requirements of the experimental apparatus, length of the experimental session, length of the 
experiment, and cost. Restriction to some percentage of a free-feeding weight may be 
necessary for initial training or for study of certain experimental questions, but the particular 
percentage necessary may vary across individual monkeys. Nonhuman primate species differ 
in their nutrient and energy (gross kilocalorie per kilogram of body weight) requirements. 
Familiarity with requirements for the species is important if food restriction is to be used, 
particularly if feeding will consist primarily of food pellets formulated for use as reinforcers for 
monkeys. Some species may need a vitamin supplement. Nonhuman primates require a 
dietary source of vitamin C; providing a supplement of fresh fruit or vegetables daily or a 
couple of times a week helps prevent vitamin C deficiency and also serves as a means of 
environmental enrichment. 
GENERAL PROCEDURES AND CONSIDERATIONS 
Unless specific protocols require exemption, allowing most laboratory animal species to feed 
at least once per day is consistent with standards of humane care, and is required for species 
covered by USDA regulations (see review of research by Toth and Gardiner, 2000). 
Information on the daily caloric, nutrient, and water requirements of many species is 
published in the ILAR Report, Nutrient Requirements of Domestic Animals Series (ILAR, 1995). 
Balanced animal diets, which consider these recommendations, are available commercially as 
pellets for reinforcement for a variety of species. As long as the expiration dates are heeded, 
the diet is all that is needed to feed laboratory animals appropriately under free-feeding 
conditions. Under restricted feeding conditions, however, vitamin supplements may be used, 
depending on the species. Supplements also may be appropriate when feeding is not 
particularly restricted but amount consumed is likely to decrease as a function of some 
experimental manipulations, such as surgical interventions or administration of some drugs. 
Constant access to water typically is provided under food control regimens. There is 
interdependency between food and water intake (e.g., food-restricted animals may drink less 
water), but species differ in their patterns of drinking during the day and in their response to 
food restriction. 
Food-restricted animals typically are weighed frequently, usually before experimental sessions. 
Species whose weights change slowly under minimal restriction regimens may be weighed 
less often if some form of anesthesia (e.g., ketamine) is required to accomplish this. However, 
animals on food restriction must have their body weight recorded on a regular schedule. 
Once animals are trained under many behavioral procedures, they may continue to serve as 
subjects over their life spans. A factor to consider is whether there will be a return to 
unrestricted food in periods between studies. Practices vary and there are several 
considerations. These include (1) the extent to which weight was restricted below an ad 
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