Oecomys nanus, Voss & Fleck & Jansa, 2019

Voss, Robert S., Fleck, David W. & Jansa, Sharon A., 2019, Mammalian Diversity And Matses Ethnomammalogy In Amazonian Peru Part 5. Rodents, Bulletin of the American Museum of Natural History 2024 (466), pp. 1-180 : 69-71

Oecomys nanus , new species

Figures 26C, 26F

Oecomys species: Patton et al., 2000: 126 (an unnamed form distinguished from other congeners by size and cytochrome b sequence comparisons).

TYPE MATERIAL AND TYPE LOCALITY: The holotype, MUSM 23815, consists of the skull, fluid-preserved body, and frozen tissues of an adult male collected by Jessica Amanzo (original number JAA 842) at Jenaro Herrera on 21 June 2003. The frozen tissues of the holotype are preserved in the Ambrose Monell Cryo Collection at the American Museum of Natural History with catalog number M-280569.

OTHER VOUCHER MATERIAL (N = 11): Jenaro Herrera ( AMNH 276699, 276713, 276722; MUSM 5452, 5462, 16001, 16003, 23814, 23816– 23818). Two additional specimens from Jenaro Herrera ( ZMMU S-151007, -151012) examined by the late G.G. Musser probably correspond to this species (see below).

OTHER SPECIMENS EXAMINED (N = 3): Brazil — Amazonas, Colocação Vira-Volta on left bank of Rio Juruá (MVZ 200906), Lago Vai-Quem-Quer on right bank of Rio Juruá (MVZ 200905), Seringal Condor on left bank of Rio Juruá (MVZ 200904).

DESCRIPTION: Oecomys nanus is one of the smallest species in the genus, with recorded nonpregnant adult weights consistently less than or equal to 30 g. The adult dorsal pelage is grizzled reddish-brown and 6–7 mm long at midback. The ventral pelage is self-white (the hairs white to the roots) from chin to anus, but two specimens ( MUSM 23816 , MVZ 200905) have narrow (ca. 3 mm wide) lateral zones of gray-based hairs between the fore- and hind legs. The hind feet are covered dorsally with short, pale hairs, but most specimens have indistinctly darker metatarsal markings ; the ungual tufts are neither dense enough nor long enough to conceal the claws in our Peruvian material, but one Brazilian specimen ( MVZ 200905 ) has long and dense ungual tufts on digits II– V. The tail is unicolored (dark above and below), about 115% of head-and-body length (on average) and bears a pencil of apical hairs that are 6–9 mm long. We counted 17–19 scale-rows/cm on four fluid-preserved specimens. In four dissected adult male specimens ( AMNH 276699 , 276722 ; MUSM 23814, 23815) the preputial glands are very large, extending beyond the ventral flexure of the penis to lie between the skin and the abdominal musculature ; measured from the free margin of the prepuce to their distal margin, these glands are 9–12 mm long, and they are visible externally as paired subcutaneous swellings just anterior to the penis.

The skull is diagnostically small in most measured dimensions (table 21). In dorsal cranial view the base of the rostrum is flanked by very shallow zygomatic notches and, as in other congeneric species, the interorbital region is beaded and strongly convergent anteriorly. Postorbital processes are absent, and the temporal crests (marking the dorsalmost origin of the temporalis muscle) are only weakly developed. In ventral view the incisive foramina are moderately long relative to diastemal length (LIF/LD = 0.63 ± 0.02), and sphenopalatine vacuities are absent (except in MVZ 200905). The alisphenoid strut (separating the buccinator-masticatory and accessory oval foramina) is bilaterally absent in seven adult skulls and bilaterally present in four others. The carotid circulation is complete, with a well-developed supraorbital ramus of the stapedial artery (pattern 1 of Voss, 1988). The tegmen tympani usually does not contact the squamosal, subsquamosal fenestrae are consistently present and patent (opening into the endocranial space without bony obstruction), and the mastoid capsules are usually fenestrated. The molar dentition qualitatively resembles those of other congeneric species and seems to lack any diagnostically useful traits.

Three Brazilian specimens karyotyped by Patton et al. (2000: 127) had 2 n = 86 chromosomes, but an even higher diploid number (2 n = 90) was reported by Sokolov and Malygin (1994) from a taxon they identified as “ Oecomys bicolor ?” collected at Jenaro Herrera. The late G.G. Musser (personal commun.) measured two ZMMU specimens from Jenaro Herrera that might have been the ones karyotyped by Sokolov and Malygin; both had short (3.4–3.6 mm) molar toothrows—within the range of dental variation we attribute to O. nanus . However, we have not personally examined this material, so the attribution of Sokolov and Malygin’s 2 n = 90 karyotype to the present species is uncertain.

VARIATION: We refer the three Brazilian specimens that Patton et al. (2000) called “ Oecomys species ” to O. nanus despite modest-to-substantial mtDNA sequence divergence (ca. 2.9%–7.1%, uncorrected, at the cytochrome b locus) because those specimens form a strongly supported clade with our material (fig. 27), and because the associated voucher material is morphometrically and qualitatively similar (with the apparently minor exceptions noted above). Although we acknowledge the possibility that the Brazilian and Peruvian specimens differ karyotypically (see above), it seems useful to have a name for the clade to which they both belong.

COMPARISONS: Oecomys nanus is smaller than sympatric O. bicolor (although there is some overlap in all measured dimensions; table 21), and it has longer fur (6–7 mm versus 4–6 mm) and a longer pencil of apical caudal hairs (6–9 mm versus 2–5 mm). Additionally, these species differ conspicuously in preputial gland morphology (very large in O. nanus , but macroscopically absent in O. bicolor sensu stricto), karyotypes (2 n = 86–90? versus 2 n = 80), and in mtDNA sequence comparisons (ca. 7.8%, uncorrected, at the cytochrome b locus; table 18).

The only currently recognized congener that resembles Oecomys nanus in size is the Guianan species O. rutilus Anthony, 1921 . In fact, despite their wide geographic separation, O. nanus and O. rutilus are morphometrically indistinguishable, with homologous measurement means that differ by only a few tenths of a millimeter (table 22). Oecomys rutilus also resembles O. nanus in external appearance, with long (6–8 mm) reddish-brown dorsal fur, self-white ventral fur, and a long-penciled tail (Voss et al., 2001: 109–116). The only craniodental difference we observed in side by side comparisons is that the zygomatic notches are so shallow in O. nanus as to be almost inapparent in dorsal view, whereas the zygomatic notches are visibly deeper and wider in O. rutilus (although not sufficiently so for the difference to be measurable). Like the specimens we dissected of O. nanus , dissected male specimens of O. rutilus (AMNH 266561, 267584, 267586, 267588–267590) have large preputial glands. Despite such morphological similarities, karyotyped specimens of O. rutilus have 2 n = 54 chromosomes ( Gomes et al., 2016)—16 fewer diploid pairs than the Brazilian specimens of O. nanus karyotyped by Patton et al. (2000) and 19 pairs fewer than the karyotype reported from Jenaro Herrera by Sokolov and Malygin (1994) — and phylogenetic analyses of mtDNA sequence data suggest that these species are not closely related ( Suárez-Villota et al., 2018). 22

Curiously, our phylogenetic analysis (fig. 27) recovered Oecomys nanus as the robustly supported sister taxon of an unnamed species repre-

22 Oecomys nanus is represented in Suárez-Villota et al.’s (2018: fig. 1) phylogenetic results by the cytochrome b sequence labelled “ Oecomys sp. JUR 354,” which was obtained by Patton et al. (2000) from the specimen now cataloged as MVZ 200905. Note, however, that none of the internal nodes separating O. nanus and O. rutilus in Suárez-Villota et al.’s phylogeny is strongly supported.

sented by two specimens from southeastern Peru (FMNH 170599, 170604). The latter are much larger than O. nanus in all external and craniodental dimensions (e.g., LM = 4.6–4.7 mm), and they have partially gray-based ventral fur, very long (9–11 mm) dorsal fur, long ungual tufts, and narrow interorbital regions. The average pairwise uncorrected sequence difference at the cytochrome b locus between O. nanus and this unnamed taxon is 6.9%. No other taxon appears to be closely related to O. nanus based on our mtDNA sequence analyses.

ETYMOLOGY: The species name is Latin for “dwarf,” a noun standing in apposition to the generic name.

ETHNOBIOLOGY: This species is not known to occur in Matses territory.

MATSES NATURAL HISTORY: The Matses have no knowledge of this species.

REMARKS: Of the eight specimens of Oecomys nanus collected in 2003 at Jenaro Herrera, seven were taken in pitfalls in swampy primary