lower jaw are covered in soft skin, forming the bill. 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 , with males being larger than females. Males average in total length, while females average , with substantial variation in average size from one region to another. 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 rather than the 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 teeth in each of the maxillae (one premolar and two molars) and dentaries (three molars), which they lose before or just after leaving the breeding burrow; adults have heavily keratinised pads called ceratodontes in their place, which they use to grind food. The first upper and third lower cheek teeth of platypus nestlings are small, each having one principal cusp, while the other teeth have two main cusps. 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 pre mammalian 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. As in many other aquatic and semiaquatic vertebrates, the bones show osteosclerosis, increasing their density to provide ballast. 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 on its front feet, to protect the webbing between the toes. Venom While both male and female platypuses are born with ankle spurs, only the spurs on the male's back ankles deliver venom, composed largely