Atretogenic action of estrogen in rhesus monkeys: Effects of repeated treatment

The effects of 17β-estradiol (E₂), administered in Silastic capsules for 24 hours at intervals of 10 or 14 days, on follicular development and menstrual cycle characteristics were studied in 13 rhesus monkeys. In seven monkeys receiving E₂ at l0-day intervals for 50 treatment periods, new follicles frequently developed between treatments but usually regressed. In seven instances, the follicles persisted longer than expected but were steroidogenically suppressed and regressed spontaneously. Ovulation occurred in only two instances. In six monkeys receiving E₂ at 14-day intervals, new follicles developed regularly, with seven ovulations occurring in 37 treatment periods. A persistent anovulatory follicle was noted in only one instance. Menstruation occurred with equal frequency, and the interval from treatment to onset of menstruation was not significantly different regardless of treatment or the occurrence of ovulation; the intervals between menstruation approximated those of normal menstrual cycles. In general, following termination of treatment, menstrual cycles returned to normal quickly. These data indicate that E₂ administered intermittently at 10-day intervals effectively suppresses ovulation, and they provide new insight into the actions of E₂ on folliculogenesis in primates.     

A new approach to evaluating troop deployment in wild Japanese Monkeys

In winter when the mountain slopes are covered with deep snow, it is easy to obtain quantitative data on the two-dimensional deployment of members of a troop of wild Japanese monkeys. We observed the deployment of a troop on a slope from the opposite side of a river. The deployment patterns, evaluated on the basis of the relative distance from the central point (centroid) of the troop, were different for each sex and age category. Adult females, infants, and 1-year-olds tended to be grouped together and were concentrated near the center of the troop. On the other hand, adult males were randomly spaced. These tendencies suggest that the deployment reflects the social structure of the duplicate concentric-circle model originally proposed by J. Itani (1954).     

Visitors excite primates in zoos

Fifteen species of primate were observed to assess the effects of zoo visitors on their social behavior. When visitors were present primates were less affiliative, more active, but more aggressive. These changes were particularly marked in arboreal monkeys, especially in smaller species, and were reduced by 50% by lowering the height of spectators. Detailed observations of a group of mandrills indicated that with increasing numbers of visitors the monkeys showed a linear increase in attention to visitors, in activity, and in stereotyped behavior. All of these effects are consistent with an interpretation that visitors are a source of stressful excitement rather than of enrichment.     

Spectral sensitivity of vervet monkeys (Cercopithecus aethiops sabaeus) and the issue of catarrhine trichromacy

Several genera of platyrrhine monkeys show significant polymorphism of color vision. By contrast, catarrhine monkeys have usually been assumed to have uniform trichromatic color vision. However, the evidential basis for this assumption is quite limited. To study this issue further, spectral sensitivity functions were obtained from vervet monkeys (Cercopithecus aethiops sabaeus) using the technique of electroretinographic flicker photometry. Results from a chromatic adaptation experiment indicated that each of the twelve subjects had two classes of cone pigment in the 540/640 nm portion of the spectrum. That result strongly suggests that this species has routine trichromatic color vision. Comparison of the spectral sensitivity functions obtained from vervets and from similarly-tested humans further indicates that the cone complements of the two species are very similar. Results from this investigation add further support to the idea that there are fundamental differences in the genetic mechanisms underlying color vision in platyrrhine and catarrhine monkeys.     

Abnormal behaviors in nursery-reared squirrel monkeys (Saimiri sciureus)

Seventeen squirrel monkeys (Saimiri sciureus), 11 nursery-reared and six mother-reared, were monitored to clarify the type and degree of deviant behaviors that result from rearing this species in the nursery. Two observation periods were used: the first when the subjects averaged 6.7 months of age, the second when they averaged 15.5 months. Thirteen activities involving variations of nonnutritive orality, stereotypic posturing, and agitated behaviors were seen in the nursery-reared subjects, but never in the mother-reared subjects. No consistent gender, subspecies, or age differences were evident. Squirrel monkeys, like chimpanzees and some Old World monkeys, do develop atypical self-directed behaviors when deprived of normal stimulation during early development.     

Diet and dominance among female vervet monkeys (Cercopithecus aethiops)

Behavioral observations of free-ranging female vervet monkeys (Cercopithecus aethiops) suggest that priority of access to food enhances individual fitness. Seventeen adult females living in two social groups were observed for 26 months in the Samburu/Isiolo Game Reserves, northern Kenya. Priority of access to feeding and sitting sites was associated with significant differences in diet and reproduction. Food species distribution altered rank-related differences in diet. Rank-related differences in diet occurred when food items were clumped in distribution, but not when they were randomly distributed. The relation of individual food items to reproduction influenced rank-related differences in reproduction. Rank was significantly correlated with birth rate when a preferred food species was clumped in distribution.     

Subspecies differences in hormonal and behavioral responses after group formation in squirrel monkeys

The behavioral and hormonal responses of squirrel monkeys of the Bolivian and Guyanese subspecies were compared after a group formation procedure. Two groups of each subspecies, consisting of five females and three males (later reduced to two) were observed daily during the week of group formation and for nine weeks following removal of a single male from each group. Measures of plasma cortisol were examined in the females after the initial group formation and after the groups were reduced by one male. The levels of plasma testosterone were assessed in all the males during the initial week of group formation. Linear dominance hierarchies were determined both within and across the sexes in both subspecies. The frequency and directionality of low-level aggressive interactions indicated that females of the Bolivian subspecies were dominant to the males, while the males of the Guyanese subspecies ranked over all the females. Additionally, the Bolivian squirrel monkey females showed an elevation of plasma cortisol on the day of group formation, which declined 48 hr later, then reelevated after the groups were reduced by one male and declined gradually over a nine-week period. Guyanese females showed little change in cortisol levels during both periods. This suggests fundamental hormonal, as well as behavioral, differences between the two subspecies. The change in plasma testosterone levels in the males during the initial week of group formation was positively correlated with social status. Furthermore, differences in the dynamics within individual groups for each subspecies were reflected by the levels of plasma cortisol of the female members.     

Assessment of individual differences in the preferred proximity to a human feeder by partitioned raisin test, with two species of macaque monkeys

All animals must acquire food and mates by approaching them despite possibilities of accompanying risks and thus are frequently encountered with approach-avoidance conflicts in daily lives. Behavioral individual differences in such situations may be considered as one of the most biologically fundamental personality trains. “Partitioned raisin test” was devised to assess this trait with macaque monkeys. It involved throwing raisins into groups of monkeys and observing the preferred distance of each from the human feeder, a source of possible harm. The test was administered to 4 groups of Japanese monkeys (30 total) and 3 groups of rhesus monkeys (19 total), all l-yr-old and matched in history. Individual differences in the preferred proximity to the feeder, as expressed by the Proximity Index (PI), were found in both species.PI was not correlated with a measure of dominance over the raisins. Individual differences inPI were also not due to territorial effects unrelated to the location of the feeder.PI was stable in five of the six monkeys re-tested after one year of interval in a newly organized group, where there supposedly had been a change in their social structure. Partitioned raisin test was shown to be capable of depicting individual differences related to differential approach bias in an approach-avoidance conflict situation. Although possible confounding effects by social factors need to be delineated in the following studies, the method may provide a handy and widely applicable way for the assessment of this trait with monkeys.     

Role of spatial memory in primate foraging patterns: Saguinus mystax and Saguinus fuscicollis

From field data collected in the Amazon Basin of northeastern Peru, I present evidence that moustached (Saguinus mystax) and saddle-back (Saguinus fuscicollis) tamarins maintain detailed knowledge of the distribution and location of many tree species in their home range. During the wet season months of October through December 1984, fruits and exudates from 20 tree species and over 150 individual trees accounted for 75% of plant feeding time. These trees exhibited a patchy distribution; mean nearest-neighbor distances between trees of the same species averaged 148 meters. Moustached and saddle-back tamarins visited an average of 13 trees per day, concentrating their feeding efforts on 4–11 individual trees from a small number of target species. In 70% of all cases the nearest tree of a given species was selected as the next feeding site. Movement between these sites was characterized by relatively straight-line travel. It is argued that S. mystax and S. fuscicollis offset the patchiness component of the fruit and exudate part of their diet through goal-directed foraging and an ability to compare accurately the distances and directions from their present location to a large number of potential feeding trees.