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The first specimen of Euchoreutes naso was collected by N. M. PRZEWALSKI 1879 in Central Asia, in Dzungaria. SCLATER described the species in 1890. In Mongolia Euchoreutes was found in 1924. 50 years later new records were published by SOKOLOV et al. (1978). A taxonomic revision was done by ŠENBROT et al. (1992, 1995). The species was related to the family Sminthidae (Rodentia/Mammalia). We found the most eastern locality in Mongolia at the longitude 106° 47' E. The reproduction rate was 3.1 (n = 8 females). In the laboratory we noticed a litter of 4 young. The body mass of newborns was less than 2 g. The body mass of adults was in July in the middle 31 g. The tail was with 145-180 mm more than 50% larger than the head-body-distance (95-107 mm). Insects are the preferred nutrition of this rodent. Telemetry studies have shown that settled burrows of one individual can have a distance up to 833 m. In July the short-term investigated animals have occupied minimum areas between 2 ha and 14.5 ha. Expectation of life in captivity can be registered with several years.

The Eurasian beaver, Castor fiber L., suffered extreme demographic reduction through overhunting until the end of the 19th century. However, active protection measures have led to a powerful recovery in range and population numbers. The vast majority of beavers (83%) now occur in the former Soviet Union. The present study investigates the geographic distribution of genetic variation of C. fiber in this eastern part of the species range (former Soviet Union and Mongolia), with special emphasis on small isolated populations of the Asian subspecies C. fiber pohlei, C. fiber tuvinicus, and C. fiber birulai. The analysis yielded 12 different haplotypes, all of which were population specific. Results indicate that C. fiber displays great population structuration (FST = 0.985), coupled with an overall low level of genetic divergence (mean number of pairwise differences 7.262 ± 3.435). In particular, the autochthonous populations in Mongolia or Siberia do not appear significantly different from samples from the European part of Russia, despite the great geographical distance. C. f. birulai appears as the most divergent member, a fact that could result from its longer genetic isolation in an enclosed watershed. Examination of our data suggests a single recent origin of the present beaver population in eastern Europe and Asia.