Horizontal tooth replacement in the Amazonian manatee (Trichechus inunguis)

Abstract

Studies of captive Amazonian manatees (T. inunguis) have provided the 1st quantitative documentation of the long-recognized fact that manatees produce unlimited numbers of supernumerary molars that are replaced horizontally throughout the animal''s life. Three teeth (corresponding to deciduous premolars) are fully erupted in each jaw quadrant about the time of birth; this number increases with growth of the mandible, reaching 7 or 8 in adults. All teeth after the first 3 are true molars. As the manatee is weaned and begins to chew solid food, the mandibular symphysis fuses and the anteromost tooth in each quadrant is lost; the toothrows then begin to migrate forward at rates on the order of 1 mm/mo., presumably by continuous resorption and deposition of bone in the interalveolar septa. As each tooth moves forward, its roots are gradually resorbed, and it is finally lost on reaching the front of the toothrow, while new teeth are formed and erupt at the rear. The rate of movement is directly proportional to the amount of plant food the rear. The rate of movement is directly proportional to the amount of plant food (especially grass) consumed, and is probably controlled by the mechanical stress of chewing. Movement rate was also directly correlated with degree of wear on the teeth at the time of loss, implying that in the food consumed by the animals studied the qualities of toughness and abrasiveness were combined, as would be expected for grass containing siliceous phytoliths. The dentition and replacement process of T. inunguis seem best adapted to a mixture of grass and softer aquatic plants; this is consistent with available natural history data. On the other hand, a relatively nonfibrous diet contaminated with abrasive matter such as sand might be expected to result in slow replacement, rapid wear and consequent severe attrition of the dental battery; this seems to occur in Florida T. manatus. Thus, the replacement process automatically adjusts itself to toughness of diet, but abrasivenesscan be dealt with only through evolutionary adaptation of the tooth morphology.