In 2022 the present authors described the mitochondrial lineage “Altai” (“A”), the most recent of the three phylogroups of the common hamster, occurring in Russia and Kazakhstan. However, the limits of its current distribution and population size required clarification. In the course of our special expedition, tissue samples of the common hamster were collected in 12 regions of Russia and Kazakhstan, including areas at the potential boundary between the ranges of phylogroups A and E (“Europe”). Analysis of the nucleotide sequences of the cytochrome b (cytb) gene of the trapped animals has shown that the boundary between these phylogroups runs between Turinsk and Yekaterinburg, next between Chelyabinsk and Kurgan, and between Omsk and Tatarsk, and diverts southward, heading away from the village of Turgai and the city of Temirtau in the west and the village of Bayanaul in the east. The structure of territorial genetic clades, belonging to phylogroups A and E, and the history of their formation were also analyzed. Most of the territorial clades detected formed during the Late Pleistocene and Holocene. The analysis of population dynamics in phylogroups A and E showed its decline in eight of the 12 regions surveyed.

For the first time, the allelic diversity of exon 2 of the DRB1 gene (class II of the major histocompatibility complex, MHC) was determined by NGS sequencing in natural populations of two species of the genus Allocricetulus: Eversmann's hamster (A. eversmanni) and the Mongolian hamster (A. curtatus). These species are genetically close, but allopatric and demonstrate different habitat preferences. Eversmann's hamster inhabits the steppes and is often associated with agrocenosis, while the Mongolian hamster occures semi-deserts and deserts, where anthropogenic pressure is practically absent. Our analysis revealed a significant effect of positive selection on the diversity of alleles of the DRB1 gene in both study species. However, Eversmann's hamster is characterized by a large average distance between alleles, but a smaller proportion of heterozygous individuals, while the Mongolian hamster is characterized by a larger individual variety of alleles. Thus, despite the extensive range of Eversmann's hamster, we noted significantly less functional diversity of adaptive genes, which may be due to an increased anthropogenic exposure.

clarification. In the course of our specially organized expedition, tissue samples of the common hamster were collected in 12 regions of Russia and Kazakhstan, including areas at the potential border of the ranges of phylogroups “A” and “E” (“Europe”). Analysis of the nucleotide sequences of the cyt b gene of the captured animals has shown that the boundary between these phylogroups runs between Turinsk and Yekaterinburg, further between Chelyabinsk and Kurgan, between Omsk and Tatarsk, and turns to the south, leaving the village of Turgai and the town of Temirtau in the west and the village of Bayanaul in the east. The structure of territorial genetic clades belonging to phylogroups “A” and “E” and the history of their formation were also analyzed. Most of the detected territorial clades were formed during the Late Pleistocene and Holocene. The population dynamics of phylogroups “A” and “E” was analyzed and its decline was shown in 8 of the 12 surveyed regions.

Based on the results of targeted sequencing on the Illumina platform we compared the allelic diversity of exon 2 of the DRB gene in urban (city of Simferopol) and rural populations of the common hamster. The urban population significantly differs from the rural one in terms of the composition and diversity of gene alleles. For individuals living in the city, a larger number of alleles, higher values of haplotype and nucleotide diversity indices, and a smaller proportion of individuals with a homozygous genotype were noted. Both populations are characterized by a significant excess of non-synonymous substitutions over synonymous ones and almost every allele of a gene corresponds to a different amino acid sequence. However, the influence of positive selection on the diversity of variants of antigen-binding sites in the alleles of the DRB gene in urban conditions is much more pronounced. The data suggest that resistance of common hamsters living in specific and varied conditions of the urban environment to various kinds of pathogenic load is higher than in rural populations.

The genus Allocricetulus at presently includes three taxa. The taxonomic status of two of them, A. eversmanni (Eversmann's hamster) and A. curtatus (Mongolian hamster), is now beyond doubt. A third one, A. e. pseudocurtatus, was described as a karyotypically isolated subspecies of A. eversmanni, and has been interpreted ambiguously. In the present study, we analyze the taxonomic and geographical structure of the genus based on 28 craniometric characters using multivariate analysis. In total, 160 specimens from 76 localities were examined. Hierarchical cluster analysis based on the Mahalanobis distance and the maximum likelihood clustering demonstrates the existence of three morphological groups corresponding to three taxa. Both A. eversmanni and A. e. pseudocurtatus form sister clades, while A. curtatus occupies a position basal to them.

The genus Allocricetulus presently includes three taxa. The taxonomic status of two of them, A. eversmanni (Eversmann's hamster) and A. curtatus (Mongolian hamster), is now beyond doubt. The third one, A. e. pseudocurtatus, was described as a karyotypically isolated subspecies of A. eversmanni, and has been interpreted ambiguously. In the present study, we analyze the taxonomic and geographical structure of the genus based on 28 craniometric characters using multivariate analysis. In total, 160 specimens from 76 localities were examined. Hierarchical cluster analysis based on the Mahalanobis distance and maximum likelihood clustering demonstrates the existence of three morphological groups corresponding to three taxa. Both A. eversmanni and A. e. pseudocurtatus form sister clades, while A. curtatus occupies a position basal to them.

Using the data-compression method we revealed a similarity between hunting behaviors of the common shrew, which is insectivorous, and several rodent species with different types of diet. Seven rodent species studied displayed succinct, highly predictable hunting stereotypes, in which it was easy for the data compressor to find regularities. The generalist Norway rat, with its changeable manipulation of prey and less predictable transitions between stereotype elements, significantly differs from other species. The levels of complexities of hunting stereotypes in young and adult rats are similar, and both groups had no prior experience with the prey, so one can assume that it is not learning, but rather the specificity of the organization of the stereotype that is responsible for the nature of the hunting behavior in rats. We speculate that rodents possess different types of hunting behaviors, one of which is based on a succinct insectivorous standard, and another type, perhaps characteristic of generalists, which is less ordered and is characterized by poorly predictable transitions between elements. We suggest that the data-compression method may well be more broadly applicable to behavioral analysis.

In laboratory experiments we investigate hunting behavior towards insects in Phodopus sungorus (Pallas 1773), P. сampbelli (Thomas 1905), Allocricetulus eversmanni (Brandt 1859), and A. curtatus (Allen 1925). In these species purposeful inter-relations with insects in the predator–prey manner have been revealed and described in details. In all species investigated, the patterns of hunting behavior are comparable with those of specialized predator hamsters. In a quarter of all cases, hamsters start attacks by seizing the prey with their paws, which is considering more evolutionary progressive than capturing with their teeth. The Djungarian hamster P. sungorus demonstrates the simplest pattern of hunting, whereas in other species some features of specialization have been revealed, such as different forms of manipulation activity. A. curtatus displays the swiftest and most effective attacks towards prey. We speculate that the obligatory innate patterns of hunting in both Allocricetulus species are connected with more specialized predatory behavior of members of this more evolutionary young group as compared to members of the genus Phodopus, with their optional hunting patterns which need experience to be completed.

Abstract—According to the Dobzhansky–Muller model (Dobzhansky, 1937; Muller, 1942), in the absence of gene flow, conspecific populations can become reproductively isolated, and postzygotic isolation should increase proportionally to the square of divergence time. On the example of four pairs of allopatric species of the subfamily Cricetinae, which is characterized by different levels of divergence, the extent of the formation of reproductive barriers was established by behavioral and physiological methods. The potential prezygotic isolation was assessed by the results of behavioral experiments where female olfactory stimuli were exposed to males. A crossbreeding analysis was performed to detect postzygotic isolation. The divergence time was determined by molecular genetic methods. It was shown that complete reproductive isolation formed in a pair of allopatric species of the genus Phodopus (P. roborovskii and P. sungorus) approximately for 5 million years. The sterility of F1 males and conspecific preferences with slight differences in chromosome morphology formed in hamsters (P. sungorus and P. campbelli) in course of 0.8–1.0 million years. In the pair of species belonging to the genus Allocricetulus, which diverged only 0.3–0.4 million years ago, postzygotic isolation mechanisms and conspecific preferences for a number of traits have already partially formed on the background of differences in the structure and number of chromosomes, although in laboratory conditions it is possible to obtain fertile hybrids (F1 and F2). In the species (chromosomal forms) of the superspecies Cricetullus barabensis sensu lato, which diverged later than the pair of species of the genus Allocricetulus (0.16–0.2 million years ago), the reproductive barriers are less pronounced. Thus, the example of the closely related species of Cricetinae shows that reproductive barriers can develop in allopatry and are more pronounced in species that remain isolated for a longer time. Prezygotic barriers in allopatry can develop as rapidly as postzygotic ones, which makes the mechanism of “reinforcement,” proposed by F.G. Dobzhansky, not so relevant (in case of secondary contact zones).

Contrary to the pre-existing concepts based on morphological and karyological data on the impossibility of hybridization between hamster species of the genus Allocricetulus (A. curtatus and A. eversmanni), F1 hybrids in both combinations, and backcrosses, were obtained for the first time. Some restrictions of hybridization between females of A. eversmanni and males of A. curtatus were detected. Analysis of the synaptonemal complex in the hybrids produced by crossing a female of A. curtatus with a male of A. eversmanni demonstarted that a significant portion of the cells was subject to arrest and selection at the stage of meiosis prophase I. Nevertheless, some spermatocytes still generate viable spermatozoa. Thus, the possibility of hybridization in the laboratory between two Allocricetulus species with different numbers of chromosomes may testify to a relatively low divergence between them.

The hibernation of the Eversman hamster is proven for the first time. The body temperature measured with a thermoaccumulator implanted into the abdominal cavity dropped down to +5°C several times during the winter. This species differs from other hibernating mammals in the peculiarities of its seasonal fluctuation of steroid hormones (cortisol, progesterone, estradiol, and testosterone) and in the absence of fat deposits before hibernation.

The hibernation of the Eversman hamster is proven for the first time. The body temperature measured with a thermoaccumulator implanted into the abdominal cavity dropped down to +5°C several times during the winter. This species differs from other hibernating mammals in the peculiarities of its seasonal fluctuation of steroid hormones (cortisol, progesterone, estradiol, and testosterone) and in the absence of fat deposits before hibernation.