Dentition and Ageing of the Hippopotamus

I. S ummary The present paper is based on the examination of lower jaws from some 3000 individuals of Hippopotamus amphibius L. which were collected in the course of cropping operations in the Queen Elizabeth Park, Uganda between 1961 and 1966. It extends an earlier study by W. M. Longhurst, who described 20 relative age groups based on tooth replacement and wear. The age groups, slightly modified, are described and illustrated here and should serve as a useful field guide. Mean chronological ages from 0–43 years have been allocated to the groups. Checks which confirm the validity of the ages allotted are presented and discussed. These include correlation with the mandibular age group of six known‐age animals; findings on the age‐related incidence of rinderpest‐neutralising antibodies; the orderly progress of fusion of the mandibular symphysis and rate of loss of the first premolar. Growth layers present in the teeth are discussed. The growth of the eye lens is also described. After an initial phase of rapid growth the lens continues to grow throughout life and follows a rectilinear pattern from an estimated age of eight years onwards. This is expected from the findings of similar studies on other species for which known‐age specimens are available. Variability of lens dry weight at age is relatively small and indicates that the estimated ages are reasonably precise. Growth of the mandible is analysed and is not inconsistent with the ages allotted. A marked sex difference in mandible weight at age allows the sex of found jaws to be determined at ages above eight years. The growth of the teeth is described. Both canines and incisors show well‐marked sex differences in growth rate and size. The post‐canine teeth do not show sex differences. Cycles of growth, wear and resorption of these teeth are discussed and it is concluded that mechanical senescence of the teeth is a major factor in mortality at ages above 30 years. Growth in body length is briefly discussed and provides further confirmation of the validity and precision of the age criteria. Growth equations are presented. Finally a survivorship curve derived from the ageing of 207 jaws assumed to represent natural deaths is presented. Population models are constructed from the survivorship curves by calculation of estimated natality rates (obtained by applying data on age at first breeding and annual pregnancy rate to the survivorship data) and life tables are constructed. The shape of the survivorship curve and the percentage recruitment agree with expectation and provide further evidence of the consistency of the age criteria.