Parturition in rhesus monkeys (Macaca mulatta)

During five years of birth season fieldwork, we observed two births and three peri-birth instances of behavior of free-ranging rhesus living in Kathmandu, Nepal. These constitute the first two recorded free-ranging rhesus births, and we compare them to the behavior which has been observed in captivity and expected in the wild. The free-ranging parturition behavior was characterized by a general lack of contact with other troop members and by overall inconspicuousness. In the first birth we observed, the troop moved about 70 m up the hill, leaving the laboring female behind on an open hillside. Two males, a female and a juvenile returned and rested about 20 m from the delivering female. During the second delivery, the female stayed with the troop and seemed to participate normally in most troop interactions, although she avoided physical contact with other troop members. We propose that this near-normal behavior may help to assure successful parturition by allowing the female the needed social isolation and inconspicuousness without any loss of troop protection. It may also partly explain why births have not been observed during previous studies. Observations of rhesus parturition behavior reported from studies of captive monkeys and reports from other free-ranging primate studies are compared with our data.     

Hydranencephaly in two rhesus monkeys (Macaca mulatta)

Spontaneously occurring hydranencephaly was diagnosed at necropsy and confirmed histologically in two stillborn fetuses that were delivered from young female rhesus macaques (Macaca mulatta).     

Skeptical responding in rhesus monkeys (Macaca mulatta)

Signalers that misinform sufficiently open may become devalued as sources of information; however, “skepticism” and any comparison involved in testing reliability entail a cost that involves delays and energy expenditure. Skepticism may be less costly though, if, as a rule, animals are not equally skeptical of the signals of all conspecifics. Animals with the ability to recognize individual conspecifics and to recall past encounters with them may have the capacity to restrict skepticism to subsets of animals that are most likely to benefit from deception. We played tape-recorded alarm calls of high- and low-ranking rhesus monkeys(Macaca mulatta) to their groups in a feeding context once daily over 8 consecutive days at the Yerkes Primate Center Field Station. Over the sequence of playbacks, response was greater to the calls of high-ranking monkeys, adult response patterns were different from those of juveniles, and for adults especially, decline in responsiveness was punctuated by partial resurgences of response. These differences may be the consequence of the adults’ more extensive histories of interaction with group members that, though generally reliable, vary with respect to the potential benefits of deceptive signaling.     

Varying responses to feeding competition in a group of rhesus monkeys (Macaca mulatta)

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.     

Sociability and responses to video playbacks in adult male rhesus monkeys (Macaca mulatta)

Studies of several primate species have suggested the existence of a personality dimension typically labeled ‘Sociability,’ which reflects an animal’s tendency to interact with others. The hypothesis that Sociability is related to social skill was tested in the present study by exposing six high- and six low-Sociable adult male rhesus macaques to videotaped presentations of unfamiliar males displaying aggressive or affiliative behaviors. Low-Sociable animals displayed higher frequencies of yawn, lower activity and tended to have higher durations of watching during the presentations that displayed social signals, and significantly fewer lipsmacks to presentations that depicted no social behavior. In response to viewing threats, tooth-grinds, yawns, and lipsmacks, both low- and high-Sociable animals’ most frequent response was to gaze avert; low-Sociable animals, however, had nearly twice the latency to gaze avert than did high-Sociable animals. The low-Sociable animals’ greater tendency to ‘sit and stare’ during the videotaped playbacks suggests that low-Sociable animals have poorer social skills. The possible developmental origins of variation in Sociability, and the functional consequences of such variation for survival and reproduction are discussed.     

Feeding tactics in juvenile rhesus monkeys (Macaca mulatta)

In a group of rhesus monkeys, feeding tactics of juveniles were studied in a competitive situation in which food presentation had been modified, and where food was supplied in a feeding trough six times a day. Juvenile offspring of high-ranking mothers remained longer in the feeding area gathering food. In contrast, juvenile offspring of low-ranking mothers went less often to the feeding area, primarily to collect food. Low-ranking juveniles also gathered the food faster than did dominants and used longer feeding bouts. High-ranking juveniles interrupted their feeding more spontaneously than did subordinate ones. Low-ranking juveniles did so more often as a result of avoiding and being startled. Juvenile males entered the feeding area to satisfy their food requirements more often than females, but were also aggressively expelled more than females. No relation was found between age in months of juveniles and any feeding parameters or causes of interruption. Nor were significant differences found between young and old juveniles. Juvenile rhesus appear to use different tactics in accessing food according to their sex and to their mothers' dominance rank.     

Determination ob biologically active LH in the serum of male rheusus monkeys (Macaca mulatta)

The in-vitro bioassay for LH, using mouse Leydig cells, has been modified for the direct measurement of serum LH in the male rhesus monkey. Validation of the assay demonstrated good reliability in terms of accuracy, precision and sensitivity (1.5 mi.u./ml). Basal LH concentrations in laboratory-maintained monkeys with and without anaesthesia were not significantly different from those in free-ranging, feral monkeys. LH-RH (50 micrograms i.v.) elicited a 30-fold increase in LH concentrations after 30 min. LH levels in castrated adult monkeys were approximately 50 times the normal levels. Intact and castrated juvenile males had only very low LH levels. LH from the serum of an adult male castrate was further characterized by Sephadex G100 column chromatography. The in-vitro bioassay provides a preferable alternative to the heterologous radioimmunoassay method for the routine determination of LH in the rhesus monkey.     

Estrous behavior of free-ranging rhesus monkeys (Macaca mulatta)

The sexual behavior of a group of free-ranging rhesus monkeys (Macaca mulatta) was studied for 13 consecutive months in an attempt to determine whether or not sexual activity occurred year-round, and the importance of sexual attraction to rhesus monkey social organization.Estrous behavior was seen both inter-menstrually and peri-menstrually, producing a shorter mean estrous cycle length than reported by other workers. New data was gathered on the interrelationships among age, dominance rank, and sexual activity; son-mother and brothersister matings; and sexual favoritism among free-ranging rhesus monkeys.A few females who failed to conceive during the fall breeding season showed cyclic estrous behavior throughout the entire annual cycle. Hypotheses are given as to possible physiological bases for birth season sexual cycles.Several forms of inter-animal bonding, including sexual bonding, are enumerated, and their importance to rhesus monkey social organization discussed.     

Testosterone changes during the period of adolescence in male rhesus monkeys (Macaca mulatta)

Testosterone levels of 59 male rhesus monkeys were monitored over a period of 5 years. Longitudinal comparisons revealed consistent rises in mid-morning levels of circulating hormone in successive years from age 2.5 to 6.5 years of age, whereas cross-sectional comparisons failed to detect significant differences among the older subjects. The first mid-morning hormonal elevation could be detected in some males as young as 2.5 years of age, whereas other males showed no detectable rises until age 5.5 years. Males showing first rises at later ages did not show hormonal levels consistently below age peers who had shown earlier rises. Extreme month-to-month variability and a failure to manifest the seasonal normal curve of fully adult males was characteristic of younger males, but some of these males, nonetheless, proved capable of fertilizing females. Although hormonal and agonistic dominance measures failed to show consistent correlations, the alpha male in an age cohort significantly more often had the highest testosterone levels. These data are used to argue that adolescence is a process that takes place over several years and that classification of adolescent animals as adults, based on a single criterion like fertility, has confounded many prior studies involving cross taxa comparisons as well as developmental variables.     

Activated coagulation time for rhesus monkeys (Macaca mulatta)

The activated coagulation time test provided a rapid yet accurate measurement of the intrinsic clotting system in rhesus monkey (Macaca mulatta) whole blood. Other advantages of this test included reproducibility, no requirement for control samples, low cost and commercial availability. The mean activated coagulation time value for 60 normal rhesus monkeys was 96 seconds with a range of 77 to 125 seconds. There were no significant differences due to sex, venipuncture site and time of blood collection.     

Determinants of behavioral rhythmicity during artificial menstrual cycles in rhesus monkeys (Macaca mulatta)

Reproductive success in many mammals depends on synchrony between copulation and ovulation, which is insured by the phenomenon of heat in the female. Certain anthropoid primates including rhesus monkeys do not show heat but may copulate throughout the menstrual cycle, especially when pairs are isolated from conspecifics. In social groups, however, mating mostly occurs around midcycle. We wished to test the hypothesis that copulations are more closely linked to ovulation when males have simultaneous access to several females in different cycle phases. Artificial menstrual cycles were therefore induced by giving hormones to ovariectomized female rhesus monkeys observed in small social groups that each consisted of four females and one male. The cycles of two hormone-treated femlab in each group were either made to synchronize or be offset by 7-day increments so that the estradiol peak of one female occurred 7 days before, and 7 and 14 days after, the estradiol peak of the other. Radioimmunoassay of plasma samples (N = 224) confirmed the timing of the estradiol peaks. Results from eight unique male-female groups (4 males, 8 females, 16 male-female pairs, 718 tests) fully supported the hypothesis. Compared with synchronized cycles, the amplitudes of rhythmic changes in offset cycles were reduced for ejaculations made by males but greatly enhanced for ejaculations received by females. We propose that this socio-hormonal integration of behavior in the group is highly adaptive and enhances the reproductive success of both males and females.     

Seasonal weight changes in male rhesus monkeys (Macaca mulatta)

Adult male rhesus monkeys lose weight during the breeding season and regain it during the nonbreeding season. The annual pattern of maximum weight gain just prior to the onset of breeding resembles the seasonal “fattening” seen in squirrel monkeys, but the period of weight gain is less discrete. The magnitude of weight change is less in younger males, in that sexually immature males gain weight in both seasons, but significantly less during the breeding season. Females do not lose weight during the breeding season. Post hoc analyses revealed no significant correlations between male testosterone levels, dominance ranks, weights, or weight changes. The heaviest animals as juveniles were predictably the heaviest as adolescents. The timing of seasonal changes in testosterone did not correlate with the timing of changes in weight; weight losses followed the rise in testosterone, and weight gains continued until early in the breeding season after testosterone levels had already begun to rise. It is suggested that seasonal hormonal changes may influence activities in individuals and that changes in the activities of particular group members may alter the activity patterns of other group members. This alteration of activity patterns due to group influences on individuals as well as individual influences on the group may explain why hormonal regulation of seasonal weight appears to be indirect and why individuals (juveniles) experiencing no seasonal hormonal changes nonetheless show differences in activity patterns and seasonal weight changes.