Odontochronologies in male and female mandrills (Mandrillus sphinx) and the development of dental sexual dimorphism

Objectives: We examine how dental sexual dimorphism develops in mandrills, an extremely sexually dimorphic primate. We aimed to (a) establish the chronology of dental development (odontochronology) in male and female mandrills, (b) understand interindividual and intersex variation in odontochronologies, and (c) determine how dental sexual dimorphism is achieved. Materials and Methods: We prepared histological ground sections from the permanent teeth of four female and four male mandrills from the semi-free ranging colony at the Centre International de Recherches Médicales, Franceville, Gabon. We used the microscopic growth increments in the sections to create odontochronologies. We compared ages at crown initiation, crown formation times (CFT) and crown extension rates (CER) between individuals and sexes to assess interindividual and intersex variation. Results: All mandrill teeth are sexually dimorphic in size. Dental sexual dimorphism in mandrills is achieved via sex differences in the duration of growth (bimaturism) and in growth rates. We also found interindividual and intersex variation in the ages at initiation and completion of crown formation. Discussion: Our results show that the rate of ameloblast differentiation varies between individuals and that selection for both the age at tooth initiation and CER has occurred independently in males and females to ensure that the teeth develop at appropriate times relative to the growth of the sexually dimorphic jaws. They also show that canine dimorphism is achieved through differences in both CER and CFT, unlike extant great apes or Cantius. Given at least three mechanisms for achieving canine dimorphism, we need more information to trace the evolution of this trait in primates.