Oecomys roberti (Thomas, 1904)

PATTON, JAMES L., DA SILVA, MARIA NAZARETH F. & MALCOLM, JAY R., 2000, Mammals Of The Rio Juruá And The Evolutionary And Ecological Diversification Of Amazonia, Bulletin of the American Museum of Natural History 2000 (244), pp. 1-306 : 128-130

Oecomys roberti (Thomas, 1904)

TYPE LOCALITY: ‘‘ Santa Anna de Chapada , a village situated at an altitude of about 800 m ; on the Serra de Chapada , some thirty miles N.E. Of Cuyabá [= Cuiaba´],’’ Chapada dos Guimarães, Estado do Mato Grosso, Brazil .

DESCRIPTION: This is the smallest of the three large­bodied species that occur within the Rio Juruá basin, averaging 287 mm in total length and with a tail 127% of head and body length (table 29). The tail is clothed in fine, short hairs (two scale rows in length), but appears naked when viewed by eye, and only a very slight pencil of fur extends beyond tip. It is paler below than above; dorsal color light brown. The hind feet are short and broad, orangish in color above. The dorsal pelage is bright reddish orange; the venter is white at the midline from chin to anus, but with broad lateral bands of white­tipped, gray­based fur. Dorsal and ventral coloration are sharply demarcated. This species has a relatively long skull (fig. 85; CIL, 29.4 mm with relatively short and broad rostrum (11.2 × 6.3 mm, length to width); heavy supraorbital ledges diverge strongly and extend onto the parietals; upper incisors are opisthodont bullae are small and uninflated; tegmen tympani is not in contact with the squamosal hamular processes of squamosal are short and broad (although not as much as in O bicolor ), with subsquamosal foramen very small or completely closed; an alisphenoid strut is variably present (13 of 34 individuals at least on one side); the incisive foramen is short and teardrop in shape; the molar toothrows are relatively short (MTRL 5.0 mm) posterior palatal pits are small but often divided, at least on one side; the mesopterygoid fossa is relatively narrow, and usually square at its anterior end; sphenopalatine vacuities are occasionally present as narrow slots along the presphenoid; and the parapterygoid fossae appear deeply excised.

SELECTED MEASUREMENTS: See table 29:

NONGEOGRAPHIC VARIATION: As with O. bicolor , this species also exhibits no sexual dimorphism in any external or cranial variable (p> 0.05 in all cases) although age contributes substantially to differences among individuals (table 32). Also, in a pattern similar to that found in O. bicolor , these variables increase with advancing toothwear age (Pearson product moment correlation coefficients for variables that exhibit significant age effects are all positive, ranging from 0.399 –0.418 at p <0.05; 0.502 –0.591 at p <0.01; and 0.580 –0.742 at p <0.001; table 32), reinforcing the conclusion that differences in age distributions should be considered when morphometric comparisons are to be made between localities for any species of Oecomys .

GEOGRAPHIC VARIATION: Strong structure in the mtDNA cytochrome­b gene sequences also occurs within O. roberti , although in this species linkage is between samples from the Lower Central and Mouth regions relative to those from the Headwaters and Upper Central ones (fig. 89). The average level of sequence divergence is not as great as it is in O. bicolor (4.1% versus 5.4%). Despite the degree of molecular differentiation, however, not one dimensional variable of either the skin or skull exhibited a significant difference in comparisons between pooled samples of each molecular clade. Oecomys roberti appears to be a markedly uniform morphological entity throughout its distribution within the Rio Juruá river basin.

DISTRIBUTION AND HABITAT: We took this species on both banks and in all geographic regions along the river, except for the Headwaters. However, it was found primarily in várzea forest on our standardized plots (44 of 50 specimens, or 88%), on the edge of igapó forest (2 of 50), or in highly disturbed second growth edges to seasonally flooded forest (4 of 50). The same number of individuals (n = 24) were taken on the ground and in the canopy, but this simple comparison is misleading since the terrestrial trap effort was nearly twice as great as that in the canopy (see table 2). We caught only two individuals in traps placed at approximately 1.5 m above the ground.

REPRODUCTION: All young individuals of both sexes (toothwear age class 2) were non­ reproductive (nulliparous females and nonscrotal males), and two of five age class 3 females were also nulliparous. Only a single pregnant female was taken, with two embryos, but two females exhibited recent placental scars that numbered either two or three Time to reproductive maturity appears somewhat prolonged, in relation to other genera such as Oligoryzomys and Oryzomys , as do pregnancy rates. The pregnant females and those with obvious scars were all taken early in the dry season, during late August or mid­ September.

KARYOTYPE: 2n = 80, FN = 114 (fig. 90) To our knowledge, the karyotype of this species has not been reported to date. We have data for 20 specimens representing seven separate localities in the Upper and Lower Central and Mouth sample regions (locality

5, n = 4; locality 6, n = 2; locality 8, n = 4; locality 9a, n = 1; locality 11, n = 4; locality 12, n = 3; and locality 14, n = 2). The autosomal complement consists of 15 pairs of medium to small metacentric and submetacentric elements, three pairs of subtelocentric ones, and 21 pairs of acrocentric chromosomes. The X­chromosome is a very large submetacentric, the largest in the entire complement; the Ychromosome is a very small uniarmed element. This karyotype is the same in diploid number (2n = 80) and similar in fundamental number (FN = 114) to that described above for O. bicolor (FN = 140) and to that of O. superans from eastern Perú (FN = 108; Gardner and Patton, 1976; originally identified as O. concolor ).

SPECIMENS EXAMINED (n = 50): (5) 11m, 3f — JLP 15743, MNFS 577–580, 589–591,

622, 638, 669–670, 676–677; (6) 1m, 1f — MNFS 532, 537; (7) 1m, 1 unknown — JLP 15241, 15413; (8) 8m, 7f — JLP 15402 15404, 15432, 15448, 15466–15469, JUR 9 MNFS 430, 433, 508, 514 516; (h) 1f — MNFS 323; (j) 1f — JLP 15200; (9a) 1m 1f — JLP 16033, MNFS 948; (10) 1f — MNFS 955; (11) 4m, 2f — MNFS 681, 692 695, 702, 708, 769; (12) 4m — JLP 15916– 15917, MNFS 725, 823; (14) 1m, 1f — JUR 532, 534.