Rat societies stratify themselves into 50% exploited, 50% exploiters

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http://www.canal-u.tv/producteurs/les_amphis_de_france_5/dossier_programmes/sciences_de_la_vie/faits_comme_des_rats

Labs rats are normally kept in an environment of unnatural abundance – no effort is required for the rats to acquire food. In this experiment, rats were placed in environment where food required work – to acquire food the rats had to swim through an aquarium, grab the food and swim back.

In around 99% of cases, half the rats regularly swam across to grab food and the other half attacked the swimmers and stole their food, never swimming themselves. The thieves went as far as pushing the swimmers into the water. 

In further experiments, the groups were mixed up and divided into groups consisting solely of either swimmers or thieves. Each group established a new hierarchy so that half became swimmers and half became thieves. Similarly, when the rats were placed in much larger groups, the rats still established a 50/50 hierarchy.

The scientists did not identify any way of predicting which rats would become swimmers and which would become non-swimmers – no rates were innate swimmers or thieves. The key factors seemed to be the initial interactions. If the initial swims or thefts were successful, the rats were likely to remain in those roles.

Links on this subject:http://www.france-info.com/chroniques-info-sciences-2010-05-03-faits-comme-des-rats-437132-81-165.html
http://www.filmdechercheur.eu/spip.php?article121