What is the primary purpose of the Arrector pili muscle?

The primary purpose of the Arrector pili muscle is to cause hair to stand up. This small, smooth muscle is attached to hair follicles and is responsible for the phenomenon commonly referred to as "goosebumps." When the body experiences cold temperatures or emotional responses such as fear or excitement, the Arrector pili muscles contract. This contraction pulls the hair follicle and causes the hair to stand upright, which can create a layer of insulation to retain heat or make an individual appear larger to threats.

The other options do not accurately describe the primary function of the Arrector pili muscle. While hair growth is influenced by various factors including genetics and hormones, the muscle itself does not play a direct role in this process. Additionally, the muscle does not have a function related to fat storage or protection against bacteria. Its primary action is specifically focused on affecting the position of hair in response to external stimuli.