The Effects of Methoxychlor on the California Vole, Microtus californicus.

摘要

The effects of endocrine disrupting substances on wildlife populations present a challenge for wildlife conservation. Toxicology analyses identifying physiological effects in individual animals cannot predict long term and indirect effects on populations and associated biotic communities. In an approach to better elucidate the relationship between responses in individual animals and population level effects, this research project identified physiological and behavioral effects of an estrogenic pesticide, methoxychlor, in a wild rodent species, the California vole, and using computer generated simulations, extrapolated those results into effects on long term vole population dynamics.;Methoxychlor is a chlorinated pesticide that is a reproductive toxin in exposed rodents. The California vole, Microtus californicus, is an herbivorous rodent that occurs across a wide range of habitats throughout California. M. californicus is conducive to determining plant structure and composition in the habitats in which it occurs and is important prey species for both diurnal and nocturnal predators. I conducted a multi-generation laboratory study of the effects of methoxychlor, which functions physiologically as an estrogen agonist, on reproduction and behavior in M. californicus. Data generated from the studies was used to construct stage structured matrix population models to examine the potential effects of methoxychlor on M. californicus population dynamics under varying exposure scenarios.;The physiological effects of methoxychlor on reproductive parameters varied depending on the exposure route and dose. Through both direct and in utero/lactation exposure, the lowest dose of methoxychlor significantly stimulated productivity relative to controls, while essentially inhibiting reproduction at a higher dose. This type of non-monotonic dose response is consistent with reported physiological effects of exposure to endocrine disrupting substances and indicates that linear, low-dose extrapolations for these substances are not appropriate for all species. Methoxychlor exposure also resulted in a highly skewed sex ratio in the non-exposed offspring of F1 generation voles that were exposed in utero and through weaning only. A long term alteration in metabolism of the F1 generation is postulated as a possible mechanism for the effect on the offspring sex ratio.;Behavioral analyses of methoxychlor exposure were conducted on three hormone-mediated behaviors: activity levels, parenting behavior, and aggression, measured as running wheel activity, infanticide, and territoriality, respectively. The effects on behavior were consistent with exposure to estrogen. Methoxychlor significantly increased running wheel activity by in utero/lactation exposed female M. californicus when tested as adults. This result indicates that methoxychlor or its metabolites caused a permanent disruption to the CNS with respect to estrogen metabolism. Infanticide was significantly reduced in both parental adult dosed dams and by F1 in utero/lactation-exposed male/female pairs. The parental reduction in infanticide is postulated to be due to methoxychlor binding to estrogen receptors in the brain of the adult dams, whereas the long term effect on the F1 generation is suggestive of a permanent, epigenetic alteration. Territorial behavior/aggression of F1 in utero/lactation-exposed males tested as adults did not differ significantly between treated and control voles. Disruptions to activity levels and hormonally-mediated behaviors that regulate the social systems of Microtus spp. could alter the demographics of the population as well as impact long term population dynamics.;The modeled population trajectories demonstrated that methoxychlor exposure could result in adverse impacts to long term vole population dynamics in natural habitats. The simulations identified increased population variability, calculated as the range of variation in abundance, associated with increased fecundity, which occurred in the methoxychlor-exposed populations. Fecundity input values were increased as a result of two distinct mechanisms: (1) reduced infanticide, which increased survival and (2) reduced (female-skewed) sex ratio. Maximum population variability occurred when the effect of increased activity level, which also was associated with methoxychlor exposure, was incorporated into the models. Variation in the standard deviation of the carrying capacity resulted in population extinctions for exposed populations, indicating that exposed populations could be vulnerable to local extinction as a result of increased environmental stochasticity.;The results of my research indicate that the reproductive and behavioral effects of estrogenic substances like methoxychlor could result in significant negative impacts on vole population dynamics in natural habitats. These studies have demonstrated that multiple approaches, including multigenerational, behavioral, and population level analyses, provide useful insights for the identification of potential population level effects in wild species. Due to the importance of voles in the habitats in which they occur, alterations to vole population dynamics could have indirect effects on the productivity and stability of the associated ecological communities.