Oryzomys - Description

Description

Oryzomys contains medium-sized, semi-aquatically specialized oryzomyine rodents. They have long, coarse fur that is grayish to reddish on the upperparts and white to buff on the underparts. The marsh rice rat superficially resembles the introduced species black rat and brown rat, but has larger differences in color between the upper- and underparts. The vibrissae (whiskers) are short and the ears are small and well-haired. The tail is usually as long as or longer than the head and body and is sparsely haired, but the hairs on the lower side are longer than those above. Females have eight mammae, as in most oryzomyines. The hindfeet are broad and have the first and fifth digits notably shorter than the middle three. The upper surface is hairy, but the underside is naked and covered with small irregularities (squamae). The pads are generally poorly developed, as are the ungual tufts. Interdigital webbing may be present, but its development is variable within the genus.

The karyotype has been recorded in various populations of the marsh rice rat and O. couesi and is apparently stable within the genus at 56 chromosomes, with the fundamental number of chromosomal arms ranging from 56 to 60 (2n = 56, FN = 56–60). In both species, the stomach has the characteristic pattern of sigmodontines (unilocular-hemiglandular): it is not split in two chambers by an incisura angularis and the front part (antrum) is covered by a glandular epithelium. Furthermore, the gall bladder is absent, a synapomorphy of Oryzomyini.

Oryzomys species have a large skull with a short rostrum and high braincase. The interorbital region, located between the eyes, is narrowest to the front and is flanked by well-developed beads at its margins. The zygomatic plate is broad and has a well-developed zygomatic notch at its front. The zygomatic arch is robust and contains a small but distinct jugal bone. The interparietal bone, part of the roof of the braincase, is narrow and short; its narrowness is a synapomorphy for O. couesi plus the marsh rice rat according to Weksler's analysis. The incisive foramina are long, with their back margin at the front of the first molars or further back. The palate is also long, extending beyond the back margin of the maxillary bone, and is perforated near the third molars by well-developed posterolateral palatal pits. There is no alisphenoid strut, an extension of the alisphenoid bone that in some other oryzomyines separates two foramina in the skull. The auditory bullae are large. The condition of the arteries in the head is highly derived. In the mandible (lower jaw), the coronoid process, a process at the back, is well developed and the capsular process, a raising of the mandibular bone housing the root of the lower incisor, is conspicuous.

As usual in oryzomyines, the molars are pentalophodont (have the mesolophs and mesolophids, accessory crests, well developed) and bunodont, with the cusps higher than the connecting crests. The cusps on the upper molars are arranged in two longitudinal series, not three as in the black and brown rats. The front cusps of the first upper and lower molar (anterocone and anteroconid, respectively) are broad and not divided completely by an anteromedian flexus or flexid. Behind the anterocone, the anteroloph (a smaller crest) is complete and separated from the anterocone. On both the second and third lower molars, the anterolophid (a crest on the inner front corner) is present, a putative synapomorphy of the genus. The first molars have additional small roots in addition to the main ones, so that the upper first molar has four and the lower has three or four roots.

As is characteristic of Sigmodontinae, the marsh rice rat and O. couesi have a complex penis, with the baculum (penis bone) displaying large protuberances at the sides. The outer surface of the penis is mostly covered by small spines, but there is a broad band of nonspinous tissue. The papilla (nipple-like projection) on the dorsal (upper) side of the penis is covered with small spines, a character these two species share only with Oligoryzomys among oryzomyines examined. On the urethral process, located in the crater at the end of the penis, a fleshy process (the subapical lobule) is present; it is absent in all other oryzomyines with studied penes except Holochilus brasiliensis. Both traits are recovered as synapomorphies of O. couesi plus the marsh rice rat in Weksler's analysis.