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Various methods are used to identify representatives of the genus Sylvaemus, most of which are metric. In order to exclude anomalies in further calculations and to minimize allometric variations associated with the growth of animals, it is necessary to take into account age as an important factor of variability. Usually, a number of criteria are used to determine the age and describe the growth of mice: measurements of body and skull parameters, body weight, the degree molar wear, and the weight of the lens of the eye. Length is a more consistent guide for determining the age of mice than body weight, which usually ceases to be directly related to age after reaching adult size. Age can also be determined by the fusion of the epiphysis with the diaphysis in the bones of the limbs and by the degree of development of the thymus. Most often, teeth are used to determine age. Among such age criteria, we distinguish eruption and replacement of teeth sets, tooth growth in length, overgrowth of the tooth pulp cavity, tooth wear, and annual layers in the tooth tissues. Usually, the age of mice is determined by the degree of molar wear. Some authors do not consider this method universal due to individual feeding habits of animals and other environmental factors. The order of wear of tooth rows can sometimes change, and the degree of wear of the right and left tooth rows can also differ. The craniological collection of small mammals of O. V. Zorya, collected in the territory of Kharkiv Oblast, Ukraine, was studied. In total, 198 specimens of three species of the genus Sylvaemus were analysed: Sylvaemus uralensis, Sylvaemus sylvaticus, and Sylvaemus tauricus. Analysed were 14 odontometric and 23 craniological characters. Among the metric characters, four odontometric (LM2, WM1, LM2, LM123) and nine craniological characters (LIOC, LD, LFI, LPP, WCH, GLS, DI, LLM, LM) made the greatest contribution to the age group differentiation of mice of the genus Sylvaemus. Among the features that contributed the most to the differentiation, length measurements prevailed. LLM, LM123, LPP, LIOC, and GLS are the least variable metric characters. It is not possible to divide the sample of mice of the genus Sylvaemus into age groups based on odontometric and craniological character since the ranges of values of metric characters overlap.

The study of the species composition, biological and ecological features of small mammals was carried out for five years (2018–2022) in the surroundings of the village of Rozsohuvata (Kharkiv Oblast, Ukraine). During the study period, nine species of mouse-like rodents and three species of insectivorans were detected. The re-search area is represented by a ravine-beam system, agrocoenoses, field-protecting forest strips, riparian vege-tation around artificial reservoirs and streams, dry meadows, and pastures. Trappings of small mammals were carried out on thirty lines of traps. During the entire period of trapping, the dominant species was Sylvaemus uralensis and in some seasons Sylvaemus sylvaticus dominated. Starting from 2018, the relative abundance increased in autumn, reached a peak in 2020 and declined in the following years. In general, 2019 was the year of maximum abundance and most species were recorded in the summer of this year. The greatest number of species was found in the ecotone between riparian vegetation and field (10 species), as well as in the area of the multi-grass steppe in the beam (8 species). The eurytopic species include Sylvaemus uralensis and Syl-vaemus sylvaticus, whereas the stenotopic species is Mus spicilegus. According to the obtained abundance scores, only one species—Myodes glareolus—occurs randomly, whereas the rare species here are Mus spicile-gus, Sorex minutus, and Crocidura suaveolens. Six species are non-abundant: Apodemus agrarius, Sylvaemus tauricus, Mus musculus, Microtus levis, Cricetulus migratorius, and Sorex araneus. Only Sylvaemus sylvaticus belongs to common species, and only Sylvaemus uralensis is abundant. It should be mentioned that Cri-cetulus migratorius, which usually belongs to the category of random or rare species, turned out to be non-abundant in the study region, and it was recorded regularly in almost half of the studied biotopes. Myodes glareolus, which usually prefers oak forest, was extremely rare. The abundance of Apodemus agrarius, which usually belongs to common species in river valleys, was also low. Sylvaemus showed greater ecological valence, dominated in abundance and inhabited most biotopes, except for Sylvaemus tauricus, which began to occur only in the last two years and mainly in forest strips.