Validation of skeletochronology to determine age of freshwater crocodiles (Crocodylus johnstoni)

Abstract

Age was estimated for wild Australian freshwater crocodiles from skeletochronology of growth marks in postoccipital osteoderms. Growth marks were distinct and counted reliably in unstained calcified thin sections (60–80 µm) viewed by Nomarski interference microscopy. The periodicity of growth marks was validated directly from crocodiles of known age (up to 19 years) and from individuals sampled sequentially in different seasons. Growth marks comprised annuli deposited in winter and zones deposited in summer. Errors in age estimation for small crocodiles resulted from difficulty in assigning the growing edge as either a zone or an annulus. No osseous remodelling was noted for immature animals. Age estimates for adult females became unreliable as previous annuli were obscured by osseous remodelling, presumably resulting from calcium mobilization by egg-laying females. Old adult males continued to deposit annuli, but these were more difficult to distinguish, being more closely spaced than in young crocodiles. Skeletochronology permitted reliable (coefficient of variation, 3·4%) and accurate (±1 year) age estimation up to about 20 years. Osteoderm dimensions were strongly correlated with average snout–vent length, mass and age and have acceptable potential as an indirect means of age and size estimation for Crocodylus johnstoni.