Article

Selective breeding of primates for use in research: Consequences and challenges

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  • Bioculture (Mtius) Ltd
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Abstract

Primates are bred in captivity for a number of purposes, from zoo-based captive breeding programmes for conservation to breeding for biomedical research. In each case, breeding animals that are fit for purpose, either as viable candidates for reintroduction or as valid research models, has presented challenges and resulted in steep learning curves. The breeding of animals for biomedical research has become increasingly focused on the production of animals that are less stressed by captive (specifically laboratory) environments. This is because elevated, particularly chronic, stress responses can result in altered physiological, neurological and behavioural states that have the potential to compromise the validity of scientific results. Selective breeding in captivity to, for example, maximise production, select for docile temperament or specific genotypes for biomedical research, is likely to be counter to natural selective pressures for evolutionary fitness. Given that many natural selective pressures active in the wild are absent in captivity, this paper reviews the selective breeding of primates (especially Old World monkeys) in captivity, its potential negative effects, and options that exist for ameliorating these negative effects.

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... It is established knowledge that animals with low genetic diversity, such as those selectively bred for particular traits that are desirable to humans or those breeding from interrelated bloodlines, can be more prone to specific injuries or heritable disorders [141,142]. This is evident in various species, including dogs, cats and primates, as well as equids [142][143][144]. ...
... It is established knowledge that animals with low genetic diversity, such as those selectively bred for particular traits that are desirable to humans or those breeding from interrelated bloodlines, can be more prone to specific injuries or heritable disorders [141,142]. This is evident in various species, including dogs, cats and primates, as well as equids [142][143][144]. In the case of equine breeds, more than 20% have been identified as susceptible [145], and an overreliance on particular bloodlines or a small pool of the most desirable breeding stock appears to have an increased risk and prevalence of heritable disorders or specific injuries related to conformational defects within some breeds. ...
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... The importance of early experience on the welfare of primates has been well documented, especially in relation to maternal deprivation (Mineka & Suomi, 1978, Wallen et al, 1981, Pryce et al, 2005, Latham & Mason, 2008. Whilst it is recommended that weaning occurs at around 12 -18 months for macaques (IPS, 2007), it is not uncommon for this to take place at around 6 months (Honess et al, 2010), and this may affect the fear response of the animals. Marmosets however usually remain with their birth group until they are 18 months old (The Boyd Group, 2002), although earlier weaning for research purposes is not uncommon (Majolo et al, 2003b) and has been standard practice in UK breeding establishments (Buchanan-Smith, personal communication). ...
... Mills et al, 2004). Cynomolgus macaques are usually reared in large groups in gang cages once weaned, although breeding systems vary from timed breeding in solitary housing to expansive corrals (Honess et al, 2010). Further, factors such as the matrilineal dominance hierarchies of the mothers (and later the young animals themselves) may lead to some more dominant or confident individual receiving more attention than those more submissive animals (Laule, 2010). ...
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... From the available evidence, it would appear that colony managers can maximise the probability of appropriate maternal behaviour by providing each female with the optimal social environment at parturition and a developmental environment that prepares her to deliver appropriate maternal care and cope with the stressors routinely associated with breeding facilities. In general, it is advisable to leave female offspring destined for breeding in their natal group, in which case inbreeding can be managed by periodically replacing the breeding male/s, mimicking the natural situation (Honess et al., 2010). Large enclosures are needed to accommodate daughters and granddaughters recruited as breeding stock. ...
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... The risk for neonatal loss decreased with advancing maternal age, possibly due to improved parenting skills in older females 6 . ...
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The claim that dominance rank determines male mating success in polygamous primate groups is controversial. One argument against the claim is that spurious correlations are often obtained between dominance rank and mating success as a result of the confounding effects of age. Mating data from 32 studies of primate behaviour, totalling 75 study groups, were collated to test this criticism and investigate the determinants of observed variation in the correlation coefficient values. Despite conflicting reports, a reliable positive relationship between male dominance rank and mating success amongst animals of the same age class is seen. Furthermore, the variation in correlation coefficient values is significantly negatively related to the number of males in the group (the single exception being highly dimorphic primate species, where instead the correlation coefficient is positively related to the number of females). The intensity of the effect of the number of males is most severe in seasonal breeders. These results indicate that the relationship between dominance rank and mating success appears to be a function of the level of competition that males face in the group for access to cycling females. Data on genetic paternity indicate that these results hold even for absolute measures of reproductive success.
Article
Many farm, laboratory, zoo and companion animals experience some form of maternal deprivation. This is typically via separation from their mothers earlier than would happen in free-living populations, in some cases even while young are still dependent upon milk. Maternal deprivation may also occur in a qualitative way, via inadequate maternal care, perhaps caused by inexperience or by restrictive environments that limit maternal behaviours. In this paper we review evidence on the link between early separation from the mother and abnormal behaviour from a wide range of sources, including the early primate studies in this field, more recent examples from zoo and commercially reared animals, and human examples from studies of institutionalised children. We discuss factors that seem to influence the magnitude of later effects, such as developmental stage and age at separation, and whether separation is gradual (i.e. more similar to natural separation) or abrupt (as often the case in captivity). In these instances, however, maternal deprivation is just one aspect of a suite of changes that occur when infants are separated from their mothers. In the second part of the paper we therefore review the few cases where maternal loss per se has been investigated, and studies showing lasting affects of qualitative aspects of maternal care. We then look at the possible mechanisms underlying maternal deprivation-induced stereotypic behaviours including potential frustration of specific motivations, and lasting, more pervasive changes for instance in temperament or motor control. Finally, we discuss the practical and welfare implications of the effects of maternal deprivation, and suggest some topics for future research.
Article
Reviews phenomena associated with social separation from attachment objects in nonhuman primates. A biphasic protest–despair reaction to social separation is often seen in monkeys, as in human children. However, upon reunion there is generally a temporary increase in attachment behaviors rather than a temporary phase of detachment, as has been reported in the human literature. Gross factors such as age and sex do not appear to influence the responses to separation or reunion substantially. Rather, behavioral repertoires prior to separation and the nature of the separation and reunion environments appear to be more important determinants of the severity of separation reactions. These findings are consistent with the human literature. Possible long-term consequences of early separations are also discussed. Four theoretical treatments of separation phenomena are presented and evaluated: J. Bowlby's attachment-object-loss theory, I. C. Kaufman's conservation–withdrawal theory, M. E. P. Seligman's learned helplessness theory, and R. L. Solomon and J. D. Corbit's opponent-process theory. (95 ref) (PsycINFO Database Record (c) 2012 APA, all rights reserved)
Article
For most of the 18 years recorded, fewer than 50% of the adult females gave birth in any one year. The colony, of 6 social groups, showed a clear-cut breeding season. Female parity and dominance had no effect on breeding rate, though 1st infants were born earlier in the year than 2nd-born ones. Only when females gave birth in successive years were the months of giving birth correlated. Mothers and daughters may tend to give birth closer in time within a breeding season than do other females.
Article
Recent amendments to the Animal Welfare Act will, upon taking effect, require that researchers who maintain nonhuman primates in captivity house their animals in such a way as to "promote their psychological well-being." Unfortunately, no consensus presently exists in terms of how to define or identify psychological well-being in primate subjects. We propose a strategy for defining psychological well-being that includes assessment of physical health, comparison with species-normative behavioral repertoires, detection of distress, and evaluation of coping responses. This set of definitions is then used to characterize prototypical primate laboratory environments (e.g., single-cage, pair, and group housing) in terms of fostering psychological well-being. The importance of factors other than housing, such as species-specific characteristics, rearing histories, and phenotypic differences, is also emphasized in developing prescriptions for psychological well-being in captive primates. It seems unlikely that simple prescriptions will be broadly applicable across the whole range of captive primates. Instead, researchers must be sensitive to the needs of their particular subjects in order to optimize their psychological well-being, however defined. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
Article
A study to systematically evaluate the effect of separation of mother and infant in the first year of life on maternal reproduction and long term offspring health was carried out. This study utilized outdoor harem breeding groups with an average ratio of one male to seven females. Female rhesus monkeys with live born infants (N = 750) were assigned to four weaning groups: 6, 8, 10, and 12 months. Two additional groups (N = 155) were included for comparison of reproductive parameters: females whose infants died at 90 days or less and females that had abortions or stillbirths. Females with infants weaned at 6 months had higher reproductive rates than other groups over the 18-month study period. Weaned infants were removed from the dam's cage and housed with age peers. No effect of weaning age was seen on overall measures of offspring health up to 2 years of age. Mortality and morbidity rates showed no differences between infant groups, although the proportion of trauma treatments increased with weaning age. Weights at 1 year varied with weaning group. These findings showed that infant age at separation from dam affected subsequent maternal reproduction without clinical detriment to infant health in harem groups of rhesus monkeys.
Article
The display of adultlike foot-clasp mounts by male rhesus monkeys occurs during the 1st year of life in animals reared with their mothers in social groups where peer interaction is possible 24 hr per day. In contrast, this form of mount is rarely displayed if infants are separated from their mothers at 3 months of age and allowed daily .5-hr periods for peer interaction. To distinguish between maternal and peer access-time influences upon the development of this response, we housed rhesus monkeys with their mothers and allowed either .5-hr or 24-hr daily access to peers in the presence of their mothers during the 1st year of life. Male infants restricted to .5-hr periods with peers rarely or never foot-clasp-mounted peers, although 3 of 6 males foot-clasp-mounted their mothers. In contrast, males given 24-hr access to peers regularly mounted peers using the foot-clasp mount. Males from the 2 groups did not differ in total frequencies of all mount types displayed, only in the display of foot-clasp mounts. After maternal separation at the end of the 1st year, rearing effects on the display of mounting persisted into the 2nd year. Males that received 24-hr daily peer access during the 1st year continued to foot-clasp-mount peers whereas .5-hr males failed to do so, indicating that the experience of mounting one's mother with the adult posture was not sufficient to support peer-oriented foot-clasp mounting at a later age. In addition to deficits in peer-oriented foot-clasp mounting. .5-hr infants displayed higher frequencies of threat and withdrawal behaviors to peers and presented to peers less frequently than did infants from the 24-hr rearing condition. These results contradict a motor deficiency hypothesis to account for the rare display of foot-clasp mounts to peers by males receiving .5-hr daily access to peers during the 1st year of life. Instead, data support the view that .5-hr animals either failed to develop positive response patterns necessary for the execution of the foot-clasp mount, or learned negative patterns of social interaction that prevented the display of this cooperative behavior.
Article
Macaque monkeys (Macaca fascicularis) were introduced to the island of Mauritius approximately 400 years ago. This study compares the mitochondrial DNA of macaques on Mauritius with those from Indonesia and the Philippines. The goal is to measure the amount of evolutionary change that has occurred in this isolated population over 400 years, and to address questions regarding the origin of the Mauritian founders. Amplification of the control region of the mitochondrial genome via the polymerase chain reaction yielded an 1800 base pair DNA fragment which was surveyed for variation using restriction endonucleases. Fifty-two macaques were separated into 17 haplotypes by mapping the restriction sites. No haplotypes were shared among the three populations, and only two closely related haplotypes appeared in the Mauritian sample. Nucleotide variation in the mitochondrial DNA in the Mauritian sample was 10-fold less than the Indonesian and Filipino samples. In contrast, allozyme data estimates of genetic diversity on Mauritius are similar to populations from the ancestral range. The evidence of the more severe bottleneck as measured by mitochondrial data may be explained in part by almost exclusive male dispersal in this species, and may support models of founder events in which rapid population growth prevents substantial loss of nuclear variation. The mitochondrial evidence supports the morphologically and historically based hypothesis that the original founders came from Indonesia.
Article
Lines of baboons with high and low blood pressure were developed by selective breeding. Blood pressure was measured in 456 adult feral baboons under ketamine immobilization by direct arterial cannulation. Males with blood pressures two standard deviations and females with blood pressures one standard deviation above and below the cumulative mean were selected as progenitors. High males were mated with high females and low males were mated with low females. We measured blood pressure and plasma renin activity on 100 progeny, 54 males and 46 females, greater than 44 months of age with an abbreviated tether protocol and software program for data collection. Mean systolic and diastolic nighttime pressures for the high line were 126/72 and for the low line were 114/65 mm Hg. Line differences for systolic (12 mm Hg) and for diastolic (7 mm Hg) pressures were significant (p < 0.001). The line difference for plasma renin activity (1.1 [ng/mL]/hr) was not significant. Progeny pressures ranged from 84/49 to 191/126 mm Hg. There was no sex effect on blood pressure or plasma renin activity line differences. Heritability of systolic pressure was 0.46 +/- 0.19 and of diastolic pressure was 0.32 +/- 0.19. These results indicate that, by selective breeding and rigorous measurement of blood pressure, lines of baboons with significant difference in blood pressure can be developed.
Article
Among some human populations, immunogenetic similarity between mates is associated with increased risk of pregnancy loss. To investigate the relationship between histocompatibility and reproductive performance in non-human primates, 128 pigtailed macaque couples were classified as 'reproductively successful' or 'unsuccessful' according to previous breeding performance. These couples were arranged into 64 triads composed of individual females, and a 'successful' and 'unsuccessful' mate. Individuals were typed for class I MHC antigens using a microcytotoxicity technique and species-specific alloantisera. Matched-pair analysis revealed that significantly more 'unsuccessful' couples shared MnLA-A antigens than did the matched 'successful' couples. Conditional matched-pair logistic regression analysis further revealed that parental sharing of MnLA-A antigens is an even more significant predictor of pregnancy wastage than is advanced maternal age. In our study population, sharing of MnLA-A antigens predicted 72% of pregnancy loss among 'unsuccessful' couples (P < 0.009). Identification of histocompatibility-associated factors influencing pregnancy success could have profound clinical implications for chronic spontaneous abortion, intra-uterine growth retardation and birth defects in humans. Among captive primates, identification of MHC or MHC-linked genes affecting reproductive outcome could lead to more efficient colony management strategies as well as development of a model for understanding human immunologically-mediated reproductive failure.
Article
In this paper the question is posed whether it is not only better for the animal to be happy, but whether its state of mind may also have the potential to influence the scientific results derived from it. To ensure good science, the animal should have a normal physiology and behaviour, apart from specific adverse effects under investigation. There is a growing body of evidence from a wide variety of sources to show that animals whose well-being is compromised are often physiologically and immunologically abnormal and that experiments using them may reach unreliable conclusions. On scientific, as well as ethical grounds, therefore, the psychological well-being of laboratory animals should be an important concern for veterinarians, animal technicians and scientists.
Article
The gestational experience of a mother can influence the intrauterine environment she provides her own offspring, allowing prenatal events to affect pregnancy outcomes across several generations. Using a multigenerational database, we determined the reproductive consequences for rhesus monkeys descended from small-for-date and large-for-date birth weight matrilines. Both the maternal half-brothers and -sisters of large-for-date infants exhibited enhanced fetal growth, but for small-for-date probands, only the maternal half-sisters experienced significant intrauterine growth constraint. In addition, the growth-restricted females were at higher risk of poor reproductive outcomes in adulthood, and they perpetuated the matrilineal birth weight pattern by selectively constraining the fetal development of their daughters. Collectively, these findings suggest a mechanism for the intergenerational persistence of suboptimal pregnancy outcomes.
Article
(Macaca nemestrina) and baboon (Papio cynocephalus, Papio anubis, and hybrids) breeding colonies from the Primate Field Station (PFS) (Medical Lake, WA) to the Tulane Regional Primate Research Center (Covington, LA). Colony records on all 598 pigtailed macaques (Macaca nemestrina) and 157 baboons (P. c. anubis) shipped to the Tulane Primate Center from the PFS breeding colony were used for analysis of species, sex, age, origin, current status, and the number of animals born at Tulane and their status. To provide comparative statistics, colony records on all 1,002 macaques and 258 baboons alive on 1 January 1991 at the Field Station were retrieved in the same manner as the Tulane data. Overall survival rates of macaques in the months following the move (71.7%) were similar to those associated with the Arashiyama West colony move from Japan to Texas. In our colony, significantly lower survival following the move was seen only in older (10 years+) macaques, while survival in other age groups was slightly lower than in the comparison year of 1991 at the Primate Field Station. Captive-bred macaques exhibited higher survival than wild-caught animals. Infant survival at Tulane was not significantly different than in pre-move years. Baboons fared well in the move, with no significant differences in mortality or reproduction when compared with the 1991 Medical Lake baboon colony.
Article
The heritability of birth weight was estimated in 3,562 captive pigtailed macaques using 30 years of breeding and pedigree records. Based on a pedigree of over 12,000 animals, quantitative genetic analyses were performed using statistical variance decomposition methods. The model included additive genetic effects, cytoplasmic genetic effects, birth environment, shared maternal environment, and unmeasured environmental effects. The results demonstrated a strong (h(2) = 0.51) heritable component of birth weight overall, and included significant additive genetic heritability (h(2) = 0.23), and cytoplasmic heritability (h(2) = 0.09). In addition, a significant effect of birth location and cage type was identified, explaining an additional 6% of birth weight variance. The use of a nonhuman primate model for studying the effects of genes on birth weight eliminated many of the problems associated with confounding variables in human studies, and allowed for the quantification of a heritable component of birth weight.
Article
To develop a SIV-rhesus macaque (Rh) model of AIDS that more closely approximates HIV pathogenesis in humans. The pathogenesis of SIV was compared in two different types of Rh, the Chinese (Ch) and Indian (Ind) subspecies. Ch Rh and Ind Rh origin were identified genetically and infected with the SIV(mac)239 molecular clone. Plasma viral loads, depletion of intestinal lymphocytes with memory phenotype, humoral immune responses and CD4/CD8 cell ratios were compared during acute and steady-state periods of infection. Plasma viral loads from 7 days after infection through 240 days were significantly lower in Rh of Ch origin compared with Ind Rh. Viral loads in Ch Rh were closer to viral loads observed in untreated humans infected with HIV-1. Depletion of intestinal effector cells was less evident in SIV-infected Ch Rh compared with Ind Rh. An index of intestinal pathogenesis was devised that closely paralleled viral load and severity of infection. There were no rapid progressors to AIDS among 10 Ch Rh. In contrast, three of four Ind Rh progressed rapidly to AIDS. Compared with Ind Rh, SIV(mac) pathogenesis in Ch Rh was closer to HIV-1 infections in untreated adult humans. The differences were statistically significant. The Ch Rh subspecies is a suitable AIDS model and may have advantages over the rapid and highly pathogenic Ind Rh model. Moreover, Ind Rh supplies are limited and use of Ch Rh provides a new resource.
Article
The shortage of rhesus macaques of Indian origin for acquired immune deficiency syndrome (AIDS) research has prompted a search for an alternate species. As rhesus macaques of Chinese origin are more readily obtainable, we have defined the parameters of infection in seven members of this subspecies with the primary virulent isolate, SIV/delta B670. Viremic peaks and set points as determined by real time polymerase chain reaction were, in general, lower than that observed in Indian origin rhesus macaques. As expected, these values were associated with maintenance of CD4+ lymphocytes and significantly longer survival, with six of seven Chinese origin animals living significantly longer than Indian origin rhesus macaques. Interestingly, these findings were associated with a selective amplification of one of two major phylogenetic groups found within the inoculum. This observation is in contrast to Indian origin animals where both phylogenetic groups are commonly identified. Together, these data suggest prudence in the design of experimental protocols using rhesus macaques of Chinese origin where survival and rapid loss of CD4+ lymphocytes are desired endpoints.
Article
Permanent mother-infant separation prior to natural weaning is a common hus-bandry practice in monkey breeding colonies. In the United States, all eight Re-gional Primate Research Centers have such colonies. Under undisturbed conditions, Old World monkey mothers wean their infants at the age of about 1 year (Hall & DeVore, 1965; Poirier, 1970; Roonwal & Mohnot, 1977; Southwick, Beg, & Siddiqi, 1965). Natural weaning is a gradual process. It implies that the mother, over a period of several weeks or months, more and more consistently discourages her infant to suck on her breasts. Once the mother stops nursing the infant for good, the affectionate bond between the two is not broken (Altmann, Altmann, Hausfater, & McCuskey, 1977; Lindburg, 1971; Poirier, 1970; Roonwal & Mohnot, 1977). The young usually remains in the ma-ternal group at least until prepuberty. Under confinement conditions, artificial weaning is an abrupt occurrence that takes place several months prior to the biologically normal age of weaning. It im-plies that the still-nursed infant is taken away from the mother and subsequently reared alone or with other artificially weaned infants.
Article
We assessed the use of nonhuman primates and nonhuman primate biological material in research by reviewing studies published in 2001 in peer-reviewed journals. The number and species of primates used, the origin of the animals, the type of study, the area of research of the investigation, and the location at which the research was performed were tabulated. Additionally, factors related to the animals that may have affected the outcome of the experiments were recorded. A total of 2,937 articles involving 4,411 studies that employed nonhuman primates or nonhuman primate biological material were identified and analyzed. More than 41,000 animals were represented in the studies published in 2001. In the 14% of studies for which re-use could be determined, 69% involved animals that had been used in previous experiments. Published studies most commonly used nonhuman primates or nonhuman primate biological material from the species Chlorocebus aethiops (19%), Macaca mulatta (18%), M. fascicularis (9%), and Papio spp. (6%). Of these studies, 54% were classified as in vitro studies, 14% as noninvasive, 30% as chronic, and 1% were considered acute. Nonhuman primates were primarily used in research areas in which they appear to be the most appropriate models for humans. The most common areas of research were microbiology (including HIV/AIDS (26%)), neuroscience (19%), and biochemistry/chemistry (12%). Most (84%) of the primate research published in 2001 was conducted in North America, Europe, and Japan. The animals and conditions under which they were housed and used were rarely described. Although it is estimated that nonhuman primates account for an extremely small fraction of all animals used in research, their special status makes it important to report the many husbandry and environmental factors that influence the research results generated. This analysis has identified that editors rarely require authors to provide comprehensive information concerning the subjects (e.g., their origin), treatment conditions, and experimental procedures utilized in the studies they publish. The present analysis addresses the use of primates for research, including the effects of a shortage of suitable nonhuman primate subjects in many research areas.
Article
In recent years there has been a marked increase in awareness of issues involving the psychological well-being of nonhuman primates (NHPs) used in biomedical research. As a result, many facilities are starting to train primates to voluntarily cooperate with veterinary, husbandry, and research procedures, such as remaining still for blood draws or injections. Such training generally reduces the stress associated with these procedures, resulting in calmer animals and, ultimately, better research models. However, such training requires great investments in time, and there can be vast individual differences in training success. Some animals learn tasks quickly, while others make slower progress in training. In this study, we examined whether temperament, as measured by response to a novel food object, correlated with the amount of time it took to train 20 adult female rhesus macaques to perform a simple task. The monkeys were categorized as "exploratory" (i.e., inspected a novel object placed in the home cage within 10 sec), "moderate" (i.e., inspected the object within 10-180 sec), or "inhibited" (i.e., did not inspect the object within 3 min). We utilized positive reinforcement techniques to train the monkeys to touch a target (PVC pipe shaped like an elbow) hung on their cage. Temperament correlated with training success in this study (Pearson chi2=7.22, df=2, P=0.03). We easily trained over 75% of the animals that inspected the novel food (i.e., exploratory or moderate individuals) to touch the target. However, only 22% of the inhibited monkeys performed the task. By knowing which animals may not respond to conventional training methods, we may be able to develop alternate training techniques to address their specific needs. In addition, these results will allow us to screen monkeys to be assigned to research projects in which they will be trained, with the goal of obtaining the best candidates for those studies.
Article
There is considerable evidence that primates housed under impoverished conditions develop behavioural abnormalities, including, in the most extreme example, self-harming behaviour. This has implications for all contexts in which primates are maintained in captivity from laboratories to zoos since by compromising the animals' psychological well-being and allowing them to develop behavioural abnormalities their value as appropriate educational and research models is diminished. This review examines the extensive body of literature documenting attempts to improve living conditions with a view to correcting behavioural abnormalities and housing primates in such a way that they are encouraged to exhibit a more natural range and proportion of behaviours, including less self-directed and social aggression. The results of housing, feeding, physical, sensory and social enrichment efforts are examined with specific focus on their effect on aggressive behaviour and variation in their use and efficacy. It is concluded that while inappropriate or poorly distributed enrichment may encourage aggressive competition, enrichment that is species, sex, age and background appropriate can dramatically reduce aggression, can eliminate abnormal behaviour and substantially improve the welfare of primates maintained in captivity.
Article
There is considerable interest in the study of stress and aggression in primates as a model for their interpretation in humans. Despite methodological and interpretational problems associated with behavioural and physiological measurement and definition, a considerable body of literature exists on these phenomena in primates. In the course of reviewing this literature we examine examples of many of the sources of variation in stress and aggression, including species identity, sex, age, breeding and social status, individual temperament, background, learning and resource distribution. This is followed by an examination of the interaction between stress and aggression before reviewing the most important areas in which changes in both stress and aggression are measured. In particular we examine those studies covering social aspects of an animal's life, specifically relating to social isolation, crowding as well as group formation, composition and instability. This review reveals the complex and often contradictory nature of relationships, not just between an animal's physiology and its behaviour, but between its stress status and display or receipt of aggression.