Oryzomys acritus, EMMONS & PATTON, 2005

EMMONS, LOUISE H. & PATTON, JAMES L., 2005, A New Species of Oryzomys (Rodentia: Muridae) from Eastern Bolivia, American Museum Novitates 3478 (1), pp. 1-27 : 14-22

Oryzomys acritus , new species

Figures 4–13 ; tables 2 , 6

HOLOTYPE: MNK 3628 , collected 6 November 1998 by Louise H. Emmons (field no. LHE1540). A young adult male with hindquarters terminating molt from juvenile to adult pelage, prepared as a skin and skull, liver tissue preserved in ethanol. External measurements recorded on label: TL 5 261, T 5 121, HF 5 32, E 5 20, weight 5 57 g. Testes scrotal, 8 3 5 mm.

PARATYPES: USNM 588193 , 597563–79; MNK (uncataloged) field nos. IGP 45, IGP 47, IGP 55– 6, LHE 1582, NRS 09, NRS 13, NRS 16, NRS 25, VCC 01–5, VCC 08, VCC 11, VCC 15; all from the type locality; USNM 588192 , from Huanchaca II; and MNK uncataloged LHE 1680, from Los Fierros. Only specimens from localities within PNNKM are designated as paratypes.

TYPE LOCALITY: Bolivia: Departamento de Santa Cruz, Provincia Velasco, Parque Nacional Noël Kempff Mercado , El Refugio Huanchaca , an outpost with a few buildings and an airstrip on private property, but within the park (14842.553 9S, 06182.034 9W [WGS 84]; elev. 170 m); on older published maps, the locality now known as El Refugio Huanchaca is shown as Huanchaca, an estancia. In 2004 the present owners added Huanchaca to their former designation of El Refugio, which appears on specimen labels and is the same locality. Captured in a deciduous forest 3.5 km north of the outpost.

DIAGNOSIS: A medium­sized terrestrial rat. Dorsal color near Olive Brown to Saccado’s Umber (olive brown 28 of Smithe, 1975), cheeks and sides with orange tones, Buckthorn Brown to Ochraceous Tawny (clay color 26 to cinnamon 123A of Smithe); rostrum and top of head to behind eyes dusky, close to Deep Neutral Gray, lined with blacktipped hairs, contrasting with brown top of crown. Pelage long, tips of most hairs at midrump reaching 9 mm; ventral fur dense and long, 3–4 mm on chest between forelegs; hairs pale gray at base with long white tips, overall ventral appearance whitish, inner limbs white; white of inner hind legs continuous with whitish tops of feet. Hind foot pads large, interdigital and hypothenar pads wider than 1.3 mm. Cranium lacking squamosoalisphenoid groove and sphenofrontal foramen ( Musser et al. 1998); bony palatal excrescences strongly developed (fig. 4). Upper second molar with a short paraflexus and a single fossette; lower second molar with a short hypoflexid opposite a lingual fossettid (fig. 5).

DISTRIBUTION AND HABITAT: Oryzomys acritus were captured by LHE at four localities in the PNNKM and one just outside its borders (fig. 4, localities 1–5). The park is on the Brazilian Shield; its vegetation is a mosaic of humid, semi deciduous and deciduous forests and liana forests, and wet and dry savanna and cerrado formations. The biota and geology are described in detail in Killeen and Schulenberg (1998) and the vegetation also in Panfil (2001). These rats were captured on the ground with Sherman, Tomahawk, and break­back traps and a standardized bait of oatmeal and raisins flavored with fish oil and essences of vanilla, banana, and coconut, in forests that ranged from evergreen riverine or gallery forest to deciduous and semideciduous terra firme and seasonally flooded forest bordering seasonally flooded pampa. One locality (Huanchaca II) is at 700 m on top of the Huanchaca meseta, but its forest is similar in species composition of both flora and fauna to that of the lower elevation sites where the species was found within the park ( Killeen and Schulenberg, 1998). These forests either have a seasonally deciduous emergent canopy, sparse understory, and midstory varying in deciduousness depending both on the location and on the dryness of the particular year, or are liana forests, with low canopies and dense lianas at all levels. Additional specimens were identified in collections from near San Joaquín, Departmento Beni, and Guarayos, Departmento Santa Cruz. All localities identified thus far are in the area drained by western tributaries of the basin of the Río Itenez (named Rio Guaporé in Brazil: fig. 6).

ETYMOLOGY: From the Greek akritos, ‘‘confused, mixed, doubtful’’ ( Brown, 1956), with reference to the mixed characters that can be confused with those of O. megacephalus or even O. nitidus .

DESCRIPTION: A medium­sized rat ( table 2 ) of similar aspect and color to Oryzomys megacephalus sensu Patton et al. (2000) . Above brown to yellow­brown with hairs graybased, younger individuals darker and less yellowish than old animals; occasional individuals bleached slightly reddish overall. Sides paler than dorsum, yellowish to distinctly orange, brightest on cheeks and below ear; lateral color can extend from thigh forward to cheeks below eye and to whitish spot at base of mystacial vibrissae. Rostrum dusky above to behind eyes, with duskytipped cream or whitish hairs. Anteroventral base of ear with inconspicuous preauricular tuft of entirely whitish or pale gray hairs sometimes tipped with pale rufous, probably associated with a cutaneous gland. Pinnae brown, thinly clothed on the interior surface with whitish or, rarely, brown hairs. Skin surrounding eye blackish. Ventral pelage whitish; hairs quite long, pale gray at bases with long white tips; white of inner legs extending to join pure white of fore and hind feet; hair at wrists white­based; chin spot self­white. Pelage at mid­rump with majority of hairs measuring 8–10 mm (a few longer hairs may be present). Ventral pelage on chest between forelegs 3–4 mm. Tail dusky, paler below at base, but not prominently bicolored, clothed with inconspicuous short, fine, dusky hairs. Hind feet long and narrow (terrestrial type), with digits II–IV noticeably longer than I and V; soles brown but not strongly pigmented. Six footpads always present, hypothenar and fourth interdigital pads broad (fig. 7); thenar pad with strongly raised part of pad short and nearly round; squamae between and around pads inconspicuous.

Cranium similar to those of other members of the megacephalus group, with most diagnostic features as outlined by Musser et al. (1998), including no squamosoalisphenoid groove or sphenofrontal foramina; incisive foramen short and broad, teardrop­shaped (figs. 8, 9, 10); and bony palate long. Supraorbital ridge prolonged into strongly developed temporal ridge that extends across parietal to occipital suture. Parietal usually with a small but distinct contribution to lateral braincase wall exhibited by a ventrad extension of the posterolateral border of the parietal forward of the parieto­occipital suture ( Musser et al., 1998: fig. 62); area of ventral contribution appears to decrease with age. Foramen ovale large and rounded, such that the opening is visible below the alisphenoid when the skull is viewed from the side (fig. 11). Anterior rim of auditory meatus usually notched, notch depth decreasing with age (fig. 11). Second upper molar with a long paraflexus, a short hypoflexus, and a labial fossette, but no medial fossette; second lower molar with hypoflexid reaching halfway or less across tooth, opposite a lingual fossettid (figs. 5, 12).

COMPARISON WITH OTHER SPECIES

We compare O. acritus only with the geographically adjacent members of the megacephalus group, O. perenensis and O. megacephalus , and with the genetically most closely related O. laticeps . As comparisons between these and members of other Oryzomys species and species groups are detailed and illustrated in Musser et al. (1998), we mention only a few of them, and we did not compare any O. megacephalus from north of the Amazon river. Externally, O. acritus is distinguishable from lowland populations of O. megacephalus and O. perenensis by its longer dorsal and ventral pelage (however, montane O. perenensis are similarly long furred). It is distinguishable from O. perenensis and O. laticeps by its orange­toned lateral color, and from lowland O. perenensis by its whiter venter (fig. 13) and white interior ankles. Oryzomys laticeps lacks the pale preauricular tuft that is present in the other three species, and its ventral pelage can have a yellowish tinge, unlike the generally pure white ventral hair tips of the others. The dorsal pelage of both O. acritus and O. megacephalus is slightly paler and more warm toned (yellow or reddish) than that of O. perenensis and O. laticeps (fig. 13), such that some older, redder individuals could be confused in the hand with O. nitidus (as can some O. megacephalus ). Sympatric O. nitidus are larger, with a much clearer, brighter, and better defined orange lateral pelage and a bicolored tail. Oryzomys acritus has a dusky rostrum with blackish hair tips, contrasting with its brown crown, and differs from O. megacephalus , whose paler, Buffy Brown tipped rostral hairs do not contrast with the crown (fig. 13). Oryzomys perenensis and O. laticeps also have dusky rostra with dark­tipped hairs, but their crowns are darker and do not contrast with the rostral color.

The hind footpads of two taxa have distinguishing characters. We examined only three fluid specimens of O. acritus , one a juvenile, so a larger series could alter our perception of differences. Oryzomys acritus has broader first interdigital and hypothenar pads than any of the other species ( table 6 ). Oryzomys megacephalus is distinguished from all others by its small to tiny interdigital and hypothenar pads (fig. 7), a feature that is also illustrated in Voss et al. (2001: fig. 53). The plantar pads and squamae of O. perenensis are much more darkly pigmented than those of the other species. Oryzomys laticeps alone has large and prominent squamae, noticeable even on dried skins. The plantar skin of the first digit of all three O. acritus that we examined had a single complete dermal ‘‘ring’’, distal to several interrupted or broken rings (fig. 7), whereas O. megacephalus had two (N 5 3) or three rings (N 5 1), O. perenensis had two2 rings (N 5 8), and O. laticeps had one (N 5 1), two (N 5 2), or three rings (N 5 1). We examined this feature on fluidpreserved material only; larger samples are needed to confirm the constancy of these differences.

The lower second molars of both O. acritus and O. megacephalus have a short metaflexid opposite a lingual fossettid, whereas O. perenensis and O. laticeps have long hypoflexi and lack a fossettid in this tooth (fig. 12). 3 Oryzomys acritus shares with the other three species the long paraflexus of M2 that is linked to the absence of a medial fossette ( Musser et al., 1998); this configuration thus remains a character that unites all members of the species complex (fig. 12).

The crania of the four megacephalus complex taxa are difficult to distinguish by qualitative characters, although the morphometric analysis shows that they are distinct in combination of size variables (figs. 2 and 3; table 2 ). Because for all measurements, the length ranges overlap among species, no simple size key can readily assign a cranium to species. Oryzomys laticeps is distinctly the largest taxon and O. megacephalus the smallest, with the other two taxa intermediate between them. Differences between the taxa are summarized in table 7 , but we admit that specimens of O. acritus are not easy to distinguish from O. megacephalus , especially when the preparations are in poor condition.

Figures

Fig. 4. Cranium and mandible of the holotype of O. acritus (MNK 3628).

Fig. 5. Tooth wear series of O. acritus, age classes I to IV. Paratypes, from left to right (USNM 597564, 597577, 597576, 597563).

Fig. 6. Map of Bolivia showing known localities of O. acritus; see specimens examined for names and coordinates; open circle (locality 1) represents the type locality.

Fig. 7. Soles of left hind feet of (A) Oryzomys acritus (USNM 597574); (B) O. megacephalus (MVZ 197663), note tiny hypothenar pad; (C) O. perenensis (MVZ 191254); (D) O. laticeps (MVZ 197631). Interdigital pads are numbered; H indicates hypothenar pad; S indicates plantar squamae. R is adjacent to the single complete dermal ring on first digit of O. acritus; note presence of two rings in B and C and three in D.

Fig. 8. Dorsal view of crania of four species of the megacephalus complex, from top down: O. acritus (USNM 597577), O. laticeps (USNM 545056), O. megacephalus (USNM 549547), O. perenensis (USNM 530924).

Fig. 9. Ventral view of crania, same specimens as in figure 8.

Fig. 10. Lateral view of crania, same specimens as in figure 8.

Fig. 11. Crania of young adult O. acritus (USNM 597577; above) and O. megacephalus (USNM 549548; below). Arrows point to notch in rim of meatus, large foramen ovale, and strongly developed bony palatal excrescences of O. acritus.

Fig. 12. Occlusal cheekteeth of four species of the megacephalus complex, maxillary teeth above, mandibular teeth below; left to right: O. acritus (USNM 597577), O. laticeps (USNM 304571), O. megacephalus (USNM 549547), O. perenensis (USNM 549524). Note presence of fossettid (arrow) in O. acritus and O. megacephalus, and absence of fossettid and longer hypoflexid (opposite fossettid) in the two others.

Fig. 13. Anterior and lateral views of skins. Top, from left: O. laticeps (USNM 545056), O. perenensis (USNM 530924), O. megacephalus (USNM 549548), O. acritus (USNM 597565). Below, from top, O. laticeps (USNM 545056), O. perenensis (USNM 582890), O. megacephalus (USNM 549548), O. acritus (USNM 597563). Note the warmer dorsal color of O. megacephalus and O. acritus, the contrasting dusky rostrum of O. acritus, paler and more uniform head color of O. megacephalus, and the slightly whiter venter of O. acritus compared to more grayish underparts of the upper two specimens.