Significant in many of these properties is the hair follicle appendage. The key role of hair is to provide protection against heat loss. Hair traps air adjacent to the skin to provide an invisible, insulating layer. Several mammalian species produce special dense winter coats with added heat trapping properties. Otters have hair to trap a layer of air around their bodies keeping them warm and making them more buoyant while they swim.
Many species go through molting cycles in tune with seasonal changes.
Hair can provide indications of sexual development through onset of secondary sexual characteristics from development of a mane on a male lion to beard development in humans. Alternatively, it may aid in camouflage for survival where mute tones or dappled color blend with an animal's environment. Specialized hair such as eyebrows and eyelashes protect the eyes by channeling or sweeping away fluids, dust and debris. Nasal hair plays an important role in trapping air borne foreign particles before reaching the lungs. Hair fiber may also increase the surface area for faster evaporation of sweat from neighboring apocrine glands.