Gibbons, known for their acrobatic movements through forest canopies, exhibit sexual dimorphism, meaning distinct differences between males and females of the same species. While not always immediately obvious, gibbons do possess subtle variations in appearance and, more notably, in their vocalizations, which allow for differentiation between the sexes.
Subtle Physical Differences
While gibbons are often described as having minimal physical differences between sexes, some subtle traits can distinguish males from females. Males are typically observed to be slightly larger or heavier than females, though this size difference is generally not as pronounced as in many other primate species. For instance, northern white-cheeked gibbons show very little sexual dimorphism in size, with males averaging around 7 kg and females slightly less.
However, fur color provides a more striking form of sexual dimorphism in certain gibbon species, a trait known as sexual dichromatism. In some Nomascus species, such as the northern white-cheeked gibbon, adult males are predominantly black with white cheek stripes, while females are a pale golden color with a dark brown patch on their heads. Similarly, the pileated gibbon (Hylobates pileatus) displays clear sexual dimorphism, with males having purely black fur and females exhibiting a white-grey body with black on the belly and head. Other species like the white-handed gibbon (Hylobates lar) show varied fur colors across individuals that are not specific to sex. The Bornean gibbon (Hylobates muelleri) also shows little to no pronounced sexual dimorphism in fur coloration, with both sexes appearing similar.
Vocalizations as Key Identifiers
For many gibbon species, vocalizations serve as the most reliable method for identifying an individual’s sex, often surpassing visual cues. Gibbons are known for their elaborate “songs,” which are loud, complex vocalizations typically performed in the early morning. These songs are often structured as coordinated duets between a pair-bonded male and female.
In these duets, the female generally leads with a characteristic series of notes known as the “great call,” a loud, conspicuous phrase that can accelerate in tempo and pitch. The male contributes with specific, less complex notes or a multi-note “coda” section that typically follows the female’s great call. These species-specific songs are distinctive, making them a crucial tool for researchers to identify different gibbon species in the wild. The nuances within these calls, including variations in frequency and temporal patterns, allow for the identification of individual gibbons and their sex, even when direct visual observation is difficult.
Ecological Significance of Dimorphism
The dimorphic traits in gibbons, particularly their prominent vocal differences, play an important role in their natural environment. The complex duets performed by mated pairs serve as a primary mechanism for territorial defense, allowing them to announce their presence and deter rival groups. These loud vocal displays are an efficient way to defend resources without direct physical confrontation.
Beyond territorial defense, these distinct vocalizations reinforce the pair bond between male and female gibbons and advertise their mated status. The unique songs also aid in species recognition, helping gibbons find suitable mates and preventing hybridization where different gibbon species overlap geographically. The subtle physical dimorphism, especially the minimal size difference compared to other primates, aligns with their largely pair-bonded or monogamous social structure, where intense male-male competition for mates is less prevalent. This contrasts with species where males compete fiercely, often developing larger body sizes or weaponry.