The effect of breakfast on social behavior and brain amine metabolism in vervet monkeys

Two groups of vervet monkeys were fed, on alternate days, either before or after a morning observation period. This enabled us to determine changes in behavior when the animals were fed a nutritionally balanced breakfast of monkey chow. Feeding did not alter the proportion of behaviors that were social or non-social, but had a marked effect on individual behaviors. Feeding increased active behaviors among the adult animals except for the vervets who were lowest in the social hierarchy in each cage. For some of the individual behaviors that were altered by feeding, the changes were most marked early on in the observation period, when the animals were still feeding. Other behavioral changes were seen only later in the observation period, a time course consistent with a food-mediated change in brain biochemistry. A parallel biochemical experiment showed that feeding decreased the levels of tryptophan and 5-hydroxyindoleacetic acid in the CSF. Our data indicate that feeding can influence both brain biochemistry and behavior. The behavioral changes may be influenced by social and psychological factors as well as changes in brain biochemistry.