Piloerection, noun (pi·lo·erec·tion): “bristling of hairs due to the involuntary contraction of small muscles at the base of hair follicles that occurs as a reflexive response of the sympathetic nervous system especially to cold, shock, or fright”
-Merriam Webster

For humans, we are familiar with piloerection as goosebumps or when the hair on the back of our neck stands up. Since chimpanzees have longer, thicker hair that covers their entire bodies, we can really notice when their arousal levels change.

Here is Jamie working up to a display. In addition to grabbing her barrel so she can bang her fists on it and push it around, her hair starts to stand on end to make herself look bigger (and more intimidating):

Here is a pilo Annie showing the loose neighbor’s cow just how scary she can be:

And an upset Negra getting reassurance from Jamie (who is also pilo):

Here is Foxie looking a little “fluffy”:

And Missy who is clearly a bit chilly:

Burrito spends a great deal of his time with his hair standing up:

This morning Jody rushed up to Missy with extremely pilo hair. This set off an urgent game of chase around Young’s Hill: