Sexual swellings of female gibbons

Female gibbons were observed for labial swellings from March 2002 to August 2003. Data were collected on length of time each female was swollen, changes in swelling period as females matured, and cycle length (number of days between onsets of consecutive swellings). Data were also collected on the behaviour of the males when the females were swollen. There was a significant relationship between age of the female gibbon and number of days she had labial swelling. There was no significant relationship between the age of the gibbon and the cycle length, nor between species (Hylobates agilis albibarbis and Hylobates muelleri) and number of days swollen. Copulation was more common when females were swollen. Swelling in female gibbons may be related to copulatory behaviour and serve to provide the male with some certainty of paternity through fertilisation insurance.