Sexual swellings in wild white-handed gibbon females (Hylobates lar) indicate the probability of ovulation

Conspicuous sexual swellings in the females of some primate species have been a focus of scientific interest since Darwin first wrote about them in 1871. To understand these visual signals, research focused on exaggerated sexual swellings of Old World primates. However, some primate species develop much smaller sexual swellings and it is as yet unclear if these smaller swellings can serve similar functions as those proposed for exaggerated swellings, i.e. advertising fertility to attract mates. We studied the temporal patterns of sexual swellings, timing of ovulation and female reproductive status in wild white-handed gibbons (Hylobates lar) at Khao Yai National Park, Thailand, where this species has a variable social organization. We established fecal progestogen profiles in fifteen cycles of eight cycling females and, to detect swellings outside the menstrual cycle, five pregnant and six lactating females. In 80% of menstrual cycles, ovulation and maximum swelling phase (duration: ∅ 9.3 days; 42.8% of cycle length), overlapped tightly. The probability of ovulation peaked on day 3 of the maximum swelling period. Nevertheless, the temporal relationship between maximum swelling and probability of ovulation varied from day − 1 to day 13 of the swelling period and three times ovulations fell outside the maximum swelling phase. The different swellings phases occurred in similar proportions in cycling and pregnant, but not lactating females, which were rarely swollen. Despite their smaller size, gibbons' sexual swellings probably serve functions similar to those suggested for exaggerated swellings by the graded-signal hypothesis, which predicts that sexual swellings indicate the probability of ovulation, without allowing males to pinpoint its exact time.