In this report, the fifth and last of our monographic series on mammalian diversity and ethnomammalogy in the Yavarí-Ucayali interfluvial region of northeastern Peru, we document the local occurrence of 40 species of rodents, including 5 sciurids, 17 cricetids, 1 caviid, 1 cuniculid, 2 dasyproctids, 1 dinomyid, 2 erethizontids, and 11 echimyids. The following substantive taxonomic results, among others, are reported: (1) We discuss current issues of sciurid classification and treat all New World tree squirrels (Sciurini), except North American Tamiasciurus, as members of the genus Sciurus; the proposed subgeneric classification is monophyletic, and it conserves longstanding binomial usage for most species. (2) We describe a new species of squirrel, Sciurus (Hadrosciurus) pachecoi, which had previously been identified as a distinct lineage by molecular analyses. (3) We discuss the nominal taxa currently synonymized with S. (H.) pyrrhinus and comment on the application of names to phenotypes and mitochondrial haplogroups. (4) The currently accepted type locality of S. (H.) spadiceus (Cuiabá) cannot be correct; instead, documentary evidence suggests that the holotype must have been collected near Santarém. (5) Sciurus flaviventer appears to be the only valid species of Microsciurus (sensu lato) that occurs in the Amazonian lowlands; Amazonian records of taxa previously reported in the literature as M. sabanillae and M. “species 2” appear to be based on erroneous geographic coordinates and unexplained genotype/ phenotype discordance, respectively.

(6) We discuss and illustrate the diagnostic morphological characters of Nectomys apicalis and N. rattus, which have broadly overlapping distributions in northern Peru. (7) We analyze cytochrome b sequence data from 143 specimens of Oecomys from western Amazonia and summarize evidence for multiple unnamed lineages; of these, three from the Yavarí-Ucayali interfluve are described as new species. (8) We question the recognition of O. tapajinus as a species distinct from O. roberti due to the lack of unambiguously diagnostic characters and the doubtful identity of the holotype of tapajinus. (9) We confirm sympatry between two species of Scolomys and provide revised diagnostic criteria for S. melanops and S. ucayalensis. (10) We report the only specimen of Dinomys branickii accompanied by definite locality data from Loreto department. (11) Proechimys quadruplicatus and P. steerei, closely related species previously thought to occur on opposite banks of the Peruvian Amazon, are both present in the Yavarí-Ucayali interfluve; diagnostic characters are tabulated for the six species of Proechimys now known to occur in our region.

Despite intensive and methodologically complementary faunal-sampling efforts, our rodent inventory is probably incomplete; at least four additional species could be expected to occur in our region based on geographic range data. If all four do occur there, then our inventory is about 90% complete. Documented sympatric species richness at intensively sampled sites in our region is substantially less than the regional total, but because of methodological omissions, no site is believed to have been completely inventoried for rodents. In the absence of known barriers to mammalian dispersal within the Yavarí-Ucayali interfluve, however, local (sympatric) species richness is probably constrained only by habitat availability.

Matses knowledge of rodents is richly detailed for primary game species (Cuniculus paca and Dasyprocta fuliginosa) but is less detailed for less culturally important subsets of the fauna. As previously documented for other mammals (e.g., primates, xenarthrans, and ungulates), important game species are known by multiple names (including synonyms and hyponyms), whereas less culturally important but still salient species (e.g., squirrels) have single names, and many inconspicuous (e.g., small, nocturnal, and morphologically indistinguishable) species do not have unique identifiers.

With the rodents treated in this report, the mammalian fauna of the Yavarí-Ucayali interfluve is now known to include at least 201 species, but >20 additional species (mostly bats) could still be expected in the region based on geographic range data. Despite the probable incompleteness of our inventory, the Yavarí-Ucayali interfluve is the only part of western Amazonia with an extensively documented mammal fauna. Therefore, the completion of this monographic series provides a unique taxonomic resource for urgently needed mammalogical research in this ecologically intact but increasingly vulnerable region.