Significance of Vietnam's Da (Black) River as a biogeographical boundary: Insights from small terrestrial mammal distributions

We surveyed the distribution of small terrestrial mammals along the Da (Black) River in the Bac Yen district, Son La province, Vietnam. We recorded 21 species in the area. Species identifications were carried out using morphological and genetic methods in accordance with up-to-date taxonomy. The Da River is traditionally considered a zoogeographic boundary dividing different biogeographic regions in northern Vietnam. Our study was aimed at testing whether it can really be considered as such. Based on the observed distributions, we conclude that the water course is not the distribution boundary for 14 species out of 21 found in the area. Out of the seven remaining species, only those which are associated with the Chinese mountain-forest faunal complex demonstrate their southern boundary along the Da River. Thus, it is unjustified to consider this river as a biogeographic boundary for mammals.