Anyway, enough with the atmospheric description, this is science, so let’s talk some. Horripilation, also known as goosebumps, is derived from the Latin word ‘horripilare’, meaning to bristle with hair. It is a subconscious reflex caused by either sudden increases in stress or a rapid temperature change. The function would have allowed other apes to appear larger to scare predators and trap air in their fur to provide insulation. The same reflex can be seen in porcupines that when threatened will raise their quills to deter predators. There is also a surprisingly successful film series starring Jack Black.