Microhydromys argenteus, Helgen & Leary & Aplin, 2010

Helgen, Kristofer M., Leary, Tanya & Aplin, Kenneth P., 2010, A Review of Microhydromys (Rodentia: Murinae), with Description of a New Species from Southern New Guinea, American Museum Novitates 3676, pp. 1-22 : 13-17

Microhydromys argenteus , new species

HOLOTYPE: BBM-NG 60202 , adult male, study skin with accompanying skull and mandible in excellent condition ( figs. 2 , 3 ), from Sirinumu Dam , 550 m, on the Sogeri Plateau (Central Province, Papua New Guinea), collected 28 October 1968 by M. Nadchatram.

REFERRED SPECIMENS: AM M14166 , adult female, flat skin with accompanying skull and mandible in good condition, from Mt. Sisa (5 Mt. Haliago), 1450 m (Southern Highlands Province, Papua New Guinea), collected 18 September 1979 by P. Dwyer (original number E1005). Dwyer (1990: 210) identified this specimen as ‘‘ Microhydromys cf. richardsoni .’’ The skull and skin of AM M14166 were figured by Flannery (1989: 217–218).

UPNG 4310, adult male ( fig. 4 ), preserved whole in fluid (fixed in 10% formalin and preserved in 70% ethanol), from 10 km NE of the village of Faia on the Darai Plateau (7.130 ° S, 143.611 ° E, Gulf Province, Papua New Guinea; figs. 1 , 9 ), 380 m, collected 26 July 2003 by T . Leary and M. Pennay (original number DAR22 ) .

DIAGNOSIS: Microhydromys argenteus differs externally from M. richardsoni in having paler, uniformly gray-brown pelage, both dorsally and ventrally (darker grayish-black fur, above and below, sometimes with white flecking on the belly, in M. richardsoni : fig. 2 ); a proportionally shorter tail, measuring 91%– 95% of head-body length (101%–111% of head-body length in M. richardsoni : table 1); a long pale terminal tip, measuring at least one-third of total tail length, with pale mottling extending beyond, to at least to the midpoint of the tail, on both the dorsal and ventral surfaces (versus a shorter discrete pale tail tip, less than one-quarter of tail length and without mottling or with less extensive mottling in M. richardsoni ); and slightly larger hind feet and pinnae (table 1). Compared to M. richardsoni , the skull of M. argenteus has proportionally wider zygomata, broader incisors, slightly narrower molars, shorter incisive foramina, and larger auditory bullae.

ETYMOLOGY: The epithet argenteus references the grayish pelage of this species, which allows for its immediate discrimination from the blackish M. richardsoni . We recommend the English common name ‘‘southern groovetoothed moss-mouse’’. Leary (2004) noted that the local name Mirin is used for this species in the Kasere language on the Darai Plateau. According to Dwyer (1990), Etolo speakers from Mt. Sisa used the name Ebele Ebele Mano for this species, a designation that indicated their impression that it was a juvenile specimen of a larger murid—‘‘i.e., the animal was said to be a baby rat; the intention may have been that it was a baby Melomys ’’ ( Dwyer, 1990: 210).

DISTRIBUTION: Microhydromys argenteus has been recorded from three localities along the southern slopes of the Central Cordillera in south-central and southeastern Papua New Guinea (Southern Highlands, Gulf, and Central Provinces ), situated from 380 to 1450 m ( fig. 1 ). The mean elevation of these three trapping sites is 793 m. Though rarely collected, the available captures indicate that it is likely to be widespread in hill forest and lower montane habitats along the southern margin of the Cordillera .

DESCRIPTION: Microhydromys argenteus is a tiny, grayish-brown mouse with very small eyes. It has a gray tail, slightly shorter than the head and body length, bearing a prominent white distal tail tip. The pelage is soft and very short, measuring 4–5 mm on the midback, as in M. richardsoni ( Tate, 1951) . The coloration of the body is distinctly paler both above and below than the more blackish fur of M. richardsoni . The dorsal pelage is pale gray, with pure gray fur bases strongly tipped with brown tones. The venter (from lower lips to tail, including the underside of the limbs) is paler and less brownish, uniformly colored apart from a barely indicated thin whitish line on the midbreast in the holotype. The tail, proportionally shorter than in M. richardsoni (table 1), is gray proximally (blackish in M. richardsoni ), with a long distal white tail tip one-third to one-half as long as the tail, sometimes accompanied by further mottling above or below (versus a short, discrete white tail tip about one-quarter of total tail length in extent in M. richardsoni ). There are 17 tail scales per cm in the midsection of the tail, compared to 15–16 in M. richardsoni . The dorsal surfaces of the manus and pes are pigmented pale and covered in very short white hairs (compared to short dark hairs in M. richardsoni , which thus has darker feet). The hind feet average longer than in M.

(UPNG 4310). Photograph by Allen Allison.

richardsoni (table 1) and appear broader in direct comparisons of skins. Judging from the few available specimens, the ear averages slightly larger than in M. argenteus (table 1), and is paler gray than in M. richardsoni . The few available body weights suggest that M. argenteus is a slightly heavier mouse than M. richardsoni .

As previously discussed by Flannery (1989), the skulls of the two species of Microhydromys are very similar ( fig. 3 ), and detailed comparisons are rendered challenging by the limited material in collections. Only three skulls of M. richardsoni are available in world museums, and in two of these the braincase is broken posteriorly by characteristic snap-trap damage. Similarly, only two intact skulls of M. argenteus are yet available. Nevertheless, several compelling craniodental distinctions are apparent in this material, which complement the clear-cut external distinctions that characterize these taxa. The two species of Microhydromys are more or less equivalent in overall skull size, with M. argenteus averaging only slightly larger (table 1). This similarity in cranial size renders certain proportional differences between the species obvious. The zygomata are more robustly expanded in M. argenteus than in M. richardsoni ( fig. 3 , table 1). Microhydromys argenteus has larger auditory bullae than M. richardsoni , a difference perhaps better appreciated in direct comparisons ( fig. 3 ) than in tabulated measurements (table 1). Compared to M. richardsoni , the incisors are more robust in M. argenteus , though the molars average a touch smaller. In Microhydromys argenteus the nasals average longer than in M. richardsoni , though the incisive foramina are slightly shorter (table 1).

NATURAL HISTORY: According to its label, the holotype of Microhydromys argenteus was snap-trapped in ‘‘secondary forest – eucalypt savannah’’ on the Sogeri Plateau, an interesting indication that this species is not confined to mature forest formations ( Flannery, 1995).

The specimen of M. argenteus from Mt. Sisa was live-trapped (in an Elliot/Sherman trap) by Dwyer on the ‘‘Magidobo grid’’ (Dwyer, field notes), a locality described as ‘‘primary forest with some tree fall dating from a storm two years before’’ ( Dwyer, 1984: 30). The Magidobo grid was trapped by Dwyer for three or four consecutive nights on 11 occasions between February 1979 and February 1980, with a total of 80 traps set per night (equal numbers of wire mesh and Elliot type traps, ‘‘nearly always baited with sweet potato’’; ibid.). No other moss-mice were collected at the site, which was revisited by Aplin in 1985 and characterized as typical lower montane forest (K. Aplin, field notes). Only one other moss-mouse, Pseudohydromys sandrae Helgen and Helgen, 2009 , is recorded from Mt Sisa (a distinctive species known only from the holotype, obtained by Dwyer at around 850 m in hill forest; Helgen and Helgen, 2009). Syntopy between this species and M. argenteus seems likely but remains to be demonstrated. Further information on Mt Sisa and its various forest communities was provided by Dwyer (1990).

The Darai Plateau (lying within Southern Highlands, Western, and Gulf provinces of Papua New Guinea) is an extensive limestone plateau with karst corridors and elevated flattopped limestone surfaces of very low relief that runs from immediately south of Mt Bosavi, almost to the Gulf of Papua ( Bellamy and McAlpine, 1995). It lies on the western edge of the Kikori Integrated Conservation and Development Project area. The associated outlying karst ridges range from 300 to 700 m above sea level. The area is replete with steep karst pinnacles and dolines that are undermined by karst caves that result in numerous sinkholes. Vegetation on the plateau has been described as lowland hill forest with polygonal karst geology and adverse soil conditions ( Paijmans, 1976). Recently, Gebia and Balun (2004) characterized the vegetation of the region in greater detail. The vegetation at the Darai site is possibly the most diverse site ever surveyed in PNG, with 368 and 389 species recorded in the two 0.2 hectare transect belts surveyed by Gebia and Balun (2004). The forest structure at the Darai site was distinctly layered, comprising emergents, upper canopy, subcanopy and shrub layers. Most emergents and many canopy trees had wide crowns with tall straight boles. Emergents were between 40 and 45 m in height, whilst the upper canopy was generally 30–35 m high. The main emergents were Pterocymbium beccarii , Sloanea forbesii , and Terminalia complanta . Upper canopy species included Elaeocarpus nouhuysii , Dysoxylum spp. , Cryptocarya spp. , and Chisocheton spp. Subcanopy species included Myristica fatua var. papuana , Haplolobus floribundus , Canarium sp. , Pouteria sp. , and Palaquium sp. The lower story was extremely dense and primarily composed of Ficus spp. , Syzygium spp. , and species of Annonaceae , Meliaceae , and Rubiaceae . The forest floor vegetation was dense and consisted of species of Zingiberaceae , Urticaceae , and Gesneriaceae . The specimen of Microhydromys argenteus from the Darai Plateau was caught in an Elliot trap, baited with canned fish, in primary forest. The trap was placed in a drainage line that flowed during heavy rain (although water dissipated through limestone crevices soon after the rain ceased). This specimen was captured alive in the trap, but died within two hours of being brought to the camp. The area where this specimen was captured was situated at an elevation of approximately 380 m ( figs. 1 , 9 ). The village of Faia, on the Hawoi River, is located approximately 10 km to the south west from the trapping site. The trapping site is on a walking route between Faia and the village of Kiam, and is periodically used for hunting by local landowners. One of us (TL) has surveyed small mammals with similar trapping protocols at nine other sites situated in similar habitats and elevations in Gulf and Southern Highlands provinces over the past decade (e.g., Leary and Seri, 1997; Leary and Mamu, 2004), but the paratype from the Darai Plateau is the only specimen of Microhydromys encountered to date. This provides a further indication that this species is naturally rare and/or difficult to trap.

The three documented trapping localities for M. argenteus (Sogeri Plateau, Mt. Sisa, and the Darai Plateau) indicate that the species probably has a relatively broad distribution in the foothills and lower montane forests along the southern slopes of the Central Cordillera, and is likely to occur in similar habitats across the region where trapping efforts have been less intensive. Mammal species with similar recorded global geographic and elevational distributions to M. argenteus include the dasyurid Myoictis leucura Woolley, 2005 ; the macropodids Dendrolagus spadix Troughton and Le Souef, 1936 and Dorcopsulus macleayi (Miklouho- Maclay, 1885); the murine Leptomys elegans Thomas, 1897 ; the hipposiderid Hipposideros muscinus (Thomas and Doria, 1886) ; and the emballonurid Emballonura furax Thomas, 1911 ( Flannery, 1995; Flannery et al., 1996; Helgen, 2007; Musser et al., 2008; Woolley, 2005). The southern foothill forests of the Cordillera are generally less well explored for mammals in the west (in Indonesian New Guinea) than they are in the east (in Papua New Guinea). Although many of these species, like M. argenteus , have only been recorded in Papua New Guinea to date, we suspect that this entire suite of southern hill forest taxa may have a broader actual distribution that extends also into appropriate habitats in southern Indonesian New Guinea ( Helgen, 2007).

Figures

Fig. 2. Skins of M. argenteus, n. sp. (A, B, holotype: BBM-NG 60202, adult male, Sogeri Plateau) and Microhydromys richardsoni (A9, B9: BBM-NG 105829, adult female, Telefomin).

Fig. 3. Skulls of Microhydromys argenteus, n. sp., left (holotype: BBM-NG 60202, adult male, Sogeri Plateau), and M. richardsoni, right (BBM-NG 105829, adult female, Telefomin).

Fig. 4. Above, head of Microhydromys argenteus, n. sp. (UPNG 4310), from the Darai Plateau, taken in the field, photograph by Michael Pennay. Below, body of the same, dorsal (left) and ventral (right) views, photographs by Allen Allison. Image of head (above) previously figured by Leary and Mamu (2004) as ‘‘Microhydromys richardsoni.’’

Fig. 1. Map of New Guinea, showing recorded localities (see Gazetteer) for Microhydromys richardsoni

Fig. 9. Lowland rain forest on the Darai Plateau, one of the three trapping sites for M. argenteus