Toxic effects of molybdenum on forest small mammals at the Endako mine in north-central British Columbia
Mathieu, Susan F.
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Molybdenum is an essential element for both plants and animals but high dietary levels can result in molybdenum toxicity in some mammals. This study documented the spatial pattern of molybdenum in the environment surrounding an active molybdenum mine in north-central British Columbia, and also examined the potential toxic effects of molybdenum in small rodents, including the red-backed vole (Clethrionomys gapperi), the deermouse (Peromyscus maniculatus), and the meadow vole (Microtus pennsylvanicus). Results from this study indicated that molybdenum mining increased molybdenum concentrations in the water and soils surrounding the mine site. Elevated molybdenum concentrations in soils were mainly found within the first 5 m (riparian zone) on either side of the streams at both mine-influenced (treatment) and control sites. In contrast, this study demonstrated no relationship between the concentration of molybdenum in the livers of herbivorous small mammals and the spatial pattern of molybdenum measured in soils. Molybdenum concentrations in small mammals were not higher in individuals captured in treatment areas than in those captured in control areas. Nor were liver molybdenum concentrations higher in small mammals captured within the riparian zone than in areas farther from the stream. Individuals with elevated concentrations of molybdenum in their livers were found in both treatment and control areas. Thus the effects of molybdenum on small mammals had to be investigated by comparing those individuals with low concentrations of molybdenum in their livers (5 mg/kg), regardless of area of capture. In this study symptoms of molybdenum toxicity were not observed in small mammals with elevated concentrations of molybdenum in their livers although these levels were similar to those in related species which exhibited toxic effects in laboratory tests. Elevated concentrations of molybdenum in the liver of small mammals did not appear to reduce growth in body mass or cause abnormal long bone development. In males with elevated liver molybdenum concentrations, both the mass of reproductive organs and accessory glands, and sperm production were not affected. Similarly for females, the incidence of pregnancy, ovulation rates, litter size, and the incidence of resorption were unrelated to molybdenum concentration in their livers. It should be noted though, that there was a trend for reduced ovulation rates and litter sizes in C. gapperi with elevated molybdenum concentrations, but these trends were not statistically significant. This study illustrates three critical points for conducting field toxicological studies. First, this study illustrates the importance of determining contaminant loading in individuals rather than composite sampling. Composite sampling may not be sensitive enough to determine the pattern of contaminant loading in wildlife. Secondly, it also illustrates the importance of avoiding the assumption that contaminant loading of individual animals will follow the overall pattern observed in the environment when designing field toxicity studies. Thirdly, there were no toxic effects observed although the concentrations of molybdenum in the livers of small mammals were similar to those in rats which exhibited toxic effects in controlled experiments. The lack of observed toxic effects may be because different species of rodents respond differently to the same exposure level of molybdenum. It may also be possible that the populations of small mammals found around the mine area have developed a resistance to the toxic levels of molybdenum over evolutionary time.