At present little is known about the internal morphology of the nostrils (Auffenberg 1988) but the size and position of the nostril openings reflect the range of habitats and feeding techniques utilised by monitor lizards. The nostrils of species that spend a lot of time in water are often equipped with flaps of skin that prevent water entering whilst the animals are submerged (e.g. Dumeril's monitor, the mangrove monitor, the water monitor, Menens' monitor (Mertens 1942, Krebs 1979). In species such as the water monitor and the mangrove monitor the nares are situated towards the front of the snout, allowing the lizards to be able to keep almost all of the head below the water and still be able to breath. In Mertens' monitor, the openings are uniquely situated on the top of the snout rather in the fashion of crocodilians, so that the animals can breath even when the entire head is submerged. In other aquatic monitors (such as Dumeril's monitor and the Nile monitor) and in many ground dwelling species (such as the Bengal monitor and Bosc's monitor) the round openings are replaced with slits and situated closer to the eye than the tip of the snout. All of these lizards retrieve a lot of their food from below the ground. They use their remarkable sense of smell to detect prey and uncover it by pushing their snouts into the earth. The narrow slit-like opening prevents the entry of most of the debris that would otherwise longest the nostrils. In some species this type of foraging behaviour seems to be restricted to the older animals, and once again we see examples of major change in the transition to adulthood. Both Bengal and Nile monitors have rounded nares as juvenile, which become narrower as the animals age. These species spend most of their time in the comparative safety of trees as juveniles and only seek food on the surface when they become larger. Bose's monitor, on the other hand, roots for prey with the snout virtually from birth and the shape of the openings does not change significantly with age.
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