Oecomys roberti (Thomas, 1903)

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 : 82-84

Oecomys roberti (Thomas, 1903)

Figures 30C, 30F

VOUCHER MATERIAL (N = 7): Jenaro Herrera (AMNH 276701), Nuevo San Juan (AMNH 268259, 268262, 273112, 273119; MUSM 11215, 15340).

UNVOUCHERED RECORDS: None.

OTHER SPECIMENS EXAMINED (N = 46): Brazil — Amazonas, Altamira on right bank of Rio

Juruá (MVZ 200907), Barro Vermelho on left bank of Rio Juruá (MVZ 200908), Barro Esperança on right bank of Rio Juruá (MVZ 200909, 200910), Colocação Viravolta on left bank of Rio Juruá (MVZ 200912), Condor on left bank of Rio Juruá (MVZ 200915), Igarapé Nova Empresa on left bank of Rio Juruá (MVZ 200917, 200918), Jainu on right bank of Rio Juruá (MVZ 200927, 200928), Penedo on right bank of Rio Juruá (MVZ 200947), Sacado on right bank of Rio Juruá (MVZ 200934, 200935); Mato Grosso, Serra de Chapada (BMNH 3.7.7.67 [holotype of roberti ]), Serra do Roncador (BMNH 1981.410– 1981.417); Pará, Santa Rosa on Rio Jamanxim (BMNH 9.3.9.9 [holotype of tapajinus ]). Guyana — Pomeroon-Supenaam, Supenaam River (BMNH 10.5.4.23–10.5.4.28 [type series of guianae ]), Potaro-Siparuni, Iwokrama Forest (ROM 111776), Upper Takutu-Upper Essequibo, Gunn’s Strip (ROM 106779), Quarter Mile Landing (ROM 98125). Peru — Amazonas, Huampami on Río Cenepa (MVZ 153528, 155005); Loreto, 25 km S Iquitos (TTU 98797), Collpa Salvador on Río Pucacuro (MUSM 17631, 17685), Nina Rumi (MUSM 44981–44984), Ullpayacu on Río Pastaza (MUSM 16424); Madre de Dios, 15 km E Puerto Maldonado (KU 144333, 144335, 144336, 144338).

IDENTIFICATION: We recognize Oecomys roberti in the broad sense of Carleton and Musser (2015) —including the nominal taxa tapajinus Thomas, 1909 , and guianae Thomas, 1910 , as junior synonyms—despite conspicuous geographic variation in ventral pelage coloration and deep phylogeographic structure revealed by mtDNA sequence analyses. This is a moderately large (ca. 45–65 g) species with brownish dorsal fur and a unicolored (all dark) tail that is substantially longer than the combined length of the head and body (LT/HBL = 1.21 ± 0.10) and lacks a terminal pencil of long hairs. Most specimens from south of the Amazon and west of the Rio Tapajós have a continuous streak of self-white ventral fur that extends from chin to anus, but specimens from eastern Amazonia (including those referred by authors to guianae and tapajinus ) have almost completely gray-based ventral fur, as do the specimens we identify as O. roberti from the Yavarí-Ucayali interfluve and northern Loreto.

Other morphological traits that we associate with Oecomys roberti include 7–9 mm long dorsal fur, short and sparse ungual tufts that do not conceal the claws on digits II–V of the hind feet, shallow zygomatic notches, absence of postorbital processes, incisive foramina that do not extend posteriorly to the level of the M1 alveoli, absence of an alisphenoid strut (in most specimens), a complete stapedial arterial circulation (pattern 1 of Voss, 1988), and a distinctive range of morphometric variation (table 25). Several fluid-preserved specimens had either 17 or 18 caudal scale rows/cm, and a single specimen that we dissected (AMNH 273112) had small preputial glands that did not extend anteriorly beyond the ventral flexure of the penis. Diploid chromosome counts of 2 n = 80–82 have been reported for specimens of O. roberti from central and western Brazil (Amazonas, Mato Grosso, and Rondônia; Patton et al., 2000; Langguth et al., 2005; Suárez-Villota et al., 2018).

Although Rocha et al. (2018) recognized Oecomys tapajinus as a valid species, we are not persuaded by their results. Their multigene topology recovered a cluster of four sequences that they identified as O. tapajinus , as did separate analyses of two mitochondrial genes (CYTB and COI), but it is not clear whether the nuclear locus they sequenced (intron 7 of beta fibrinogen) supported the same topology when analyzed separately. Because analyses of single-locus data (the mtDNA genome is a single-locus marker) cannot distinguish gene trees from species trees, evidence from other sources—multiple loci, karyotypes, and morphology—is important for confident species delimitation. Unfortunately, the mtDNA clades that Rocha et al. (2018) recovered were not morphometrically distinguishable, karyotypic data were not discussed, and the qualitative character differences that the authors attributed to roberti on the one hand and tapajinus on the other are inconsistent. 25 Lastly, the application of the name tapajinus is problematic because the holotype (BMNH 9.3.9.9) has the ventral coat-color phenotype that Rocha et al. attributed to roberti (a continuous midventral streak of self-white fur; Thomas, 1909), the molars are too worn to assess the allegedly diagnostic occlusal morphology of M1, and the type

25 Oecomys roberti was said to have either an all white venter or to have a continuous midventral streak of self-white fur, but some western Amazonian specimens with the sequence characteristics of roberti (see below) have entirely gray-based ventral fur. Similarly, although tapajinus usually has entirely gray-based ventral fur, some specimens have a self-white ventral midline ( Rocha et al., 2018). We were unable to replicate Rocha et al.’s observations about the dental difference they observed between roberti (supposedly without an anteromedian flexus on M1) and tapajinus (supposedly with an M1 anteromedian flexus) due to variation and scoring ambiguities in both taxa.

locality ( Rocha et al., 2018: fig. 1) is in a region where both roberti and tapajinus are said to occur and to be difficult to distinguish.

Phylogenetic analysis of sequence data from the western Amazonian material that we identify as Oecomys roberti recovered several strongly supported haplogroups (fig. 34). Haplogroup A includes Patton et al.’s (2000) specimens from western Brazil (Amazonas) and a smaller series from southeastern Peru (Madre de Dios) ; most specimens from this clade have self-white ventral fur, as does the holotype ( BMNH 3.7.7.67) and other material we examined from Mato Grosso (but KU 144338 with gray-based buffy ventral fur is an exception). Haplogroup B includes 10 specimens from several localities north of the Amazon, and haplogroup C includes four specimens from the Yavarí-Ucayali interfluve ; by contrast with most specimens from haplogroup A, specimens from haplogroups B and C have completely gray-based ventral fur. Average uncorrected pairwise sequence differences at the cytochrome b locus (table 26) suggest substantial genetic divergence among these mtDNA clades, but our measurement data document broad morphometric overlap (table 27), and we were unable to discover any phenotypic differences other than the ventral pelage traits described above.

ETHNOBIOLOGY: The Matses do not recognize this species and so have no particular beliefs about it.

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

REMARKS: Of the six specimens collected at Nuevo San Juan, four were taken in seasonally flooded primary forest and two were taken in upland primary forest close to a stream. Four specimens were trapped on lianas 1.0– 1.5 m above the ground, one was shot at night on a liana close to the ground, and one was trapped on the ground. Few as they are, these data seem consistent with the observations of other authors ( Patton et al., 2000; Rocha et al., 2018) that Oecomys roberti (sensu Carleton and Musser, 2015; i.e., including tapajinus ) is primarily associated with seasonally flooded forests.