent their female partners from turning into rival males.

Cleaner wrasse (Labroides dimidiatus) are hermaphrodites that are all born female but can turn into males when they become the biggest fish in their group. They get the name "cleaner fish" from the fact that they will clean the skin of other fish by feeding on those "client fishes' " parasites.

Cleaner wrasse feed in male-female pairs, with both fish removing parasites from the larger client fish. Most of the time, the feeding cleaner fish service their client responsibly, but if a cleaner fish gets greedy, it may take a bite out of the client fish's protective mucus coat as well. Result: The agitated client fish swims away, leaving the cleaner fish pair without the rest of their meal.

Scientists have known male cleaners will punish greedy females by chasing and harassing them, but they didn't know whether the severity of this punishment differed with the situation. [Gallery: Stunning She-Males of the Animal World]

To find out, Nichola Raihani of the Zoological Society of London and colleagues placed pairs of cleaner fish in tanks with a simulation of what a client fish offers — a plate holding prawns (representing the client's mucus coat) and flakes (representing the client's parasites). As soon as one of the female fish "cheated" by snagging a prawn, the researchers removed the plate and then recorded the number of aggressive chases by the male in the next 30 seconds.

This and follow-up experiments revealed male punishment was most severe for the loss of high-value clients (a bigger plate) and when the females were large. When the female was relatively small, males didn't seem to adjust the punishment level.

It turns out, the males were ensuring females didn't balloon too much in size and transform into a rival male, the researchers said.

"Our research shows that male cleaner fish are sensitive to their female partner's size," lead researcher Raihani said in a statement. "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."

Cleaner fish live in groups, with one dominant male and a harem of up to 16 females. During feedings, the male leader will usually partner with the largest female fish in the harem so that he can monitor her food intake to make sure that she doesn't "cheat" while eating and grow too big.

The results have parallels in the human realm. "Cleaner fish and humans may not share many physical traits, but cleaner fish punish cheating individuals, just as we punish people who step outside of the law," Raihani said. "In both situations, harsher punishment may serve as a stronger deterrent against future crimes."

The study will be published online in Proceedings of the Royal Society B in November.

This article was reprinted with permission from LiveScience.

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