4.1. Barren Cage

Solitary imprisonment is a dreaded punishment for human primates who suffer from apathy, depression, frustration and behavioral pathologies when they are kept alone on a long-term basis (Figure 6a; Scott and Gendreau, 1969; Sluga and Grünberger, 1969; Wells, 1974; Bach-Rita, 1974; Yaroshevsky, 1975; Walters et al., 1963; Grassian, 1983; Suedfeld, 1984; Grassian and Friedman, 1986; Gamman, 1995; Andersen et al., 2000; Andersen et al., 2003; Arrigo and Bullock, 2007). It stands to reason that nonhuman primates, who are also highly evolved social creatures, suffer when they are forced to live permanently alone in barren cages.

Seeing the inside of a primate research facility for the first time was a shocking experience for me, not only as a psychologically healthy person but also as a scientist who has been trained to rigorously control extraneous variables that might influence research data. There were hundreds of animals kept in barren single-cages with nothing to do but stare at bleak walls and wait for their turn to be subjected to life-threatening procedures (Reinhardt and Reinhardt, 2001).


Figure 6a,b Solitary imprisonment is distressing not only for human primates (a) but also for nonhuman primates (b).

4.1.1. Signs of Distress and Impaired Well-Being

Being permanently imprisoned in a barren cage is distressing and impairs the well-being of nonhuman primates for the following reasons:

  1. Primates have a biological need for companionship (Figure 5). Without other conspecifics, a monkey or ape has no chance of long-term survival in the wild. To be with and interact with at least one companion is a fundamental condition for the well-being of primates. When they are kept alone on a permanent basis primates tend to:
    (a) suffer from apathy, depression (Figure 6b; Luck and Keeble, 1967; Erwin and Deni 1979; Lilly et al., 1999), extreme boredom and frustration (Figure 7) resulting in the development of compulsive hair-pulling and self-biting (Figure 2 & 4a,b; Erwin et al., 1973; Gluck and Sackett, 1974; Anderson and Chamove, 1981; Russell and Russell, 1985; Line et al., 1990; Watson, 1992; Platt et al., 1996; Lutz et al., 2000a; Kaufman et al., 2002; Marshall et al., 2002; Tully et al., 2002; Novak, 2003; Baumans et al., 2007), and
    (b) become more susceptible to disease (Shively et al., 1989; Reinhardt, 1990a; Schapiro and Bushong, 1994; Poole et al., 1999).

    Figure 7 Hatty has been imprisoned in a barren cage for many years. The hyper-aggressive gesture suggests that Hatty is frustrated of being unable to change her species-inappropriate living condition.

  2. In their natural habitat, nonhuman primates spend a major portion of the day foraging (Figure 8). They have a biologically inherent need to do so; it keeps them alive. Even though primates kept in research laboratories have no real need to forage, since their daily food ration is usually freely presented, they are strongly motivated to work for their food anyway. Experiments conducted with gibbons (Markowitz, 1979), stump-tailed macaques (Anderson and Chamove, 1984; Washburn and Rumbaugh, 1992; O'Connor and Reinhardt, 1994; Chamove, 2001), long-tailed macaques (Evans et al., 1989; Watson et al., 1999), rhesus macaques (Line et al., 1989; Reinhardt, 1994a), chimpanzees (Menzel, 1991), vervet monkeys (Pastorello, 1998) and marmosets (de Rosa et al., 2003; Bjone et al., 2006) have revealed that the animals will spend a considerable amount of time and effort to retrieve food that is hidden behind a barrier, even though the same food is also freely accessible next to them. From this it can be inferred that they are highly motivated to forage, with the engagement in foraging activities serving as primary reinforcement.

    Figure 8 In their natural habitat, baboons and all other nonhuman primates spend a major portion of their time foraging, i.e., searching for, retrieving and processing food. Photo by Malino on Flickr.

    Foraging has a distinct survival value for primates. Therefore, it can be assumed that the animals' well-being is promoted when they are given the opportunity to engage in food searching, processing and food retrieving activities.

    Figure 9a-b Nonhuman primates are arbnoreal animals; a) Vervet monkeys; Photo by Doug Mons on Flickr; b) Rhesus macaques; Photo by Natash Secret on Flickr.


  3. In the wild, nonhuman primates spend the night and a major portion of the day well above the ground in trees, on rocky outcroppings or cliffs. Access to the vertical dimension is a basic condition for them to escape and to be safe from predators during periods of affiliative and playful social interaction, rest and sleep (Figure 9a-c). Most primates also forage in trees (Figure 9d). Without access to the vertical dimension, they are restricted to a terrestrial lifestyle to which they are not adapted (Figure 10).

    Figure 10 Nonhuman primates are not adapted to a terrestrial lifestyle, yet these rhesus macaques are imprisoned in a bottom-row cage without elevated structure.

    When they are confined in barren cages with no possibility of retreating to a high, safe place, nonhuman primates are literally cornered when they are approached by human primates who, after all, are their natural predators. This common situation is likely to distress the animals in research laboratories on a daily basis (Figure 11a,b).


    Figure 11a,b This female rhesus macaque feels distressed because a fear-inducing investigator (a) is approaching her cage and she has no option to retreat to a high, quasi-safe refuge (b).

4.1.2. Refinement

Refining aspects of housing, husbandry, enrichment and socialization helps alleviate or prevent distress (National Research Council, 2008, p 55). Companionship

In the wild, primates benefit from each other's survival skills, such as avoiding predators, fleeing from predators, and finding species-appropriate food-stuff. A socially isolated primate would have no chance of long-term survival. Primates have a strong need for companionship. Taking the example of capuchin monkeys, it has been demonstrated that the animals perceive a companion as a necessity at a level similar to that of food (Dettmer and Fragaszy, 2000). Their social disposition is underscored by the observation that individually caged animals often try to touch and interact with their neighbors, despite substantial physical restriction and no visual access (Chamove, 1989, Figure 7; Baker, 1999).


Figure 12 In their natural habitat, macaques spend a major portion of their time grooming each other. Photo by Keithng on Flickr.

Studies of wild populations indicate that Old World primates spend 5 to 25 percent of the day interacting with each other, with grooming being the prevalent social activity (Figure 12; long-tailed macaques: Leon et al., 1993; McNulty et al., 2004; rhesus macaques: Lindburg, 1971; Teas et al., 1980; Chopra et al., 1992; Japanese macaques: Hanya, 2004; chimpanzees: Wrangham, 1992; baboons: Hall and De Vore, 1965). Comparative data on New World primates have yet to be published. Previously Single-Housed Animals Can be Transferred to Social-Housing Arrangements Without Undue Risks
Line et al. (1990) established four pairs of previously single-caged adult female long-tailed macaques (cynomolgus macaques, Macaca fascicularis) by introducing the potential companions in double-cages without any preliminaries. All four pairs were compatible and no fighting occurred during a two-week follow-up period.

Crockett et al. (1994) pre-familiarized the partners of 15 adult female and 15 adult male long-tailed macaque pairs via transparent cage dividers, allowing visual but not physical contact. After two weeks, pairs were formed by removing the divider. On the first day of introduction, partners were separated after 90 minutes. On each of the next 12 days, they were housed together for seven hours and separated during the remaining 17 hours to allow for collection of urine samples. Under these circumstances, only 53 percent of the male pairs turned out to be compatible. Within the first two weeks, 47 percent (7/15) of them had to be separated because of repeated fighting and serious lacerations. None of the female pairs had to be separated; they were all compatible.

Lynch (1998) applied a less disruptive pair formation strategy to 34 adult male long-tailed macaques. Potential partners were also first given the opportunity to get to know each other during a non-contact familiarization period, but they were introduced to each other - in a different double-cage to avoid possible territorial antagonism - only after they had established a dominance-subordinance relationship. Once paired, they were allowed to stay together uninterruptedly throughout the day and night. Under these conditions, 94 percent (16/17) of the pairs turned out to be compatible over follow-up periods of 12 to 42 months (Figures 13). Serious fighting at the time of introduction occurred in only one incompatible pair.


Figure 13 Long-tailed macaques Ted and Tom have lived together as compatible companions for more than three years. Photo by Richard Lynch.

Clarke et al. (1995) established a trio of previously single-caged adult male long-tailed macaques by:

  1. Exposing each subject to a mirror to provide an intermediate form of social stimulation during a two-week period.
  2. Exposing each male to each other in a pair-wise arrangement that allowed visual, auditory and olfactory access to each other, but no opportunity for physical contact during a two-week period.
  3. Introducing the three males into a group cage, one at a time, in rapid succession.

The formation of the trio was not associated with serious fighting. Group members spent much of the time grooming each other during the first two weeks, and relationships between them appeared to be relaxed. The primarily affiliative and submissive behaviors shown by the three males suggest that they were able to establish a dominance hierarchy and peaceful relations quickly and easily. They were living peacefully together during a follow-up period of three years.

Byrum and St. Claire (1998) established 12 pairs of previously single-caged adult female pig-tailed macaques (Macaca nemestrina) after partners had established dominance-subordinance relationships during a one-week non-contact familiarization period. No injurious fighting occurred, neither at the moment of introduction nor during a two-year follow-up period.
Gust et al. (1996) released eight previously single-caged adult female pig-tailed macaques and one adult male simultaneously into a compound and encountered no problems. The animals established dominance-subordinance relationships within the first week without engaging in overt aggressive interactions.


Figure 14 Food sharing is one factor that distinguishes rhesus macaques Sissi and Jill as compatible companions five years after pair formation.

Reinhardt et al. (1988a) placed previously single-caged adult female rhesus macaques (Macaca mulatta) pair-wise in double-cages, with partners being separated from each other by a wire mesh partition permitting non-contact communication. The animals were familiarized in this manner for seven days. Partners were paired on day eight only if they had not been seen threatening each other across the grated partition. The actual introduction then took place in a different double-cage to avoid the risk of possible territorial antagonism. A total of 27 dyads were tested. Partners threatened each other during the familiarization situation in nine (33 percent) cases. Reciprocal threatening was not witnessed in the other 18 dyads and the partners were, therefore, paired with each other. They were compatible in 83 percent (15/18) of cases during a follow-up period of five to six years. Absence of serious aggression, as well as food sharing distinguished partner compatibility (Figure 14); this implied that subordinate animals showed the same body weight gains, as did their dominant partners (Reinhardt et al., 1988b). Pairs were incompatible in 17 percent of cases, with one animal inflicting a serious injury on the other in one case, and one partner showing signs of social distress in the other two cases. These three dyads were permanently separated on days four, five and 15, respectively.

Subsequent work with female and male rhesus showed that the two partners of compatible pairs do not differ in their serum cortisol concentrations, indicating that living with a compatible companion does not constitute a distressing situation for either the subordinate or the dominant partner (Figure 15; Reinhardt et al., 1990a; Reinhardt et al., 1990b). The same findings have been made in squirrel monkeys (Gonzalez et al., 1982), and they may apply to all other primate species when animals are housed on a long-term basis as compatible pairs.

Figure 15 Mean serum cortisol concentrations of the dominant and subordinate partner of five compatible male and five compatible female adult rhesus macaque pairs. The animals were trained to cooperate during venipuncture; blood samples were taken from the males at 12:00 and from the females at 13:15 (Reinhardt et al., 1990b).

Eaton et al. (1994) applied a similar pre-familiarization technique with female rhesus. Of 21 pairs tested, 86 percent (18/21) were compatible throughout a follow-up period of more than three years, 14 percent of the pairs were incompatible and had to be separated because of serious fighting during the first hour (two cases) or after three months (one case). The partners of compatible pairs spent 40 percent of the time during the day in close proximity, and 80 percent of the time during the night. They did not show any differences in body weight gains, clinical morbidity, reproduction and immune response. This suggests that subordinate animals were not hindered by their dominant companions to obtain the appropriate share of the daily food ration, nor was their health and general well-being jeopardized by their dominant cage mates.

In order to minimize the potential risk of injurious fighting, Reinhardt (1989a) refined this pair formation protocol for adult rhesus males by making it a condition that potential partners must establish a dominance-subordinance relationship during non-contact familiarization, so that they will have no reason to fight over dominance when they are introduced to each other. Seven pairs were tested. Two of them failed to establish a clear-cut dominance-subordinance relationship. Five did establish such a relationship, with one of the partners showing unidirectional submissive gestures. When the partners of these five pairs were introduced to each other in a different double-cage, not a single incident of fighting occurred, but the animals reconfirmed their already established rank positions with subtle gestures involving no physical contact (Figure 16).


Figure 16 Rhesus macaque Mike grooms his dominant cage mate Bob after they have reconfirmed their rank relationship with subtle gestures.

This pair formation technique was subsequently implemented at a research facility as a standard procedure for adult rhesus macaques, including 24 to 35 years old animals (Reinhardt, 1991b; Figure 17). When 77 female pairs and 20 male pairs were established on this occasion, fighting occurred in only 2 percent of the 97 pairs: two female pairs, no male pair (Reinhardt, 1994b).

Figure 17 Twenty-six-year-old Sissa grooms her 35-year-old companion Senila shortly after pair formation. These two aged rhesus macaques have lived most of their lives alone in barren single-cages.

Doyle et al. (2008) familiarized the potential partners of four adult rhesus macaque pairs in cages in which partners were separated by a panel consisting of bars spaced 2 cm apart. The eight males were all implanted with biotelemetry devices for remote heart rate monitoring. After 24 hours, as neither persistent aggression nor wounding was observed, each pre-familiarized pair was introduced into full contact by removing the barred panel. All four introductions were successful and subjects showed no physiological (fecal cortisol concentration and heart rate) or behavioral signs (pathological behavior) of stress, or psychological indices of distress (depressive/anxiety-related behavior) not only during the introduction process but also over a follow-up period of 18 months. No overt aggression was displayed at all during the first two hours following pair formation. Aggressive interactions were minimal thereafter. Only one bite laceration was incurred 14 weeks after pair formation. The partners of this pair were maintained in the home cage with the barred panel to allow wound healing; they were subsequently placed again into full contact with no further complications.

Roberts and Platt (2005) paired adult rhesus males who had cranial implants. Potential companions were familiarized and their compatibility was carefully evaluated over a period of five weeks. In order to be physically introduced in the same test cage, partners had to establish a clear-cut dominance-subordinance relationship during the first week, when the animals were separated by transparent cage dividers. During the next four weeks, partners were allowed to live together intermittently for progressively longer periods of time. After the fifth week, they finally lived together continuously. Of 13 pairs tested in this manner, 92 percent (12/13) were compatible. Only one pair was deemed incompatible because of continued non-injurious aggression during the sixth week. This pair was separated.



Figure 18a,b Rhesus macaques Matt (a) and George (b) hold and huddle their infant cage mates Jimmy and Billy, who show no signs of depression. Both males are very protective of their little companions; they yawn because they feel uncomfortable being observed.

Reinhardt et al. (1987) and Reinhardt (1991) examined the practicability of pairing adult rhesus macaques with infants. Naturally weaned, 12 to 18 months old infants of both sexes were removed from two breeding troops to avoid overcrowding and placed, without any preliminary precautions, pair-wise with unfamiliar single-caged, 7 to 33 years old adults of both sexes. A total of 40 pairs were tested: 12 adult female-infant female pairs, 11 adult female-infant male pairs, 11 adult male-infant male pairs, and six adult male-infant female pairs. The pairs were compatible in 92 percent (37/40) of cases with:

Figure 19a-d Adult rhesus macaque Cora allows her infant companion Gina get her share of food treats. Note that Gina has a cranial implant.

Compatibility was dependant neither on the sex of the adult and infant, nor on the age of the adult partner. Three pairs were incompatible. One female grabbed the female infant immediately upon her arrival; she continued to do this repeatedly during the next 30 minutes, after which the infant was removed. One male bit the female infant on the fourth day of introduction. The youngster was slightly injured, although not bleeding. When the infant started to consistently avoid the adult, the pair was split. Another male often grabbed his male infant companion, even though he gently groomed him and the two huddled with each other regularly. Gradually, however, the infant showed more and more avoidance behavior, and the two were finally separated after nine days.

Several attempts have been described to transfer single-caged adult rhesus macaques to compatible group-housing arrangements, but none of them were successful enough to be recommended as a safe standard procedure. Whether future group members are strangers or have been carefully pre-familiarized with each other, and whether they are introduced simultaneously or sequentially as a new group, vicious and even deadly fighting and persistent aggressive harassment seem to be unavoidable (Bernstein and Mason, 1963; Erwin, 1979; Jensen, 1980; Line et al., 1990a; Reinhardt, 1991b; Clark and Blanchard, 1994).

Reinhardt (1994c) transferred ten adult female and six adult male stump-tailed macaques (Macaca arctoides) from single-housing to isosexual pair-housing by first allowing potential partners to establish dominance-subordinance relationships without risk of injury during a three-day non-contact familiarization phase, and then introducing them to each other in a new home cage. All five female and all three male pairs established clear-cut dominance relationships while they were familiarized with each other. Following subsequent introduction, all eight pairs showed signs of compatibility. Female partners reconfirmed their rank relationships within 30 minutes with subtle gestures, never by overt aggression. Male partners engaged in hold-bottom rituals (de Waal and Ren, 1988) upon being introduced to each other. The partners of two pairs reconfirmed their rank relationships within 30 minutes with gestures, while the third pair resorted to a brief non-injurious dominance reconfirming fight which was followed by another reconciliatory hold-bottom ritual. All eight pairs remained compatible, with no signs of injurious aggression throughout a six-month follow-up period (Figure 20).


Figure 20 Stump-tailed macaques Roger and Paul get along well with each other six months after pair formation.

Bourgeois and Brent (2005) established four pairs and two trios of previously single-caged 3 to 4 years old male baboons (Papio sp.) by sedating potential companions and having them wake up together in the same cage. Rough-and-tumble wrestling occurred and dominance positions were quickly established, with all disputes followed by bouts of grooming. Transfer to social-housing was successful in each instance, and no injuries or overt aggression were observed during a follow-up period of two weeks (Figure 21).


Figure 21 These three baboons are a compatible trio. Photo by Edgar Thissen on Flickr.

Fritz and Fritz (1979) and Fritz (1994) developed a protocol to introduce previously single-caged chimpanzees (Pan troglodytes) to unfamiliar peers. The newcomer is first moved into a specially designed social unit and kept next to the cage of a selected member of an already established group. The two chimpanzees have full olfactory, visual and auditory contact as well as limited tactile contact. The selected group member is moved in as a cage mate for the newcomer as soon as friendly interactions through the separating cage mesh are consistently observed. After several days, another group member is introduced to the pair in this same way, then another is introduced to the trio, and so on until the newcomer has met all members of the group and is then fully integrated. A total of 59 of 60 chimpanzees - of both sexes and all age classes - were successfully re-socialized to compatible group-living in this manner without a single incidence of serious fighting (Figure 22; Fritz, 1989).


Figure 22 Living in a compatible group allows previously single-caged chimpanzees to express their social needs. Photo by Wayne/Flickr.

Gwinn (1996) used a pole-housing system to identify compatible adult male squirrel monkeys (Saimiri sp.) before introducing them as pairs:

Pole-housing allows several primates to interact or retreat to safety. First the animals are habituated to collar, leash and pole. During this time, the animals cannot physically interact with others. When they have adapted to the pole system, they are moved closer to one another. They are observed for aggression or fighting at frequent time intervals. When two animals exhibit compatibility, having been observed interacting positively for one week, they are pair housed. Eight monkeys are currently housed as pairs.

The percentage of pairs exhibiting compatibility in the pole-housing arrangement is not indicated. Compatible Companionship Enhances Well-Being by Addressing the Need for Social Contact and Social Interaction
Compared to wild animals, captive pair-housed primates spend more time engaged in social activities - especially grooming each other - probably because there is little else for them to do.

Reinhardt and Reinhardt (1991) kept 15 adult female rhesus macaque pairs in double-cages that were each equipped with a privacy panel allowing the partners to stay in different halves of the cage without maintaining visual contact with each other. During one-hour observations, companions spent 76 percent of the time in the same half of the cage. Obviously, they had a need for companionship and preferred not be alone, even though this implied a relative reduction of the available cage space. They were engaged in grooming and hugging each other on average 37 percent of the time.

Basile et al. (2007) observed 25 adult female rhesus pairs in double-cages with privacy panels for two 30-minute sessions. Companions spent 52 percent of the time in the same half of the cage, and they engaged in affiliative interactions 24 percent of the time.

Eaton et al. (1994) established 11 pairs of adult female rhesus macaques and recorded their behavior during 10-minute sessions, three times per week during a six-month period. Companions spent on average 35 percent of the time engaged in species-typical social behavior, with grooming being the predominant interaction (31 percent). There was no indication that companions lost interest in each other over time.

Ranheim and Reinhardt (1989) took two 30-minute behavioral records of six pairs of adult female rhesus macaques who had lived together for 30 months. Companions spent on average 35 percent of the test sessions interacting with each other, primarily in the form of grooming (30 percent). Apparently, partners had not become bored with each other during the two and a half years of uninterruptedly living together in the same cage.

Reinhardt and Hurwitz (1993) paired three 30 to 35 years old female rhesus macaques - who had lived most of their lives alone - with compatible adult female partners. During three one-hour sessions conducted 16 months after pair formation, the three aged animals were grooming and hugging their companions on average 29 percent of the time (Figure 17).

Baker (2007) observed 13 adult male rhesus pairs during 12 half-hour sessions. Partners spent an average of 18 percent of the time in affiliative interactions.

Line et al. (1990a) formed five pairs of adult female long-tailed macaques. During approximately seven hours of observation distributed over the first two weeks, partners spent approximately 31 percent of the time grooming each other.

Crockett et al. (1994) recorded the behavior of 15 female and 8 male pairs of adult long-tailed macaques 13 days after the pairs were formed. During a 90-minute test session, female companions spent an average of 35 percent of the time while male companions spent an average of 17 percent of the time grooming each other (Figure 13).

Reinhardt (1994c) established five pairs of adult female and three pairs of adult male stump-tailed macaques. During one-hour observations conducted six months later, females interacted with each other on average 24 percent of the time, males interacted with each other 17 percent of the time (Figure 23a,b). Grooming (77 percent) and hugging (22 percent) were the salient social activities.

 Figure 23a,b Stump-tailed macaques Claudia and Clara are engrossed in reciprocal grooming. Companionship Buffers Fear and Anxiety
Like human primates (Arsenian, 1943; Schachter, 1959; Wrightsman, 1960), nonhuman primates have a reassuring, anxiety-reducing effect on each other in distressing situations.

Rowell and Hinde (1963) exposed 17 rhesus macaques of both sexes and all age classes to a mildly stressful situation, i.e., being looked at by a person with a grotesque mask, for three minutes alone or with several familiar group members. When they were tested alone, the animals showed significantly more signs of fear (threatening, hair raising), anxiety (yawning) and tension (scratching), than when they were exposed to the stressor in the company of other monkeys.

Gunnar et al. (1980) captured five infant rhesus macaques from their social group and placed them in an unfamiliar environment for 24 hours, either alone or with another infant from the same group. When tested alone, the animals exhibited significantly more signs of distress (agitation and distress vocalization) than when they were tested with a companion, indicating that the companion had a stress-buffering effect.

Mason (1960) placed 12 infant rhesus macaques into a strange environment, either alone or with another familiar or unfamiliar same-aged peer. Subjects showed significantly fewer signs of emotional disturbance (crouching and self-clasping) when they were tested in the company of another monkey. The distress-buffering effect was not dependent on the familiarity of the accompanying partner.

Due to repeated traumatic experiences with humans, caged monkeys often become alarmed when a person enters the room (Malinow et al., 1974; Manuck et al., 1983; Hassler et al., 1989; Arluke and Sanders, 1996; Capitanio et al., 1996; Schnell, 1997; Bowers et al., 1998; Boinski et al., 1999; Crockett and Gough, 2002; Lueders, 2004). During such frightening situations, paired animals often exhibit behavioral responses that suggest that they reassure and calm one another (Figures 24a-c).

Figure 24a-c Rhesus macaques Bobby and Circle comfort each other while an investigator catches another animal of the room for an experimental procedure.

Hennessy (1984) observed eight pair-housed squirrel monkey infants when they were transferred to an unfamiliar cage alone or with the companion. The animals vocalized significantly less when they were tested together, suggesting that the companion moderated the fear response to the unfamiliar environment.

Coe et al. (1982) confronted 14 male adult squirrel monkeys for 60 minutes with a snake behind a mesh barrier and noticed that the animals' behavioral distress responses (alarm vocalization, fear reactions and agitation) were significantly buffered when they were tested in company of another male than when they were tested alone. Companionship Buffers Physiological Distress
The physiological stress and distress response to challenging situations is mitigated by a social partner in human primates (Kissel, 1965; Epley, 1974; Lynch et al., 1977; Witcher and Fisher, 1979; Drescher et al., 1980; Kamarck et al., 1990; Gerin et al., 1992; Lepore et al., 1993; Gerin et al., 1995; Kirschbaum et al., 1995; Uchino et al., 1996; Christenfeld et al., 1997; Thorsteinsson et al., 1998; Fontana et al., 1999; Gallo et al., 2000; Uno et aal., 2002). This seems to be true also for nonhuman primates.

Vogt et al. (1981) exposed 24 adult squirrel monkeys, who lived in four heterosexual groups, to a caged snake alone versus in the company of the other group members. The adrenocortical activation evoked by such a potent fear stimulus was significant when the animals were tested alone, but it did not occur when they were tested as a group.

Gonzalez et al. (1982) exposed six single-housed and six pair-housed adult female squirrel monkeys to the stress of capture followed by anesthesia and cardiac puncture, and found that the 30-minute plasma cortisol increment was significantly lower in subjects housed with a companion (38 percent) than in subjects housed alone (60 percent).

Coelho et al. (1991) measured blood pressure via arterial catheter implants of four tethered adult male baboons who were kept in a test room either alone or in a double-cage in which they had visual, tactile and auditory contact with a familiar companion through a wire mesh partition. Mean resting blood pressures were consistently lower when the baboons were able to interact with a neighboring baboon, suggesting that companionship buffered distress arising from imprisonment in an unfamiliar environment (Figure 25).

Figure 25 Mean arterial blood pressures of four tethered baboons when caged alone (red line) versus with social contact (blue line) in an unfamiliar environment (Coelho et al., 1991).

Doyle et al. (2008) assessed fecal cortisol levels and monitored heart rates of eight adult biotelemtry device-implanted male rhesus macaques (a) after they had lived alone in single-cages for several months and (b) after they were paired with each other and had lived together for more than four months. Both stress/distress parameters were significantly lower in the pair-housing versus the single-housing condition, indicating that the males experienced less distress in the company of another male than when they lived alone.

Gust et al. (1994) transferred seven adult female rhesus monkeys from their group to an unfamiliar environment, either alone or together with a preferred group member. During both conditions, subjects were initially equally distressed, as measured in alterations of cell-mediated immune parameters, but they recovered significantly quicker when they had the social support of a companion.

Drug testing can be a distressing experience that is often reflected in the subjects' gradual loss in body weight. Gwinn (1996) noticed during nine treatments with an identical test compound that adult male squirrel monkeys lost significantly less weight when they were caged with a companion (n=4) than when they were caged alone (n=4).

It has been demonstrated in some species, especially human primates, that contact with friendly individuals of another species can have a calming, stress- and disterss-buffering effect (Gantt et al., 1966; Lynch and Gantt, 1968; Lynch et al., 1974; Astrup et al., 1979; Hemsworth et al., 1981; Friedmann et al., 1983; Baun et al., 1984; Wilson, 1987; Vormbrock and Grossberg, 1988; Siegel, 1990; Allen et al., 1991; Barnett et al., 1994; Pedersen et al., 1998; Allen et al., 2001; Allen et al., 2002; Barker et al., 2005; Coppola et al., 2006; Cole et al., 2007) and enhance resistance to pathophysiological processes (Friedmann et al., 1980; Nerem et al., 1980; Todd-Schuelke et al., 1991/92; Anderson et al., 1992; Friedmann and Thomas, 1995; Craig et al., 2000; Cole et al., 2007).

There seems to be a general consensus that positive contact - not necessarily tactile contact - with personnel has a stress-mitigating effect on nonhuman primates in research laboratories (Figure 26; Anchel, 1976; Wolfle, 1987; Institute for Laboratory Animal Research, 1992; Canadian Council on Animal Care, 1993; National Research Council, 1998; American Association for Laboratory Animal Science, 2001; Bayne, 2002; Prescott, 2002; Primate Research Institute, 2003; Abney et al., 2006; Baumans et al., 2007). Studies have yet to be published to provide supportive data for this very plausible assumption.


Figure 26 Regular affectionate interaction with attending personnel fosters a trust-based human-animal relationship that is likely to help the animal subject cope with distressing situations, such as being chair-restrained during a neurophysiological experiment. Companionship Promotes Health
Schapiro and Bushong (1994) examined the health records of 98 rhesus macaques who were 1 to 2 years old when they were individually caged; they were 2 to 3 years old when they were subsequently kept in opposite-sex pairs; they were 3 to 4 years old when they were finally kept as breeding groups or male-only groups. Veterinary treatments were necessary:

The incidence of veterinary treatment was conspicuously low when the animals were pair-housed. This was probably related to the fact that pair housed monkeys required significantly fewer veterinary interventions for diarrhea than did single or group housed monkeys (Schapiro et al., 1997, p 147), and fight injuries requiring treatment were relatively common when the animals lived in groups. In a subsequent study, Schapiro et al. (2000) compared the cell-mediated immune response of 12 adult rhesus macaques who lived either alone, in pairs, or in breeding groups. Based on significant differences in the animals' immunological responses, it was contended that strong social relationships, particularly the affiliative interactions that characterize pair housed monkeys, may diminish the likelihood of severe infection with potentially diarrhea-inducing agents (p 79).

Reinhardt (1990) assessed the clinical records of a rhesus macaque colony consisting of 237 single-housed and 382 pair-housed animals of both sexes and all age classes. The incidence of non-research-related veterinary treatment was more than twice as high for single-caged than for pair-housed animals (Figure 27), indicating that the animals were healthier when they lived with a companion.

Figure 27 Percentages of a colony of 237 single-housed and 382 pair-housed rhesus macaques requiring veterinary treatment in the year 1989 (Reinhardt, 1990a).

Shively et al. (1989) compared clinical data of female long-tailed macaques consuming an atherogenic diet and housed either alone (n=15) or with three or four other females (n=24). The extent of atherosclerosis was four times greater, on average, in females who lived alone than in those living with social companions (Figure 28). These findings corroborate with human primate studies indicating that lack of social support (House et al. 1982) is associated with an increased risk of coronary heart disease (Manuck et al., 1986; Lynch, 1987; Shumaker and Czajkowski, 1994) and other health issues (Kaplan et al. 1977; Berkman, 1985; Cohen and Syme, 1985; Broadhead et al., 1983; House et al., 1988; Christenfeld and Gerin, 2000; Hays et al., 2001; Spiegel and Sephton, 2001; Richmond et al., 2007).

 Figure 28 Mean coronary artery atherosclerosis extent as measured by intimal area in group-housed and single-housed adult female long-tailed macaques (Shiverly et al., 1989). Companionship Alleviates or Eliminates Behavioral Pathologies
In a colony of about 650 mother-reared, single-caged adult rhesus macaques, self-biting was witnessed in four males and three females. This behavior pattern was predictably exhibited whenever one of the subjects was approached by personnel; the animal would show signs of intense excitation and start repeatedly biting a particular body part while staring and/or charging at the person. The self-biting resulted in no visible trauma in one female and two males; two females showed abrasions on the bitten hand; two males required surgical treatment, one of a lacerated thigh, the other of a lacerated arm. All seven subjects were successfully transferred from single- to compatible pair-housing arrangements with same-sex adult partners (six cases) or with an infant (one case). This had a therapeutic effect in all seven subjects: The conspicuous excitation and self-biting in the presence of personnel was abandoned immediately on the day of pair formation by three animals, or gradually within two months by the other four animals (Figure 29a-c). This pathological behavior pattern was no longer witnessed thereafter in any of the seven subjects (Reinhardt, 1999).

Figure 29a-c Rhesus macaque Paul (left) required two surgeries on self-inflicted bite lacerations (a,b). Being paired with Peter cured Paul of this behavioral pathology (c). In the course of a three-year follow-up period, Paul has not engaged in any noticeable self-biting.


Weed et al. (2003) vasectomized six single-caged rhesus males, who engaged in persistent self-injurious biting, and paired them with adult females. Three of these males stopped the self-biting after being transferred to social-housing, and self-biting was no longer noticed during a one to six-month follow-up period. Socialization had a moderating but not healing effect in the other three males.

Alexander and Fontenot (2003) established 19 isosexual groups with 80 previously single-caged adult male rhesus macaques. Thirty-one (39 percent) of these animals had at least one prior incidence of self-injurious biting. During the year before group formation, the clinical history of the subjects included a 13 percent incidence of self-biting requiring wound care. No self-biting was noted during the first four months after the groups were formed.

Line et al. (1990a) paired five long-tailed macaques, who had a history of self-biting, with compatible female companions. Pair-housing corrected the behavioral problem and no further self-biting occurred in the course of a five-month follow-up period.

Reinhardt et al. (1987) transferred an adult female rhesus macaque from single-housing to pair-housing with a surplus infant from a breeding troop. While she was caged alone, Chewy predictably chewed and bit her left thumb whenever she was approached by personnel. She stopped this compulsive behavior within the first month of living with her companion Cute (Figure 49b), and she did not resume it during a one-year follow-up period.

Baumans et al. (2007) refer to a case of three self-biting adult male rhesus:

The animals were treated with various drugs - diazepam, fluoxetine, guanfacine - which did alleviate but not eradicate the self-biting. Once the treatments were discontinued, the animals resorted to self-injurious biting (SIB) as before. All three males self inflicted repeatedly serious lacerations that required surgical care. When it was considered to euthanize these males, because the SIB could not be stopped with pharmacological therapy, permission was finally given to pair them with other compatible companions. This "treatment" brought the self-biting to an end in all three cases. Carl, however, had a relapse when his buddy was removed for research-assignment reasons after 14 months. Fortunately, the investigator was considerate enough to drop the companion from the research protocol. Once re-united with his companion, Carl promptly stopped again biting himself.

Fritz (1989) reports of three male and one female individually housed chimpanzees who stereotypically mutilated themselves. The animals were carefully socialized in compatible group settings that caused all four of them to gradually stop injuring themselves.

Minkel (2007) gives an account of a long-tailed macaque who was cured from compulsive hair-pulling by being paired with another conspecific:

At a previous institution we had a cyno - "Grandpa" - who suffered from severe hair-pulling. He had removed practically all hair from his body; all that was left was a patch in the middle of his back that he could not reach! He was not shy about hiding his idiosyncratic behavior and would contort into strange positions to do it. The veterinarians tried various treatments to alleviate the problem to no avail. We tried all the various enrichment devices we could find; they would only keep him occupied for a day or so. We increased the space of his cage; no luck. We were reluctant to pair him as he was an older male who had been singly housed for so long, but there was no other treatment option left.
      We tried two unsuccessful pairings and finally settled on a newly acquired juvenile male who was very rowdy and active; Grandpa was quite the opposite, relaxed and sedate. The little guy himself was on his second pair attempt; during his first one - all he did was try to start a fight. To our great relief the new pair worked out just fine. This truly "odd couple" got along great from the start. Grandpa responded correctly, brought the little guy in line, and actually perked up. The most surprising part, however, was that Grandpa stopped hair-pulling. He stopped completely, and all his hair had grown back in the course of several months. Grooming Opportunities

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