External morphology Drymoreomys albimaculatus is a medium-sized, long-tailed, short-eared, short-footed rodent. It is quite distinct from other oryzomyines and has a number of unique traits. In 11 adults from
Parque Natural Municipal Nascentes do Garcia in Santa Catarina, head and body length was , tail length was , hindfoot length was , ear length was , and body mass was . The fur is long and dense and consists of thin, short, woolly underfur and long, thick overfur. Overall, the fur of the upperparts is orange to reddish-
buff. In the closely related
Eremoryzomys, the upperparts are grayish. The hairs of the underfur, which are long, are grayish for most of their length and orange or brown at the tip. In the overfur, the
cover hairs (which form the main body of the fur), are long and brown at the tip, with an orange band below the tip, and the longer, sparse
guard hairs are red to dark brown in the half closest to the tip and are long. The sides are reddish brown. On the underparts, the hairs are grayish at the base and white at the tip, except on the throat, chest, and (in some specimens) groin, where the hairs are entirely white—a trait unique among the oryzomyines. In overall appearance, the underparts are grayish, with white spots where the hairs are completely white. The small, rounded ears are covered with dense golden hairs on the outer and with reddish brown hairs on the inner surface. The
mystacial vibrissae (whiskers on the upper lip) are long, usually extending a little beyond the ears when laid back against the head, but the
superciliary vibrissae (whiskers above the eyes) are short and do not extend beyond the ears. The upper surface on the forefeet is covered with brown fur, and there is white or silvery fur on the
digits.
Ungual tufts (fur around the bases of the claws) are present on the second through fourth digits. On the short, fairly broad hindfeet, the upper side is covered densely with silvery to white hairs near the tips of the feet and toes, and with brown fur otherwise. No other oryzomyine has such brown fur on its hindfeet. The second through fourth digits have long silvery-white ungual tufts, but those on the first digit are short. On the sole, the
pads are very large. Among oryzomyines, only
Oecomys and the extinct
Megalomys have similarly large pads between their digits. There is a dense cover of short brown hairs on both the upper and lower sides of the tail. Unlike in
Eremoryzomys, the tail is the same color above and below. The tail ends in a tuft, an unusual feature among oryzomyines.
Skull In the skull, the rostrum (front part) is relatively long. The
nasal and
premaxillary bones extend in front of the
incisors, forming a
rostral tube, which is shared among oryzomyines only with
Handleyomys. The
zygomatic notch (a notch formed by a projection at the front of the
zygomatic plate, a bony plate at the side of the skull) is shallow. The
interorbital region (between the eyes) is narrow and long, with the narrowest part towards the front. The crests on the
braincase and interorbital region are weakly developed.
Eremoryzomys has larger crests on its interorbital region. The
incisive foramina (openings in the front part of the
palate) are long, sometimes extending to between the first
molars (M1). The bony palate is broad and short, with the posterior margin between the third molars (M3).
Nephelomys levipes is the only other oryzomyine with such a short palate, although that of
Eremoryzomys polius is only slightly longer. The
posterolateral palatal pits (openings in the back part of the palate near the M3) vary from small to fairly large and are located in slight
fossas (depressions). In
Eremoryzomys, these fossas are deeper. The roof of the
mesopterygoid fossa, the opening behind the palate, is completely closed or contains small
sphenopalatine vacuities. The vacuities are much larger in
Eremoryzomys. The
alisphenoid strut, a piece of bone that separates two
foramina (openings), is present in all
Drymoreomys specimens examined, except in one juvenile specimen. The
mandible (lower jaw) is long and low. The
coronoid process, the frontmost of the three main
processes (projections) at the back of the jawbone, is large and about as high as the
condyloid process behind it. The
angular process, below the condyloid, is fairly short and does not extend further backwards than the condyloid. There is no noticeable
capsular process (a raising at the back of the jaw that houses the root of the lower incisor).
Dentition The upper incisors are
opisthodont (with the cutting surface oriented backwards) and have orange to yellow
enamel. The upper molar rows are either almost parallel or slightly convergent with each other toward the front.
Holochilus and
Lundomys are the only other oryzomyines with non-parallel molar rows. The valleys between the
cusps of the upper molars extending from the inner and outer sides overlap slightly across the midlines of the teeth. The molars are high-cusped, almost
hypsodont. On M1, the
anterocone (the front cusp) is divided into two cuspules on the lingual (inner, towards the tongue) and labial (outer, towards the lips) sides of the teeth. The
mesoloph, a crest near the middle of the labial side of the tooth, is long and well developed on each of the three upper molars. On the lower molars (m1 to m3), the cusps on the labial side are located slightly in front of their lingual counterparts. The
anteroconid, the front cusp on the m1, is divided in two. The m1, m2, and usually m3 have a
mesolophid, a crest corresponding to the mesoloph but located on the lingual side. Each of the lower molars has two roots.
Other anatomy There are 12 ribs and 19
thoracolumbar (chest and abdomen), four
sacral, and 36 to 38
caudal (tail) vertebrae. There are three digits at the tip of the penis, of which the central one is the largest. The two lateral digits are not supported by mounds of the
baculum (penis bone). There is only one
spine on the
papilla (nipple-like projection) on the upper side of the penis. On the
urethral process, located in the crater at the end of the penis, a fleshy process at the side, the lateral
lobule, is present. The
preputial glands (glands in front of the genitals) are large. The lack of lateral bacular mounds, presence of a lateral lobule, and size of the preputial glands are all unique traits among the oryzomyines.
Karyotype The
karyotype of
Drymoreomys albimaculatus is 2n=62,
FN=62: the animal has 62
chromosomes, and 29 pairs of
autosomes (non-sex chromosomes) are
acrocentric (with one arm so short as to be almost invisible) and one small pair is
metacentric (with two equally long arms). Both sex chromosomes are
submetacentric (with one arm noticeably longer than the other), and X is larger than Y. Blocks of
heterochromatin are present on all autosomes and the long arm of Y.
Telomeric sequences are found near the
centromeres of the sex chromosomes. Aspects of this karyotype—with a high number of mostly acrocentric chromosomes and the presence of heterochromatin on the Y chromosome—are consistent with the pattern seen in other oryzomyines. However, no other oryzomyine has exactly the same karyotype as
D. albimaculatus. Other species in clade D have fewer chromosomes, down to 16 in
Nectomys palmipes, although the karyotype of
Eremoryzomys polius is unknown. This suggests an evolutionary trend of decreasing chromosome number within the clade. ==Distribution and ecology==