Looking the River Horse in the Mouth: New Insights on Incisors, Canine Curvatures and Mulling the Molars
Authors: Naples Virginia , Haji-Sheikh Misty 
Published: April 2020

Tooth numbers, types, eruption patterns, occlusion, and timing of tooth replacement in the living hippopotamus species, Hippopotamus amphibius (common or Nile hippo) and Choeropsis liberiensis (pygmy hippo), were examined. We discovered inaccuracies in the published dental formula of H. amphibius, and present the corrected dental formula in this study. By examining crania and mandibles ranging in chronological age from late fetal or neonatal to aged adult hippos, we also identified the relative timing of tooth eruption and growth rates for both deciduous and permanent dentitions. Living hippos have ever‐growing incisors and canines that shape the anterior rostrum for foraging and intraspecific competition. Despite their highly differential use of the canines, with H. amphibius using these teeth for intraspecific aggression and display while pygmy hippos use them in foraging, the canine teeth of both living taxa are greatly enlarged in comparison to other components of the dentitions. In fact, Hippopotamus amphibius has some of the largest canine teeth among mammals. In addition to thegosis, or wear caused by tooth‐tooth contact, foraging behavior and tongue wear cause incisor attrition, albeit deriving from different species‐specific tooth usage. As individuals of H. amphibius grow, the erupting canines show lateral displacement, which correlates with anterior rostral elongation. This dramatic change in both length and width of the face in H. amphibius allows the roots of the enlarging canine teeth to occupy increased space in the maxilla and mandible. Despite facial elongation, enlargement of the recurved roots of the ever‐growing canines drastically reduces space for the alveolus of the first and sometimes the second premolar. In some cases this results in the elimination of one or more premolars from the adult dentition. These losses may not be symmetrical in either maxilla or mandible. In other cases, all deciduous premolars may be retained into adulthood, resulting in the published conclusion that the number of premolars in H. amphibius is variable, a phenomenon that is rare among mammals. In contrast, in C. liberiensis the canines enlarge to a relatively lesser degree and do not displace the deciduous premolars; therefore pygmy hippos retain the full premolar complement, explaining the dichotomy in dental formulae between the two species. This study also expands a previous rubric for chronological age determination in H. amphibius, wherein mandibles were divided into twenty stages to include characterization of the dentitions of the correlated skulls (N = 34). The crania examined represented thirteen of twenty H. amphibius age classes. These specimens are housed in the osteological collections of the Smithsonian Institution (USNM) and the Field Museum of Natural History (FMNH). This chronological aging method was also adapted to provide a relative age of skulls and mandibles (N = 15) representing ten of the twenty age classes of C. liberiensis from the same collections. Observations reveal that an increased time interval between successive molar eruptions in H. amphibius may reflect adaptation to a lifespan of up to sixty years, a phenomenon not seen in C. liberiensis.

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