Penguins' waterproof feathers

Penguins maintain their plumage by spreading a water-repellent oil from the uropygial gland across their feathers. Dense, waterproof feathers help retain insulation and reduce heat loss; air and bubbles may remain between the body and plumage in water. In many species, subcutaneous fat provides additional insulation. Together these adaptations allow penguins to swim for long periods in cold water, though their roles and thickness vary by species and conditions.