Cyclicity in tumescence of the perineal labia of female lowland gorillas.

The first group norms on the periodic occurrence of tumescence in the perineal labia of female lowland gorillas were obtained in 9 captive subjects. Individual mean cycle lengths for 8 adult females in 2 experiments ranged from 25.9-36.3 days with overall means of 31.1 and 32.5 days. The one subadult female had a mean cycle length of 40.7 days. The median and modal duration of maximal labial tumescence were 1.8 and 1 day, respectively. Data from 3 animals on which menses were recorded during 6 cycles yielded relatively consistent postswelling phase durations of 8-11 days and durations of menstruation of 1-2 days. The data from 2 conceptions of 1 female suggested that time of ovulation was closely associated with the day of detumescence. These data are discussed in terms of certain differences as well as similarities which exist between the gorilla and its closest relative, the chimpanzee. The contribution of these data to the area of comparative reproduction is enhanced because of the significant taxonomic affiliations of the gorilla species.