Marmosops impavidus (Tschudi, 1844)

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 : 53-56

Marmosops impavidus (Tschudi, 1844)

TYPE LOCALITY: ‘‘Der mittleren und tiefen Waldregion’’; interpreted by Cabrera (1957 16) as ‘‘ Montaña de Vitoc , cerca de Chanchamayo,’’ Departamento de Junín, Perú .

DESCRIPTION: This is a moderate­sized murine opossum averaging 279 mm in total length, 32.6 mm in condylo­incisive length of the skull, and 41 g in weight when fully adult (all four molars fully erupted). The dorsal coloration is rich grayish­brown (Snuff­ Brown to Saccardo’s Brown; Ridgway 1912). The tail is dark but with a tendency to become paler distally in most specimens Fur of the venter is creamy white along the midline bordered by a distinct but narrow band of gray­based, silver­tipped hairs which typically constrict the white in the ab­

dominal and inguinal region (fig. 41, second and third from left); the width of the selfcolored portion in the abdominal region is> 15 mm. The skull is delicate, with large anterior and posterior palatal vacuities (fig. 45), and the interorbital region bears supraorbital ledges that are only weakly evident in young animals but become increasingly apparent in older­age individuals. In the oldest animals, these ledges are weakly V­shaped, more so than in any other species from the Rio Juruá (fig. 46). The anterior opening of the infraorbital foramen lies dorsal to the anterior root of PM4. The maxillary toothrow averages 13.5 mm; upper M1­M4 molar series length averages 6.7 mm. The canine appears proportionately long and narrow, with its width at the base about 55% its length in unworn specimens.

COMPARISONS: Intermediate in overall size between the larger M. noctivagus and the smaller M. parvidens (see table 8), M. impavidus is readily distinguished from these two species by the white venter bordered by a narrow but distinct band of gray­based, silver­tipped hairs (as in M. parvidens ) instead of a broad white venter lacking the border (as in M. noctivagus ; fig. 41); dorsal coloration is similar to but darker and grayer than that of M. noctivagus ; it lacks clearly developed supraorbital beading (in older individuals) present in M. noctivagus , but does have weakly developed ledges (fig. 46), as opposed to the lack of any beading in M. parvidens ; relatively long and narrow canines in comparison to those of M. noctivagus ; presence of posterior palatal vacuities, as in M noctivagus , but which are lacking in M. parvidens . Compared with M. neblina , M. impavidus differs in its brighter dorsal coloration and broader white ventral area with narrower and less obvious lateral bands of graybased hairs (fig. 41). It has less swollen paroccipital processes, more swollen mastoid processes, somewhat more well­developed supraorbital ledges (fig. 46), and somewhat longer and narrower upper canines. All palatal vacuities are larger with the posterior ones placed more distally on the palate (fig 45). The two also differ in those qualitative features noted by Gardner (1989), but cannot be distinguished from each other on the basis of external or cranial dimensions (tables 8 9).

DISTRIBUTION AND HABITAT: Despite the limited number of individuals, this species was taken at scattered localities in each of the four sample regions along the Rio Juruá (Headwaters, Upper and Lower Central, and Mouth). All individuals were taken in undisturbed or second­growth terra firme forest Three were taken at heights of 1.5 to 2 m the remainder were taken on the ground

None were collected in the canopy platform traps.

REPRODUCTION: All adult females (M1–M4 fully erupted) collected in the Headwaters (locality 1) and Mouth regions (locality 14) during the wet season were parous, with clearly evident discolored orange inguinal areas and enlarged nipples, although none had attached young. The one female taken during the dry season (November, at Altamira, locality 9) was a juvenile. These data suggest that M. impavidus breeds during the wet season, but whether breeding extends into or through the dry season is unknown.

KARYOTYPE: 2n = 14, FN = 24 (fig. 47A) Chromosomal preparations are available from two specimens (MNFS 1191 and 1779) All members of this genus apparently have

the same diploid number and similar autosomal and sex­chromosome complements, although there appears to be slight differences in centromere position in some of the smaller autosomal elements and in the size of the Y­chromosome (Reig et al., 1977; Palma and Yates, 1996; Svartman, 1998) The autosomal complement of M. impavidus is comprised of four pairs of large meta­ and submetacentrics and two pairs of small biarmed autosomes, one of which is clearly metacentric and the other subtelocentric. The X­chromosome is a small metacentric, small­ er than the smallest autosome, and the Y is a somewhat smaller acrocentric.

SPECIMENS EXAMINED (n = 11): (1) 2m, 2f — MNFS 1191, 1235, 1319, 1366; (6) 1 m — JLP 15633; (9) 1 m — JUR 197; (12) 1 m — MNFS 760; (14) 2m, 2f — MNFS 1686, 1757, 1779, JUR 550.