Juvenile White - Handed Gibbon (Hylobates lar) Social Development

White-handed gibbons (Hylobates lar) are lesser apes endemic to the tropical forests of Southeast Asia. Most gibbon species are monogamous, but live in small family groups characterized by complex social relationships that evolve throughout their lifetime. Due to poaching and habitat fragmentation, white-handed gibbons are listed as a critically endangered species. A Gibbon Species Survival Plan (SSP) was established to oversee white-handed gibbon populations' breeding, transfers, and management in AZA-accredited institutions. However, gibbons' arboreal nature makes them challenging to study in the wild, and there is a lack of studies on their natural social behaviors and life history timelines in situ. Improper socialization early in life could hinder social development, leading to dysfunctional social groups and unsuccessful breeding efforts.

In March 2024, an infant male white-handed gibbon, Eros, was born at the Philadelphia Zoo. This observational study, approved by the Gibbon SSP, aimed to describe and quantify Eros's behavioral changes during weaning—the critical transitional period from exclusive reliance on maternal care and nutrition to complete independence. This study observed an abrupt stop of weaning at around 15 months of age, different from previous literature, which suggested weaning as a gradual process occurring over several months. Additionally, a transition from Eros's nursing behavior with his mother to social behaviors with his sister was observed throughout the study. This is consistent with previous literature, which suggested a "two-stage learning model" for primates.