[Back to Scientific Articles]
Male Homosexual Behaviour Accompanied by Ejaculation in a Free-Ranging Troop of Japanese Macaques (Macaca fuscata)
Folia Primatol 1998; 69: 364-367
Homosexual behaviour among males in free-ranging Japanese macaques (Macacafuscata) has not been reported, whereas that among females has frequently been reported under both free-ranging and captive conditions ,[ 2]. Mountings between males, commonly seen in Japanese macaques, have not been regarded as 'sexual behaviour'. In most cases, such interactions consist of a single mount and are not accompanied by ejaculation .
In this paper, I analyse observed cases of male-male mountings with ejaculation among a free-ranging troop of Japanese macaques and examine whether this behaviour can be classified as 'sexual behaviour'. The causal factors and effects of such behaviour are discussed.
Study Troop and Method
The study was conducted in the Arashiyama Monkey Park, Kyoto prefecture, Japan, and involved members of the Arashiyama E and F troops. All the monkeys have been individually identified since 1954, and birth dates and matrilineal kin relations are known except for those individuals which came from outside Arashiyama . Soybeans and wheat grains were given to the monkeys by the park staff 4 times a day.
Observation periods encompassed two mating seasons, from September 1993 to February 1994 (period 1) and September 1994 to February 1995 (period 2). Total observation days were 162. The number of individuals in each period was 172 and 181 for the E troop, and 134 and 129 for the F troop, respectively. The adult sex ratio (number of males over 7 years old/number of females over 5 years old) was 0.36 in period I and 0.32 in period 2 for the E troop. and 0.35 and 0.33 for the F troop, respectively.
Table 1. List of males who engaged in mounting events with ejaculation
Four males were observed to engage in male-male mounting events with ejaculation (table 1). Two of them (Mino-6369748389 and Mino-637590) were prepubescent (i.e. just before testicular descent). They were born in the E troop and usually foraged near their mothers in the central unit of the E troop. Momo-677584 was born in the F troop and transferred to the E troop during period I. Glance-647188 was a peripheral male of the F troop. Momo-677584 and Glance-647188 were sexually mature but had not yet reached their full adult body size.
Eleven mounting events with 152 mountings were observed in three pairs: Momo-677584 and Mino-6369748389 (pair 1); Glance-647188 and Mino-637590 (pair 2); Momo-677584 and Mino-637590 (pair 3). All these pairs were combinations of a young adult male and a prepubescent male. No member of a pair had shared maternal kinship. In the cases of pair 1 and pair 3, both participants mounted each other, while in the cases of pair 2 only Glance-647188 mounted Mino-637590.
When the males mounted they showed penile erection and thrusting and young adult males ejaculated in 3 cases (1 case for each dyad). The mountee's penis also erected while he was mounted. Momo-677584 sometimes reached back and stimulated the mounter's penis manually. No anal penetration was confirmed.
Prepubescent males showed several characteristics which are typical of ovulating females; they had bright red faces and emitted vocalizations similar to 'ovulating calls'. Additionally, the prepubescent male tended to sit in front of the young male with ventrodorsal contact during the mounting events as is seen in male-female mounting events. In fact, when a prepubescent male exhibited the above behavioural and physiological characteristics, he and his partner strongly resembled the mating pair of a young male and ovulating female.
The pairs formed consort relationships which lasted several days. They foraged together, groomed each other, attacked other monkeys together and engaged in mounting events. None of the adult males and females seemed to pay any attention to these consort pairs. It was once observed that Momo-677584 chased the consort pair of Glance-647188 and Mino-637590 and consequently forced them to terminate the mounting event.
In addition to the above observation, 2 other males were noted to have congealed semen on their lower back on one occasion in the study period (table 1). It is obvious that they had been mounted and ejaculated on by other males. Mino-6369748389 and Mino-637590 were observed to engage in sexual behaviour with troop E females.
Mounting events observed in this study included most of the behavioural elements typical of heterosexual interactions of Japanese macaques (multiple mountings, pelvic thrust, ejaculation, consortship), and shown by physical and behavioural characteristics of one of the participants mimicking characteristics during ovulation. Also these interactions occurred during the mating season. These interactions might be classified as sexual behaviour.
It is difficult to examine the causal factor (i.e. abnormal hormonal state , ) of these behaviours since there is no physiological information available. All the pairs were combinations of a male from troop E and a male from troop F, so it is unlikely that the participants had formed abnormally affiliative relations as juveniles as reported by Erwin and Maple .
There are two hypotheses about homosexual behaviour:
Although the sex ratio of adults of the Arashiyama troops during the study period was not biased towards males compared to other troops , it is difficult for peripheral males to gain access to ovulating females, since the central adult males would attack the peripheral males if they tried to do so. Females did not tend to prefer prepubescent males. In group-living rhesus macaques, the males engaged in homosexual behaviour when their access to females was physically limited . Thus in these cases, it is possible that the males engaged in homosexual behaviour as an alternative to heterosexual intercourse, because their access to ovulating females was socially limited.
I wish to thank Prof. T. Nishida for his guidance and encouragement. Drs. J. Yamagiwa, Y. Takahata. H. Ihobe and D.S. Sprague for stimulating discussion and valuable advice, Dr. S. Suzuki. Ms. L.A. Turner and other members of the Laboratory of Human Evolution Studies of Kyoto University for discussion and helpful criticism and Mr. N. Asaba and other staff of the Iwatayama Monkey Park for their support during the field study.
 Wolfe LD. Japanese macaque female sexual behaviour: A comparison of Arashiyama east and west: in Small M (ed): Female Primates: Studies by Women Primatologists. New York. Liss. 1984, pp 141-157. [Back]
 Huffman MA: History of the Arashiyama Japanese macaques in Kyoto, Japan: in Fedigan L. Asquith P (eds): The Monkeys of Arashiyama: Thirty-five Years of Research in Japan and the West. Albany, State University of New York Press, 1992. pp 21-53. [Back]
[Back to Scientific Articles]