Fish diet to avoid fights

People diet to look more attractive. Fish diet to avoid being beaten up, thrown out of their social group - and getting eaten as a result.

That is the fascinating conclusion of the latest research into fish behavior by researchers from the ARC Centre of Excellence for Coral Reef Studies and James Cook University, described in the journal Current Biology.

The same team who recently revealed fish use the threat of punishment to keep competitors in the mating game in line, have taken the work a step further to discover that subordinate fish deliberately go on a diet to avoid posing a challenge to their larger rivals.

“In studying gobies we noticed that only the largest two individuals, a male and female, had mating rights within the group,” explains Dr Marian Wong. “All other group members are non-breeding females, each being consistently 5-10 per cent smaller than its next largest rival. We wanted to find out how they maintain this precise size separation.”

The reason for the size difference was easy to see. Once a subordinate fish grows to within 5-10 per cent of the size of its larger rival, it provokes a fight which usually ends in the smaller goby being expelled from the group – and the safety of the coral it occupies. More often than not, the evicted fish is then gobbled up by a predator.

It appeared that the smaller fish were keeping themselves small in order to avoid provoking the boss fish. Whether they did so because of stress caused by the bullying of larger fish - or voluntarily, by restraining how much they ate was not clear.

The team of Dr Wong, Dr Philip Munday, Dr Peter Buston and Professor Geoff Jones decided to try to fatten up some of the subordinate gobies to see what happened. To their surprise, the gobies simply refused the extra tidbits they were offered, apparently preferring to remain small and avoid fights, over having a feast.

The discovery challenges the traditional scientific view of how dominant individuals retain their position in a hierarchy. Previously it was thought that large individuals simply used their weight and size to terrorise their subordinates and grab more of the food for themselves, so keeping their rivals small.