Marmosa (Stegomarmosa) andersoni

Rediscovery of Marmosa (Stegomarmosa) andersoni

Four juvenile opossums (MUSM 14151–14153, USNM 582777) were collected, by hand, on the night of 27 November 1997, in a large stand of bamboo at Cashiriari 3 well-site (11°52’ S, 72°39’ W; 694 m). Ranges of external measurements were: TL, 182–195; LT, 105–110; HF, 15–18; E, 14–16; W, 9 g. Although immature (MUSM 14151 falls into age class 2 of Pine et al. 1985a), their external characters (warm brownish coloration, black eye-rings, long distal caudal hairs) suggested membership in the genus Gracilinanus, and, accordingly, they were reported as G. agilis (Burmeister) (see Solari et al. 1998). The following year, two adult (of age class 5 of Pine et al. 1985a) female mouse opossums of similar appearance to each other, but unlike any known species in the area, were secured at two localities, Cashiriari 3 and San Martín 3 well-sites. One of these, MUSM 14154, carrying five attached young = 11 mm crown-rump, was taken on 31 August 1998 from a sticky-trap set around a branch of a medium-sized tree near the forest edge, by the herpetological team, at the Cashiriari 3 well-site. The other one, MUSM 14155, was caught inside a bamboo cane (Guadua sp.) by a native guide, on 9 November 1998, at the San Martín 3 well-site (11°47’ S, 72°42’ W; 474 m). Both of the new localities (Fig. 1) are within an area ecologically similar to the Cosñipata Valley, which contains the type locality.

All four juveniles were found climbing around in a single bamboo thicket, probably close to their nest. At the Cashiriari 3 well-site, other mouse opossums (Marmosa quichua Thomas, Marmosops noctivagus [Tschudi], and Micoureus demerarae [Thomas]) were collected in bamboo thickets, where they nested using dead leaves. None of our M. andersoni specimens was observed or caught more than 3 m above the ground.

Skull and external characters of the adults matched those given by Pine (1972) in his description of Marmosa (Stegomarmosa) andersoni. Although the type specimen (FMNH 84252) is of an adult male, the two adult female specimens resemble it in most essential details, such as the enormous postorbital processes, the strongly constricted interorbital area (Fig. 2), the presence of a conspicuous fringe of relatively long silvery bristles on each side of the naked caudal prehensile surface, and an obvious gular gland. Sexual dimorphism is evident, however, in that the male holotype, but not the females, possesses the lateral carpal tubercle (see Lunde & Schutt 1999). The three adults are remarkably similar in their dimensions (Table 1). After the adults were identified as M. andersoni, the four juveniles were re-identified as belonging to the same taxon (see Solari et al. 2001). Differences between the juveniles and the adults are no more than would be expected within a single species but involving animals of different ages. Diagnostic features used in assigning the juveniles to M. andersoni include various of those seen in adults, such as the skull profile, large orbits, the color pattern (although juvenile MUSM 14151, at least, shows somewhat browner, less reddish dorsal coloration), the incipient and beaded postorbital processes (highly unusual for such young animals), and the characteristic silvery bristle fringes on each side of the caudal prehensile surface. The six new specimens extend the geographic range of this rare and apparently endemic opossum by almost 200 km to the northwest of the type locality (Fig. 1).

At the type locality, Villa Carmen, three other species of mouse opossums were also reported by Pine (1972): Marmosa rubra Tate, Marmosops impavidus (Tschudi), and Marmosops cf. parvidens (Tate) — this last specimen actually represents M. bishopi (Pine) as defined by Voss et al. (2001). Didelphid species sympatric with M. andersoni at the two new localities include: Didelphis marsupialis Linnaeus, Gracilinanus cf. agilis, Hyladelphys kalinowskii (Hershkovitz), Marmosa quichua, Marmosops noctivagus, Metachirus nudicaudatus (É Geoffroy), Micoureus demerarae, Monodelphis emiliae (Thomas), and Monodelphis peruviana (Osgood). In the LUR Valley, which includes the aforementioned two well-sites plus two others (Cashiriari 2 and Pagoreni, see Alonso et al. 2001), 17 species of didelphids were recorded (Solari et al. 2001), thus giving this area one of the most diverse marsupial faunas in the Neotropics.

The predominant vegetation of the region was lowland tropical rainforest; descriptions of the vegetation at each well-site have been provided by Comiskey et al. (2001) and are summarized here. The terra firme forests were characterized by a relatively low abundance of trees> 10 cm dbh (diameter at breast height), but a high canopy, ascending to> 30m. These primary forests were dominated by Iriartea deltoidea (Arecaceae) and Matisia cordata (Bombacaceae). Other important terra firme forest species included Chimarrhis sp. and Pentagonia parvifolia (both Rubiaceae). The most defining feature of the secondary forests was the overwhelming dominance of Senefeldera inclinata (Euphorbiaceae), which occurred under no other conditions. These secondary forests were characterized by a high abundance of trees> 10 cm dbh and a low canopy. The mixed upland forests were structurally and compositionally intermediate as compared to the primary and secondary forests; however, they had the lowest stature of all the sites. The arborescent bamboo, Guadua sarcocarpa, was abundant at San Martín 3, and at Cashiriari 3, while it was absent from the Cashiriari 2 and Pagoreni wellsites. Important species at Cashiriari 3 included Iriartea deltoidea (Arecaceae), Miconia triplinervis (Melastomataceae), and Pseudolmedia laevis (Moraceae). In general, human disturbance was negligible throughout the study area. Temperatures across this area were typically warm and showed little annual variation. Mean temperatures ranged from 23.5 to 24.5 °C and relative humidity normally exceeded 80%. The region experienced distinct wet and dry seasons. Mean annual rainfall ranged between 3,000 and 3,500 mm and occurred mostly from the beginning of October through the end of April (Alonso & Dallmeier 1998).

According to NWMSG (1996), M. andersoni is “critically endangered” on the basis of being: “Severely endangered or known to exist at only a single location” and “Continuing decline, observed, inferred or projected, in…area, extent and/or quality of habitat.” Our new information has increased the known localities from one to three, but has not appreciably changed the extent of our knowledge concerning these criteria visa-vis M. andersoni. Only one adult was taken at each new locality, and the 4 juveniles were caught together. All the localities are within a narrow strip along the base of the Andes, in very similar pre-montane forests below 1000 m. However, the actual distribution and abundance of this animal may be quite different than the limited information currently available would seem to indicate. The actual conservation status of this species, like that of so many known from very few specimens, is unknown.