Oecomys Thomas, 1906

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 : 61-62

Oecomys Thomas, 1906

Species of Oecomys are semiarboreal cricetids that occur throughout the forested Neotropical lowlands from Costa Rica to northern Argentina ( Carleton and Musser, 2015). Multiple sympatric species occur in local faunas throughout Amazonia ( Voss and Emmons, 1996; Patton at al., 2000; Voss et al., 2001; Hice and Velazco, 2012), which appears to be the center of diversity for this taxonomically challenging genus. Although progress has been made in sorting out the species that occur in eastern Amazonia (Voss et al., 2001; Rocha et al., 2018; Suárez-Villota et al., 2018), the identification of western Amazonian Oecomys remains problematic.

Species of Oecomys can be distinguished from members of other oryzomyine genera by the following combination of morphological traits: (1) long mystacial vibrissae (laid back alongside the head, these sensory hairs always extend well beyond the tips of the pinnae); (2) unwebbed hind feet with a long, semiprehensile fifth digit and large, fleshy plantar pads; (3) soft (nonspinous) fur; (4) beaded, ridged, or crested supraorbital margins that are always anteriorly convergent; (5) a zygomatic plate that is variable in width but that never has a spinous dorsal process; (6) a long-wide palate; and (7) pentalophodont, brachydont-bunodont molars that are incipiently lophodont (with shallowly interpenetrating labial and lingual flexi). Species of Oecomys are sometimes confused in the field with species in other oryzomyine genera—especially Euryoryzomys and Hylaeamys —but the morphology of the hind foot (illustrated by Voss et al., 2001: fig. 53) is diagnostic.

Morphological characters that vary among species of Oecomys include (inter alia) size; dorsal fur length; ventral fur pattern and coloration; length and density of ungual hairs at the bases of digits II–V of the hind foot; caudal scale size (inversely correlated with scale-row counts per centimeter); presence/absence of a distinct pencil of long hairs on the tail tip; depth of the zygomatic notches on either side of the rostrum; presence/absence of postorbital processes; length of the incisive foramina relative to diastemal length (as quantified by the ratio LIF/LD); presence of a bony strut of the alisphenoid (separating the buccinator-masticatory foramen from the accessory oval foramen); carotid arterial morphology; presence/absence and size of the subsquamosal fenestra; overlap between the tegmen tympani and the posterior edge of the squamosal; and the presence, absence, and size of preputial glands. Although taxonomic variation has sometimes been described in the width and shape of the mesopterygoid fossa, the number and depth of the posterolateral palatal pits, shape of the zygomatic arches, and details of molar occlusal morphology, substantial individual (intraspecific) variation in all these features makes it difficult to use them for diagnostic purposes.

Karyotypes are also highly variable in the genus, with known diploid numbers ranging from 54 to 90 chromosomes ( Sokolov and Malygin, 1994; Patton et al., 2000; Gomes et al., 2016). Unhappily, many specimens of Oecomys (including the ones we collected in the Yavarí-Ucayali interfluve) are not accompanied by karyotypic data, so this useful source of taxonomic information is not consistently available.

Sympatric species of Oecomys can usually be distinguished morphologically, but diagnostic features are often subtle, and associating local phenotypes with name-bearing material from distant localities is sometimes difficult. To supplement our morphological comparisons, we analyzed cytochrome b gene sequences from specimens collected throughout western Amazonia, including new sequence data obtained by us and additional sequences downloaded from GenBank. In total, we analyzed sequence data from 143 specimens of western Amazonian Oecomys , of which 17 were collected in the Yavarí-Ucayali interfluve (appendix 3).

Based on maximum-likelihood analysis of these data and our examination of voucher material and other specimens, we recognize a total of eight species in western Amazonia, of which at least five occur in the Yavarí-Ucayali interfluve (fig. 25). Of the five species in our region, two are members of widespread complexes ( O. bicolor , O. roberti ), and three are described as new in the following accounts ( O. galvez , O. nanus , O. makampi ). Of the three additional western Amazonian species, one is apparently restricted to the north bank of the Amazon ( O. “ paricola ” sensu Carleton and Musser, 2015), one is widespread north and south of the Amazon and might eventually be found in our region ( O. superans ), and one is only known from scattered localities along the base of the Andes ( O. osgoodi , currently treated as a synonym of O. trinitatis sensu Carleton and Musser, 2015 ). 18 Average uncorrected pairwise (p) distances among these taxa range from 7.4% to 11.1% (table 18).

Of the five species of Oecomys known to occur in our region, three are small and have self-white ventral fur (the hairs entirely white from tips to roots), whereas two others are substantially larger and have gray-based ventral fur. In the following accounts we treat the small species with self-white ventral pelage first.