sumed to the exclusion of nutritionally balanced foods, and nutrient distortion of the diet can be serious. The use of vitamin and mineral supplements is not a dependable way to correct these problems, because supplements might be inappropriately formulated for the purpose (not all supplements are alike), and they are often administered without measuring, so there are risks of overdosing or underdosing.
With respect to selecting foods to extend foraging time, novel colors, sizes, shapes, smells, and textures and the presence of shells or husks that require removal can be sensory stimulants (Schapiro et al., 1996; Noonan, 1998). If cognitive tasks are required to acquire food, the associated mental stimulation appears to be rewarding beyond the acquisition of calories (Reinhardt, 1997). Singly housed marmosets foraged for up to 6 h when food was mixed with sawdust (Scott, 1991). Juvenile patas monkeys have been reported to leap several feet in the air to reach fruit stuck on branches in a zoo, even when adequate amounts of fruit were available on the ground (McGovern, 1994). A comparative study found that cynomolgus monkeys preferred foraging activities more than other enrichment methods (Bryant et al., 1988). When various foraging enrichment devices were presented to squirrel monkeys, the devices that increased foraging times and that were manipulated the most were a capped polyvinyl chloride (PVC) pipe with food-dispenser holes and food dispensers made from 2-L plastic beverage containers (Boinski et al., 1994). Provision of a PVC feeding device and more frequent feeding reduced abnormal behaviors in singly housed baboons (Brent and Long, 1995). Puzzle feeders requiring manipulation to acquire treats were more effective than treats alone in reducing locomotor stereotypies in singly housed rhesus macaques. However, the effects lasted only as long as the manipulation time (about 1 h) that was required to acquire the treats. If puzzle difficulty was increased, the monkeys tended to give up (Novak et al., 1998). Thus, it might be important to distinguish between provision of treats with little or no foraging activity and promotion of foraging activity for acquisition of principal food sources.
Different species use different foraging techniques, and promotion of foraging activity in captivity should consider species differences. Are the primates principally frugivores, folivores, insectivores, gummivores, or omnivores? Are they principally terrestrial or arboreal foragers? Are they manually dextrous? Do they use their hands or tools? What is their relative cognitive ability? Those factors are all relevant in determining which foraging enhancements may be most effective.
Specialized foraging adaptations and food preferences of several species have been described. Ring-tailed lemurs have been reported to prefer fresh new leaves in the wild, whereas brown lemurs preferred mature leaves. Lemurs processed fruit very little and licked the open end of bananas rather than peel them (Jolly, 1985). Golden-lion tamarins forage for insects by manipulation; they sift through substrate, search for insect holes, remove bark, and break open wood in their quest for food. But cotton-top tamarins feed opportunistically on insects that they encounter as they move through dense tangles of branches and vines (Steen, 1995). Lorises are able to capture only slow-moving and often relatively unpalatable prey, whereas galagos capture more rapid and more palatable prey (Charles-Dominique, 1977). In captivity, patas monkeys preferred browse from poplar trees (Populus spp.) but used it more for bark-chewing than leaf-eating; for effective use, it was necessary to mount the browse in a metal sleeve to hold it in a natural, upright position (McGovern, 1994). Surfaces of novel devices containing food are inspected by sniffing, touching, and licking by captive squirrel monkeys but are more likely to be persistently manipulated by capuchins (Fragaszy and Adams-Curtis, 1991). Great apes, baboons, macaques, and capuchins explore the properties of objects and appear to have the cognitive ability to relate them to each other. These skills are basic to tool use, something that wild chimpanzees practice regularly in nut-cracking and in foraging for termites and ants. Although other apes and the monkey species mentioned above rarely use tools in the wild, they adapt readily to use of tools when they are provided in captivity (Tomasello and Call, 1997).
Food can be used to enrich a captive environment by manipulating foraging opportunities in time and space. If outdoor exhibit or holding areas are available, food can be placed on the ground or in trees, as it might be found in a wild environment. Foraging time also can be extended in inside areas by scattering food in a substrate, such as leaf litter, straw, hay, wood shavings, or shredded paper. Hidden foods can include the primary nutrient source, usually an extrusion, and low-density items, such as popcorn and some dry breakfast cereals. Whole fruits and vegetables that require husking or peeling before eating can also be useful. Time spent in feeding was increased when lion-tailed macaques were presented whole versus chopped foods (Lindburg and Smith, 1988). Stereotypies in singly caged baboons were reduced by offering corn on the cob (Bennett and Spector, 1989).
It is particularly important to place food in multiple locations in group enclosures so that aggression and food monopolization are minimized. Studies with Diana monkeys (Cercopithecus diana), Allen’s swamp monkeys (Cercopithecus nigroviridis), lion-tailed macaques (Macaca silenus), and Hamlyn’s monkeys (Cercopithecus hamlyni) in