Splitting Hairs

Hairs in mammals and feathers in birds are two of the smallest and most fascinating organs in the body. These keratinized skin appendages serve the purposes of thermoregulation, sensation, and social interaction, and in the case of feathers, flight. Early stages of feather and hair development have both common and distinct features, so comparative studies provide useful ideas about how both structures form (thoroughly reviewed in Chuong 1998). The variation of these organs in nature is impressive. Otter skin has 125,000 hairs per square centimeter to provide insulation and a waterproof exterior, whereas human hair is relatively sparse and mouse foot pads are devoid of hair. Peacock feathers, with their extraordinary colors and patterns, make up about 17% of the birds’ body weight. How is all this controlled?