Nutrition in Primates

Considerable variability in feeding and digestive strategies is found across primate taxa. Many primate species can be categorized based on feeding habits (eg, as folivore or insectivore). However, feeding strategies of wild primates are affected by their environment, which includes spatial and seasonal aspects. For dietary management of captive primates, the goal should be formulating a substitute diet in which nutritional requirements, foraging ecology, and gut characteristics of primates are considered.

Because specific information on natural diets is often incomplete or not known and natural food items are not commercially available, primates traditionally have been provided with diets of inappropriate or insufficient food items and/or nutrients. Captive frugivorous primate species have been provided with diets high in commercially available fruit for decades. Food items such as fruit in the natural diets of primates, however, differ considerably in nutritional content compared with commercially available fruits. Incorrect substitution of cultivated fruits in captive primate diets has therefore led to diets high in nonstructural carbohydrates and low in fiber, protein, and calcium; these deficits contribute to the many physical health problems seen in captive primates. Wild fruits more closely resemble cultivated vegetables, rather than cultivated fruit, and therefore herbivorous primate species should be fed a fruit-free diet. Moreover, several studies in lemurs have indicated that fruit-free diets not only are beneficial for physical health but also have a positive effect on behavior and welfare (1).

Nutrient requirements for nonhuman primates, including science-based information for the development of primate diets based on estimated minimal nutrient requirements, were published by the NRC. Based on their feeding ecology and digestive morphology, primates can be divided into several model categories in order to formulate proper feeding plans.

Primate diets should include a commercial pellet to provide adequate amounts of vitamins and minerals. Several primate pellets are commercially available, including high-fiber pellets specifically developed for New World monkeys (insectivorous primates or callitrichids). In addition, most primate species require a diet high in structural fibers, provided by a diet high in (green) vegetables and/or browse. Feeding greens and green vegetables, which are more comparable to the natural diet, should be encouraged. Monkey biscuits, high-fiber primate pellets, and canned products should comprise 20% of the dry-matter intake of gorilla and orangutan diets; fruits and treat items should comprise ≤ 10%, depending on the species, and green vegetables and browse should be at least 40% of the diet. Gorillas and orangutans should get browse as well; alfalfa hay can also be offered.

Dietary crude protein ranges for Pongidae, such as gorillas, are 14–16% and for New World primates, such as squirrel monkeys, 18–22%. The unsaturated fatty acid composition should be at least 2.5% (two-thirds n-6 fatty acids). Neutral detergent fiber (NDF) in dry matter ranges from at least 10% for macaques and marmosets to 20% for lemurs, chimpanzees, and gorillas and 30% for howler monkeys and langurs. Acid detergent fiber (ADF) ranges from 5% for macaques, marmosets, and tamarins to 10% for lemurs and Pongidae and 15% for howler monkey and langurs.

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 that diets for squirrel monkeys, marmosets, and tamarins should contain at least 2,400 IU vitamin D. Vitamin C levels 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.

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. In addition, New World primates require higher levels of dietary vitamin D. 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. 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. For larger primates, high-fiber biscuits should comprise at least 50% of 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 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.

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 diet specifically formulated for marmosets. Little (< 10%) to no fruit, which contains high levels of easily digestible sugars and which can cause diarrhea and obesity, should be fed, and moderate amounts of carrot, sweet potato, and apple, should be offered, depending on the species. Monkey biscuits, high-fiber primate pellets, and canned products should comprise 20% of the dry-matter intake of gorilla and orangutan diets; fruits and treat items should comprise ≤ 10%, depending on the species, and green vegetables and browse should be at least 40% of the diet. Gorillas and orangutans should get browse as well;. alfalfa hay can also be offered.

Obesity should be prevented; therefore, it is important to weigh animals regularly or, at minimum, to perform body condition scoring.

Energy-dense products, (eg, seeds, nuts, and several insects) should be fed 2 or 3 times a week at most, not every day. The energy amount of these enrichment items should not exceed 5–10% of the total energy consumed by the animal.

Traditionally, great apes were often provided with dairy and meat products. Based on primates' natural feeding strategies, those products should not be included in the diet. Moreover, because hypercholesterolemia occurs in many captive gorillas, feeding meat may be contraindicated. 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 (2). Most zoos have discontinued the feeding of mouse pups to these New World primates.

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 4 times the amount of vitamin D₃ required by other New World primates; consequently, commercially available marmoset diets contain high levels of vitamin D₃. Because of potential vitamin D toxicity, commercially available marmoset diets should be fed only to marmosets and not to other primates. If noncommercial mixtures of cut apples, bananas, and cereal products with added vitamins and minerals are fed to smaller primates, 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 UVB from sunlight, captive animals may rely entirely on a dietary source of vitamin D. Infants at weaning appear particularly at risk for rickets, because vitamin D levels in milk are probably quite low and many foods young primates begin to eat are not vitamin D–fortified. Exposing infants or juveniles to natural sunlight may be the best solution, because ensuring that a dietary supplement is consumed by a young primate may not be possible.

Because vitamin D can be stored in the body, vitamin D supplementation can potentially lead to toxicosis and should therefore be closely monitored. Whereas excessive solar UV radiation exposure does not lead to toxic vitamin D concentrations, excessive oral intake can cause toxic concentrations. The maximum daily intake of 4,000 IU for human adults is often used as a guideline in primates.

In captive populations of marmosets and other callitrichids, marmoset wasting sydrome is a major problem. Clinical signs include severe weight loss, muscular atrophy, and chronic diarrhea, resulting in high mortality rates. Marmoset wasting syndrome seems to be multifactorial, and several hypotheses exist concerning its causes. These include dietary factors (eg, gluten intolerance), stress, and poor husbandry.

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 sufficient. Lights that emit energy in the UVB range can be practical for use with primates, provided precautions are taken to prevent primates from being able to touch UVB lamps.

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 pellets, meaning it will not degrade substantially within 6 months of milling. Vitamin C supplementation is done because the amount of vitamin C primates consume via ingestion of green vegetables, oranges, multivitamins, fruit juice, or fruit juice powders may not be sufficient.

Folivorous primate species, primates whose natural diets consists primarily of leafy material, have highly adapted digestive systems. Feeding inappropriate diets (eg, diets high in nonstructural carbohydrates) to these primate species has resulted in many health problems from GI disorders. Dietary management of folivorous primate species should therefore aim to increase dietary fiber and limit the amount of easily fermentable carbohydrates.

Members of the subfamily Colobinae (colobus monkeys, red colobus, and langurs) are folivores and perhaps represent 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 (subfamily Alouatta) have a hindgut fermentation comparable to horses. In the wild, howler monkeys spend a great deal of time foraging, and leaves and high-fiber fruits and nuts form a major part of their high-fiber diets. Offering these monkeys rich, rapidly consumable monkey biscuits and fruit (containing easily digestible sugars and starches), a diet radically different from what they typically consume in the wild, can frequently cause GI problems. In addition, some evidence suggests that a high percentage of colobus monkeys may also be sensitive to dietary starch and gluten (3).

Commercial, preferably gluten-free, high-fiber monkey biscuits (25–50% NDF and up to 15–35% ADF) have been developed to feed captive colobines. A daily diet of 10–20% of a palatable high-fiber biscuit, ≥ 70% green vegetables, and high amounts of fresh browse is recommended for colobines. Only high-fiber primate biscuits, rather than typical lower-fiber commercial monkey biscuits, are recommended 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 should always be made gradually to allow their gastric microflora time to adapt.

Wild-living primates spend a large proportion of their active time foraging and feeding. Because captive primate species are presented with limited food selection, feeding management of captive primates should aim to stimulate feeding behavior. Therefore, food should be presented in such a way that primates must work to access it (eg, trough feeding devices). In addition, increasing the number of feeding moments per day or cutting the size of food items can also help stimulate feeding behavior.