1. When in water, the outer layer adheres closely to the body, compressing the dry underfur.

2. The guard hairs are hollow, trapping air within and between them, and the underfur also traps air.

3. 8 The guard hairs are hollow, trapping air within and between them, and the underfur also traps air.

4. The denseness of fur can increase an animal's insulation value, and arctic mammals especially have dense fur; for example, the musk ox has guard hairs measuring 30 cm (12 in) as well as a dense underfur, which forms an airtight coat, allowing them to survive in temperatures of −40 °C (−40 °F).