Ontogenetic changes in bone compactness in male and female Nile monitors (Varanus niloticus)

Abstract

The compactness of bone tissue was measured in the shaft of the femur, tibia and fibula in 390 male and female Nile monitors Varanus niloticus from three distinct regions of Sahelian Africa. In males, bone compactness increases during ontogeny in the three bones. Conversely, in females, it tends to decrease in the femur and, to a lesser extent, the tibia, but not in the fibula. In addition to this long-term trend, gravid females have lower femoral compactness than mature non-gravid ones. Moreover, important differences are noticeable between local samples, indicating that the variation in bone compactness during ontogeny is a flexible or even a facultative process. Histological observations reveal the existence of an extensive remodelling by resorption and reconstruction on the walls of the marrow cavity in the femur and tibia, and the complete absence of Haversian substitution. It is concluded from these data that: (1) during ovogenesis, female Nile monitors recycle skeletal calcium through cortical resorption in certain bones, if the calcium supply from food is not enough; (2) this remodelling process is not balanced, which creates a long-term tendency towards a deficit in reconstructive bone, and a consecutive decrease in the compactness of the skeletal elements involved. The comparative and functional aspects of these results are discussed.