Forest loss is affecting the facial features of orangutans

Photo credit: Zac Mills, the Wildlife Collective

All male orangutans begin life looking quite similar. But during puberty, some males develop large cheek pads (flanges) and throat sacs, while others remain unflanged. Despite being fully adult, unflanged males can delay their physical transformation for decades or skip it entirely. What triggers this “developmental arrest” remains a mystery.

Initially, scientists thought unflanged males would suffer higher levels of stress due to fear of dominant, flanged males. However, flanged males actually showed higher levels of cortisol: the stress hormone. This sparked a new theory: unflanged males experience less severe stress in childhood, making “developmental arrest” a strategy to avoid the physical strain of chronic stress.

Researchers found that flanged males have much deeper tooth grooves than unflanged males. In fact, they have the deepest grooves recorded in any great ape. This shows that flanged males suffered far more severe childhood stress. While unflanged males still experienced stress, it was much less intense. However, whether high stress triggers flanging (or low stress prevents it) remains unclear.

How does this relate to forest loss? Wild orangutans have high levels of nutritional stress, and are often faced with fruit shortages. As habitat loss worsens, the life of a young orangutan will become even harder. If childhood stress triggers flanging, increasing forest fragmentation could drive more males to flange at puberty, causing unflanged males to disappear entirely.