Emperor penguins have spear-shaped, stiff, short feathers. They have the highest feather density among birds: about 100 feathers per square inch. In addition to a favorable shape and density of feathers, emperor penguins also have oil glands in their skin, which provide a water proof cover to the outer layers of feathers. This tight and dense body cover protects the birds against temperatures as low as -60 degrees Fahrenheit, allowing them to swim under cold waters.
Unlike most birds, penguins have wings in the shape of stiff flippers, which are adapted for swimming instead of flying. While most flying birds exert pressure only in the down-stroke when flying, emperor penguins actively exert propulsion during the upstroke and down-stroke movements, when swimming underwater. Emperor penguins have an average 30-inch wingspan.
Penguins do not have hollow or pneumatic bones, which are typical of most flying birds. The solid bones of emperor penguins help decrease buoyancy when diving, allowing them to reach great depths in a short time. Emperor penguins have flattened bones in their wings, which help reduce water resistance when swimming.
Hemoglobin is the substance responsible for carrying oxygen from the lungs to all body cells, through the blood stream. Emperor penguins carry a different type of hemoglobin, which allows them to survive long periods underwater without breathing. When diving, emperor penguins have their heart rate reduced to 15 beats per minute, when in normal conditions it is about 70 beats per minute.