Apomys (Megapomys) gracilirostris Ruedas 1995

Heaney, Lawrence R., Balete, Danilo S., Duya, Mariano Roy M., Duya, Melizar V., Kyriazis, Christopher C., Rickart, Eric A., Steppan, Scott J. & Rowsey, Dakota M., 2025, Three new species of Philippine forest mice (Apomys, Muridae, Mammalia), members of a clade endemic to Mindoro Island, Zootaxa 5647 (1), pp. 1-26 : 14-17

Apomys (Megapomys) gracilirostris Ruedas 1995

Apomys gracilirostris Ruedas. Proceedings of the Biological Society of Washington 108: 305.

Holotype. Originally cataloged as Cincinnati Museum of Natural History ( CNMH) 650; transferred to National Museum of the Philippines ( NMP) and cataloged as M-3482. Original field number NMP / CNMH 1136 . Collected 12 June 1992. Prepared as stuffed study skin with clean skull.

Type locality. Philippines: Mindoro Island : Municipality of San Teodoro : North Ridge approach to Mt. Halcon, ca. 1580 m elevation, ca. 13 o 16’ 48”N, 121 o 59’ 19”E, in a place locally known as Hanglo .

Measurements. Tables 1 and 4.

Specimens examined. (n = 25). Philippines: Mindoro Island: Oriental Mindoro Province: Baco Municipality : 4.75 km N, 0.8 km E Mt. Halcon peak, 1650 m elevation, 13.30687 o N, 121.0009 o E ( FMNH 222069–222080 , 222316–222321 ) ; 0.95 km N, 1.45 km W Mt. Halcon peak, 2150 m elevation, 13.27222 o N, 120.980 o E ( FMNH 222064 ) ; 0.5 km N, 1.1 km W Mt. Halcon peak, elevation 2310 m, 13.26745 o N, 120.9838 o E ( FMNH 222065– 222068 , 222314 , 222315 ) .

Etymology. From the Latin gracilis and rostrum, referring to the long and slender rostrum.

Diagnosis and Description. For an extensive description, see Ruedas (1995). We note the following features that together diagnose this species: large size among Megapomys , basioccipital length x̄ = 34.8 mm (33.6–36.1 mm, Table 4); HB x̄ = 142 mm (range 134–150 mm), weight = 83 g (range 70–102 g; Table 1); pelage dark brown dorsally and ventrally, slightly paler ventrally with slight ochraeous wash at tips ( Fig. 4A ); mystacial vibrissae up to 55 mm long; no evidence of a postauricular tuft of elongated or pale hair; tail dark brown dorsally and medium brown ventrally, often with a white tip (1–15 mm), and with short but visible hairs along its entire length dorsally and ventrally; tail proportionately longer than most Megapomys , TV/HB about 1.11 ( Table 1), thick at the base and tapering to tip; ears dark brown and short (x̄ = 19.5 mm) absolutely and relative to HB ( Table 1); forefeet with dark hairs extending to base of digits dorsally, with unusually long, sturdy digits and claws; hind feet long (x̄ = 38.5 mm) but broad and proportionately slightly short ( Table 1) with six large plantar pads, heavily pigmented ventrally; skull and teeth as described above for the genus Apomys ; upper incisors narrow, laterally compressed relative to depth ( Fig. 5A ); lower incisors long and slender, and unpigmented on anterior surface; rostrum long and slender, slightly up-turned at the tip; braincase more globose than other Megapomys ; molars narrow and toothrows short; post-palatal region long; foramen present in the bullae for stapedial artery; interparietal bone usually wide; mandible long and slender with short coronoid process and short, blunt angular process.

Comparisons. Apomys veluzi sp. nov. ( Fig. 2B ) is easily distinguished externally from A. gracilirostris ( Fig. 2A ). The former is smaller in all respects; e.g., HB x̄ = 129 mm vs. 142 mm, BOL x̄ = 31.79 mm vs. 34.78 mm ( Tables 1 and 4). The pelage of A. gracilirostris is dark brown, slightly less dark ventrally, with a hint of ochraceous wash dorsally and ventrally, and short, black guard hairs are visible dorsally; that of A. veluzi sp. nov. is a rich ochraceous brown dorsally and pale cream ventrally with a slight ochraceous wash. Ears of A. gracilirostis are both absolutely and proportionally to HB shorter than those of A. veluzi sp. nov. ( Table 1), and are dark brown vs. medium brown. Hair behind the ears is the same color and length as other dorsal pelage; that of A. veluzi sp. nov. is pale sandy-orange, slightly longer than surrounding fur. On A. gracilirostris , dark brown fur extends onto the tops of fore and hind feet; forefeet are lightly pigmented ventrally, hind feet are dark ventrally; on A. veluzi sp. nov., the fore and hind feet are covered dorsally with fur that is pale cream, and with pale skin ventrally. Forefeet of A. gracilirostris have sturdy digits and claws that are long and curved; those of A. veluzi are proportionately and absolutely shorter, and less strongly curved. The tail of A. gracilirostris is dark brown dorsally and paler brown ventrally, and a small (1–15 mm) white tip is usually present; that of A. veluzi sp. nov. is dark dorsally and white ventrally, sometimes with a slightly paler tip. The tail of A. gracilirostris has short but visible hairs along its entire length dorsally and ventrally; that of A. veluzi sp. nov. has visible hairs only toward the tip. The tail of A. gracilirostris is absolutely longer (x̄ = 159 mm vs. 151 mm), but shorter relative to the body (TL/HB x̄ = 1.1 vs. 1.2; Table 1) than that of A. veluzi sp. nov. The skull of A. gracilirostris ( Fig. 5A ) is larger in all respects than that of A. veluzi sp. nov. (e.g., BOL x̄ = 31.79 mm vs. 34.78 mm) except having nearly equal breadth of M1 (x̄ = 1.76 mm vs. 1.81 mm), and with a notably narrower zygomatic plate (x̄ = 2.58 mm vs. 3.22 mm; Table 4). The anterior surface of lower incisors in A. veluzi are pigmented orange; those of A. gracilirostris are white or nearly so. Lower incisors of A. gracilirostris much longer relatively than those of A. veluzi sp. nov.

Apomys crinitus sp. nov. is easily distinguished from A. gracilirostris in being externally smaller in all respects (HB x̄ = 128 mm vs. 142 mm, WT x̄ = 63 g vs. 83 g; Table 1), having bright ochraceous-brown dorsal fur and pale cream ventral fur rather than dark brown dorsally and ventrally ( Fig. 2C vs. 2A), tail longer proportionate to HB (x̄ = 1.2 vs. 1.1), and white ventrally vs. medium to dark brown; ear longer both absolutely and proportionately ( Table 1); elongated tuft of pale sandy to nearly white fur behind ear, rather than the same length and dark brown color as surrounding fur. Forefeet of A. gracilirostris have sturdy digits and claws that are long and curved; those of A. crinitus sp. nov. are proportionately and absolutely shorter, and less strongly curved. Skull smaller in all respects (e.g., BOL x̄ = 31.56 mm vs. 34.78 mm) except having a longer incisive foramina (x̄ = 5.36 mm vs. 5.10 mm) and notably narrower zygomatic plate (x̄ = 2.79 mm vs. 3.22 mm; Table 4). The anterior surface of lower incisors in A. crinitus are pigmented orange; those of A. gracilirostris are white or nearly so. Lower incisors of A. gracilirostris much longer than those of A. crinitus ( Figs. 5A and 6A ).

Apomys minor sp. nov. is easily distinguished from A. gracilirostris in being externally smaller in all respects (HB x̄ = 122 mm vs. 142 mm, WT x̄ = 57 g vs. 83 g; Table 1), with dorsal pelage that is medium brown rather than dark brown; ventral pelage that is pale brown with distinct ochraceous wash rather than dark brown with a slight ochraceous wash; hind feet that are white dorsally rather than having dark brown hairs covering most of the foot; ear longer both absolutely and proportionately ( Table 1); tail that is medium brown to white ventrally rather than dark brown. Forefeet of A. gracilirostris have sturdy digits and claws that are long and curved; those of A. minor sp. nov. are proportionately and absolutely shorter, and less strongly curved. Much smaller skull in all respects (BOL x̄ = 30.79 mm vs. 34.78 mm, Table 4) except in having equal length of incisive foramina (5.12 mm vs. 5.10 respectively) and notably narrower zygomatic plate (x̄ = 2.65 mm vs. 3.22 mm; Table 4). The anterior surface of lower incisors in A. minor sp. nov. are pigmented orange; those of A. gracilirostris are white or nearly so. Lower incisors of A. gracilirostris much longer than those of A. minor sp. nov. ( Figs. 5A and 6B ).

Distribution and Ecology. Known only from Mt. Halcon, northern Mindoro (study area 3 in Fig. 1 ). Ruedas (1995) documented this species from 1250 m to 1950 m elevation; we recorded them from 1650 m, 2150 m, and 2310 m ( Fig. 1B ), in old-growth mossy forest dominated by gymnosperms (e.g., Dacrycarpus , Falcatifolium , Phyllocladus , and Podocarpus ) and myrtles (e.g., Leptospermum and Syzygium ), where leaf litter and humus formed thick layers on the ground and epiphytes were abundant on the trees. We did not capture them at our sampling areas from 500 m to 1110 m ( Fig. 1B ), in regenerating forest over limestone, regenerating lowland forest, and montane forest, at all of which leaf litter and humus were virtually absent. Other native small mammals we documented from 1650-2310 m were Crocidura mindorus , Anonymomys mindorensis , Apomys microdon , Apomys musculus , Chrotomys mindorensis , Maxomys sp. , and Rattus mindorensis . Although three non-native species ( Suncus murinus , Rattus exulans , and Rattus tanezumi ) were present at lower elevations, they were absent where A. gracilirostris occurred.

As noted by Ruedas (1995), “the gracile skull, long rostrum, reduction in relative size of upper incisors, and extreme elongation of lower incisors”, to which we add the presence of large forefeet with long, sturdy digits and claws that are likely to be associated with extensive digging in leaf-litter and humus, “ ... all point to a diet of softbodied invertebrates”, perhaps similar to the diet of Rhynchomys (Heaney et al., 2016; Petrosky et al., 2021).

Ruedas (1995) noted one female that carried three embryos. Females that we captured with tiny, inconspicuous mammae weighed 70 to 79 g, including two weighing 70 g and 77 g that had no uterine scars or embryoes. Those weighing 74 to 89 g had larger, pigmented mammae. Twelve males we captured weighing 70 to 102 g had testes 6 x 4 to 7 x 5 mm.

Figures

FIGURE 1. A, Map of Mindoro Island showing topographic features and the locations of areas intensively surveyed by the authors. 1 = Mt. Calavite; 2 = Mt.Abra de Ilog; 3 = Mt. Halcon; 4 = Mt. Tallulah; 5 = Mt. Mangibok; 6 = Mt.Wood; 7 = Barangay Morente; 8 = Mt. Masulong; 9 = Sta. Teresa Hill and Mt.Alibug; 10 = Ilin Island. Apomys gracilirostris were captured in area 3; A. veluzi sp. nov. in area 2; A. crinitus sp. nov. in areas 4 and 5; and A. minor sp. nov. in areas 6 and 9. Members of the subgenus Megapomys were not detected in areas 1, 7, 8, and 10. B, Elevational ranges of the four species of Megapomys on Mindoro based on specimens reported here. For each species, black circles indicate elevations at which the given species was documented; open circles are elevations along the same transects where the relevant species was not documented; short horizontal lines indicate the maximum elevation along the given transect. For A. gracilirostris, open triangles indicate elevations at which Ruedas (1995) reported that species. Dark vertical lines connect localities along a single transect where the given species was documented.

FIGURE 2. Drawings of four species of Apomys (Megapomys) from Mindoro. A = A. gracilirostris (from study area 3 in Fig. 1), B = A. veluzi sp. nov. (from study area 2 in Fig. 1), C = A. crinitus sp. nov. (from study areas 4 and 5 in Fig. 1), D = A. minor sp. nov. (from study areas 6 and 9 in Fig. 1).

FIGURE 4. Results of principal components analysis of 18 craniodental measurements (Table 3) of 19 adult Apomys (Megapomys) from Mindoro. Loadings and eigenvalues are shown in Table 5.

FIGURE 5. Photographs of the skulls of A Apomys gracilirostris (FMNH 222320) and B Apomys veluzi sp. nov. (holotype, FMNH 222928).

FIGURE 6. Photographs of the skulls of A Apomys crinitus sp. nov. (holotype, FMNH 222954) and B Apomys minor sp. nov. (holotype, FMNH 228798).