Microhydromys Tate and Archbold, 1941

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 : 5-13

Microhydromys Tate and Archbold, 1941

TYPE SPECIES AND CONTENT

The type species of Microhydromys is M. richardsoni Tate and Archbold, 1941 . Natural history museums currently hold material of one additional species, described here as M. argenteus , n. sp. ( figs. 2–4 ).

‘‘ Microhydromys ’’ musseri Flannery, 1989 ( fig. 5 ) is a distinctive moss-mouse species known from a single specimen collected in mossy upper montane forest on Mt. Somoro in the Torricelli Mountains (part of Papua New Guinea’s North Coastal Range). Flannery (1989) originally described this species within Microhydromys , a genus previously known only by the type species, M. richardsoni , but did not provide a redefinition of the genus to accommodate this decision. Ziegler (1982: 883) had initially identified this taxon ( musseri ) as ‘‘ Pseudohydromys (undescribed species),’’ and subsequent reviewers have queried its placement in Microhydromys ( Musser and Carleton, 1993, 2005; Helgen, 2005b). Certainly, ‘‘ Microhydromys ’’ musseri lacks the most conspicuous (and synapomorphic within the context of Australo- Papuan murines) features that characterize both M. richardsoni and M. argenteus , n. sp. — extremely small size (condyloincisive length # 20.5) and longitudinally grooved upper incisors. On the other hand, it shares several derived traits with the species of Pseudohydromys (including Neohydromys and Mayermys ) to the exclusion of M. richardsoni and M. argenteus , n. sp., including reduced auditory bullae, a distinctively shortened interparietal bone, and a derived pattern of cephalic arterial circulation as reflected by the size and configuration of foramina in the basicranial region (for assessments of polarity cf. Musser, 1982, 1987; Musser and Heaney, 1992; Helgen and Helgen, 2009). It also bears an intimate craniometric resemblance to certain species of Pseudohydromys , as document- ed by multivariate morphometric comparisons ( Helgen and Helgen, 2009), rather than to Microhydromys spp. or any other hydromyinan taxa. As Helgen and Helgen (2009) note, ‘‘In combination … these morphometric and qualitative morphological comparisons reject a sister relationship between musseri and the species of Microhydromys and suggest instead that musseri is more appropriately recognized as a member of Pseudohydromys .’’

DIAGNOSIS AND DESCRIPTIVE NOTES

The species of Microhydromys can be distinguished from all other murines by the following combination of characters: (1) very small body and cranial size, with condyloincisive length measuring # 20.5 mm; (2) short, soft, blackish or grayish pelage; (3) a dark tail with a pale tail tip, slightly shorter to slightly longer than the head and body; (4) orthodont upper incisors, each marked by a distinct longitudinal groove ‘‘separating [the] anterior face of [the] tooth into an outer third and an inner two-thirds’’ ( Tate and Archbold, 1941: 2), a trait unique among Australo-Papuan murine genera; (5) two relatively small molars in each quadrant of the jaw (i.e., M1/m1 and M2/m2), all with ‘‘basin’’-shaped surfaces (i.e., with occlusal topography extremely similar to that in Hydromys and Baiyankamys —e.g., see pictures in Musser and Heaney, 1992; Misonne, 1969); (6) a basic cranial conformation denoted by weakly expanded zygomata, a short and rather blunt rostrum featuring nasals that protrude sharply over the premaxillae to overhang the upper incisors, short subparallel incisive foramina, a broad and high braincase sloping sharply but linearly downward from crown to nasals, a weakly constricted interorbital region, and relatively large auditory bullae; (7) osseous reflections in the basicranium (e.g., a large stapedial foramen and short groove in the ventral surface of the pterygoid plate in which the infraorbital branch of the stapedial artery courses) indicating retention of the plesiomorphic murine cephalic arterial pattern (see diagrams in Musser and Heaney, 1992; Musser et al., 1998); (8) a gracile dentary with long and slender incisors, tiny molars, a tiny coronoid process, and slender condyloid and angular processes; and (9) small, unspecialized feet (with plantar and palmar surfaces bearing the usual complement of pads—four metacarpal pads, a thenar, and smaller hypothenar on the forefeet; conspicuous interdigital pads, large thenar, and small hypothenar on the hind feet), without extensive interdigital webbing— denoting terrestrial habits (cf. Tate and Archbold, 1941; Helgen and Helgen, 2009) and without excessively elongate hind feet (e.g., as seen in the partly saltatorial hydromyinan genera Leptomys and Paraleptomys ; Musser et al., 2008).

The number of teats in Microhydromys is not previously recorded in the literature ( Flannery, 1995) in part because the first three specimens of Microhydromys to be collected were male ( Tate and Archbold, 1941; Flannery, 1989). Only two females have ever been collected. Peter Dwyer’s field notes (original copy held at ANWC) for AM M14166 , a female of M. argenteus , n. sp., from Mt. Sisa, includes the comment ‘‘tts med – 0 + 2, perf’’ which signifies moderately prominent teats, with pectoral and abdominal teats absent, two inguinal teats present on each side, and a perforate vagina. (The skin of this specimen is preserved as a flat preparation and the teats are no longer apparent.) A female specimen of M. richardsoni from Telefomin (BBM-NG 105829) includes a well-prepared study skin. The accompanying skin label notes that the animal was lactating when captured but does not record the mammary formula. Two pairs of inguinal teats are clearly apparent on the study skin, and there is a faint indication of a pair of axillary teats (probably nonfunctional if truly present). We suspect that the typical mammary conformation for both species of Microhydromys , as for most hydromyinans ( Helgen and Helgen, 2009), is two pairs of inguinal teats.

Microhydromys richardsoni Tate and Archbold, 1941

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 9 S, 139 ° 13 9 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).

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 graybrown 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 groovetoothed 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 X width of M1, 1.57 mm X 0.86 mm; length X width of M2, 0.83 mm X 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 indicat- ed by C 14 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.

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 in a mountain-locked valley dominated on the other side by a 1600 m. crest on the range on which the higher camps of this series were situated. Swampy in parts, and drained by a considerable stream named Araucaria Creek for stands of Araucaria … that occurred on its lower course, the valley bottom broadened to approximately 0.5 km. above the camp. Below camp the mountains gradually closed in and the creek became a succession of rapids, pools, and small waterfalls…. Rain-forests filled the valley bottom and extended up the slopes to Nothofagus - forest on both sides. But on the south slope, between 870 m. and 950 m. altitude, some broad ridges, at best poorly drained and in parts swampy, carried a distinctive type of forest dominated by Agathis ….

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.

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. 5. Skull of the holotype and only known specimen of Pseudohydromys musseri (Flannery, 1989) (BBM-NG 101737, Mt. Somoro, Torricelli Mountains), originally described as a species of Microhydromys (Flannery, 1989; Helgen and Helgen, 2009). Scale bar 5 1 cm.

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.

Fig. 8. An isolated maxilla of Microhydromys richardsoni with M1–2 (ANWC CM29994) from the Oenake Range, West Sepik Province, Papua New Guinea, in ventral view (CLM [M1–2] 5 2.30 mm).

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