Reproductive physiology and behaviour of gorillas

Research on reproduction of gorillas focussed on 3 related issues, cyclicity in sexual behaviour, hormonal regulation of sexual behaviour and restriction of mating to the periovulatory phase of the menstrual cycle. A study of female genital swelling or perineal labial tumescence demonstrated that this tumescence exhibited a pattern that was similar to that of the female chimpanzee, although maximal tumescence was for only 1-2 days of the 32-day cycle. Female gorillas were the initiators of sexual activity and copulation was restricted to a relatively brief, 1-2 day period, associated with maximal or near-maximal labial tumescence. Gorillas exhibited menstrual cycle hormone patterns more similar in certain respects to the chimpanzee and human than to other primates. Labial tumescence increased with increasing concentrations of oestradiol-17 beta during the follicular phase and detumescence occurred after the midcycle surge in LH at the time oestradiol was declining and progesterone was increasing. Sexual behaviour of individual gorillas was not restricted to the time of the midcycle peaks in oestradiol-17 beta or testosterone. The pattern for the entire group, however, indicated increased copulation with increasing oestradiol values and a maximal frequency of copulation on the day of the testosterone peak. When females controlled mating, copulation occurred primarily during the periovulatory period, whereas mating temporally dissociated from the periovulatory period seemed to result from the male's intimidation of the female. The data suggest that hormones exert a significant influence on the regulation of sexual behaviour by gorillas, but that environmental and social factors can moderate that influence.