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 ®bula 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 ®bula. 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¯exible 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 de®cit 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.

Evolution of humeral microanatomy and lifestyle in amniotes, and some comments on palaeobiological inferences

“…Finally, a certain proportion of error is inevitable because many factors influence bone microanatomy (Castanet et al ., 2001). These include phylogenetic position (Cubo et al ., 2005, Germain & Laurin 2005; Laurin et al ., 2004), biomechanical constraints (de Margerie et al ., 2005; Meers, 2002), ontogeny, pathology (Castanet et al ., 1993; Castanet et al ., 2001), metabolism (de Ricqlès, 1983) or reproductive status (de Buffrénil & Francillon‐Vieillot, 2001).…”

Microanatomical diversity of the humerus and lifestyle in lissamphibians

“…Bone microanatomy also depends on environmental constraints, such as temperature or food and water availability (de Buffrénil and Francillon‐Vieillot 2001; Meers 2002). Thus, Meers (2002) noted that captive crocodilians exhibit unusual cross‐sectional geometric properties (such as a larger cortical area relative to wild conspecifics) that may result from different nutrition or different locomotor activity pattern.…”

“…Physiological factors also influence bone microanatomy. De Buffrénil and Francillon‐Vieillot (2001) demonstrated that bone compactness in the rear limbs of the Nile monitor ( Varanus niloticus ) is largely influenced by the sex of the specimen and the reproductive status. In males, femoral compactness increases during ontogeny, whereas in females, it tends to decrease.…”

“…Finally, several studies showed that bone structure variation depends on the identity of the skeletal element considered (de Buffrénil and Francillon‐Vieillot 2001; Laurin et al . 2004; de Margerie et al .…”

Longevity and growth strategies of the desert tortoise (Gopherus agassizii) in two American deserts

“…Non-mechanical factors also need to be taken into account. For example in Nile monitors {Varanus niloticus), bone in juvenile males and females share the same mechanical properties and density, but upon sexual maturity the endosteal cavity of females enlarges with each subsequent egglaying cycle (de Buffrenil and Francillon-Viellot, 2001). Bone lost while producing eggs is not fully regained before the next egglaying period.…”