Nectomys apicalis Peters, 1861

Voss, Robert S., Fleck, David W. & Jansa, Sharon A., 2019, Mammalian Diversity And Matses Ethnomammalogy In Amazonian Peru Part 5. Rodents, Bulletin of the American Museum of Natural History 2024 (466), pp. 1-180 : 55-58

Nectomys apicalis Peters, 1861

Figures 18B, 18E, 22A, 23A, 24A

VOUCHER MATERIAL (N = 12): Nuevo San

Juan (AMNH 268255, 268256, 273135; MUSM

11203–11207, 15332), San Pedro (MUSM 22344, 22345; UF 30787).

UNVOUCHERED OBSERVATIONS: Field identifications of this species cannot be accepted as valid without supporting voucher material.

OTHER SPECIMENS EXAMINED 15 (N = 136): Brazil — Amazonas, Barro Vermelho (MVZ 190374*), Jainu (MVZ 190373*). Ecuador — Orellana, 42 km S Pompeya Sur (ROM 104495, 106067, 106081); Sucumbios, Limoncocha (USNM 513585*), Zamora-Chinchipe, Los Encuentros (USNM 513584*). Peru — Amazonas, Huampami (MVZ 153539*, 153568, 153569, 155018, 155019*–155022, 155024–155029), La Poza (MVZ 157813–157827), San Antonio (MVZ 153534*), 46 km SE Bagua Grande (LSUMZ 19296); Ayacucho, Hacienda Luisiana (LSUMZ 16711*), San José (LSUMZ 16715*), Santa Rosa (LSUMZ 15694); Cusco, Hacienda Cadena (FMNH 65687–65694), Hacienda Villa Carmen (FMNH 84290–84295), Kiteni (USNM 19297), Ocobamba (USNM 86917, 86918), Río Comberciato (USNM 194830), San Fernando (USNM 194832, 194833, 194835–194837), San Pedro (FMNH 172263–172265), Santa Ana (USNM 194907, 194909, 194910, 194912– 194915), Uvini (USNM 194831), 40 km E Quincemil (USNM 19298), 15.9 km SW Pilcopata (FMNH 175091); Huánuco, Hacienda Vista Alegre (FMNH 24131), Tingo Maria (FMNH 46101, 46102); Junín, Perené (AMNH 63857), Río Tulumayo (USNM 507262*); Loreto, Boca Río Curaray (AMNH 71909–71911, 71913, 71915–71920), Boca Río Peruaté (FMNH 88912), 25 km S Iquitos (TTU 98642, 98994, 100883, 124939, 124940), Puerto Indiana (AMNH 73351, 73352), San Jacinto (KU 158188, 158189); Madre de Dios, confluence of Río La Torre with Río Tambopata (LSUMZ 24593), Hacienda Amazonía (FMNH 139832, 139833, 139835), 2.75 km E Shintuya (FMNH 170593), Puerto Maldonado (KU 144301, USNM 390136); Pasco, San Ramón (AMNH 213534); Puno,

15 Specimens marked with asterisks (*) are those karyotyped by Gardner and Patton (1976), Patton et al. (2000), or Bonvicino and Gardner (2001).

Carabaya (USNM 137520, 172961), San Juan (FMNH 78378–78383), Santo Domingo (= “Inca Mines”; AMNH 15801, 16057; FMNH 18201, 29452); Ucayali, Balta (LSUMZ 12321, 12324, 12326–12329, 14377, 16708; MVZ 136639– 136641*), Lagarto (AMNH 76512, 76520), Santa Rosa (AMNH 76057).

IDENTIFICATION: Peruvian specimens of Nectomys can be sorted into two taxa that we identify as Nectomys apicalis and N. rattus based on qualitative and morphometric trait differences. We have not examined the type material of either species (in Berlin and Vienna, respectively), but we provisionally accept the application of these names as recommended by Bonvicino and Weksler (2015). As currently understood, Nectomys apicalis is a western Amazonian taxon characterized by diploid counts of 38–42 chromosomes, whereas N. rattus is a widespread (pan-Amazonian) species with 2 n = 52–56 chromosomes. According to Patton et al. (2000), these karyotypes can be associated with different cranial phenotypes: specimens with 2 n = 38–42 have interparietals that are deep (anteroposterior dimension) in proportion to their width (transverse dimension), whereas specimens with 2 n = 52–56 have interparietals that are proportionately shallower. Although Patton et al. (2000) suggested that the ratio of interparietal depth ( IPD) to interparietal width ( IPW) could be used as a diagnostic criterion, our measurements of karyotyped specimens—those marked with asterisks in lists of examined material (above and below)—resulted in narrowly overlapping values of this ratio ( IPD / IPW). Fortunately, other morphological traits allow confident identification of these species (table 16).

Several qualitative craniodental characters described by Chiquito (2015) are correlated with interparietal shape and, in combination, can be used to sort western Amazonian specimens into two distinct groups: (1) The dorsal exposure of the lacrimal bone at the anteromedial corner of the orbit is usually small in specimens with deep/narrow interparietals (fig. 22A), but the lacrimal tends to be conspicuously larger in specimens with shallow/wide interparietals (fig. 22B). (2) The suture between the parietal and the squamosal on the posterolateral braincase—above and behind the squamosal root of the zygomatic arch—is produced as a low crest in most specimens with deep/narrow interparietals (fig. 23A), but a sutural crest is absent in most specimens with shallow/wide interparietals (fig. 23B). (3) The incisive foramina in specimens with deep/narrow interparietals are parallel sided or taper anteriorly without interruption, and the median septum that divides the left and right foramina is uniformly narrow (fig. 24A); by contrast, in specimens with shallow/wide interparietals, the incisive foramina are laterally constricted anterior to the premaxillary/maxillary suture, and the median septum is conspicuously flattened or dimpled at a point corresponding to the lateral constriction (fig. 24B).

Adult specimens 16 of Nectomys from western Amazonia with deep/narrow interparietals, small lacrimals, crested parietal/squamosal sutures, and unconstricted incisive foramina include six with 2 n = 38–42 chromosomes ( LSUMZ 16711, 16715; MVZ 155019, 190373, 190374; USNM 513585), whereas western Amazonian adults

16 Some of these traits are not consistently developed in juveniles.

with shallow/wide interparietals, large lacrimals, uncrested parietal/squamosal sutures, and constricted incisive foramina include three with 2 n = 52 chromosomes ( LSUMZ 14372–14374). Both phenotypes occur in Loreto department, where they are also morphometrically divergent, especially in molar toothrow length (table 17). Unfortunately, we have not discovered any external character by which these species might be confidently identified in the field.

Peters’ (1861) illustration of the holotype skull of Nectomys apicalis is consistent with the application of this name to the western Amazonian species with deep/wide interparietals, small lacrimals, and unconstricted incisive foramina (the morphology of the posterolateral braincase is not sufficiently resolved in Peter’s figure to confirm the presence of a parietal-squamosal crest). As noted by Hershkovitz (1944: 25–26), the type was purchased at Guayaquil (presumably from a dealer in natural history curiosa), but it was almost certainly collected in eastern Ecuador. According to Chiquito’s (2015) revision, N. apicalis occurs in eastern Colombia, eastern Ecuador, eastern Peru, and eastern Bolivia. We additionally recognize Patton et al.’s (2000) material from the Rio Juruá in western Brazil as N. apicalis , whereas Chiquito refers their Juruá material to an unnamed taxon (“ Nectomys sp. B ”) that also includes two MVZ specimens from Balta (in Ucayali department, Peru). Molecular sequence data would be helpful in resolving inconsistencies between Chiquito’s restricted usage of N. apicalis and our more inclusive application of this name.

ETHNOBIOLOGY: The Matses name for the water rat is maka tanun (“gray rat”). No subtypes are recognized by the Matses, and they have no synonyms for this species. The water rat is a very minor game animal for the Matses. It is sometimes killed in deadfall traps baited with manioc. While some Matses eat them, others do not, and those who do eat them discard any that are infested with botfly larvae. Water rats are pests that feed on crops in swiddens surrounded by forest and on crops planted next to houses in