Final answer:
The process of raising individual hairs in mammals for insulation is called piloerection, accomplished by arrector pili muscle contractions that trap warm air to increase insulation.
Step-by-step explanation:
The raising of individual hairs in mammals to increase the insulation capacity of the fur is a process known as piloerection. This is achieved through the contraction of small muscles in the skin called arrector pili. When mammals are cold, the arrector pili muscles contract, causing the hairs to stand up and trap a layer of warm air close to the skin, effectively increasing insulation. In humans, this reaction is evident as goosebumps. The insulating properties of hair or fur are greatly beneficial for maintaining a constant body temperature, especially in colder environments or for smaller animals with a larger surface area relative to their size. Filled with air and breaking up convection, fur is an exceptional lightweight insulator, as the low conductivity of air within it prevents rapid heat loss.