Age Determination of Living Nile Crocodiles from the Cortical Stratification of Bone

Vital staining experiments with captive, known-age Nile crocodiles, Crocodylus niloticus, in Zimbabwe, show that all skeletal elements have laminae formed by a broad zone deposited in the hot season of rapid growth and a narrow annulus deposited in the cool non-growing season. Each lamina indicates the passage of one year. To age living crocodiles, a method was developed to estimate age from laminae in osteoderms. Internally, the osteoderms of all animals, except breeding females, have well represented laminae with only the most deep-seated having been eroded. The osteoderms of breeding females show considerable internal remodeling. They probably act as a store for calcium used during oogenesis. Age based on number of laminae in osteoderms was tested in adult animals of known ages up to 46 yr. Errors in the most satisfactory method of estimation varied between 9 and 15%. This technique was applied to a population of wild crocodiles at Ngezi, Zimbabwe.