Feathers & Fur
Animal furs generally have at least 2 kinds of hair: guard hair (longer, straighter, colored) and ground hair (shorter, denser, and often curly).
A similar distinction exists for bird feathers. Bird wings are optimized for flight, but they also serve as insulation, among other things.
Mammal hair varies from straight to curly. For aquatic mammals, hair shape allows an air layer to be maintained within the fur during submersion, increasing water repellency.
After being soaked, many animals shake themselves to rid their bodies of water. They do so at optimal frequencies. The bigger an animal gets, the slower it shakes. Whereas a mouse shimmies at 27 hertz – back and forth 27 times per second – a bear shakes at 4 hertz. Loose skin helps by oscillating to release more water than if skin were tighter.
Shedding water efficiently is important. It could otherwise cost 25% of daily caloric intake in maintaining body temperature, and risk hypothermia and death.
For land mammals, fur features are optimized for the thermal habitat. That often means keeping out the cold.
Polar bear fur is only 5 cm thick yet impenetrable enough to keep bear bodies warm, even when temperatures reach a frigid –40 °C.
In adapting to the Arctic climate, polar bears got stinky feet. Pungent paws leave scent tracks by which polar bears can find one another; because sheer white-on-white in a vast wilderness would be a surefire formula for species extinction.
It is commonly assumed that feathers and fur keep animals warm by trapping a layer of air that slows thermal conduction. Instead, radiation shielding often plays a larger role.
Bird feathers are often barbed, with long appendages that diffuse thermal radiation. Barbed feathers and the individual hairs of fur are precisely arranged to repeatedly backscatter infrared light. These create a radiative shield that provides thermal insulation.
If trapping heat were the point, polar bears would be black. Instead, their white fur maximizes reflectivity. This keeps polar bears warm, as it creates the most effective radiative shield.