White-handed gibbons (Hylobates lar) are a species of lesser ape that use vocalizations as their primary mode of communication. These vocalizations include female-specific, stereotyped sequences known as great calls. To better understand the function of great calls, we examined if the onset time of great calls in wild white-handed gibbons differed in the presence or absence of predator model stimuli. Vocalizations were recorded from fifty-two wild gibbons in Khao Yai National Forest in Thailand. Duration measurements in minutes:seconds (MM:SS) were taken from the start of the vocalization bout to the start of the first great call note. The average duration measurements from the start of calling to the first great call note is 11:16 ± 9:58 for clouded leopard responses, 2:38 ± 4:22 for duets, 15:28 ± 9:22 for snake responses, and 14:51 ± 11:52 for tiger responses.

The duet calls had the shortest average onset timing. Based on our preliminary analysis there was a statistically significant difference between the treatment groups (p-value = 0.003). A shorter onset time in duet calls is potentially due to predator warning calls including a referential sequence of notes at the beginning of each call bout. Future research will focus on whether the variability in onset timing is correlated with the strength of the pair bond.