Ronald D. Nadler

Sexual behavior of captive orangutans.

Opposite-sex pairs of orangutans were tested for sexual behavior during the intermenstrual period of the female. The male orangutan was the primary initiator of sexual interactions and initiated copulation forcefully on a daily basis, irrespective of female resistance. However, although single copulations occurred daily, copulations beyond the first occurred most frequently during midcycle. Other evidence of cyclicity in behavior was the midcycle decrease in female avoidance of the male and increased grooming, proceptivity, and masturbation by the female. Comparative analysis suggests that differences in sexual cyclicity among the great ape species are related to interspecies differences in sexual assertiveness of males and females. Sexual activity in the cycle is relatively brief when the female controls mating, more prolonged when the male is in control. This finding in mans closest taxonomic affiliates suggests that similar social factors may also influence the distribution of sexual interactions in the human cycle.

Sex and species differences in captivereared juvenile chimpanzees and orang-utans

Abstract The behavior of nursery-reared juvenile chimpanzees and orang-utans was studied during intraspecific group interactions in large outdoor compounds. Chimpanzees spent more time on the ground, more time in proximity to another and exhibited more slap-hit behavior than the orang-utans. Both groups exhibited only species-typical vocalizations. Males of both species were more assertive in social interactions, whereas females interacted more with inanimate objects. Positive correlations between body weight and the frequency of some social interactions partially confounded interpretation of the sex differences. The results indicate that early experience with adults is not necessary for the development of certain sexually dimorphic and species-typical behavior patterns in juvenile apes. However, early experience with peers seems to facilitate development of these patterns. The results are discussed in relation to available data on the early hormonal and social determinants of behavior in the higher Primates.

Sexual behavior of captive lowland gorillas.

One of the more enduring issues in the study of sexual behavior concerns the degree to which this activity is regulated by hormones. In his early review of this subject, Beach (1942) concluded that, across phylogeny, species with relatively greater encephalization developed greater cognitive control over behavior and a decreased behavioral dependence on hormones. At the time Beach proposed his hypothesis, there had been few systematic laboratory studies conducted on the more advanced species and among the apes, only the chimpanzee having been investigated (Yerkes and Elder, 1936; Yerkes, 1939). The results of these latter studies indicated that chimpanzees, in general, copulated at all phases of the females sexual cycle, but also that the preponderance of sexual activity occurred during a 10-day midcycle period of maximal female genital swelling. The concordant increase in copulation with genital swelling suggested that hormones were influencing sexual behavior of this species, whereas the occurrence of copulation at phases other than.midcycle indicated at least some degree of behavioral independence from hormones. Therefore, based on the chimpanzee data, the great apes appeared to display an emancipation from hormonal regulation of sexual behavior commensurate with their taxonomic affiliation with man. Clearly, evaluation of the phyletic status of a taxonomic family, such as the great apes, requires data on more than a single species in order to define which of the characteristics of interest pertain to the family as a whole and which pertain only to the particular species studied. There is still a paucity of information in the literature related to sexual behavior of the other two great apes, gorilla and orangutan, and that which pertains to the gorilla was derived primari-

Menstrual cycle patterns of hormones and sexual behavior in gorillas

Oppositely sexed pairs of gorillas were tested behaviorally during the menstrual cycle to determine the relationship between hormone concentrations of the female and the frequency of sexual activity by the pair. Five females were tested individually during two cycles with each of two males, but serum samples for hormone assay were obtained from each female only during the first cycle of testing. There was no clear relationship between hormones and behavior for the single cycle in which the serum samples were obtained, with the exception that no copulations occurred after the early luteal phase, when progesterone was greater than 5 ng/ml. Normalized behavioral data from all four test cycles for all pairs suggested that female-solicited copulations were restricted primarily to the periovulatory period. Male sexual initiative (by one of the males) accounted for most copulations temporally dissociated from the periovulatory period. Normalized hormone data for all of the females suggested that (1) attractivity was associated with estradiol concentrations during the follicular phase, (2) proceptivity with estradiol and testosterone at midcycle, whereas (3) receptivity was not associated with hormone patterns or cycle phase. The data suggest that hormones are one of several variables that contribute to the regulation of sexual behavior in gorillas.

Behavioral Responsiveness of Young Chimpanzees (Pan troglodytes) to a Novel Environment

Behavioral responsiveness to a novel environment was documented in 22 chimpanzees grouped according to age; 6-months, 1-year, 2-years and 5-years. An attachment figure, a human caretaker, accompanied each subject during the 15-min test sessions so as to preclude confounding of responses to novelty with separation responses. Extreme distress reported previously for chimpanzees and human children when tested alone in a novel situation was rarely observed in these tests when an attachment figure was present. Stereotyped rocking, an indication of mild distress occurred more frequently in the younger animals. Younger animals engaged in distal visual exploration of the environment while remaining close to the attachment figure, whereas the older animals locomoted more frequently and explored the environment directly with their hands. Repeated exposure to the environment reduced the differences among the 6-month, 1-year and 2-year groups. The 6-month group, however, continued to locomote least and least frequently engaged in tactile exploration. These data on chimpanzees resemble data on human children which suggest that an attachment figure: attenuates the distress exhibited by young individuals of these species when exposed to novel stimuli, and thereby provides a secure base which supports the exploration of novel stimuli, a prerequisite to behavioral adaptation.

Periparturitional behavior of a primiparous lowland gorilla

The behavioral interactions of a primiparous lowland gorilla and her newborn female infant were observed and recorded during the birth of the infant and the subsequent 27 hours. The mothers behavior, initially unresponsive to the infant, progressed rapidly through a series of interactions that are described as preliminary to, and perhaps essential for the ultimate development of appropriate maternal behavior. The relationship between different critical activities, such as: (1) the ingestion of birth fluids by the mother and the establishment of initial contact with the infant, (2) the progression from impersonal contact to ventral-ventral contact, and (3) the coordination between the mothers and infants vocalizations and activity preceding nursing are described, analyzed, and interpreted.

Mother-infant separation and reunion in the great apes

One mother-infant dyad of each species of great ape, orang-utan, chimpanzee and gorilla, with an additional conspecific cagemate, were studied for the effects of maternal separation and reunion. High levels of agitation were observed in all infants immediately upon separation, followed by a longer period of behavioral depression with continuing, intermittent agitation. A compatible cagemate seemed to attenuate, to some degree, the behavioral depression reaction. Initial detachment following reunion with the mother occurred for all the great ape infants studied. Subsequent intensification of mother-infant attachment occurred during reunion for all three species. The findings of intermittent agitation during separation and the initial detachment upon reunion with the mother are not generally reported for monkeys, but are reported for human children. These data suggest that certain responses to maternal separation and reunion occur, to some degree, among all primates that have been studied, whereas other responses seen among apes and humans are not generally reported for monkeys.

Influence of Male Aggression on Mating of Gorillas in the Laboratory

The frequency of sexual interactions between oppositely-sexed pairs of gorillas tested in the laboratory was directly related to the frequency of male aggression directed toward the female. The data suggest that male aggression stimulated female presenting and copulation, and accounted for mating temporally dissociated from the periovulatory period. Among the great apes tested in the laboratory, the male primarily accounts for mating that is unlikely to contribute to reproduction.

Maternal transport and infant motor development in a captive group of lowland gorillas

Infant development and mother-infant interactions were studied in three group-living lowland gorilla infants through the first 18 months of life. Ten modes of maternal transport of infants were identified. Of these, tripedal walk and back-ride were used by all mothers. The other forms of transport were more idiosyncratic. Almost all forms of maternal transport had ceased by 18 months. Infant motor development was found to progress rapidly through the first six months of life, with most motor behaviors exhibited by this age. Beyond this, more specialized (e.g., aggressive) motor patterns appeared.

Sexual behavior of maternally separated gibbons (Hylobates)

The effect of early maternal separation on the sexual behavior of captive gibbons was investigated because (a) maternal separation compromises sexual behavior of some nonhuman primates and (b) adequate sexual behavior is essential to species propagation. Most of the maternally separated gibbons (24/31) were sexually proficient. Sexual behavior overall did not differ significantly in relation to species, sex, origin (wild-vs. captive-born), or type of rearing facility (home vs. zoo). Sexual proficiency was not related to the age at separation from the mother, but it was associated with introduction within 19 months of age to a conspecific of less than 3 years of age and an absolute age difference of less than 2 years. Sexual proficiency was associated with rearing and adult housing in relatively large enclosures. Gibbons that were isolated from conspecifics between 6 months and 2 years of age were strongly attached to humans, but this did not prevent sexual proficiency. A greater proportion of males than females were adversely affected sexually by prolonged early social isolation. Inadequate sexual behavior was associated with fearfulness of conspecifics, which probably interfered with compatible social relationships, including duetting. Inadequate sexual behavior was but one aspect of a more general behavioral deficiency resulting from inadequate early socialization. Early maternal separation in gibbons is compatible with species-typical sexual behavior under the conditions described above. It is not necessary for gibbons to learn sexual and parental behavior by observing experienced adult conspecifics.