CHACO FAT-TAILED OPOSSUM Thylamys pusillus FAUNA PARAGUAY

Thylamys pusillus (Desmarest, 1804)
TAX: Class Mammalia; Subclass Theria; Infraclass Metatheria; Order Didelphimorphia; Family Didelphidae; Subfamily Didelphinae, Tribe Monodelphini (Myers et al 2006, Gardner 2007). The genus Thylamys was defined by Gray, 1843 and Palma et al (2002) concluded that the genus was differentiated during the Pleistocene. The species currently recognised as Thylamys correspond roughly to the "elegans-group" as defined in the monography by Tate (1933). There are nine known species according to the latest revision (Gardner 2007) two of which are present in Paraguay. The genus Thylamys is from the Greek meaning "pouched mouse". The species name pusillus is Latin meaning "very small or tiny". The species is monotypic, though Gardner (2007) notes that the taxon requires revision. No holotype was designated (Brown 2004). The species was described by Desmarest (1804) as Didelphis pusilla, based on de Azara´s (1801: 290) account of two males which he called "Le Micouré nain" and with type locality "Saint Ignace-Gouazou". Tate (1933) restricted this to San Ignacio Guazú, Departamento Misiones, Paraguay. De Azara´s original description was vague and fails to conclusively identify this species based on our current knowledge of Didelphids. Additionally the type locality given is in eastern Paraguay where this species apparently does not occur, or at least where it has never been conclusively recorded. It seems that de Azara´s description may have been referring to another species, but due to its vagaries it was was later applied by Desmarest and others to what we now know to be Thylamys pusillus. Much of the confusion in the literature of the late 19th and early 20th Century can be attributed to O.Thomas (1888) who misapplied the name pusillus to the unrelated Gracilinanus microtarsus (Wagner 1842). Tate (1933) used the name Marmosa marmota verax in the same sense that Thylamys pusillus is used here and his Marmosa pusilla appears to be based at least in part on this species. Cabrera (1958) included Marmosa marmota as a synonym of Marmosa grisea (Bertoni 1914) =Thylamys macrurus. To further complicate matters O.Thomas (1896) used Marmosa marmota for material from Corrientes, Argentina which included an immature individual which he had previously described as "Micoureus griseus Desmarest 1827". He later received more material from the same location that he initially attributed to Marmosa marmota, but then later used as the basis of his Marmosa citella which is now considered a junior synonym of Marmosa marmota, which in turn is a junior synonym of Thylamys pusillus. Hershkovitz (1959) considered the species to be conspecific with Thylamys macrurus under the name Marmosa pusilla. Palma et al (2002) concluded from their molecular analysis that this species was derived from the more primitive Thylamys macrurus following a migration of that species across the Rio Paraguay. Synonyms adapted from Gardner (2007):
Didelphis pusilla Desmarest 1804:19. Based on de Azara (1801). Type locality "Saint Ignace-Gouazou" =San Ignacio Guazu, Departamento Misiones, Paraguay.
[Didelphys] nana Illiger 1815:107. Nomen nudum.
Didelphys nana Oken 1816:1140. Name unavailable.
[D]idelphys. nana Olfers 1818:206. Based on Desmarest (1804).
Sarigua pusilla Muirhead 1819:29. Name combination.
Didelphys [(Grymaeomys)] pusilla Burmeister 1854:140. Name combination.
Philander pusillus Cope 1889:130. Name combination.
Micoureus pusillus Ihering 1894:11. Name combination.
Micoureus griseus O.Thomas 1894:184. Not Didelphis grisea Desmarest (1827) and later redescribed as Marmosa citella O.Thomas (1912).
M[armosa]. pusilla O.Thomas 1896:314. Name combination.
Marmosa marmota O.Thomas 1896:313-314. Based on Didelphys marmota Oken (1816) but indication to O.Thomas (1894).
[Didelphys (Marmosa)] pusilla Trouessart 1898:1240. Name combination.
Marmosa citella O.Thomas 1912:409. Type locality "Goya, Corrientes, Argentina" =Micoureus griseus.
[Didelphis (Thylamys)] citella Matschie 1916:271. Name combination.
[Marmosa (Marmosa)] pusilla Cabrera 1919:38. Name combination.
[Marmosa (Thylamys)] citella Cabrera 1919:40. Name combination.
[Marmosa (Thylamys)] marmota Cabrera 1919:40. Name combination.
Marmosa verax O.Thomas 1921:520. Type locality "Mission west of Concepción, Paraguay"
Marmosa marmota verax Tate 1933:220. Name combination.
Marmosa janetta pulchella Cabrera 1934:126. Type locality "Robles, Santiago del Estero, Argentina"
[Marmosa (Thylamys)] pusilla Cabrera 1958:32. Name combination.
M[armosa]. pusilla verax Wetzel & Lovett 1974:206.Name combination.
[Thylamys] pusillus Reig, Kirsch & Marshall 1987:7. First use of modern name.
Thylamys pusilla Brown 2004:193. Incorrect gender.
ENG: Chaco Fat-tailed Opossum, Chacoan Thylamys (Gardner 2007), Common Fat-tailed Mouse Opossum (IUCN 2009), Small Fat-tailed Mouse Opossum (IUCN 2009), Common Mouse Opossum (Canevari & Vaccaro 2007), Small Fat-tailed Opossum (Canevari & Vaccaro 2007).
ESP: Marmosa común (Massoia et al 2000, Redford & Eisenberg 1992), Comadrejita enana (Chebez 1996, Massoia et al 2000), Comadreja enana (IUCN 2009), Comadrejita común (Canevari & Vaccaro 2007), Comadrejita enana común (Canevari & Vaccaro 2007).
GUA: Anguyá guakí (Massoia et al 2000, Canevari & Vaccaro 2007).
DES: A small mouse opossum with short, dense, smooth fur. This species is almost identical in external appearance to Thylamys macrurus and is best separated by body and tail measurements. Dorsally they are darkest, being greyish-brown, fading to light grey on the flanks. Darker dorsal colouration extends down the centre of the head as a medial line. Ventrally they are creamy-white or pure white. Eyes large, surrounded by a poorly-defined narrow black patch that extends only slightly in front of the eye and onto the cheeks. Ears large, naked and reddish-brown in colour. Tail prehensile and c1.14x head and body length. It appears essentially naked without magnification. Colour brownish-grey on the dorsal side basally, greyish distally and pale greyish on the ventral side for its entire length and lacks a whitish tip. Tail scales are tiny rounded or square in shape and arranged in rings with three hairs pair scale. The prehensile tip lacks hairs. Fat is stored in the tail and some degree of incrassination is usually visible throughout the year. Limbs are grey dorsally and whitish ventrally, those of the forelimbs paler dorsally. Feet are small and white with ungual tufts and short digits. The toes are long with short claws that do not extend beyond the apical pads of the forefeet, but are slightly longer on the hindfeet. Six separate pads are present on each foot, with large granules and dermatoglyphs on the palmar and plantar surfaces. Females lack a marsupium and have bilaterally symmetrical rows of 15 teats, most inguinal in position with two pectoral pairs. (Canevari & Vaccaro 2007, Carmignotto & Monfort 2006, Solari 2003). CR - Skull robust, with broad zygomatic arch and slender rostrum. Brain case moderately broad. Nasals long and rounded posteriorly. They show slight expansion at the maxillo-frontal suture, narrowing slightly behind it and the lateral margins are subparallel. Supraorbital crests well-developed and pronounced postorbital constriction. Palate long. Posterolateral palatal foramina are large and exceed the lingual apices of the protocones of M4. Tympanic bullae well-developed. Carmignotto & Monfort (2006) give the following measurements for 7 specimens (6 males, 1 female) from Paraguay without distinguishing between the sexes: Basal Length: 26.31mm (+/-0.83); Greatest Cranial Length: 26.83mm (+/-0.94); Greatest Cranial Height: 9.20mm (+/-0.28); Width of Braincase: 10.45mm (+/-0.26); Greatest Zygomatic Width: 15.06mm (+/-0.46); Length of Nasals: 11.01mm (+/-0.4); Width of Nasals: 2.47mm (+/-0.21); Palate Length: 13.76mm (+/-0.79); Palate Width: 8.24mm (+/-0.32); Interorbital Constriction: 4.00mm (+/-0.29); Postorbital Constriction: 4.59mm (+/-0.24); Width of Rostrum: 4.38mm (+/-0.42); Width Across Bullae: 9.95mm (+/-0.34); Width Between Bullae: 4.16mm (+/-0.26); Bullae Width: 2.60mm (+/-0.08); Minimum Pterygoid Bridge Width: 2.32mm (+/-0.17); Mandibular Length: 19.49mm (+/-0.80). Mares & Braun (2000) give the following combined measurements for 5 specimens (3 males, 2 females) from Argentina: Greatest Cranial Length: 27.4mm (25.8-30.4mm); Condylobasal Length: 27.3mm (25.4-29.8mm); Width of Braincase: 10.5mm (9.9-11mm); Greatest Zygomatic Width: 15.5mm (14.9-16.5mm); Palate Length: 13.7mm (12.3-15.7mm); Interorbital Constriction: 4.2mm (3.7-4.7mm); Mandibular Length: 19.8mm (18.5-21.7mm). DF: I5/4 C1/1 P 3/3 M 4/4 = 50. Molar rows convergent and compressed antero-posteriorly. Canines well-developed but short. Carmignotto & Monfort (2006) give the following measurements for 7 specimens (6 males, 1 female) from Paraguay without distinguishing between the sexes: Length of Upper Toothrow: 10.13mm (+/-0.29); Length of Upper M1-M4: 5.09mm (+/-0.08); Length of Mandibular Row of Molars: 5.65mm (+/-0.11); Upper Canine Length: 1.97mm (+/-0.30); Upper Canine Width: 1.14mm (+/-0.18); Upper P3 Length: 1.26mm (+/-0.05). Mares & Braun (2000) give the following combined measurements for 5 specimens (3 males, 2 females) from Argentina: Length of Upper Toothrow: 9.9mm (9.2-10.9mm); Length of Mandibular Row of Molars: 10.2mm (9.5-11.6mm). CN: 2n=14; FN=20. X chromosomes are small submetacentrics and Y chromosome absent in somatic cells (Palma 1995).
TRA: No information.
MMT: The smaller of the two Paraguayan Thylamys. Carmignotto & Monfort (2006) give the following measurements for 7 specimens (6 males, 1 female) from Paraguay without distinguishing between the sexes: HB: 10.23cm (+/-0.93); TA: 11.65cm (+/-0.96) approximately 1.14x head and body length; FT: 1.38cm (+/-0.08); EA: 2.14cm (+/-0.16); WT: 25.03g (+/-6.15). Mares & Braun (2000) give the following combined measurements for 9 specimens (4 males, 5 females) from Argentina: TL: 20.29cm (17.3-27cm); HB: 9.63cm (7.7-12.5cm); TA: 10.66cm (9.1-14.5cm); FT: 1.32cm (1.1-1.6cm); EA: 2.14cm (1.5-2.6cm); WT: 18.7g (13-29g).
SSP: Members of the genus Thylamys can be distinguished from other mouse opossums by their noticeably bicoloured pelage, being darker dorsally and paler laterally, and by the densely-granulated soles of the feet. Furthermore female Thylamys have the teats arranged in bilaterally symmetrical rows and not in a circular pattern as in other mouse opossums. The only other member of the genus present in Paraguay is Thylamys macrurus, which is principally distinguished by its larger size, being typically >40g in weight and with a tail typically greater than 135mm in length (compared to <35g and tail much less than 135mm in this species). The two species of Thylamys are apparently allopatric, this species being so far only conclusively recorded in the Chaco west of the Rio Paraguay, whilst T.macrurus appears to be confined to eastern Paraguay. Note that this species does not show a whitish tip to the tail which is usually present in T.macrurus. Cranially the posterolateral foramen exceeds M4 in this species, though it does not in T.macrurus.
DIS: The geographical range of this species is poorly known and further confused by various taxonomic changes and errors in the published literature. Carmignotto & Monfort (2006) consider the species to be adequately documented only from southern and eastern Bolivia, the Chaco of Paraguay, northwestern Argentina and Uruguay. Their examination of the specimens proved that published records of this species from northeastern and central Brazil (Gardner 1993, Eisenberg & Redford 1999) were actually referrable to Thylamys karimii, which was formerly considered conspecific, whilst Schaller´s (1983) records of "Marmosa pusilla" from Matto Grosso do Sul, are in fact Gracilinanus agilis. They consider that the species has yet to be documented for Brazil. They examined eight specimens from Paraguay from the following locations, all in the Paraguayan Chaco: 11km N of Filadelfía, Departamento Boquerón (FMNH 164095 & FMNH 164096); 295km NW by road from Villa Hayes, Departamento Presidente Hayes (MVZ 144310); 410km NW by road from Villa Hayes, Departamento Boquerón (MVZ 144312 & MVZ 144313); 460km NW by road from Villa Hayes, Departamento Boquerón (MVZ 144311); Estancia Toledo, 35km W of Filadelfía, Departamento Boquerón (FMNH 164097); Dr. Pedro P Peña, Departamento Boquerón (USNM 390027). The type locality is San Ignacio-Guazú, Departamento Misiones, Paraguay. This locality is in eastern Paraguay apparently outside the known range of this species, and no bona fide specimens have ever been found east of the Rio Paraguay. See the taxonomic section for notes on the vagaries of de Azara´s original description and the reasons for suspecting that de Azara may have been referring to a different species given our current understanding of Didelphid taxonomy. Though Gardner (2007) maps the species for Provincia Misiones, Argentina its occurrence there was considered hypothetical by Chebez (1996) who noted that the only evidence for its occurrence are remains of a species similar to Thylamys recovered from the pellets of a Barn Owl (Massoia et al 1989). Mares & Braun (2000) note that in Argentina the species is known from Provincias Chaco, Formosa, Santiago del Estero, Corrientes, Entre Rios and Salta.
HAB: This species is confined to xeric, wooded, scrubby and frequently thorny habitats in the Paraguayan Chaco. It is semi-arboreal in behaviour (Cannevari & Vaccaro 2007).
ALI: Considered primarily an insectivore with omnivorous tendencies (Cannevari & Vaccaro 2007), though no detailled information has ever been published on the diet of the species. Fruit likely plays a significant part in the diet at least seasonally. The species is able to accumulate fat in the tail during times of plentiful resources, which help it to survive leaner times in its harshly seasonal environment.
REP: Little published information. Canevari & Vaccaro (2007) state that females apparently reproduce twice a year giving birth to 14 or 15 young per brood. Mares & Braun (2000) note lactating females in January and November in Provincia Santiago del Estero
BEH: Activity Levels Considered nocturnal and arboreal in behaviour (Cannevari & Vaccaro 2007). Specimens have been taken on the ground in pitfall traps as well as in trees and shrubs (Gardner 2007). Roosts This species spends the day in burrows holes or abandoned nests (Cannevari & Vaccaro 2007). One burrow was located under a cactus and in Salta, Argentina the species continued to be active even when there was snow on the ground. (Eisenberg & Redford 1999). Parasites Limardi (2006) listed the ectoparasite Didelphoecius squammatus (Acari: Astigmata: Atopomelidae) from Brazil.
VOC: No information.
HUM: None.
CON: Globally considered to be of Low Risk Least Concern by the IUCN, click here to see their latest assessment of the species because of its wide distribution, occurrence in protected areas and large population. The species is apparently common in the Paraguayan Chaco, though rarely encountered other than by trapping studies and given the isolated nature of its habitat and low human population pressure it is currently under not threat in Paraguay. It probably occurs in most if not all of the protected areas in the Dry Chaco, but to date has been documented only for PN Rio Negro, Departamento Alto Paraguay (questionably) and the now defunct PN Tinfunque, Departamento Boquerón/Presidente Hayes (Yanosky 1998). However the species has likely declined as a result of deforestation in the areas around the Mennonite colonies in the Central Chaco and will continue to do so as the agricultural frontier expands in the region.
Citable Reference: Smith P (2009) FAUNA Paraguay Online Handbook of Paraguayan Fauna Mammal Species Account 32 Thylamys pusillus.
Last Updated: 30 June 2009.
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ACKNOWLEDGEMENTS
Special thanks to Juan Carlos Chebez for providing important literature and Nilton Cáceres for very kindly reviewing texts and providing a copy of his book Os Marsupiais do Brasil.