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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 a team of Australian scientists．
The research team has discovered that subordinate(低一等的)fish voluntarily diet to avoid challenging their larger competitors．“In studying gobies we noticed that only the largest two individuals, a male and female, had breeding(繁殖)rights within the group,” explains Marian Wong．“All other group members are nonbreeding females, each being 5-10％ smaller than its next largest competitor．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％ of the size of its larger competitor, it causes a fight which usually ends in the smaller goby being driven away from the group．More often than not, the evicted fish is then eaten up．
It appeared that the smaller fish were keeping themselves small in order to avoid challenging the boss fish．Whether they did so voluntarily, by restraining how much they ate, was not clear．The research team decided to do an experiment．They tried to fatten up some of the subordinate gobies to see what happened．To their surprise, the gobies simply refused the extra food they were offered, clearly preferring to remain small and avoid fights, over having a feast．
The discovery challenges the traditional scientific view of how boss individuals keep their position in a group．Previously it was thought that large individuals simply used their weight and size to threaten their subordinates and take more of the food for themselves, so keeping their competitors small．
While the habits of gobies may seem a little mysterious, Dr．Wong explains that understanding the relationships between boss and subordinate animals is important to understand how hierarchical(等级的)societies remain stable．
The research has proved the fact that voluntary dieting is a habit far from exclusive to humans．"As yet, we lack a complete understanding of how widespread the voluntary reduction of food intake is in nature," the researchers comment．"Data on human dieting suggests that, while humans generally diet to improve health or increase attractiveness, rarely does it improve long-term health and males regularly prefer females that are fatter than the females' own ideal．"
