Exposure of sub-adult Nile crocodiles (Crocodylus niloticus) to extreme lead concentrations: a 48-week experimental study with implications for wild populations

Lead (Pb) poisoning poses a significant threat to wildlife. A primary cause of Pb poisoning is the unintentional ingestion of Pb ammunition fishing weights, which are still used for hunting and fishing in numerous regions globally. While the effects of Pb poisoning on birds and mammals are well-established, impacts on reptiles are less well documented and difficult to assess under field conditions. In this study, we investigated the effects of extreme Pb exposure on captive sub-adult Nile crocodiles (Crocodylus niloticus; n = 18). We administered Pb dosages in the form of fishing weights (54–215 g) and monitored changes in blood lead levels (BPb), packed cell volumes (PCVs), growth, and body condition over a 48-week period. Crocodiles exhibited a remarkable tolerance to exceptionally high Pb exposure over the duration of the study. Despite the lack of obvious clinical signs of Pb toxicity, elevated BPb concentrations were linked to lower PCVs, indicating anaemia across all treatment groups by week eight. However, crocodiles showed a sustained erythropoietic response which may be contributing to their resilience to acute Pb toxicity. While Pb exposure did not significantly affect body condition, it was associated with a discernible reduction in weight gain over the duration of the study. Our estimation of a 5.8 to 7.3-year timeframe for complete dissolution of the Pb fishing weights in the experimental crocodiles' stomachs carries significant implications for wild populations, which are likely to be exposed to Pb for far longer than 48-week duration of this study.