Fish dish out punishment to fit the crime

By Jessica Campion 24 June 2011
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Male cleaner wrasse dish out measured punishment to females so they toe the line, new research shows.

ADJUSTING THE SCALES OF crime and punishment is a slippery business but apparently fish know how to weigh up the stakes. Scientists have revealed one fish species, like humans, not only uses punishment to maintain law and order but adjusts punishment to fit the crime.

Alpha male bluestreak cleaner wrasse (Labroides dimidiatus), use punishment to enforce cooperation and maintain their underwater social hierarchy, and now a team of biologists from Switzerland, London and Queensland have shown that the amount of punishment is tailored to suit the offenders and the severity of their misconduct.

Co-author of the research Dr Lexa Grutter, a marine biologist from the University of Queensland, told Australian Geographic the study is the first to demonstrate measured discipline in a non-human species. 

“This is the first time any animal has been shown to adjust punishment levels depending on the level of crime – punishment being something that you do to somebody that is costly but makes them behave better in the future,” she says.

Warding off a sex change

The bluestreak cleaner wrasse feed off parasites and dead tissue found on the skin of larger fish, which scientists refer to as ‘clients’, performing an important cleaning role. One of several cleaner wrasse species found on coral reefs in the Indian and Pacific oceans, this species typically consists of groups with one head male and 8 to 16 females; they usually feeds in pairs – one female, one male.

Gender and rank in this fish species is determined by size – all cleaner wrasses are born female but morph into rival males if they grow as large as the harem male. To stay head of the pack, the male cleaner wrasse keeps his females in constant check to ensure they don’t eat too much and grow too big, researchers say.

Previous study has proved cooperation is crucial in the life of a male cleaner wrasse, Lexa says.

“Cooperative relationships have to be maintained between the cleaner and the client fish, which are two different species, and also between the cleaning pair,” she says. “The male is able to maintain the cleaning service and his position at the top of his harem by making sure his females cooperate and don’t end the interaction by cheating the parasite removal service and biting the client or eating the client fish’s mucus – their favourite but forbidden food.”

Punishment fits the crime

Previous studies have shown that punishment maintains this cooperation because the punished females tend to be better behaved in future interactions, Lexa says.

The new study is the first to demonstrate that male wrasse adjust the level of punishment to fit the crime when females do not cooperate. The research shows the intensity of the reprimand is determined by the size and threat of offending females.

“If the females eat their client’s mucus, the client fish get upset and leave – and this causes conflict between the cleaner pair,” says Lexa. “If the offending female is similar in size to the male and becoming a potential rival, the offence is greater and there will be even more conflict. The male will chase the female around and be really aggressive.”

The severity of the punishment dished out to the offending female, the study found, also depends on the value of the lost client. “High-value clients in the wild could be a larger fish, one with more food available to the cleaners,” Lexa says. “We found the male punished females more when the females both cheated a high-value client and was similar in size to the male.”

Marine biologist Dr Marian Wong of Boston University in the US is a fish behaviour expert and has researched coral reef fish in Queensland. She says the study builds on the growing realisation that fish use punishment to preserve group hierarchy.

“The study demonstrates that conflict over rank seems ubiquitous in fishes, and that punishment has evolved as a useful method of promoting cooperation in lots of different fish species including anemonefish, angelfish, and the coral dwelling goby,” she says.