Morphological data
Results of the PCA of morphometric data (Fig. 1a, b) are congruent with the genetic data in identifying two putatively allopatric groups in Ecuador, one (V) formed by specimens collected on the eastern slopes of the Andes and the other (M) formed by specimens collected on the western slopes of the Andes. In addition, a second western group, distributed north near Antioquia, Colombia, was also recovered (there is currently no genetic data available for this group). The eastern and western groups marginally overlap. This overlap decreased when age groups and sexes were examined separately (Fig. 1a, b, respectively). The two western subgroups overlap more than the eastern and western groups but still are well separated. When the three groups were examined using LDA (Fig. 1c, d), the separation was complete in both age and sex analyses (Fig. 1c, d).
Genetic data
Both phylogenetic analyses of mitochondrial and nuclear DNA sequences (Fig. 2) depict a strongly monophyletic Sigmodontinae (clade I; PP = 1.00). Importantly, the tribe Ichthyomyini also is recovered as monophyletic (clade II; PP = 1.00); Rheomys appears sister to Neusticomys. Within Neusticomys, two distinct groups are indicated, each highly supported (PP = 1.00). The groups appear to be allopatric; clade V is distributed east of the Andes and clade M west of the Andes (Fig. 3). Average genetic distances among the eastern and western groups were 8.5 % (Cytb) and 0.8 % (Rbp3); a much higher value than that observed within each group (eastern: 0.4 and 0.0 %; western: 0.1 and 0.1 %, respectively).
Taxonomic implications
Despite the small sample size examined, we consider that when taken together, available data provide sufficient evidence to justify recognition of an additional species of Neusticomys. As no name is available, we name and describe it below.
Neusticomys vossi sp. nov.
Voss’ fish-eating rat
urn:lsid:zoobank.org:act:FB716D66-3D57-40A0-9E48-0E8A5A9B402E
Holotype—QCAZ 7830, an adult lactating female collected by T. E. Lee (personal catalog number TEL1846) in August 2005 with a Sherman trap placed in a forested mountain stream near a small (1 m tall) waterfall (Fig. 4). The specimen is preserved as the skin, skull and skeleton, and frozen tissue and deposited at El Museo de Zoología, Pontificia Universidad Católica del Ecuador, Quito, Ecuador.
Type locality—12 km by road northwest of Cosanga (0° 31′ 70″ S, 77° 52′ 99″ W), Napo Province, Ecuador (1900 m).
Diagnosis—As stated by Percequillo et al. (2005), species of Neusticomys are difficult to diagnose using presumptive autapomorphies; rather, unique combinations of character states are operationally useful for species recognition. Neusticomys vossi sp. nov. is a species of Ichthyomini, that can be distinguished from other ichthyomyine species by its smaller size, reduced hallux, single cusped last molar, large interparietal bone, and an apparent less advanced aquatic specialization, and can be distinguished from other congeners (except N. monticolus) by having dull grayish rather than brownish pelage and by occipital condyles not projecting posteriorly beyond rest of occiput. N. vossi sp. nov. can be distinguished from N. monticolus by its smaller overall size and by its narrower incisors, braincase, occipital condyles, and rostrum as well as shorter condylo-incisive length, length of diastema, length of maxillary molars, zygomatic breadth, breadth of braincase, and breadth of occipital condyles.
Holotype measurements—Head body length (103 mm), tail length (108 mm), hindfoot (27 mm), ear (13 mm), breadth of nasals (2.52 mm), length of nasals (8.89 mm), depth of incisor (1.23 mm), length of incisors (3.03 mm), breadth of M1 (1.16 mm), and the length of the incisive foramina (4.45 mm).
Description—Dull grayish black pelage (Fig. 5). Ventral pelage color often slightly lighter than dorsal pelage. Mystacial vibrissae and oral margins usually silver. Tail uniformly colored with whitish hairs along ventral surface and occasionally a white tip. Some individuals may have white midpectoral blazes and irregular whitish dorsal spotting. Manus and pes covered with whitish silver hairs turning to dark at the wrist, ankles, and metapodials.
Rostrum slender with threadlike zygomatic arch (Fig. 6). Infraorbital foramina very large. Palatal foramina very large and medially broad, reaching the anterior lobe of large palate, well surpassing the posterior end of the toothrow. Square mesopterygoid fossa lacking a median palatine process. Braincase broad, flat, and smooth. Coronoid process long and narrow. No ridges on frontals or parietals. Small depression at base of nasals, plane of nasals continuous with frontals. Large interparietal bone. Occipital condyles not projecting posteriorly beyond rest of occiput, and not exposed dorsally. Well-developed gnathic process. Peglike process on maxillary roots of zygomata external to molars. Upper incisors ungrooved and orthodont. Parallel upper molar rows. Molar cusps nearly opposite. High crowned molars with deep flexes almost touching in the middle line of molars. M1 much larger than M2 and M3. M1 with three similar-sized lobes M2 with paracone-protocone pair much wider than the metacone-hypocone pair, M3 reduced.
Comparisons—Voss (1988) noted that specimens from Papallacta in the eastern Andes, referred here to N. vossi sp. nov., average slightly smaller than N. monticolus from Guarumal in the western Andes; the same is true for the specimens, including the holotype of N. vossi sp. nov., collected by us. The breadth of the occipital condyles appears nearly diagnostic between the two groups in Voss (1988) study. BOC in N. vossi sp. nov. samples is ≤6.9 mm with females being smaller than males. For the most part, N. monticolus shows a BOC of ≥6.9 mm with females being smaller than males.
Similar to Neusticomys venezuelae and Neusticomys mussoi, Voss’ fish-eating rat is differentiated from Neusticomys ferreirai, Neusticomys oyapocki, and Neusticomys peruviensis in that the posterior edge of the inferior zygomatic root lies above the anterecone of M1. N. vossi sp. nov. differs from N. oyapocki and N. ferreirai in having three upper and lower molars.
Distribution—Known from four sites on the eastern slopes of the Andes in northern Ecuador and southern Colombia ranging from 1° 58′ N, 76° 35′ W in the north to 0° 33′ N, 77° 36′ W and from 1900 to 3750 m.
Etymology—Neusticomys vossi sp. nov. is named to honor Dr. Robert S. Voss of the American Museum of Natural History. Rob Voss is the author of a large series of key contributions towards the understanding of South American mammals; in particular, he authored a now classical monograph on ichthyomyine rodents. As such, he was the first to recognize the morphological differences between the new species described here and N. monticolus. As part of an ichthyomyine review, Voss (1988) wrote the following as a comment on N. monticolus, “The series from Guarumal together with those from nearby Las Machinas and from the Rio Pita closely resemble the Volcan Pichincha specimens, but the Papallacta series exhibits some metric differences.”
Natural history—One N. vossi sp. nov. was recorded at Papallacta with two large embryos on the 15th of May (Voss 1988). The type collected in August is a lactating female. The type was taken by a small waterfall about 1 m tall, with traps exposed to the spray of water from the fall, which is congruent with the description by Tate (1931) of Neusticomys habitat. The stream was rocky and about 1 m across with fast rapids (Fig. 4). This specimen represents a low elevation record in Ecuador at 1900 m for the eastern Andes (Lee et al. 2006a). Oreoryzomys balneator and Thomasomys erro were collected in the same trap line or in nearby forests, as the type specimen (Lee et al. 2006a). Vegetation along the stream consisted of plants with large waxy leaves. Plants of the families Araceae, Arecaceae, Cecropiaceae, Chloranthaceae, Cyatheaceae, Cyclanthaceae, Flacourtiaceae, Lauraceae, Lobeliaceae, Melastomataceae, Meliaceae, Moraceae, Piperaceae, and Poaceae were found along the stream bank (Lee et al. 2006a).