Summary1. It has been hypothesized that the induction of silicon (Si)-based plant defence in response to herbivore damage may engender rodent population cycles. Many studies have also considered accumulation of Si as a process controlled by geo-hydrological factors. 2. To test these ideas, we investigated the relationship between concentration of Si in fibrous tussock sedge (Carex appropinquata) and the population density of a major sedge consumer, the root vole (Microtus oeconomus), in field enclosures in natural habitat under a variety of natural water regimes and weather conditions. 3. We found that a high density of voles at the end of summer resulted in the immediate accumulation of Si by rhizomes, followed by accumulation of Si in leaves with a 1-year lag time. The level of river flooding in the same year had an additional impact on Si concentration in rhizomes but did not affect silicification of leaves. 4. Overwinter changes in concentration of Si in sedges were influenced by fluctuations in ambient temperature and the depth of snow cover (multiple freeze-thaw cycles), thus affecting the quality of winter food available for voles. 5. Smaller voles had lower mortality during early winter than large voles, which seemed to be connected with changes in the quality of the autumn rather than the winter food base. Winter survival of voles was not associated with Si concentration in their faeces, however. 6. Our results suggest that changes in Si concentration in fibrous tussock sedge can be induced by changes in vole population density and are also additionally affected by the amount of flooding and weather conditions.

Although the biological significance of individual variation in physiological traits is widely recognized, studies of their association with fitness in wild populations are surprisingly scarce. We investigated the effect of individual phenotypic variation in body mass, resting (RMR) and peak metabolic rates (PMR) on mortality of the root vole Microtus oeconomus. Body mass and metabolic rates varied significantly among consecutive years and were also age dependent, as individuals born in late summer and autumn were characterized by significantly lower body mass and metabolic rates than animals born earlier. At the beginning of winter voles born in spring and early summer exhibited reduced body mass and metabolic rates, whereas animals born later maintained lower body mass and RMR, which may be interpreted as phenotypic plasticity enhancing the probability of survival. Body mass had no significant effect on vole survival during summer. In contrast, smaller individuals were characterized by lower mortality during early winter, whereas higher body mass was positively associated with survival later in the season. High body‐mass‐corrected RMR positively affected survival in both summer and winter. The effect of PMR was apparent only during winter, though its direction (and correlation with RMR) varied among years. Deep snow cover negatively affected the survival of voles in both early and late winter. Ambient temperature was positively associated with winter survival, except for late winter, when rising temperature caused flooding of vole habitat. We conclude that the lack of consistency in the directionality and strength of the effects of body mass and metabolic rates on winter survival does not undermine their importance, but rather demonstrates the ability of individuals to adjust metabolic rate to changing environmental conditions. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113, 297–309.

Recent studies on grasses and sedges suggest that the induction of a mechanism reducing digestibility of plant tissues in response to herbivore damage may drive rodent population cycles. This defence mechanism seems to rely on the abrasive properties of ingested plants. However, the underlying mechanism has not been demonstrated in small wild herbivores. Therefore, we carried out an experiment in which we determined the joint effect of abrasive sedge components on the histological structure of small intestine as well as resting metabolic rate (RMR) of the root vole (Microtus oeconomus). Histological examination revealed that voles fed with a sedgedominated diet had shorter villi composed from narrower enterocytes in duodenum, jejunum and ileum. Reduction in the height of villi decreased along the small intestine. Activity of the mucus secretion increased along the small intestine and was significantly higher in the ileum. The intestinal abrasion exceeded the compensatory capabilities of voles, which responded to a sedge-dominated diet by a reduction of body mass and a concomitant decrease in whole body RMR. These results explain the inverse association between body mass and the probability of winter survival observed in voles inhabiting homogenous sedge wetlands. KEY WORDS: Body mass, Plant defensive mechanism, Sedges, Silicon, Small intestine INTRODUCTIONGrasses and sedges dominate many terrestrial ecosystems and are the food base for numerous cyclical populations of small herbivores. This plant food contains a high concentration of silicon (Hodson et al., 2005), which together with fibre (Vincent, 1982;Grzelak et al., 2011), increases its abrasive properties (Montagne et al., 2003;. The abrasive properties of this plant food have been proposed as an anti-herbivore defence mechanism (Massey et al., 2008). Our previous studies showed that changes in the silicon concentration in fibrous tussock sedge (Carex appropinquata Schumacher 1801) were induced by a high density of cyclical population of the root voles (Microtus oeconomus Pallas 1776) at the end of the previous summer. We also found that smaller (lighter) voles were characterised by lower mortality during early winter Zub et al., 2014), which might be correlated with the low quality of their food base. However, the underlying proximate mechanism promoting smaller individuals is not known. Low body mass of voles feeding on a highly silicated and fibrous diet (Massey et al., 2008) may reflect the need to reduce absolute energy requirements (Ergon et al., 2004), to bring them into line with reduced digestive efficiency . This efficiency may be compromised because of a reduction of the intestinal surface and its function of nutrient absorption, stemming from the mechanical abrasion of the apex of villi where the widest mature enterocytes, which produce digestive enzymes, are located (Montagne et al., 2003;Abbas et al., 1989;Barker et al., 2008). However, as far as we know the above putative mechanism has not yet been demonstrated in the context of the ...