Some animals' fur grows thicker for staying warm in the winter.
Mammals have hair and fur.
Some animals' hair grows thicker for staying warm in the winter.
Why do some animals have hair that grows longer during the winter?
Options:
  * It expands
  * to make braids
  * to wear a scarf
  * to attract mates
  * When it's cold
  * to stay warm
  * It remains dormant
  * genetic diversity