Department of Forest Conservation, Deep Forest Field School, Inc.
March 28, 2015 4:30, Grand Ballroom E/F/G
Monogamy is relatively uniform within hylobatids but the external genitalia of females vary in structure and response to hormones. In particular, female Nomascus are characterized as having a peniform clitoris, and a membrane has been shown to occlude the vulva. To verify this diversity, a comparative study was made of living representatives of four genera: Nomascus (n=9); Bunopithecus (n=4); Symphalangus (n=3); Hylobates (n=8). Using head and body length to obtain relative sizes, the Relative Ano-Genital Distance, RAGD, was taken as a measure of masculinization and compared with the relative length of the external clitoris (RCL). Results showed that adult Nomascus have a longer ano-genital distance (RAGD = 7.0) than the other genera (RAGD = 4.5). They exhibit a relatively long clitoris (RCL = 2.3) compared to the sessile clitoris of Hylobates (RCL = 1.3) but this was similar to those of Bunopithecus (RCL = 2.3) and Symphalangus (RCL = 2.75). The clitoris of adolescent Nomascus, however, was found to be truely peniform (RCL = 4.0) and a membrane occluded the vulva so the genitalia appear to be male and intromission impossible. Resemblance to the male is enhanced by young females exhibiting the black pelage of the male rather than the golden-brown of the adult female. To masquerade as a male and prevent vaginal penetration may contribute to a female's ability to extend its adolescence within the natal group while minimizing incestuous attention from male relatives and aggressive attention from the mother.
This research was supported by the Gibbon Conservation Center. Los Angeles Zoo, Cheyenne Mountain Zoo, Denver Zoo, and Americas Teaching Zoo at Morepark College provided key access to study subjects.