Oecomys sydandersoni is an arboreal species of rodent in the genus Oecomys. It lives in forest patches in a small area in eastern Bolivia. It is a medium-sized species, weighing about 45 g (1.6 oz), with mostly grayish and brownish fur and short and broad hindfeet with well-developed pads.
First collected in 1964, it was formally described in 2009. The species may be most closely related to O. concolor and O. mamorae, which are distributed further north and south in South America. Among other characters, the three share a particular arrangement of the arteries of the head. Virtually nothing is known of its biology.
Oecomys sydandersoni is medium-sized for its genus, being larger than, for example, O. bicolor, but smaller than O. concolor and O. mamorae. Its short, soft, and fine fur is bright ochraceous brown to pale tawny on the upperparts, changing moderately abruptly into the generally gray underparts. The fur on the chin, throat, and part of the belly is entirely white. The head is more grayish than the rest of the upperparts and the eyelids are black. Short hairs thinly cover the external ears, which are brown to gray-brown in color. The short and broad hindfeet are dirty white and show some characters that are typical of the genus, including well-developed pads and tufts of hair on the digits and a long fifth digit. The tail is slightly longer than the head and body on average, but relatively short for the genus. It is uniformly brown, but with a somewhat paler portion below near its base. Sparsely haired, the tail ends in a rudimentary pencil. There is no evidence for sexual dimorphism.
The skull shows a short front part (rostrum) and a broad interorbital region (located between the eyes). The roof of the braincase is marked by supraorbital shelves. The incisive foramina, which perforate the front part of the palate, are very broad and extend between the first molars. The zygomatic plates, the flattened anterior portions of the zygomatic arches (cheekbones), are expansive and show well-developed notches at their front ends. The palate itself is flat and ends a little behind the third molars. The posterolateral palatal pits, located near the third molars, are well developed. The mesopterygoid fossa, the opening behind the end of the palate, is rounded at the frond and broad. An extension of the alisphenoid bone known as the alisphenoid strut is usually present, separating two foramina (openings), the foramen ovale accessorium and the masticatory-buccinator foramen. The roof of the tympanic cavity, the tegmen tympani, is small.
The upper incisors are opisthodont, with the chewing edge located behind the vertical plane of the incisors. As is usual for the genus, the molars are brachyodont, low-crowned, and bunodont, with the cusps higher than the connecting crests, and accessory crests such as mesolophs are present. On the upper first molar, the frontmost cusp, the anterocone, is divided into two small cusps in young individuals, connected by a crest, but the two fuse with wear.
O. sydandersoni is known from the departments of Beni and Santa Cruz in eastern Bolivia, including the Noel Kempff Mercado National Park (NKMNP), where the type locality is located. All but a few specimens come from pockets of woodland in seasonally flooded grasslands, where it is the most frequently encountered rodent; the related oryzomyine Hylaeamys acritus, the spiny rat Proechimys longicaudatus, and the opossum Marmosa murina were found in the same habitat. It is absent in other, more contiguous forests and in other grasslands without large forest patches. Thus, O. sydandersoni is a narrow habitat specialist with a limited distribution. It joins several other species with restricted ranges found in the NKMNP, including Hylaeamys acritus, the akodontine rodents Juscelinomys guaporensis and J. huanchae, and an opossum, Cryptonanus unduaviensis.
All specimens from the NKMNP were caught above the ground in vegetation, suggesting that the species is arboreal like the other members of its genus. The chigger Eutrombicula batatas has been found on O. sydandersoni. Nothing else is known about ecology, behavior, diet, reproduction, or conservation status.
