Duck-Billed Platypus: Ornithorhynchidae
A platypus, at first glance, resembles an otter with a duck's bill on its face and a beaver's tail in back. An adult platypus, about the size of a house cat, weighs from 3 to 5 pounds (1.5 to 2.5 kilograms), its adult head and body length runs 12 to 18 inches (30 to 45 centimeters), and the tail adds another 4 to 6 inches (10 to 15 centimeters). Males are larger than females.
The snout, despite its duckbill shape, is soft, moist, and rubbery in texture, not hard like a bird's beak. The bill has an upper and lower section, like that of a mammal or bird, and the jaw hinging and motions are like those of mammals. The nostrils are set close together on the top of the upper bill.
The word "platypus" means "flat feet," referring to the animal's webbed, somewhat ducklike feet. The scientific name, Ornithorhynchus anatinus, means, in Latin, "bird-snout, resembling a duck." The plural is "platypuses" or just "platypus."
Most of the body is covered with fine, soft fur. The pelt color varies from dark amber to very dark brown on the platypus's back and sides, and from grayish white to yellowish brown on the underbelly. Platypus fur is fine, soft, and dense, with up to 900 hairs per square inch of skin. The fur has two layers, an undercoat with a woolly texture and an overcoat of coarser hair. As the platypus dives, the two fur layers trap a layer of air next to the skin, thus keeping the body dry and helping to insulate it against cold while the platypus swims, often throughout the night, and sometimes in temperatures close to freezing.
The body is somewhat flattened and streamlined. The limbs are short and muscular. As in other monotremes, the limbs of the platypus are set in a permanent push-up position, the upper limb bones extending out from the sides of the body, horizontal to the ground, the lower limb bones going straight down. Although an excellent swimmer, the platypus is clumsy when trying to walk on land, and seldom does so anyway, except within its tunnels, since it burns up twice as much body energy moving about on land as it does swimming underwater.
All four feet have five claws apiece and are webbed, but the webbing of the front feet extends in a flat flange beyond the toes when the platypus swims. Back on land or in its burrow, the animal folds the extra webbing under its forefeet and walks on its knuckles. The platypus uses the forelimbs and forefeet for swimming and digging, while using the hind feet and claws as combs to keep the fur clean and waterproof.
The eyes are small and the external ears are mere holes in the skull, although the internal structure of the ears is like that of other mammals. There are two long grooves for protecting the eyes and ears, a single groove surrounding both the eye and ear on each side. These grooves are closed underwater, shutting both eyes and ears, when the platypus dives to hunt for food. Out of water, the senses of sight and hearing are sharp.
Both hind limbs of the male bear hollow, pointed, poison spurs mounted on the insides of the ankles, just above the heels. There are venom glands, one in each thigh, called the "crural glands" because they are controlled by the crural nerves, which are major motor nerves of the hindlimbs. The glands secrete venom that is passed through ducts to the sharp spurs, which the platypus can erect like jacknife blades and stab into other animals.
Both sexes have the spurs when they are young. At four months of age, male spurs are protected by a covering of whitish, chalky material that sloughs off completely by the end of the first year of age. Females bear smaller, useless spurs, without venom, that they shed by ten months of age.
The platypus's flat, beaverlike tail is used as a swimming rudder, a shovel, for fat storage, and by the mother for keeping eggs and young warm. The tail can store up to fifty percent of a platypus's total load of body fat. Female platypus use the tail to carry leaves to the nesting chamber, and both sexes use it to sweep loosened soil out of the way when digging. The tail has no fine fur, only coarse, bristly hair on its upper surface to aid in carrying or sweeping.