Age Estimation by Skeletochronology in the Nile Monitor (Varanus niloticus), a Highly Exploited Species

Abstract

The value of skeletal cyclic growth marks, or SGMs, for age estimation in the Nile monitor, Varanus niloticus, a species highly exploited for food and leather, was checked by a mark-recapture experi- ment including in vivo labelling of bone growth. The basic histological features of the SGMs are described, as well as the variability in the structure of these marks with regard to the skeletal elements considered, the technique for processing the sections, and differences between populations or individuals. Ground sec- tions from the shaft of the fibula, observed under transmitted polarized light, are best suited for counting growth marks. Bone labelling indicates that SGMs have an annual periodicity and can thus be reliable age indicators as long as somatic growth proceeds. The formation of SGMs is interpreted with reference to the ecology of the Nile monitors. The practical value of skeletochronology for population surveys is discussed. The Nile monitor, Varanus niloticus, is among the most exploited species of tetrapods in Af- rica, especially in the part of its range corre- sponding to the sudano-tropical zone (part of the continent between 10 and 16?N) where it is used as food and for the leather trade. Ten years ago, the number of skins of this species legally exported by just four countries was close to one million specimens per year (Luxmoore and Groombridge, 1990; de Buffrenil, 1993). Though