TOOTH-SHARPENING BEHAVIOR (THEGOSIS) AND OTHER CAUSES OF WEAR ON SHEEP TEETH IN RELATION TO MASTICATION AND GRAZING MECHANISMS

Four types of wear can be defined in sheep teeth: thegotic, abrasive, interproximal and corrosive. Thegosis in sheep is the innate grinding of mandibular cheek teeth against maxillary teeth which produces chara cteristic flat facets on the enamel. The freshly thegosed facets chara cteristically have clearly defined edges and fine parallel striations on the surface. Thegosis maintains sharp enamel blade systems that all ow efficient cutting of food during mastication while eating and rumin ating. Interproximal wear results from adjacent teeth rubbing together , and, with mesial drift, helps prevent impaction of food between them . Abrasive wear is caused by the movement of food or other exogenous m aterials over the surfaces of these teeth. On the cheek teeth abrasion hollows out dentine between the enamel ridges and wears away the shar p edges and parallel striations of the thegotic facets on the enamel, replacing them with randomly placed scores of uneven depth. Hollowing out serves the useful function of exposing sharp enamel blades at the occlusal surface. Abrasion on the incisors results in characteristic w ear areas on the enamel and dentine surfaces. These abrasive areas are bounded by relatively sharp edges to the labial enamel because of the particular morphology of the incisors and the grazing mechanism of sh eep, which are described in this paper. Corrosive wear is caused by ch emicals from herbage corroding the hard calcium phosphates from the te eth. The main anatomical and functional features of the cheek teeth ar e described using R. G. Every's thegotic terminology for mammalian tee th. The thegotic nomenclature uses functional concepts not implicit in the more conventional Cope-Osborn terminology traditionally used to d escribe teeth. Sheep teeth, as generally for placental herbivores, are classed as beta dentition. The main feature is the triakididrepanoid blade system. The mechanism is described by which these blade systems interrelate during mastication to achieve efficient cutting up of food material. The incidence of thegosis, rumination and eating was record ed during a continuous 24h cycle and showed that there is a major peri od of thegosis during the dark morning hours. The results provided une quivocal evidence for a tooth-to-tooth wearing behaviour (thegosis) th at always occurred at a separate time from mastication. The advantage of night-time and stress-induced thegosis is discussed.