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Look who’s dieting
Paragobiodon xanthosomus

If you thought only fashion and health conscious men and women take to dieting, you’re wrong. Fish, too, voluntarily restrict their food intake — not to look more attractive but to maintain a smaller size so as to avoid provoking and consequently being thrown out of the group by bigger, dominant individuals.

Researchers at the ARC Centre of Excellence for Coral Reef Studies and James Cook University in Queensland, Australia, reached this fascinating conclusion after studying the behaviour of the group-living reef fish, goby (Paragobiodon xanthosomus). The findings appear in the May issue of Current Biology.

Understanding why and how some individuals in a group remain smaller than the dominant ones may help explain the mechanisms underlying the hierarchy in certain animal societies, the researchers feel.

“In studying gobies we noticed that only the largest two individuals — a male and a female — had mating rights within the group. All other group members are non-breeding females, each being consistently 5-10 per cent smaller than its next largest rival,” explains Dr Marian Wong, the lead author. The researchers wanted to find out how the subordinate members maintain this precise size separation that goes on to ensure stability in the system. They tried to fatten some subordinate gobies by giving them more food. To their surprise, the animals refused to eat the tidbits offered, thus choosing to stay smaller, rather than growing in size and risking conflict with the boss.

So far, size regulation in hierarchical societies was thought to be a result of competition over limited food supplies — that is, the subordinates are bullied by the dominants, get less food and grow more slowly.

“Hierarchies function as queues for breeding. When a dominant dies, all subordinates below it grow and shift up in rank. Only when they reach the front of the queue can they breed,” write the researchers.

The research has highlighted the fact that dieting is a habit far from exclusive to humans — and appears to reach back at least to our fishy ancestors, although different species may diet for different reasons. However, the researchers are not yet sure about how widespread the self-imposed reduction of food intake is in Nature.

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