Element Concentrations in the Flesh and Osteoderms of Estuarine Crocodiles (Crocodylus porosus) from the Alligator Rivers Region, Northern Australia: Biotic and Geographic Effects

Abstract

The concentrations of Na, K, Ca, Mg, Ba, Sr, Fe, Al, Mn, Zn, Pb, Cu, Ni, Cr, Co, Se, U, and Ti were determined in the flesh and osteoderms of estuarine crocodiles (Crocodylus porosus) captured in three adjacent catchments of Kakadu National Park, within the Alligator Rivers Region of northern Australia. This study provides, for the first-time, baseline concentrations of elements in both flesh and osteoderms of wild crocodiles. Multiple linear regression analyses were used to determine the effects of total crocodile length, estimated age, gender, inferred reproductive status, physical condition, and catchment of capture on element concentrations in both tissues. The Mg concentration (log₁₀) in the flesh and osteoderms of C. porosus significantly (p ≤ 0.001) decreased with increasing length (1.7–5.0 m) and estimated age (5–40 years). Similarly, the Ti concentration (log₁₀) in flesh significantly (p ≤ 0.01) decreased with increasing length. In contrast, Zn and Se concentration (log₁₀) in flesh significantly (p ≤ 0.001) increased with increasing length and/or age, suggesting that these relationships are mediated by biological rather than environmental chemical factors. In flesh, Fe and Na concentrations (log₁₀) significantly (p ≤ 0.05) increased as the physical condition of C. porosus deteriorated. No significant (p > 0.05) effects of gender or inferred reproductive status on element concentrations in the flesh and osteoderms were found. The mean concentrations (log₁₀) of Al, Ba, Cr, Ni, and Pb in flesh and Co, Fe, Mg, Mn, and U in the osteoderms were significantly (p ≤ 0.01) different between catchments. The significant (p ≤ 0.05) effects of catchment on the concentrations of various elements indicate that C. porosus reflects the chemistry of its environmental milieu and therefore has a certain degree of catchment fidelity, even though the catchments are adjacent to one another. Such catchment-specific signals may be useful in the determination of the provenance of itinerant crocodiles. They also point to the utility of crocodiles as long-term biomonitors of their environment.