A platypus has such dense fur that its skin stays completely dry while swimming underwater.

The platypus (Ornithorhynchus anatinus) possesses one of the most specialized coats in the animal kingdom. Research indicates their fur density is approximately 600 to 900 hairs per square millimeter, which is significantly higher than most other mammals. This density is achieved through a unique follicle structure where multiple fine under-hairs emerge from a single pore alongside a sturdier guard hair.The coat consists of two distinct layers that work in tandem. The inner layer is composed of fine, kinky wool that traps a thin cushion of air directly against the skin. This air layer acts as a thermal barrier, preventing body heat from escaping into the cold water. The outer layer consists of flat, spade-like guard hairs that overlap like shingles on a roof. These guard hairs are coated in a natural oil that makes the surface hydrophobic.When the platypus dives, the water pressure pushes these flat guard hairs together to create a watertight seal. This prevents water from penetrating the woolly undercoat, ensuring the skin never actually touches the water. This evolutionary adaptation is critical for survival, as platypuses spend up to 12 hours a day hunting in water temperatures that can drop as low as 0°C (32°F). Without this high-performance insulation, their high metabolic rate would be unable to compensate for the rapid heat loss.