Akodon spegazzinii

The coloration of the upperparts is generally uniform, with some scattered darker hairs. There is a yellow ring around the eyes, which is more prominent than in A. sylvanus. The underparts are not strongly demarcated from the upperparts in color and are yellow-brown to gray. There are some scattered white hairs on the chin. Although this white spot is better developed than in A. sylvanus, A. spegazzinii lacks the conspicuous white spot seen in A. simulator. The color of the feet ranges from white and yellow-brown to gray. Ungual tufts of hairs cover the claws; these hairs are grayish-brown at the bases and whitish at the tips. The amount of hair on the tail is variable, but it is dark brown above and white to yellow-brown below. High-altitude animals tend to have hairier ears and tails.

In the skull, the front part (rostrum) is large, but not as long as in A. budini. The skull is more robust than in the very similar Akodon boliviensis, but less so than in A. simulator. The hourglass-shaped interorbital region (between the eyes) is narrower than in A. caenosus and not as squared as in A. polopi. Members of the Akodon varius group, with which Akodon oenos was formerly associated, tend to have much broader interorbital regions. The braincase is somewhat inflated and bears well-developed temporal and lambdoid crests relative to the situation in A. caenosus ; Akodon polopi has even better developed crests.

Although the zygomatic plates (plates of bone at the side of the skull) are variable, their size is generally intermediate for the Akodon boliviensis group and their front margin ranges from straight to a little concave. The zygomatic notches, projections at the front of the plates, are better developed than in A. caenosus and A. sylvanus. The incisive foramina (openings in the front part of the palate) are long, sometimes extending between the first upper molars. Tiny posterolateral palatal pits are located at the back of the palate. The back margin of the palate is squared to rounded, with a spine in the middle (medial process) sometimes present. The opening behind the palate, the mesopterygoid fossa, is of intermediate width, being narrower than in A. sylvanus, A. simulator, and A. budini but broader than in A. caenosus.

The masseteric crests (crests on the outer sides of the mandibles) reach their front ends below the front border of the first molars. Usually, the capsular process (a projection at the back of the mandible housing the root of the lower incisor) is well-developed. The enamel of the upper incisors is yellowish-orange and the incisors are orthodont (with their cutting edge perpendicular to the plane of the toothrow) or slightly opisthodont (with the cutting edge inclined backwards). In contrast, Akodon simulator has more proodont incisors (with the cutting edge oriented forwards) and Akodon neocenus has more opisthodont incisors. The molars are more hypsodont (high-crowned) than in A. caenosus, but are unlike the very hypsodont molars of A. budini. The molar rows are relatively longer than in A. polopi. There are 13 or 14 thoracic (chest), 7 or 8 lumbar (abdomen), and 23 or 26 caudal (tail) vertebrae.

The karyotype includes 40 chromosomes, with a fundamental number of 40 major chromosomal arms and resembles that of other members of the A. boliviensis group. Head and body length is 93 to 196 mm (3.7 to 7.7 in), averaging 158 mm (6.2 in); tail length is 46 to 83 mm (1.8 to 3.3 in), averaging 66 mm (2.6 in); hindfoot length is 18 to 25 mm (0.71 to 0.98 in), averaging 23 mm (0.91 in); ear length is 12 to 21 mm (0.47 to 0.83 in), averaging 14 mm (0.55 in); and body mass is 13.0 to 38.0 g (0.46 to 1.34 oz), averaging 21.6 g (0.76 oz). Like other members of the Akodon boliviensis group, individuals of Akodon spegazzinii continue to grow in adulthood.