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Gilbert Van Tassel Hamilton (1877-1943) was an American physician specialising in psychotherapy who wrote “A study of sexual tendencies in monkeys and baboons”, published in the Journal of Animal Behavior, vol. 4, no. 5, Sept.-Oct. 1914 (New York: Henry Holt & Co.) pp. 295-318, one of the earliest studies to draw attention to the common practice of pederasty by monkeys.

Hamilton was concerned with all the sexual tendencies of macaques and baboons, two kinds of Old World Monkey (Cercopithecidae, the family most closely related to hominids), but the extracts presented here are limited to his introductory comments on the relevance of his findings to understanding human behaviour, his observations of pederasty and such of the general conditions he described as may be relevant to these observations.

A study of sexual tendencies in monkeys and baboons


Montecito, California

Gilbert Van Tassel Hamilton

In spite of the considerable advance that has been made in our knowledge of sexual life since the appearance of Freud's[1] Drei Abhandlungen zur Sexualtheorie, we still lack that knowledge of infra-human sexual life without which we may scarcely hope to arrive at adequately comprehensive conceptions of abnormal human sexual behavior. For example, the possibility that the types of sexual behavior to which the term “perverted” is usually applied may be of normal manifestation and biologically appropriate somewhere in the phyletic scale has not been suffi­ciently explored. Homosexual tendencies come to frequent expression in adolescent boys and girls, thereby presenting to the mental hygienist a problem, the solution of which awaits, first of all, biological knowledge of homosexuality which only the behaviorist can supply. It is unnecessary to multiply examples in illustration of the fact that both the theoretical interests of the science of behavior and the practical needs of what we may regard as a group of applied sciences of human behavior (viz., mental hygiene, criminology, psychopathology) place upon the animal behaviorist an obligation to lay the necessary foundations for a scientific and thoroughly comprehensive investigation of sexual life. […]


The estimated or known age of each subject is given for January, 1914. Inability to identify an animal as to species is indicated by a dash after the generic name. The “pet” name of each animal is given to facilitate the reader's identification of subjects in subsequent descriptions of behavior. An animal's sex is indicated by its laboratory number – even numbers for males, odd for females. In reporting the behavior of an animal the first reference to it will include, in the order given, its labora­tory number, pet name and initial letters of the genus and species to which it belongs. [...]

Monkey 10. Timmy. M. cynomolgus. Adult. Father of Monkeys 9, 13 and 24.

Baboon 12. Sandy. Papio—. Adult. Fur black over back and forehead, grey elsewhere. “Pig” tail, long muzzle. Legs and body much shorter and thicker and those of the chacma. A very powerful animal, almost equal to the chacma in weight.

Monkey 14. Jimmy II. M. cynomolgus. Young adult.

Monkey 16. Sobke. M. rhesus. Young adult.

Monkey 28. Scotty. M—. Young adult. Probably be- longs to the cynomolgus group.


Rhesus macaque (Macaca mulatta)

My laboratory is in the midst of a live oak woods in Montecito, California, about five miles from Santa Barbara. […] The climate here is […] exceptionally mild, even for Southern California, and at no times seems to reduce the activities of macaques – except, of course, when rain drives them to shelter.     My subjects have always been in excellent physical condition, the only deaths having been due to accident or to pathological causes that were operative at the time of an animal's purchase. […]

The entire enclosure is surrounded by live oak trees, and one tree is contained within the yard. The animals, when at large, could wander to an indefinite distance from the laboratory by passing from tree to tree, but they rarely wander out of sight of the yard. […]


The outdoor conditions described above have enabled me to liberate most of the subjects in selected pairs or groups. […] At this point it may be said that, provided the macaque or baboon have a sufficiently wide range for pursuing and fleeing his enemies, playing with his fellows, etc., confinement will be found to have no perverting effects in this climate. […]

Wherever reference is made to an animal as “sexually immature” it is to be understood […] that […] males are regarded as sexually immature until they have assumed the characteristic strut that I have found to be coincident in appearance with seminal discharge on copulating with females. Sexual maturity, in this sense, antedates the attainment of adult size by at least a year. […]

Situation 7. Several males and females at large. Number of each sex sufficient to enable each animal to have frequent natural sexual relation.

Observation 11. 16-Sobke-M-r (sexually mature), Jimmy II­-M-c (sexually mature), 24-Mutt-M-cr (sexually immature), 22­-Daddy-M-c (adult eunuch), 20-Chatters-M-c (immature eunuch), 18-Baby-M-c (immature eunuch) and the following sexually mature females had been at large for several weeks: 11-Grace-­Papio-, 3-Kate-M-r, 7-Becky-M-r, 5-Maud-M-r and 9-Gertie-­M-cr. They wandered freely about the laboratory yard, through the trees and over the tops of the laboratory and the cage. During this time I was able to make a large number of observa­tions, which will be summarized here, rather than given separately and in detail. I found that, although Sobke dominated the band, and could therefore have as frequent sexual relations with any of the females as he desired, he, like all the other males, indulged in homosexual relations. His case is especially interesting, since he was acquired when quite small, and has spent much of his time at large, in company with both males and females. I cannot recall any time when he was confined without at least one female companion. Nevertheless, during the time now under discussion he would approach a smaller male, smack his lips and mount the object of his homosexual desires with evidence of considerable sexual excitement. He copulated with the females more frequently than with the males, and his homosexual behavior was of decidedly less frequent occurrence several months after he arrived at sexual maturity than it was during his sexual immaturity. The same kind of observations were made with reference to Jimmy II: even after he became sexually mature, and had free, outdoor access to females, he would use male monkeys as he used females, but this behavior became much less frequent after sexual maturity than it was during sexual immaturity. […]

During the time that these eleven monkeys were at large I was never able to observe a sexually mature (uncastrated) Monkey assume the female position for copulation with a weaker fellow. Sobke, for example, would use Jimmy II as he would use a female, but would not assume the female position for Jimmy II. [...] Sobke was always larger than Jimmy II, but before these two animals arrived at sexual maturity one was as apt to assume the female position during homosexual relations as was the other. [...]

Situation 8. Mature male and immature male, having formed a friendship for one another, are separated.

Observation 12. 14-Jimmy II-M-c (sexually mature) and 24-Mutt-M-cr (sexually immature) had been with a band consisting of females and other males. They all had access to the alley and four of the cages. Although Jimmy II cohabited with the females and the eunuchs, he and Mutt were usually seen perched upon a shelf together. Flea-hunting and affectionate embraces usually terminated in homosexual relations, Mutt always assuming the female position. After they had been together two weeks Jimmy II was removed from the band and placed in a cage with 7-Becky-M-r for one week. During this week of confinement with Becky, Jimmy II copulated with her freely. At the end of the week he was allowed to rejoin the band. As soon as he and Mutt caught sight of one another they rushed into an embrace, smacking their lips and making crooning sounds. Both animals quickly manifested marked sexual excitement, and when Jimmy mounted Mutt the latter even rotated his head to bring his lips in contact with those of his friend. I have found that friendships between immature males and mature males are of frequent occurrence, and that they seem to have a sexual basis. [...]

Hamadryas baboon (Papio hamadryas)

Situation 14. Male, threatened by larger male and unable to escape.

Observation 22. When 12-Sandy-Papio— (mature) was first admitted to the alley band, of which 24-Mutt-M-cr (immature) was a member, all of the band excepting Grace fled. Sandy, it will be remembered, is a very large baboon, and Mutt is an immature rhesus, hence very tiny in comparison with Sandy. When the band scattered on the approach of the baboon, Mutt darted into a cage and crouched in a corner, on the floor. Sandy followed him, leering at him threateningly. Mutt squealed in terror, and looked about him as if seeking an avenue of escape. Finding none, he assumed the female sexual position, but as Sandy approached, now displaying only friendliness and sexual excitement, the little monkey darted between his legs and escaped.

Observation 23. 28-Scotty-M— occupied a cage adjacent to that occupied by 10-Timmy-M-c. Scotty is sexually mature, but not yet of adult size. He is very timid and Timmy is a savage adult. Timmy forced the netting partition loose and entered Scotty's cage. Scotty fled in terror, shrieking and gnashing his teeth. When Timmy finally caught him and began to bite him Scotty assumed the female sexual position, thereby effecting a temporary escape, since this diverted Timmy sufficiently to enable his victim to dart past him. [...]

Situation 18. Infant monkey in danger of attack. [...]

Observation 30. 24-Mutt-M-cr, when 6 months old, was observed to assume the female sexual position while his mother was absent from the shelf. He was whimpering and running back and forth on the shelf, when 10-Timmy-M-c (mature) threatened him from the other side of the netting. The infant assumed the female sexual position momentarily, then fled down the side of the cage to his mother. This kind of behavior may have been manifested earlier.


Cynomolgus macaques, the most common species of monkey in Hamilton's laboratory

1. Tendencies to Seek Sexual Satisfaction.

A. Male Tendencies.

(a) Tendency to engage in typical sexual intercourse with females.

(b) Tendency to increase sexual excitement by preliminary examination of the female's genitalia, or by chasing and biting the female.

(c) Tendency to use a younger or weaker male as a female.

(d) Tendency to play the role of female to a copulating male.

(e) Tendency to attempt copulation with non-primates and humans.

(f) Tendency to masturbate (probably developed only under abnormal conditions).

2. Tendencies to Assume the Female Sexual Position as a Defensive Measure.

A. Male Tendencies

(a) Tendency to assume the female sexual position when attacked by a more powerful fellow of either sex.


At least two, and possibly three, different kinds of hunger, or needs of individual satisfaction, normally impel the macaque toward the manifestation of sexual behavior, viz., hunger for sexual satisfaction, hunger for escape from danger and, possibly, hunger for access to an enemy.

Homosexual behavior is normally an expression of tendencies which come to expression even when opportunities for hetero- sexual intercourse are present. Sexually immature male monkeys appear to be normally impelled toward homosexual behavior by sexual hunger. The fact that homosexual tendencies come to less frequent expression in the mature than in the immature male suggests the possibility that in their native habitat these animals may wholly abandon homosexual behavior (except as a defensive measure), on arriving at sexual maturity. [...]

It is possible that the homosexual behavior of young males is of the same biological significance as their mock combats. It is clearly of value as a defensive measure in both sexes. Homosexual alliances between mature and immature males may possess a defensive value for immature males, since it insures the assistance of an adult defender in the event of an attack.


[1] Freud, Sigmund. Drei Abhandlungen zur Sexualtheorie. Vienna. 1905. [Author’s footnote]




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