Have you ever noticed a photo of the chimpanzees looking larger and “fluffier” than normal? Having hair that is pilo erect means that it involuntarily stands on end due to excitement, fear, or cold. Humans become pilo when we get goosebumps, and since we have less hair, it’s less obvious. Another example of piloerection is when a dog’s hackles go up. Looking to see if a chimpanzee is pilo or not helps caregivers gauge their arousal level. I perused through some of our old photos and videos to find some good examples of each chimpanzee demonstrating piloerection.
I had a tough time finding a photo of Negra with pilo erect hair, but that’s because she’s usually in action during a fight. In the following video from a couple years ago, you can see Negra and others during a conflict. Watch for some good examples of piloerection: