Testosterone just the opposite effect on hair follicles in the face. In men it's the onset of puberty and the elevated levels of testosterone that stimulate the follicles of the face to make beard hairs, and beards are exclusively a masculine feature. Extremely old women can grow stubble such as a man's when their estrogen dries up, because women do produce small amounts of testosterone. When their supply of estrogen is completely gone, there's enough to stimulate their facial hair follicles.

Equivalent phenomena occur in animals. The "ruff" of fur around the neck is associated with some breed of dogs in intact males, along with the large and impressive mane in the male lion. In gorillas, maturation of males contributes to the development of silvery tips on the guard hairs, as well as a male that has reached a clear stage is referred to as a "silverback."

Color change with season is another hormonally-controlled aspect of the hair cycle: in this instance it's the activity of melanocytes being modulated to generate more or less pigment. The varying hare of your northern latitudes shuts its melanocytes off during the cold months, and grows white hair as camouflage inside the snow; in Spring the melanocytes increase production again and the hare turns brown, to better blend in with the foliage to the forest floor.

Other Influences

Other pursuits that influence hair growth include radio waves (especially hard gamma rays and X-rays) and several drugs. Any agent, physical or chemical, which affects rapidly dividing cells will affect hair regrowth, usually by stopping it. The loss of hair (alopecia) is a common side effect of radiation treatment and chemotherapy. Parasites, disease, and poor nutrition all affect hair growth. The condition of an animal's hair coat can be quite a very good diagnostic clue to its internal medical problems.