In general, primates can be fed a diet based on commercial monkey biscuits, high-fiber, old world and new world primate pellets, or canned primate diet. Marmosets should be fed a marmoset diet. To prevent high amounts of easy digestible sugars, which can cause diarrhea and obesity, low amounts (< 10%) or no fruits should be fed, and moderate amounts of carrot, sweet potato and apple, should be offered, depending on the species. Feeding greens and green vegetables, which are more comparable to the natural diet, should be encouraged. Monkey biscuits, high-fiber pellets for primates, and the canned products should comprise 20% of the dry-matter intake of gorilla and orangutan diets; fruits and treat items should comprise ≤10%, depending of the species, and green vegetables and browse should be at least 40% of the diet. Gorillas and orangutans should get browse. Alfalfa hay can also be offered.
Feeding only greens and vegetables, with vitamin and mineral supplements, is being tested in several species of primates. Diets should be developed and offered in such a way that enrichment is supported.
Dietary crude protein ranges for Pongidae such as gorillas are 14%–16% and for New World primates such as squirrel monkeys are 18%–22%. The unsaturated fatty acid composition should be at least 2.5% (⅔ n-6 fatty acids). Neutral detergent fiber (NDF) in the dry matter ranges from at least 10% in macaques, to 10% in marmosets, 30% in howler monkeys and langurs, and 20% in lemurs, chimpanzees, and gorillas. Acid detergent fiber (ADF) ranges from 5% in macaques, marmosets, and tamarins, 10% in lemurs and Pongidae, to 15% in howler monkey and langur diets. For all primates, the diet should contain 0.8% calcium and 0.6% phosphorus. Primate diets should contain 5,000–8,000 IU vitamin A and 800–1,500 IU vitamin D, except diets for squirrel monkeys, marmosets, and tamarins should contain 2,400 IU vitamin D. The vitamin C level should be at least 200 mg/kg dry matter. The maintenance metabolic rate (kcal/day) is 3,621 for Pongidae (gorillas, orangutans), 1,099–1,324 for baboons, 632–637 for langurs, 567–1,146 for rhesus macaques, 31–76 for marmosets, and 57–105 for tamarins. Obesity should be prevented; therefore, it is important to weigh animals regularly or to perform body condition scoring.
Monkey biscuits containing high-quality protein (18%–22.5% crude protein) should be fed to New World primates to ensure that their higher protein requirements are met. Regular monkey biscuits can be fed to Old World species depending on other components in the diet, although many larger Old World species such as gibbons, orangutans, chimpanzees, and gorillas also need high-fiber products. Laboratory primate biscuits are typically formulated with very low fiber levels (eg, 5%). Because many of the natural foods consumed by these species appear to contain very high fiber levels (eg, >20%), increasing the dietary fiber intakes of larger primate species is widely practiced. High-fiber biscuits should comprise at least 50% of the dietary dry matter, and browse should be at least 40% of the diet fed. Most of the time, however, these amounts are not available, and greens and vegetables can be a replacement for the browse. Monkey pellets should contain 10%–15 % crude fiber, 15%–20% NDF, and 5%–10% ADF. High-fiber monkey pellets can contain up to 27% crude fiber, 25%–50% NDF, and 30%-–35% ADF.
Cultivated fruits should be used sparingly for great apes and leaf-eating species because, compared with cultivated green vegetables, they are typically high in sugars and simple carbohydrates and low in protein and calcium. Monkey biscuits can be made more palatable for some species by soaking them in water or fruit juice. To prevent leaching of nutrients, the biscuits should be placed in a thin film of liquid so that the liquid is drawn up into the biscuit.
Other items commonly included in primate diets include hard-boiled egg (if cholesterol is not a concern), yogurt, and bread. Grapes, raisins, peanuts, crickets, and mealworms are treat items well liked by most species. However, these items should be fed two or three times a week at the most, not every day. The energy amount of these enrichment items should not exceed 5%–10% of the energy consumed by the animal.
Mouse pups are favored items for many smaller primates. However, callitrichid hepatitis in tamarins and marmosets has been associated with the feeding of newborn mice infected with lymphocytic choriomeningitis virus. Most zoos have discontinued the feeding of mouse pups to these New World primates. Sunflower seeds, instant rice, cracked corn, and shredded coconut can be scattered around the exhibit or holding areas to promote foraging activity. The amount of energy used for these enrichment feeds should not exceed 5%–10% of planned dietary energy. Hay should be provided for nesting materials and diversion, and it can act as a foraging substrate.
Many zoos offer meat to their great apes; although meat is often relished by the animals, there is no evidence it is necessary if the diet is properly balanced. Because hypercholesterolemia is seen in many captive gorillas, the feeding of meat may be contraindicated. For most primates, meals should be offered at least twice daily. Smaller species may benefit from even more frequent feedings.
New World primates use vitamin D2 poorly. It is particularly important that these species receive an adequate source of stabilized vitamin D3 (cholecalciferol) in their diet if they are not exposed daily to direct sunlight. Marmosets require up to four times the amount of vitamin D3 required by other New World primates. Because of potential vitamin D toxicity, commercial marmoset diets should be fed only to marmosets. In noncommercial mixtures for smaller primates (mixtures of cut apples, bananas, and cereal products with vitamins and minerals added), vitamin D3 should be included; however, care should be taken to prevent vitamin D toxicity.
Several cases of rickets in some Old World species at weaning have been reported. This may be because barred, outdoor primate exhibits have been replaced with indoor, albeit more naturalistic, exhibits. Although most free-ranging primate species probably satisfy their requirement for vitamin D by exposure to ultraviolet B (UVB) from sunlight, captive animals may rely entirely on a dietary source. Infants at weaning appear to be particularly at risk because milk levels of vitamin D are probably quite low, and many foods the young begin to eat are not fortified with this vitamin. Exposing the infant or juvenile to natural sunlight may be the best solution, because assuring that a dietary supplement is consumed by a young primate may not be possible.
In marmosets and other small primates, marmoset wasting syndrome may be caused by a gluten intolerance; some facilities feed these primates gluten-free pellets as a precaution. Instead of a pellet form, other zoos feed their small primates a cake-like diet made from a powdered base specific for marmosets and tamarins that is mixed with water.
The amount of UVB from sunlight is the highest in and around the equator; in regions at latitudes farther away from the sun the amount of radiated UVB, particularly in autumn, winter, and spring, is not enough. Lights that emit energy in the UVB range can be practical for use with primates, provided precautions are taken to prevent the primates from being able to touch the UVB lamp.
All primates require a source of vitamin C, which is added to commercial monkey biscuits. Most of the time, stable vitamin C is added to the pellet, meaning it will not undergo significant destruction within 6 months of milling. Supplementation is done, because the amount of vitamin C consumed via green vegetables, oranges, multiple vitamins, chocolate, fruit juice, or fruit-juice powders may not be sufficient.
Members of the subfamily Colobinae are perhaps the greatest challenge in proper feeding of captive primates. Pregastric fermentation, similar to that in ruminants, occurs in the complex stomach of these species. Howler monkeys (belonging to the subfamily Alouatta) have a hindgut fermentation comparable to horses. In the wild, leaves and high-fiber fruits and nuts form a major part of the diets. Therefore, natural diets are high in fiber, and animals spend much time foraging. Offering a rich, rapidly consumable diet of monkey biscuits and fruit (easily digestible sugars and starches) in captivity presents a situation quite different from that typically found in the wild, and it may frequently cause GI problems. In addition, some evidence suggests that a high percentage of colobus monkeys may be sensitive to starch and gluten.
Commercial, preferably gluten-free, high-fiber monkey biscuits (25%–50% neutral detergent fiber and up to 15%–35% acid detergent fiber) have been developed to feed captive colobines. A diet consisting of 30% of a palatable high-fiber biscuit and ≥70% green vegetables and fresh browse is recommended for most colobines. Only a high-fiber primate biscuit and browse is the preferable food for colobines. If the biscuit is not readily accepted, adding a limited amount of applesauce or banana flavor can increase palatability. Also, alfalfa pellets and a good-quality alfalfa hay can be provided in limited quantities. If a gluten-sensitive enteropathy is suspected, any product that contains wheat, barley, rye, or oats should be removed from the diet. In colobines, dietary changes always should be made gradually to allow their gastric microflora time to adapt.