Description
The body and the broad, flat tail of the platypus are covered with dense, brown fur that traps a layer of insulating air to keep the animal warm. The fur is waterproof, and the texture is akin to that of a mole. The platypus uses its tail for storage of fat reserves (an adaptation also found in animals such as the Tasmanian devil and fat-tailed sheep). It has webbed feet and a large, rubbery snout; these features appear closer to those of a duck than to those of any known mammal. The webbing is more significant on the front feet and is folded back when walking on land. Unlike a bird's beak (in which the upper and lower parts separate to reveal the mouth), the snout of the platypus is a sensory organ with the mouth on the underside. The nostrils are located on the dorsal surface of the snout, while the eyes and ears are located in a groove set just back from it; this groove is closed when swimming. Platypuses have been heard to emit a low growl when disturbed and a range of other vocalisations have been reported in captive specimens.
Weight varies considerably from 0.7 to 2.4 kg (1.5 to 5.3 lb), with males being larger than females; males average 50 cm (20 in) in total length, while females average 43 cm (17 in), with substantial variation in average size from one region to another, and this pattern does not seem to follow any particular climatic rule and may be due to other environmental factors, such as predation and human encroachment.
The platypus has an average body temperature of about 32°C (90°F) rather than the 37°C (99°F) typical of placental mammals. Research suggests this has been a gradual adaptation to harsh environmental conditions on the part of the small number of surviving monotreme species rather than a historical characteristic of monotremes.
Modern platypus young have three-cusped molars, which they lose before or just after leaving the breeding burrow; adults have heavily keratinised pads in their place. The platypus jaw is constructed differently from that of other mammals, and the jaw-opening muscle is different. As in all true mammals, the tiny bones that conduct sound in the middle ear are fully incorporated into the skull, rather than lying in the jaw as in cynodonts and other premammalian synapsids. However, the external opening of the ear still lies at the base of the jaw. The platypus has extra bones in the shoulder girdle, including an interclavicle, which is not found in other mammals. It has a reptilian gait, with the legs on the sides of the body, rather than underneath. When on land, it engages in knuckle-walking to protect the webbing between its toes.
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