Why Cleaner fish punish their partners for putting on weight

Largest fish in harem will turn into rival male

June 2011: Telling your partner to watch her weight is not recommended - unless you're a male cleaner fish, reports a new study.

Cleaner fish feed in male-female pairs by removing parasites from larger ‘client' fish. While providing this cleaning service, cleaners may get greedy and bite clients rather than sticking to parasites. This cheating by cleaners causes mealtimes to come to an abrupt end as the irritated client fish swims off.

RICH PICKINGS: Cleaner fish remove parasites from larger fish - but are not averse to sometimes taking a bigger bite. Picture: Joao Paulo Krajewski

But it's not just the client fish that is disgruntled. Females males that bite clients receive aggressive punishment from their male partners for such greedy behaviour. And with good reason.

Greedy females kept in check with harsh punishments
Scientists from the Zoological Society of London (ZSL) and a number of other leading scientific institutions have now shown that male fish lose more than just a meal from their partner's big appetite - they also risk the female becoming so large that she will turn into a rival male.

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.

Deterrent alters the female's behaviour
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.

The female fish will respond to this punishment by providing better service to high value clients in the future. This is the first non-human example of where punishment fits the crime and results in the offender adjusting their behaviour according to the potential penalties.

Dr Nichola Raihani says: ‘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. In both situations, harsher punishment may serve as a stronger deterrent against future crimes.'

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