Heape[92] has most fully and carefully described menstruation in monkeys.  He found at Calcutta that the Macacus cynomolgus menstruated regularly on the 20th of December, 20th of January, and about the 20th of February.  The Cynocephalus porcaria and the Semnopithecus entellus both menstruated each month for about four days.  In the Macaci rhesus and cynomolgus at menstruation “the nipples and vulva become swollen and deeply congested, and the skin of the buttocks swollen, tense, and of a brilliant-red or even purple color.  The abdominal wall also, for a short space upward, and the inside of the thighs, sometimes as far down as the heel, and the under surface of the tail for half its length or more, are all colored a vivid red, while the skin of the face, especially about the eyes, is flushed or blotched with red.”  In late gestation the coloring is still more vivid.  Something similar is to be seen in the males also.

Distant, who kept a female baboon for some time, has recorded the dates of menstruation during a year.  He found that nine periods occurred during the year.  The average length between the periods was nearly six weeks, but they occurred more frequently in the late autumn and the winter than in the summer.[93]

It is an interesting fact, Heape noted, that, notwithstanding menstruation, the seasonal influence, or rut, still persisted in the monkeys he investigated.

In the anthropoid apes, Hartmann remarks that several observers have recorded periodic menstruation in the chimpanzee, with flushing and enlargement of the external parts, and protrusion of the external lips, which are not usually visible, while there is often excessive enlargement and reddening of these parts and of the posterior callosities during sexual excitement.  Very little, however, appears to be definitely known regarding any form of menstruation in the higher apes.  M. Deniker, who has made a special study of the anthropoid apes, informs me that he has so far been unable to make definite observations regarding the existence of menstruation.  Moll remarks that he received information regarding such a phenomenon in the orang-utan.  A pair of orang-utans was kept in the Berlin Zooelogical Gardens some years ago, and the female was stated to have at intervals a menstrual flow resembling that of women, and during this period to refrain from sexual congress, which was otherwise usually exercised at regular intervals, at least every two or three days; Moll adds, however, that, while his informant is a reliable man, the length of time that has elapsed may have led him to make mistakes in details.  Keith, in a paper read before the Zooelogical Society of London, has described menstruation in a chimpanzee; it occurred every twenty-third or twenty-fourth day, and lasted for three days; the discharge was profuse, and first appeared in about the ninth or tenth year.[94]