M.A. Theses: Suyoko Anne Tsukamoto, 2003

The Prevalence and Timing of Enamel Hypoplasia in the Bonobo, Pan Paniscus

Systematic analysis of prevalence and timing of developmental stress in the bonobo, Pan paniscus, is accomplished through examination of enamel hypoplasia (EH), a non-specific stress-induced defect permanently recorded in the developing dentition.

Bonobo crania (n=182) housed at the Royal Museum for Central Africa in Tervuren, Belgium, are grossly examined under natural and artificial oblique lighting for presence of EH. Three methods are employed to determine the timing of linear enamel hypoplasia (LEH). (1) Imbricational enamel formation schedules are created for bonobo teeth (n=6) by counting the number of perikymata from the cemento-enamel junction to the occlusal surface. (2) Caliper measured defects are calculated as a proportion of crown height and multiplied against crown formation times for bonobo and chimpanzee to establish age at which defects form (n=490 teeth). (3) Using scanning electron microscopy, tooth crown surfaces (n=22) are examined and perikymata counted within and between enamel defects.

Bonobos are very stressed; 100% of adults (n=58) are affected with LEH – the highest prevalence reported for any ape. Bonobos also yield a high prevalence of pitting and localized hypoplasia of the primary canine but not for Darcy’s Defect, indicating these primates are more impacted by or susceptible to physiological stressors than other apes. Distribution, however, is consistent with other apes, i.e., anterior teeth demonstrate a higher degree of EH; LEH and pits are common in permanent and deciduous dentitions, respectively. No significant differences exist between sexes.

Imbricational enamel formation times for the bonobo are 2.9 and 3.4 years for the maxillary central (n=3) and lateral incisors (n=1), respectively, 3.3 years for the mandibular lateral incisor (n=1), and 5.2 years for the mandibular canine (n=1). Onset of LEH occurs around 2 to 3 years of age and continues throughout crown formation. LEH is found to be episodic, averaging 6-month intervals, and stress lasts 6 to 8 weeks. The periodic nature of LEH is attributed to seasonal rainfall and disease cycles (e.g., parasitic infection) rather than nutritional stress. Teeth picked up milder microscopic episodes of stress suggesting a chronic problem exacerbated at various times, including seasonal moisture cycles.