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Jordan, K. E., & Brannon, E. M. (2006). Weber's Law influences numerical representations in rhesus macaques (Macaca mulatta). Anim. Cogn., 9(3), 159–172.
Abstract: We present the results of two experiments that probe the ability of rhesus macaques to match visual arrays based on number. Three monkeys were first trained on a delayed match-to-sample paradigm (DMTS) to match stimuli on the basis of number and ignore continuous dimensions such as element size, cumulative surface area, and density. Monkeys were then tested in a numerical bisection experiment that required them to indicate whether a sample numerosity was closer to a small or large anchor value. Results indicated that, for two sets of anchor values with the same ratio, the probability of choosing the larger anchor value systematically increased with the sample number and the psychometric functions superimposed. A second experiment employed a numerical DMTS task in which the choice values contained an exact numerical match to the sample and a distracter that varied in number. Both accuracy and reaction time were modulated by the ratio between the correct numerical match and the distracter, as predicted by Weber's Law.
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Beran, M. J. (2007). Rhesus monkeys (Macaca mulatta) succeed on a computerized test designed to assess conservation of discrete quantity. Anim. Cogn., 10(1), 37–45.
Abstract: Conservation of quantity occurs through recognition that changes in the physical arrangement of a set of items do not change the quantity of items in that set. Rhesus monkeys (Macaca mulatta) were presented with a computerized quantity judgment task. Monkeys were rewarded for selecting the greater quantity of items in one of two horizontal arrays of items on the screen. On some trials, after a correct selection, no reward was given but one of the arrays was manipulated. In some cases, this manipulation involved moving items closer together or farther apart to change the physical arrangement of the array without changing the quantity of items in the array. In other cases, additional items were added to the initially smaller array so that it became quantitatively larger. Monkeys then made another selection from the two rows of items. Monkeys were sensitive to these manipulations, changing their selections when the number of items in the rows changed but not when the arrangement only was changed. Therefore, monkeys responded on the basis of the quantity of items, and they were not distracted by non-quantitative manipulations of the sets.
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Lacreuse, A., Martin-Malivel, J., Lange, H. S., & Herndon, J. G. (2007). Effects of the menstrual cycle on looking preferences for faces in female rhesus monkeys. Anim. Cogn., 10(2), 105–115.
Abstract: Fluctuations of ovarian hormones across the menstrual cycle influence a variety of social and cognitive behaviors in primates. For example, female rhesus monkeys exhibit heightened interest for males and increased agonistic interactions with other females during periods of high estrogen levels. In the present study, we hypothesized that females' preference for males during periods of high estrogen levels is also expressed at the level of face perception. We tested four intact females on two face-tasks involving neutral portraits of male and female rhesus monkeys, chimpanzees and humans. In the visual preference task (VP), monkeys had to touch a button to view a face image. The image remained on the screen as long as the button was touched, and the duration of pressing was taken as an index of the monkey's looking time for the face stimulus. In the Face-Delayed Recognition Span Test (Face-DRST), monkeys were rewarded for touching the new face in an increasing number of serially presented faces. Monkeys were tested 5 days a week across one menstrual cycle. Blood was collected every other day for analysis of estradiol and progesterone. Two of the four females were cycling at the time of testing. We did not find an influence of the cycle on Face-DRST, likely due to a floor effect. In the VP however, the two cycling individuals looked longer at conspecific male faces than female faces during the peri-ovulatory period of the cycle. Such effects were absent for human and chimpanzee faces and for the two noncycling subjects. These data suggest that ovarian hormones may influence females' preferences for specific faces, with heightened preference for male faces during the peri-ovulatory period of the cycle. Heightened interest for stimuli of significant reproductive relevance during periods of high conception risk may help guide social and sexual behavior in the rhesus monkey.
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Brennan, J., & Anderson, J. (1988). Varying responses to feeding competition in a group of rhesus monkeys (Macaca mulatta). Primates, 29(3), 353–360.
Abstract: The behaviour of members of a group of rhesus monkeys was observed in experimentally created, competitive feeding situations. Socially dominant members of the group tended to start eating before lower-ranking subjects, and generally ate more. Dominants sometimes used aggression to control access to food, but overall, intermediate-ranking monkeys were involved in most agonistic episodes. Non-dominant subjects improved their feeding performance when food was presented in three piles rather than one pile, often by snatching food and consuming it away from the pile. These general patterns were less evident when realistic snake models were placed on some of the food piles. Feeding was disrupted by the presence of snakes, but notably, subordinates risked feeding in these conditions. Piles containing preferred foods and snakes were eaten from, but a low-preference food (carrot) under snakes went untouched by all subjects. The results suggest that group-members evaluate potential risks and benefits of competing for a restricted resource, and that dominance status, while an important factor, is only one element in the equation.
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Cook, M., Mineka, S., Wolkenstein, B., & Laitsch, K. (1985). Observational conditioning of snake fear in unrelated rhesus monkeys. J Abnorm Psychol, 94(4), 591–610.
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Owren, M. J., Dieter, J. A., Seyfarth, R. M., & Cheney, D. L. (1993). Vocalizations of rhesus (Macaca mulatta) and Japanese (M. fuscata) macaques cross-fostered between species show evidence of only limited modification. Dev Psychobiol, 26(7), 389–406.
Abstract: Two rhesus and two Japanese macaque infants were cross-fostered between species in order to study the effects of auditory experience on vocal development. Both the cross-fostered and normally raised control subjects were observed over the first 2 years of life and their vocalizations were tape-recorded. We classified 8053 calls by ear, placed each call in one of six acoustic categories, and calculated the rates at which different call-types were used in different social contexts. Species differences were found in the use of “coo” and “gruff” vocalizations among control subjects. Japanese macaques invariably produced coos almost exclusively. In contrast, rhesus macaques produced a mixture of coos and gruffs and showed considerable interindividual variation in the relative use of one call type or the other. Cross-fostered Japanese macaques adhered to their species-typical behavior, rarely using gruffs. Cross-fostered rhesus subjects also exhibited species-typical behavior in many contexts, but in some situations produced coos and gruffs at rates that were intermediate between those shown by normally raised animals of the two species. This outcome suggests that environmentally mediated modification of vocal behavior may have occurred, but that the resulting changes were quite limited.
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Brannon, E. M., & Terrace, H. S. (1998). Ordering of the numerosities 1 to 9 by monkeys. Science, 282(5389), 746–749.
Abstract: A fundamental question in cognitive science is whether animals can represent numerosity (a property of a stimulus that is defined by the number of discriminable elements it contains) and use numerical representations computationally. Here, it was shown that rhesus monkeys represent the numerosity of visual stimuli and detect their ordinal disparity. Two monkeys were first trained to respond to exemplars of the numerosities 1 to 4 in an ascending numerical order (1 --> 2 --> 3 --> 4). As a control for non-numerical cues, exemplars were varied with respect to size, shape, and color. The monkeys were later tested, without reward, on their ability to order stimulus pairs composed of the novel numerosities 5 to 9. Both monkeys responded in an ascending order to the novel numerosities. These results show that rhesus monkeys represent the numerosities 1 to 9 on an ordinal scale.
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de Waal, F. B., & Luttrell, L. M. (1986). The similarity principle underlying social bonding among female rhesus monkeys. Folia Primatol (Basel), 46(4), 215–234.
Abstract: Twenty adult female rhesus monkeys (Macaca mulatta) were observed over a three-year period. They lived in a mixed captive group with kinship relations known for three generations. The study's aim was to test Seyfarth's [J. theor. Biol. 65: 671-698, 1977] model of rank-related grooming and to investigate two other possible determinants of social bonding, i.e. relative age and the group's stratification into two social classes. Data on affiliation, coalitions, and social competition were collected by means of both focal observation and instantaneous time sampling. Whereas certain elements of the existing model were confirmed, its explanatory principles were not. Social competition did not result in more contact among close-ranking females (the opposite effect was found), and the relation between affiliative behavior and coalitions was more complex than predicted. Based on multivariate analyses and a comparison of theoretical models, we propose a simpler, more encompassing principle underlying interfemale attraction. According to this 'similarity principle', rhesus females establish bonds with females whom they most resemble. The similarity may concern genetical and social background, age, hierarchical position and social class. Effects of these four factors were independently demonstrated. The most successful model assumed that similarity factors influence female bonding in a cumulative fashion.
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de Waal, F. B., & Johanowicz, D. L. (1993). Modification of reconciliation behavior through social experience: an experiment with two macaque species. Child Dev, 64(3), 897–908.
Abstract: Reconciliation, defined as a friendly reunion between former opponents shortly after an aggressive encounter, is common in the stumptail macaque (Macaca arctoides) but rare in the rhesus macaque (M. mulatta). Juveniles of the two species were cohoused for 5 months, after which they were observed with conspecifics only. Control rhesus monkeys, matched in age and sex to the experimental subjects, went through the same procedure without exposure to the other species. A threefold increase in the proportion of reconciled fights was measured in the rhesus subjects. The difference emerged gradually during cohousing with the tutor species and was sustained following removal of this species. Other behavior, such as grooming and aggression, decreased over time. It is suggested that the social attitude of the subjects was affected through contact with a species characterized by a more relaxed dominance style.
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Judge, P. G., & de Waal, F. B. (1994). Intergroup grooming relations between alpha females in a population of free-ranging rhesus macaques. Folia Primatol (Basel), 63(2), 63–70.
Abstract: Intergroup affiliation among female rhesus macaques, Macaca mulatta, was examined in the captive free-ranging colony of Morgan Island, S.C., USA. The provisioned colony has many social groups (35) and is maintained at a relatively high population density (21 animals/ha) with a relatively low adult male to female ratio (1:8.8). Focal and ad libitum samples were collected on 32 adults (3 males and 29 females) from two groups. Although infrequent, grooming was observed between adult females from different groups, and alpha females were the main participants in these interactions. Colony records indicated that none of the intergroup grooms was between females formerly from a common group. Relations between familiar neighboring groups may be maintained by a combination of both affiliative and aggressive behavior.
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