Selenium (Se) is required at a fundamental physiological level in all animals. Adequate levels of Se are necessary for proper bone metabolism, iodine metabolism, immune function, reproductive success, and recruitment. Selenium is a component of enzymes which scavenge oxidative free radicals that would otherwise degrade cell membranes. Severe deficiency results in obvious symptoms such as white muscle disease in ungulates. However, more frequently, deficiency may be chronic and subclinical. Individuals then display no obvious signs of malady, yet performance suffers until their populations decline without apparent cause or through proximate factors which obscure underlying primary factors. Although well known in domestic stock, the link between population performance and Se deficiency in wild populations has been difficult to firmly establish. Confounding factors include the role of vitamin E, which also acts as an antioxidant to mitigate the need for Se under some circumstances; changing Se requirements at changing times in animal life history; changing Se requirements in relation to pollution levels and other factors causing oxidative stress; and the non-uniform distribution of Se in its various chemical forms in the environment. The latter point is especially important to wild populations that have been reduced to remnant portions of their previous range. Here, we have reviewed the literature of Se in wildlife as well as provided an introduction to Se in physiology and Se behavior in the environment for the wildlife researcher and manager. We conclude that unrecognized Se deficiency may often impede optimal population performance, and we provide recommendations for habitat analysis with regard to Se which can be used in future research. Finally, evidence that the amount of available Se in the environment is decreasing from anthropogenic causes is shown.

Geophagia has been commonly reported for bighorn sheep (Ovis canadensis) and other ungulates worldwide. The phenomenon is often attributed to the need to supplement animal diets with minerals available in the soil at mineral lick locations. Sodium is the mineral most frequently cited as being the specific component sought, although this has not been found universally. In this study area, bighorn sheep left normal summer-range to make bimonthly 26-km, 2,000-m-elevation round-trip migrations, the apparent purpose of which was to visit mineral licks on normal winter-range. Lick soil and normal summer-range soil were sampled for their available mineral content and summer-range forage was sampled for total mineral content, and comparisons were made to determine the specific components sought at the lick by bighorn sheep consuming soil. It was concluded that bighorn sheep were attracted to the lick by a desire for sodium but that geophagia also supplemented a diet deficient in the trace element selenium. Where sheep are denied access to licks, populations may be limited by mineral deficiency.

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Regional extirpations of pikas (Ochatona princeps) within the last few decades have been attributed to global warming. Other recent global alterations such as increased nitrogen (N) deposition and associated selenium (Se) deficiency may further stress pika populations. In 2003 and 2004, we live-trapped pikas from three populations in Wyoming and measured Se values in their hair. We also sampled hair and liver from museum specimens collected throughout the Northern Rocky Mountains in 1987 and 1988. Our results suggest that liver and hair values were related, and that pika hair reflected the Se concentrations of the geologic parent materials. We determined that animals residing in several remote areas in the Rocky Mountain region could be Se deficient and that increase in N deposition correlated with an increase rather than a decrease in Se values in pika hair. In addition, we found no relation between Se contents in hair and body condition index, suggesting that low Se levels may not have negative effects on individual pikas. Whether Se levels influence reproductive success of pikas is unknown and should be the focus of future studies.

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