Type material and locality

The holotype of richardsoni is AMNH 152079, adult male, skin and skull, from “four kilometers southwest of” Bernhard Camp on the Idenburg River (03°29′S, 139°13′E, Papua Province, Indonesia), 850 m, northern slopes of the Snow Mountains, collected 16 March 1939 by W.B. Richardson (Tate and Archbold, 1941; Tate, 1951; Lawrence, 1993). This expedition camp (“four kilometers southwest of Bernhard Camp”) was erected on the bank of what expedition members designated as “Araucaria Creek,” a tributary of the Sahoeweri River (Archbold et al., 1942). The camp was “situated in a shut-in valley behind the fronting ridge of the mountains that rise from the Mervlakte” (Archbold et al., 1942: 239). This spot is depicted in a photograph in figure 2 of plate 11 in Archbold et al. (1942), reproduced here as figure 6; also see the description published by Brass (1941). A photograph of rain forest in the vicinity (900 m) was figured by Brass (1941: pl. 5, fig. 2), reproduced here as figure 7. It took expedition members about four hours or so to trek from “Bernard Camp” to the 850 m locality (and the habitats are very different between the two camps; Brass, 1941).

Fig. 6

“Expedition camp in flood plain rain-forest at 850 m. on Araucaria Creek, four kilometers southwest of Bernhard Camp” (Brass, 1941: 342). The holotype of M. richardsoni was collected while the 1938–1939 Archbold Expedition was stationed at this camp. Photograph previously figured by Brass (1941: pl. 6, fig. 1) and Archbold et al. (1942: pl. 11, fig. 2) and reproduced from the Archbold archives.

Fig. 7

Mossy Nothofagus rain forest in the immediate vicinity of the type locality of Microhydromys richardsoni—4 km southwest of Bernhard Camp, 900 m (see text). Photograph previously figured by Brass (1941: pl. 5, fig. 2) and reproduced from the Archbold archives courtesy of Darrin Lunde.

Diagnosis

Microhydromys richardsoni differs externally from M. argenteus, n. sp., in having darker grayish-black pelage, both dorsally and ventrally, sometimes with white flecking on the belly (paler, uniformly gray-brown fur, above and below, in M. argenteus, n. sp.: fig. 2); a proportionally longer tail, measuring 101%–111% of head-body length (91%–95% of head-body length in M. argenteus, n. sp.: table 1); a short discrete white tail tip, less than one-quarter of tail length, without extensive mottling (versus 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 the dorsal and ventral surfaces, in M. argenteus, n. sp.); and slightly shorter hind feet and pinnae (table 1). Compared to M. argenteus, n. sp., the skull of M. richardsoni has proportionally narrower zygomata, narrower incisors, slightly wider molars, longer incisive foramina, and smaller auditory bullae. We recommend the English common name “northern groove-toothed moss-mouse” for this species.

Distribution and natural history

Microhydromys richardsoni is known from three modern specimens collected at three localities scattered in the foothills and lower montane forests in northern New Guinea. Two specimens have been taken on the northern margin of the Central Cordillera—one from 850 m on the Sahoeweri River, situated on the slopes of the Snow Mountains in the Mamberamo Basin (the type locality), the other at 1500 m at Telefomin in the Star Mountains. One additional specimen derives from middle-elevation forests (670 m) at Wanuma in the outlying Adelbert Range of northeastern New Guinea. The mean elevation of these three trapping sites is 1007 m.

A fourth specimen, an isolated subfossil maxilla with M1–2, has been collected by K. Aplin in the Oenake Range, a mountain block near the Bewani Mountains that is part of the outlying North Coastal Range (fig. 8; measurements for this specimen include: breadth of the zygomatic plate, 1.13 mm; CLM (M1–2) 2.30 mm; length × width of M1, 1.57 mm × 0.86 mm; length × width of M2, 0.83 mm × 0.71 mm). The Oenake Range specimen is part of a large assemblage of small vertebrate bones taken from a 1 m deep sequence of bone-rich sediments in Paleflatu Cave, located in an uplifted coralline limestone terrace close to the present coastline and no more than 30 m above sea level. The bone accumulation is attributed to long-term use of the cave by owls (of uncertain taxonomic identity) through the mid- to late Holocene (as indicated by C¹⁴ dating; K. Aplin et al., unpublished data). Though the cave appeared to be unoccupied by owls in 2005, the presence on the surface of partially disaggregated pellets suggests a recent to contemporary age for remains lying on the surface of the deposit. To date, only one specimen of M. richardsoni has been identified in the assemblage, and this comes from the surface layer.

Fig. 8

An isolated maxilla of Microhydromys richardsoni with M^1–2 (ANWC CM29994) from the Oenake Range, West Sepik Province, Papua New Guinea, in ventral view (CLM [M1–2]  =  2.30 mm).

Together, these four records of occurrence (three modern, one subfossil) establish that M. richardsoni is widely distributed in northern New Guinea, and indicate an ecological association with forests situated on mountain slopes down to low elevation. Though captures of M. richardsoni are rare, like Tate (1951: 225) we expect that this species occurs along the length of the northern face of the expansive Central Cordillera. The records from the Adelbert and North Coastal Ranges suggest further that it is likely to occur in foothill forests in other northern New Guinea outlying ranges, such as the Foja and Cyclops Mountains of western New Guinea and the mountain ranges of Papua New Guinea's Huon Peninsula. It remains to be established whether, in addition to mountain slope habitats, M. richardsoni inhabits the expansive rainforested lowlands of northern New Guinea (principally comprising the Sepik-Ramu and Mamberamo-Idenburg-Rouffaer drainage basins) that isolate the outlying mountain ranges of northern New Guinea from the extensive montane rain forests of the Central Cordillera.

Almost nothing is known of the natural history of M. richardsoni. All three known specimens were snap-trapped, presumably on the ground. A label accompanying the Wanuma specimen notes that it was collected in “second growth,” but no explicit habitat notes accompany the holotype, or the specimen from Telefomin. Flannery and Seri (1990) provided a good account of local habitats in the vicinity of Telefomin, where one of the three modern specimens was trapped (Ziegler, 1989). Though Tate and Archbold (1941) did not record the exact context of collection for the holotype, Archbold et al. (1942: 239–243) and Brass (1941) described in detail the forests along the flood plain of Araucaria Creek and on adjacent ridges around the 850 m camp station at which the holotype of M. richardsoni was collected, capturing a range of forest formations from flood plain to ridges. According to Brass (1941: 289–290), this station lay beyond a 900 m ridge

Brass' detailed description of local habitats at this station, along with good accompanying photographs (Brass, 1941; pl. 6, fig. 1, and pl. 5, fig. 2; reproduced here as figs. 6 and 7, respectively), provide the ecological context for the collection of the holotype, allowing a glimpse into general forested environments in an area where M. richardsoni occurs.

On the northern slopes of the Central Cordillera, M. richardsoni occurs at lower elevations than most species of Pseudohydromys, New Guinea's other ‘moss-mouse’ genus (Helgen and Helgen, 2009). If M. richardsoni is wholly or largely confined to elevations of 1500 m and lower, it is likely to co-occur locally with only one species of Pseudohydromys, P. ellermani (Laurie and Hill, 1954), a Central Ranges endemic that has an elevational “floor” of 1400 m and occurs across most of the length of the Cordillera, from the Snow Mountains to the Wau Valley (Helgen, 2005a; Helgen and Helgen, 2009). We suspect that M. richardsoni may also co-occur with Pseudohydromys musseri in the North Coast Range, where both species are recorded, as the sole known trapping site for musseri (situated at 1350 m) falls within the recorded elevational range of richardsoni (i.e., from near sea level [Oenake Range maxilla] to at least 1500 m along the Central Cordillera).

The total sample of cranial and dental remains from Paleflatu Cave in the Oenake Range represents several thousand individuals and includes numerous examples of another similarly tiny-bodied murine rodent, Lorentizimys nouhuysi Jentink, 1911 (K. Aplin, personal obs.). Unless some aspect of the biology of M. richardsoni (e.g., diurnality, though the minuscule eyes and dark gray pelage of the species of Microhydromys suggest nocturnal habits) makes it largely immune to owl predation, its sparse representation in the Paleflatu assemblage is best explained by natural rarity in those habitats within the hunting range of the owls in the area (which would include various lowland and limestone hill forests, up to local topographic highs of approximately 500 m).

All other hydromyinans that have been studied to date are largely or partly animalivorous (e.g., Woollard et al., 1978; Menzies and Dennis, 1979; Voss, 1988; Jackson and Woolley, 1993; Flannery, 1995; Helgen, 2005b; Musser et al., 2008; Helgen and Helgen, 2009), and we suspect the same is true for the species of Microhydromys. Their solid orthodont incisors, tiny molars, somewhat shrewlike appearance, and terrestrial habits suggest to us a lifestyle spent preying upon invertebrates in soil and leaf litter on the forest floor in New Guinea hill forests.

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 ( =  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).

Fig. 9

Lowland rain forest on the Darai Plateau, one of the three trapping sites for M. argenteus (UPNG 4310). Photograph by Allen Allison.

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 groove-toothed 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. 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 flat-topped 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).