Individual and demographic responses of the gray-tailed vole to vinclozolin

We tested the hypothesis that vinclozolin, a fungicide that has been demonstrated in the laboratory to disrupt hormonal activity of rats and mice, would have adverse effects on a field population of voles. We conducted two field experiments to determine whether exposure to vinclozolin following a standard application rate of Curalan fungicide had adverse effects on the reproductive development and demography of the gray-tailed vole, Microtus canicaudus. In one experiment conducted during the rainy season, mean lengths of testes and seminal vesicles were smaller (p greater than or equal to 0.08), levels of plasma testosterone were lower (p = 0.025), and proportion of pregnant females was lower (p = 0.05) in treatment than in control animals; however, this difference was not sufficient to affect juvenile recruitment, sex ratio, or population growth. In a second experiment conducted under drier conditions, no differences occurred in measurements of reproductive organs or demography of treatment and control populations. The results suggest that precipitation may have increased the potency of the chemical by washing it down to ground level and (or) by increasing the uptake of toxic metabolites by the plants, resulting in greater dietary exposure to the voles. Although our results revealed no biologically significant effects of vinclozolin at the population level under the conditions of our experiments, they do suggest that multiple applications or a higher application rate of a vinclozolin product may have negative effects on male reproductive development and demography in wild populations.