Seasonal shift of diet in bank voles explains trophic fate of anthropogenic osmium?

Diet shifts are common in mammals and birds, but little is known about how such shifts along the food web affect contaminant exposure. Voles are staple food for many mammalian and avian predators. There is therefore a risk of transfer of contaminants accumulated in voles within the food chain. Osmium is one of the rarest earth elements with osmium tetroxide (OsO4) as the most toxic vapor-phase airborne contaminant. Anthropogenic OsO4 accumulates in fruticose lichens that are important winter food of bank voles (Myodes glareolus). Here, we test if a) anthropogenic osmium accumulates in bank voles in winter, and b) accumulation rates and concentrations are lower in autumn when the species is mainly herbivorous. Our study, performed in a boreal forest impacted by anthropogenic osmium, supported the hypotheses for all studied tissues (kidney, liver, lung, muscle and spleen) in 50 studied bank voles. In autumn, osmium concentrations in bank voles were even partly similar to those in the graminivorous field vole (Microtus agrestis; n=14). In autumn but not in late winter/early spring, osmium concentrations were generally negatively correlated with body weight and root length of the first mandible molar, i.e. proxies of bank vole age. Identified negative correlations between organ-to-body weight ratios and osmium concentrations in late winter/early spring indicate intoxication. Our results suggest unequal accumulation risk for predators feeding on different cohorts of bank voles.

Diabetes and myocarditis in voles and lemmings at cyclic peak densities--induced by Ljungan virus?

Although it is well-documented from theoretical studies that pathogens have the capacity to generate cycles, the occurrence and role of pathogens and disease have been poorly empirically studied in cyclic voles and lemmings. In screening for the occurrence of disease in cyclic vole and lemming populations, we found that a high proportion of live-trapped Clethrionomys glareolus, C. rufocanus, Microtus agrestis and Lemmus lemmus at high collective peak density, shortly before the decline, suffered from diabetes or myocarditis in northern Scandinavia. A high frequency of animals had abnormal blood glucose (BG) levels at the time of trapping (5-33%). In contrast, C. rufocanus individuals tested at a much lower overall density, and at an earlier stage relative to the decline in the following cycle, showed normal BG concentrations. However, a high proportion (43%) of a sample of these individuals kept in captivity developed clinical diabetes within five weeks, as determined by BG levels and a glucose tolerance test performed at that later time. A new picornavirus isolated from the rodents, Ljungan virus (LV), was assumed to cause the diseases, as LV-induced diabetes and myocarditis, as well as encephalitis and fetal deaths, were observed in laboratory mice. We hypothesize that LV infection significantly affects morbidity and mortality rates in the wild, either directly or indirectly, by predisposing the rodents to predation, and is at least involved in causing the regular, rapid population declines of these cyclic voles and lemmings. Increased stress at peak densities is thought to be an important trigger for the development of disease, as the occurrence of disease in laboratory mice has been found to be triggered by introducing stress to LV-infected animals.

Generation time and temporal scaling of bird population dynamics.

Theoretical studies have shown that variation in density regulation strongly influences population dynamics, yet our understanding of factors influencing the strength of density dependence in natural populations still is limited. Consequently, few general hypotheses have been advanced to explain the large differences between species in the magnitude of population fluctuations. One reason for this is that the detection of density regulation in population time series is complicated by time lags induced by the life history of species that make it difficult to separate the relative contributions of intrinsic and extrinsic factors to the population dynamics. Here we use population time series for 23 bird species to estimate parameters of a stochastic density-dependent age-structured model. We show that both the strength of total density dependence in the life history and the magnitude of environmental stochasticity, including transient fluctuations in age structure, increase with generation time. These results indicate that the relationships between demographic and life-history traits in birds translate into distinct population dynamical patterns that are apparent only on a scale of generations.

Type 1 diabetes in Swedish bank voles (Clethrionomys glareolus): signs of disease in both colonized and wild cyclic populations at peak density.

Colonized bank voles (Clethrionomys glareolus) originating from Sweden developed type 1 diabetes. Animals became polydipsic, glucosuric, and hyperglycemic and gradually developed a lethal ketoacidosis. Pancreas in animals with end-stage disease showed total destruction of islet cells. Interestingly, also a high proportion of wild bank voles in cyclic populations that were trapped at (or close to) the cyclic population density peak frequently showed high blood glucose levels and pathological glucose tolerance test. Extensive islet destruction was not seen in wild bank voles at the time of capture, but did develop in some of the animals over a time period of two months. Diabetes in both colonized and wild bank voles was associated with Ljungan virus (LV). LV could be isolated from the pancreas of diabetic bank voles and antigen detected at the site of tissue damage by immunohistochemistry. In addition, picornavirus-like particles were visualized in the islets of diabetic voles using thin-section transmission electron microscopy.