Chapter 28: Nonhuman Primates

Primates are the order of mammals which includes humans and monkeys. Overall, the number of nonhuman primates (NHP) in research is very small, representing only about 0.33% of all laboratory animals used in the United States. NHPs are animal models primarily for cancer, HIV and neuroscience research, and vaccine development.

Despite the small numbers of animals used, these species are very important in research due to their similarities to humans. Nonhuman primates are informally divided into three main groups: prosimians, the most primitive group, of which lemurs and lorises are the best-known examples; New World monkeys, which are typically found in South and Central America; and monkeys and apes of the Old World. New World and Old World monkeys are the NHPs primarily used in research because of their similarity to humans. There is very little research using the great apes at this time. Most primate species have a well-developed social structure, are very intelligent, and can even use tools.1

The anatomy and physiology of NHPs is similar to humans. NHPs have well-developed brains. They have opposable thumbs and grasping hands, which provide them with increased dexterity compared to most other animals. They generally have flat, expressive faces with front-facing eyes. Most monkeys are diurnal. The difference between the two groups is shown in Table 28.2.

Old World monkeys (Figure 28.1A–D) have short noses with narrow nostrils that point in a downward direction. They have cheek pouches, formed by the folds in the skin of the mouth, which can be stretched to hold feed. Some of these monkeys have callous (ischial) pads on their buttocks. They generally lack prehensile tails, are often terrestrial, and are more closely related to the hominids (apes and humans) than to the other groups of primates.

New World monkeys (Figure 28.1E–H) have a broad, flat nose. The nostrils open in an outward direction. Some New World monkeys have prehensile (grasping) tails, which allow them to grasp and hang from tree limbs by their tails. Owl monkeys, the only nocturnal NHP in this group, have highly developed eyes; they are frequently used to study vision.

NHP sex determination is based on external genitalia, with males having a pendulous penis and scrotal sac, and females having a vulva and smaller anogenital distance. Most NHPs give birth to one offspring at a time, typically at night, and mothers usually care for their young, though abandonment can occur.

NHPs are social animals, and they benefit from contact and communication with other NHPs. One type of social activity among NHPs is grooming one another. Grooming also plays an important role in their breeding behaviors.

While there are some behavior patterns common to all species, NHPs generally exhibit body language, means of communication and behavior patterns specific to their species. An observant animal technician can tell a great deal about an animal’s mental and physical health from its body language as it relates to behavior involving sexual activities, aggression, territorial marking, and group interaction.

NHPs display aggression and submission through facial expressions and posturing. Technicians should be trained to recognize these behaviors and approach aggressive animals cautiously.

Safe handling and restraint of NHPs is essential. All NHPs are strong and agile, and must be approached with caution. Risk of injury and exposure to zoonotic diseases transmitted by a bite or scratch is minimized by appropriate restraint and appropriate PPE. Restraint can be accomplished using either physical or chemical means. Only animal care staff who understand the behavior of the species, are trained in the correct techniques, and who are using the correct restraint devices should handle NHPs.

Chemical restraint is frequently used in research with NHPs. Animals are first immobilized in a squeeze cage before being injected through the cage door with an anesthetic in the leg or arm. Once the animal is sedated, it can be removed from the cage. Carry the animal in front of you with its arms secured behind its back, being careful not to use too much force (Figure 28.3).

NHPs may also be handled using physical restraint, such as the pole and collar method. This method requires no sedation, and is typically used in conjunction with a restraint chair. The animal is fitted with a rigid collar that contains a ring. A pole is used to grasp the ring; the animal can then be guided from the cage to the chair (Figure 28.4). Animals must be trained in this method of restraint; positive reinforcement is effective for teaching the animals to cooperate in going to the chair and back to the cage.

Physical restraint of nonsedated animals may be adequate for small NHPs (generally up to 5 kg). However, safety should always determine the best method of handling NHPs. PPE is also of paramount importance when working with NHPs. PPE typically worn by personnel includes a gown or laboratory coat which covers the arms, disposable gloves, a face mask, and safety glasses or a face shield. Those individuals who restrain conscious monkeys may also use puncture-resistant gloves, often covered by leather gloves, to protect from bites and scratches. NHPs are inquisitive, and will grab anything, so animal technicians should be careful to keep pens and other small objects well concealed and out of reach.

Macaques can carry the fatal macacine herpesvirus 1 (B virus), which can be transmitted through bites, scratches, and contact with contaminated materials. Any potential exposure requires immediate treatment, including scrubbing the exposure site and reporting it to a supervisor.

The primary means of identification for NHPs is a tattoo (Figure 28.5). Tattoos are performed under anesthesia and may be applied to the chest, arm, or inner thigh. Collars with an identification tag attached may also be used to identify some species.

NHPs are housed in stainless steel cages with slatted or grid floors, allowing for climbing and exploration. Cages must be strong, secure, and meet space requirements for perches, climbing structures, and enrichment devices.

Commercial primate diets with extra vitamin C are essential for primate health, as they cannot produce it themselves. Fresh water should always be available, and watering devices must be checked daily.

Unlike NHPs living in the wild, the laboratory NHP has limited opportunity to participate in foraging, exploration, or other species-specific activities. This inactivity can lead to boredom, depression, maladaptive behavior, and possibly self-injury, and ultimately lead to poor experimental data. To maintain a stimulating environment and to comply with the AWA, animal care programs must institute a plan which will “promote the psychological well-being of primates.” Environmental enrichment programs can provide NHPs the opportunity to groom, to forage for treats, and to manipulate new objects. The only exceptions to this are if the NHP is determined by the veterinary staff to be exhibiting vicious or overly aggressive behavior, is debilitated by injury or illness, or has been excluded from the program due to justified scientific objectives that have been approved by the IACUC.

Enrichment for NHPs includes toys, foraging devices, swings, perches, hammocks, and socialization with other NHPs. Pair housing is ideal, but if not possible, NHPs should be able to see and hear other NHPs, or interact with facility personnel.

NHPs are adept at hiding pain, especially in the presence of humans, so it is important to be alert to subtle signs that may indicate that the animal is sick or in pain. Watching the animals from a distance or by video can help identify clinical signs of pain or distress. Loud, persistent vocalization may be a sign of pain, but may also mean the animal is alarmed or angry. Body language is a good indication of pain and distress. An affected animal may adopt a crouched posture with its arms crossed over its chest and its head forward; the facial expression may show discomfort. The animal might also appear ungroomed, may isolate itself from its cagemates, or stop eating.

An overdose of barbiturates given intravenously is a common method used for NHP euthanasia. The overdose may be administered while the animal is undergoing a nonsurvival surgical procedure or while it is sedated at the end of an experiment. As with other laboratory animal species, death must be verified. Your institution’s SOPs may require an additional procedure to be performed for this verification. Carcass disposal should be according to the facility’s guidelines.