A negative dependence of the digestibility of grassland vegetation on the amount of grasses in the feed consumed, which is in turn associated with an increased content of silicon compounds in grasses, is found. Using the example of four species of herbivores (Przewalski’s horse, American bison, Bactrian camel, and saiga), which differ in the specifics of digestion, the selectivity of their nutrition was studied in relation to two groups of plants differing in digestibility—grasses and forbs. Saigas, like other obviously highly selective consumers of easily digestible food, strictly select forbs with a low silicon content and high digestibility. With the dominance of low-digestible grasses on the grassland and their forced consumption, the saiga population is not viable. Horses and bison, like other representatives of equines and large ruminants, due to the characteristics of the digestive system, are able to assimilate low-digestible grass feeds successfully and are highly specialized in feeding on grasses. The exclusion of forbs from the diet of horses and bison is associated with the increased toxicity of this group of plants. Camels do not discriminate in the choice of grasses and forbs of different nutritional value and currently actively use fallow areas (on former arable land) dominated by weedy plants, which are avoided by other herbivores. Different specialization in the choice of forage plants when different species of animals graze together causes their division in the use of grassland feed resources, eliminates competition between them, and acts as a necessary factor in preserving the species diversity of grassland vegetation.

The effect of organic forms of silicon contained in plant tissues on the digestibility of feed for herbivorous mammals has been studied. In the process of digestion, an extremely small part of the consumed silicon is assimilated, which makes it possible to use organic silicon naturally contained in plants as an indicator of the digestibility of plant foods. The consumed silicon reduces the digestibility of the feed, puts a limit on the fermentation of the consumed vegetable substrate and its structural components (fiber, lignin). With an increase in the part of silicon in the consumed vegetation per unit, the digestibility decreases linearly by 3–4 units. The fermentation limit is set when the silicon content in the feed is within 3–4% of the dry weight. Among the main groups of forage plants (grasses and forbs), the amount of silicon is significantly higher in grasses (1.70 and 0.91%, respectively). For this reason, the digestibility of grasses in all compared animals (camels, Przewalski’s horses, bison, saiga) is 1.2–1.4 times lower than the digestibility of forbs. With an increase in the proportion of grasses in feed per unit, the digestibility of feed in animals with different types of digestion (ruminant, monogastric) decreases linearly by an average of 0.16 units. The different digestibility of grasses and forbs causes the separation of herbivores by types of nutrition and food specialization. Consumers of various forbs (saigas) avoid the consumption of grasses and are not viable with their dominance in pasture vegetation. The specific features of the digestion of herbivores (Equidae, Bovidae) provide an increased volume of consumption, compensating for the reduced digestibility of grasses.

Among the land-dominated natural formations, like forests and grasslands, the attention of humanity is drawn at present primarily to the former. It is the forests of temperate and tropical zones that are supposed to play an important role in the regulation of biosphere processes on Earth, including the current climate. This almost completely ignores the importance of herbaceous systems widely represented by steppes and meadows. The overwhelming superiority of forests over grass ecosystems in productivity and phytomass stock is recognized. A comparative analysis of production estimates (t/ha/year) and of organic material stocks (t/ha) in the underground and aboveground parts of herbaceous and forest ecosystems accumulated in the scientific literature has shown that meadows and steppes are not inferior (and often even superior) to forests both in the mass of accumulated organic matter and in annual growth of phytomass. The only difference is that organic production is localized in forest communities mainly in the aboveground part as considerable wood (trunk) mass while it is equally or more shifted underground in herbaceous communities in the form of soil organic matter. The leaf (photosynthesizing) masses of herbaceous and forest communities are close in quantity, respectively, and their synthesis of organic products and carbon exchange with the atmosphere are also similar. The author argues that the steppe and meadow ecosystems do not lag behind forests in organic mass and participation in biosphere processes, thus deserving no less effort for their preservation.

Modern climatic changes, weakening of the anthropogenic pasture load, and an increase in steppe fires have led to the expansion and dominance of grasses (graminoids) vegetation on the steppe pastures of Russia and Kazakhstan since the end of the 20th century. This resulted in deterioration of the forage quality of pasture vegetation, a factor in the depression of the saiga population that may have affected the increase in cases of saiga die-offs. A die-off occurred in Kazakhstan in 2015, when about 200 000 animals died. A large number of different versions have been put forward about the reasons for these deaths: an infectious disease (hemorrhagic septicemia), toxic forage plants, unfavorable weather conditions, and the destructive effect of rocket fuel. In this article, we consider a different version of death associated with the negative effect of low-quality food on the sites of saiga death. It is shown that the food supply for saigas on steppe pastures consistently decreases with an increase in the abundance of grasses in forage vegetation. Suitable, nutritious food for breeding saigas include pastures in which the share of grasses in the aboveground phytomass is no more than 30%. If this level is exceeded, the pastures do not provide the necessary level of nutrition for breeding saigas and are unsuitable for them. It is possible that the saigas found themselves during spring migration in steppe regions with a predominance of turf grasses (65–75%), which exceeds their permissible (threshold) abundance (30%). This pasture condition did not provide the necessary nutrition, caused a weakening of animals during the rearing period, and promoted the activation of infectious septicemia in breeding saigas, which, with the combined action of both factors, may have led to their die-off.

Materials on the composition of forage plants in the bison diet were obtained within the enclosure of nonconsumptive use at the Rostov Nature Reserve in the summer and autumn seasons of 2018. The described area is located in the sod-forming cereal-grass steppes in the Western Manych River valley. Data on the seasonal dynamics of the composition of forage plants in the bison diet were produced via cuticular microscopic analysis of fecal samples. An electronic database was created for the plant cuticle structures typical of the species providing the floristic richness of grassland for bisons in order to identify the species of the botanical fragments in the fecal samples. Thus, the proportion of cereals in the summer and autumn diets comprised 64% and 84%, respectively. The selectivity indices of cereal species were measured as 2.1 in the summer and 4.3 in the autumn seasons. For motley grasses, it was 0.5 in the summer season and decreased to 0.2 in the autumn. The index value depends on the animals' taste preferences, the species richness in the aboveground phytomass of grassland, and the plant growth stage.

The article presents the research results the food selection and preferences of the social vole Microtus socialis in the dry steppes (northern subzone of the deserts) in the Chernye Zemli State Biosphere Reserve (north-western part of the Caspian Lowland). It is the first time when the data on seasonal variability of food composition of social voles in their permanent settlements has been obtained by means of cuticular microhistological feces analysis. The diet of the voles consists of 31 plant species. The basis of the diet is grasses (55–95%), mainly Poa bulbosa. Forbs are consumed in smaller amounts, but they play an important role in the summer life of the rodent. In spring, the tulip bulbs (Tulipa biebersteiniana) and the terrestrial parts of Chorispora tenella are of greatest importance in the nutrition of the vole. These herbivores do not eat Bassia hyssopifolia, Phlomis pungens, Carduus acanthoides at all. Voles maximize the use of forage diversity in the area of their settlements. The trophic interactions of rodents to certain plant species were evaluated through the preferences of baits containing plant extracts. The social voles consume baits with extracts from different plant species in varying degrees. In general, the consumption of baits containing grass extracts is higher than that of baits containing extracts from most species of forbs. In the group of forbs, the social vole unequivocally prefer baits containing tulip extracts. The baits with other forbs are consumed by two or more times less than those with grass extracts. The social vole gives the least preference to baits with extracts from Senecio vernalis and Tanacetum achilleifolium. Most likely, a high concentration of toxic secondary metabolites is the main reason for voles avoiding certain baits with plant extracts. How to cite this article: Scopin A.E., Dzhapova V.V., Bembeeva O.G., Ayusheva E.Ch., Dzhapova R.R., Abaturov B.D. 2020. Diet selection by the social vole Microtus socialis (Pallas, 1773) in the Northwest Caspian Lowland // Russian J. Theriol. Vol.19. No.2. P.136–148. doi: 10.15298/rusjtheriol.19.2.04.

Grasses and forbs, which form the basis of forage mass in steppe pasture ecosystems, differ in nutrient value. The species of ungulates - camel (Camelus bactrianus), saiga (Saiga tatarica), bison (Bison bison), and Przewalski's horse (Equus przewalskii), with differing types of digestion and feeding specialization, are characterized by a negative relationship between the share of grasses in the food intake and digestibility. The low and relatively constant rates of digestibility (45-59%) in all seasons are observed in Przewalski's horse and bison, whose diet almost entirely consists of grasses. Higher digestibility is characteristic for camels and saigas (up to 73%) - consumers of forbs. For all species, the choice of forage grasses and forbs is different. Saiga and camel clearly prefer more digestible and nutritious forbs. At the same time, bison and Przewalski's horse purposefully choose to eat more fibre, rough, and less digestible grasses. Differences in preference of grasses or forbs by herbivores are associated not only with the positive indicators of plant forage, but also with the natural toxicity of plants. The forbs are rich in toxic secondary metabolites, whereas in grasses these components are practically absent. Therefore, the choice of grasses and forbs by herbivores is directly related to the unequal response of the animals to the toxicity of forage plants. Bison and Przewalski's horse mainly choose for food less nutritious and feebly digestible, but lacking toxicity compounds, forbs, and almost completely avoid consumption of toxin-enriched forbs. However, many wild and domestic herbivores, in our case saiga and camel, prefer to consume more nutritious, but toxic, forbs and, on the contrary, do not consume non-toxic grasses due to their reduced digestibility. However, in this case, the mechanisms of resistance to toxic metabolites should take action. Otherwise, the risk of negative consequences of nutrition turns out to be rather high, as in saiga populations.

The fodder resources, nutrition, and food supply of free-ranging Bactrian camels, Przewalski horses, and saigas on a fenced forb-grass steppe pasture are studied. The species composition of consumed plants, food selectivity, digestibility, and amount of consumed forage are estimated seasonally. All three species differ in all diet parameters.

In 2015-2016, we studied the species composition, above-ground phytomass and feeding habits of semi-free Przewalski's horses (Equus ferus przewalskii) in a natural pasture of the steppe valley of the Western Manych. The above-ground mass of plants on the pasture varied within 146.0-168.9 g/m(2). We determined the composition of plants consumed by Przewalski's horses and their proportion in diets using a method of microscopically analysis of faeces. Cereals (Poaceae) and sedges (Cyperaceae) (80 to 100%) were the basis of the diet of the Przewalski's horse in the snow-free period. The value of the herb use varied between 0.8% and 20.5%. The main fodder plants for Przewalski horses on this pasture were the following: Festuca valesiaca, Stipa spp., Bromopsis inermis, Agropyron spp., Carex stenophylla, Kochia prostrata, while Elytrigia repens, Poa bulbosa, Chenopodium spp. were eaten in a small amount. We estimated the nutritive value of forage vegetation and nutrition indicators (daily consumption, the feed digestibility). The feed digestibility was determined by the ratio of inert (indigestible) component (silicon) in the diet and faeces. We calculated the amount of consumed feed from the mass of selected faeces and feed digestibility. A semi-free Przewalski's horse consumed 4.7 kg to 10.7-11.8 kg (dry weight) per day. The maximum intake (11.8 kg/animal per day) was observed in autumn. Feed digestibility of forages by Przewalski's horses in the snow-free period varied between 53% and 62% (average was 56%).

Abstract—This paper reports the results of investigation on the food of free-grazing Bactrian camels in a forb–grass steppe pasture, including anthropogenically disturbed areas with dominance of ruderal annuals. We identified the composition of consumed plants using a microhistological analysis of the feces; the digestibility coefficient based on inert (indigestible) silicon contained both in the diet and feces; the quantity of the forage consumed based on the mass of the feces and the digestibility coefficient; and the energy balance based on interrelations between the actual consumption and the existing requirement norms of camels and wild ungulates in energy. In spring, camels mainly consume graminoids (Stipa sp. and Festuca valesiaca), switching to forbs, largely ruderal annuals, in summer and autumn. The forage digestibility is low in spring (56%), increasing to 69–70% in summer and autumn. Low daily consumption rates of dry forage mass in winter and spring (8–9 kg) are changed to high in summer and autumn (20–26 kg), this reflecting the animals’ winter hypophagy. The amount of energy received during the warm season (1.2–1.8 MJ/kg W 0.75) considerably exceeds the requirements, thus explaining the ability of camels to accumulate a safety store of fat in the humps.

Changes in the amounts and dynamics of the deposition of carbon and nitrogen in the soil surface of anthropogenic pasture and forest ecosystems created in places of natural steppe and desert plant associations in the Northern Cis-Caspian area were studied. For two years after input, the litter of oak stands (Quercus robur) deposited 1.3 times more carbon and about the same amount of nitrogen as compared to the steppe litter, even though the mass of litter input in the forest plantation was 1.4 times less than that in the grass ecosystem. The values of carbon and nitrogen contents in the soil under steppe vegetation and under plantation were similar, but there was a redistribution of carbon and nitrogen contents in the forest soil. The carbon contents in the forest litter and 0–5 cm layer make up 33% of the contents in the litter and in 0–50 cm soil layer. The respective value was 22% under the steppe vegetation; the nitrogen contents were 2.9 and 2.0%, respectively. Due to intensive grazing, the ecosystems of microdepressions obtain 1.6–1.7 times less carbon and nitrogen than those of virgin plots. This value was 1.4 for the ecosystem of microelevations. A significant decrease in the contents of carbon and nitrogen was observed in the litter and 0–5 cm layer of meadow–chestnut soils of microdepressions under the pressure of intensive grazing.

In 2012–2013, the species composition, aboveground phytomass, and nutrition of freely ranging bison were investigated on a natural steppe pasture of the Western Manych valley. The aboveground mass of plants on the investigated pasture varied from 16.1 to 19.4 c/ha. The composition of the plants consumed by bison and their share in the bison’s diet were determined by microscopy of feces. In the snowless season, graminoids was the basis of the bison’s diet (79–87%). The use of forbs varied from 4.7 to 17.5%. Sedges were present in the forage in summer; they amounted to 6.5–15.4%. The quantitative assessment of the food selectivity was determined using the ratio of the share of a plant species in the diet to its portion in the plant community of the pasture. The most selected were Festuca valesiaca (the selectivity is 12.0), Agropyron sp. sp. (1.7), Polygonum patulum (1.2), and Carex sp. sp. (1.1). In other forage plants (Poa bulbojsa, Tanacetum achileifolium, and other forbs), the selectivity was 0.01–0.3. The nutritional value of fodder vegetation and food indexes (daily intake, digestibility of forage, consumption of metabolizable energy) were estimated. The digestibility was determined by the ratio of inert (undigestible) components (silicon, lignin) in the diet to their amount in the feces. The daily intake of forage was calculated based on the mass of released feces and digestibility of forage. In the snowless season, bison consumed from 6.3–7.0 to 9.2–9.9 kg/ind. (dry weight) per day. The maximum consumption (9.9 kg/ind.) was observed in summer. The digestibility of forage changed from 45 to 53% (the average is 50%). The consumption of metabolizable energy in different seasons varied from 0.60 to 0.91 MJ/kg0.75/day, which corresponded to maintenance energy.