Oecomys Thomas

Rocha, Rita G., Ferreira, Eduardo, Costa, Barbara M. A., Martins, Iracy C. M., Leite, Yuri L. R., Costa, Leonora P. & Fonseca, Carlos, 2011, Small mammals of the mid-Araguaia River in central Brazil, with the description of a new species of climbing rat, Zootaxa 2789, pp. 1-34 : 16-17

Oecomys Thomas

Oecomys are small to medium-sized species with the tail longer than the head and body length. The dorsal pelage varies from orange brown to brown and ventral pelage is white or yellowish white. The long tail is self-colored and is covered by dark thin hairs, usually ending in a very short tuft. Hind feet are short and broad, with a white dorsal surface. Females have four pairs of mammae.

The occurrence of Oecomys species in sympatry has already been reported in Amazonia (e.g., Patton et al. 2000; Voss et al. 2001). In this ecotonal area, we captured three Oecomys species. Oecomys paricola is relatively easy to distinguish from the other two by cranial characters, while Oecomys sp. and O. roberti are rather similar, and the former shows a high degree of morphological variation. Cranial diagnostic traits of each species are detailed in the species accounts below. Measurements and ecological notes are related to collected specimens only, because only 42 out of 106 individuals were collected and we were unable to identify them to species level in the field. Only thorough comparisons of museum specimens (mainly based on skulls) and analyses of DNA sequences allow proper identification of Oecomys species.

Phylogenetic analyses show three distinct clades of Oecomys species, occurring sympatrically at the Araguaia River ( Fig. 8 ). High bayesian posterior probabilities (BPP) support species monophyly, but interspecific relationships are not resolved, with the exception of the clade ( Oecomys sp., O. roberti ) (BPP = 0.99). Although we were unable to distinguish Oecomys sp. from O. roberti at first using morphological evaluation, the phylogenetic analysis clearly places specimens of these two species in two distinct clades, with an interspecific genetic distance of 6.5% on average. The name O. roberti is given to the clade that includes UFES 1369 and UFES 1370 from the study area, since they match the species description by Patton et al. (2000) and group with the specimen MVZ 197616, which is a topotype of O. roberti , collected at Chapada dos Guimarães, state of Mato Grosso, Brazil (type locality of the species; Musser & Carleton 2005). Concerning the other clade, we list it as Oecomys sp. since these specimens cannot be unambiguously allocated to any described species. They present high level of non-geographic morphological variation, despite very low genetic divergence within the clade (0.7%). One specimen captured in the study area (UFES 1353) shares remarkable similarities with the holotype of O. cleberi examined by one of us (YLRL), but most of the remaining specimens do not. Oecomys cleberi is known only from the holotype collected in Distrito Federal, central Brazil ( Locks 1981) and it has been associated to the bicolor species complex ( Musser & Carleton 2005). On the other hand, our series of Oecomys sp. has clear phylogenetic affinities with O. roberti . More samples are needed to clarify the taxonomic status of these Oecomys species from the mid-Araguaia River; this will be the subject of a separate paper (Rocha et al. in. prep). Like Oecomys sp., O. roberti has relatively low intraspecific divergence (0.4%), but O. paricola , on the other hand, has relatively high intraspecific divergence (3.6%), which may reflect the geographic distance of the samples from states of Tocantins (UFES 1438 and 1368) and Mato Grosso (UFMG 2841, MVZ 197507 and 197508), and the presence of geographic barriers along its distribution, such as the Araguaia and Xingu Rivers.

Figures

FIGURE 8. Bayesian tree of cyt-b haplotypes from specimens of Oecomys rooted using Hylaeamys and Thomasomys. Bayesian posterior probabilities are in bold and percentages are average Kimura two-parameter distances.