CRICETIDAE AND MURIDAE Rats and Mice FAUNA PARAGUAY

SUPERFAMILY MUROIDEA - FAMILIES: CRICETIDAE AND MURIDAE - RATS AND MICE
Thirty-four species in Paraguay, rats and mice are worldwide in distribution and make up the huge mammalian superfamily Muroidea. Cricetidae, which includes the native American Sigmodonts, was formerly considered a subfamily of a larger Muridae but genetic studies have shown that the differences deserve recognition at the familial level. Instantly recognisable, certain less obvious characters define the Rats and Mice. They lack both canines and premolars and cheek teeth are variable in cusp pattern, being laminate, prismatic or cuspid. The infraorbital foramen is small, the bulk of the masseter muscle attaching to the zygomatic arch and rostrum. The frontal lacks a postorbital process. Cusps when present are aligned in two longitudinal rows in the Sigmodontinae and three in the Muridae. Lifestyles are extremely diverse, having both diurnal and nocturnal species and filling all the major niches, terrestrial, arboreal, semi-aquatic and fossorial. Many species directly impact upon humans both as carriers of disease and because of a destructive effect on agriculture. The Paraguayan Cricetids can be split into four poorly-defined tribes whilst the Murids are all introduced commensals of man. Below is a summary of the characteristics of each tribe and their component genera.

FAMILY CRICETIDAE - "Cricetids"

Subfamily Sigmodontinae - Sigmodonts
Sigmodonts are confined to the New World. All have a double articulation on the first rib. They lack an entepicondylar foramen on the humerus and an entoglossal process on the basihyal.

TRIBE ORYZOMYINI "RICE RATS" - Pelage soft. Manual claws small and lacking keels. Sparsely-haired tail covered with inconspicuous epidermal scales, lacks terminal tuft and is at least as long as the head and body and usually longer. Eight pairs of teats in inguinal, abdominal, postaxial and pectoral pairs. Labial flexi enclosed by cingulum. Males with several reproductive gland complements. Enamel of upper incisors smoothly rounded. Molars are not usually complex, are low-crowned bunodont or terraced, and M3 is large. Mesolophs are present on all upper molars. Maxillary tooth rows are parallel. Median mure connected to protocone on M1. Paracone on M2 lacks accessory loph. Posteroflexoid present on M3. Squamosal lacks posterior suspensory process. Twelve ribs. Zygomatic plate lacks anterodorsal spiny process. Nasal bones with rounded or square posterior margins. Smooth posterior wall of orbit. Alisphenoid canal with large anterior opening. Bony palate between molars is smooth. Unilocular hemiglandular stomach and no gall bladder. Nine Paraguayan genera and fourteen species now recognised following a recent review of the group. Holochilus differs in many respects from the typical members of the tribe (Weksler et al 2006).
Oryzomys "Rice Rats" - Mouse-like. Pelage coarse but not bristly or spiny. Tail long, bicoloured and sparsely furred. Females with 8 mammae. Conspicuous ears. Baculum complex.
Oligoryzomys ·Tufted Rice Rats" - Smaller than, but similar to Oryzomys, and frequently included within it. Recognised by silvery tufts of bristles on the four longest hind toes that extend beyond the claw. Skull less robust with less marked crests and larger bullae. Externally with strong contrast between dorsal and ventral colouration. Brain case moderate with interorbital region shaped like an hourglass. Short snout.
Oecomys "Bicoloured Rice Rats" - Similar to Oryzomys and frequently included within it, but separable by karyological features. Brain case is larger and more rounded. Anterior margin of maxillary palate does not project so far forward when viewed dorsally. Feet broad, showing adaptations for climbing. Fifth hind toe is proportionately long. Paracone on M2 with accessory loph.
Sooretamys - Humid forests. Dorsal colouration sharply demarcated from ventral. Ears small not reaching eye when folded forwards. Feet with conspicuous tufts at base of claws on all digits. Soles densely-covered with squamae. Long, broad rostrum flanked with deep zygomatic notches. Interorbital hourglass-shaped. Nasals short but not extending back beyond lachrymals.
Euryoryzomys - Humid forests. Dorsal colouration sharply demarcated from ventral. Large ears reach eye when folded forwards. Hind foot with conspicuous tufts of hairs at bases of claws of digits 2-5. Tail bicoloured, dark above and pale below. Long, tapering rostrum with deep zygomatic notches. Interorbital region converges anteriorly. Occipital surface of mastoid capsule ossified.
Hylaeamys - Humid forests. Similar to Euryoryzomys but smaller and duller in colouration of dorsal pelage. Tail indistinctly bicoloured or unicoloured. Occipital surface of mastoid capsule fenestrated.
Nectomys "Neotropical Water Rats" - Nocturnal and semi-aquatic. Pelage with long, glossy guard hairs. Keel of short stiff hairs on underside of tail. Hind foot large and webbed, with middle three digits longest and a distinct fringe.
Pseudorysomys "Ratos-do-Mato" - Semi-aquatic. Moderate size. Long tail. Small ears. Fully naked soles. Well developed webbing on large hind feet. Mesolophs are not present on all upper molars. Posteroflexoid absent on M3.
Holochilus "Marsh Rats" - Semiaquatic. Short, hairy ears. Tail scarcely furred with obvious, rough-scaled rings. Three central toes longest. Nails exposed. Hind feet with interdigital membranes. Labial flexi unenclosed. Skull robust, with short, wide nasals and postorbital ridge. Interorbital elongated towards the front with well developed crest. Superorbital, temporal and lamboidal crests all well-developed. Well developed keel on palate between molars. Tympanic bullae small in comparison with other Cricetids. Zygomatic plate with anterodorsal spiny process. Median mure connected to paracone on M1. Large, strong opisthodont incisors with bright orange colouration. Enamel of upper incisors faceted. S-shaped, crowned planar molars. Mesolophs are not present on all upper molars. Maxillary tooth rows converge anteriorly. Posteroflexoid absent on M3.

TRIBE PHYLLOTINI - No one character is unique to this poorly-delimited group. Heel of hind foot is furred. Ears moderate to long. Incisive foramina long. Zygomatic arch deeply excised. Parapterygoid fossae wider than mesopterygoid fossae. Two pairs of prepucial glands. Molars tetralophodont. Molar 3 greater than half the length of Molar 2. Three genera and five species in Paraguay.
Calomys "Vesper Mice" - Nocturnal and terrestrial. Feet relatively small. Skull with elongated snout. Interorbital region with angular borders or slightly rounded. Opisthodont incisors. Tetralophodont molars with alternating tubercular crests. White patch behind ears. Tail equal to or less than head and body length. External ears moderately-sized. Superficial resemblance to introduced Mus.
Andalgalamys "Laucha Colilargas" - Terrestrial in dry habitats. Large ears. Tail equal to or longer than head and body length, bicoloured and sparsely furred. Soles of feet naked. Skull like Calomys but more slender rostrum and larger, more globular bullae.
Graomys - Scansorial and able to adapt to different habitats. Large size. Ears and feet fairly large. Ventral sharply demarcated from dorsal pelage. Well haired tail longer than head and body and with bushy ending. Skull with broad supraoccipital with protruding edges. Fairly long wide snout. Zygomatic plate with moderately developed spine. Opisthodont incisors.

TRIBE AKODONTINI - Terrestrial or semi-fossorial in behaviour. Tail shorter than head and body length. Long claws. Eyes and ears of modest size. Rounded interorbital region with a poorly developed border. Zygomatic plate well developed. Palatine is broad and ends parallel to or slightly behind the third molar. Unspecialised digestive system hinting at an omnivorous diet. Molars not high-crowned. Five genera and seven species in Paraguay.
Akodon and Thaptomys "Akodons" - Heavy bodied, vole-like mice. Small ears and short feet. Pelage soft and full. Rounded brain case with nasal bones as large as or larger than frontal. Interparietal is reduced. Mandible slender. Molars relatively small and simplified, tetralophodont. Third molar reduced in size. Female with 8 mammae.
Bolomys and Necromys "Field Mice" - Colouration typically agouti, most pronounced on sides and slightly paler below. Tail bicoloured and tipped with fine hairs. Broad hands and feet with long claws. Brain case broad and deep. Short occipital. Short snout, tapering when viewed from the side. Skull slopes forward from the middle of the parietals. Frontals longer than nasals. Parietals less than half the length of frontals and extending forward as narrow spines between frontals and temporals. Posterior palate moderately long and wide. Zygomatic plate broad and strong with anterior border perpendicular to diastema. Broad, robust molars mesodont. Lower molars with lingual cusps anterior to labial ones. Upper molars with lophs almost completely transverse.
Oxymycterus "Hocicudos" - Semi-fossorial in behaviour. Elongated and mobile snout and long foreclaws. Tail moderately furred. Hind feet and tail short. Outer digits of hind feet shorter than inner three digits. External ears small. Weak dentition. Molars with simple tetralophodont pattern. Skull elongated, nasal and premaxillaries extending forward in a form of tube. Zygomatic plate elongated. Incisors are opisthodont.

TRIBE SCAPTEROMYINI - Semi-aquatic and fossorial in behaviour. Ears short and eyes small. Skull robust with a complex molar structure. One genus and one species in Paraguay.
Scapteromys "Water Rats" - Semi-aquatic and largely nocturnal or crepuscular in habits. Thickset and heavy, resembling Rattus in appearance but longer tailed. Large head, stout body, large feet and tail with a stiff fringe of hairs. Middle toes of hind feet elongated. Long claws. Pelage long and glossy. Females have eight mammae.

Click here to view the Cricetidae Comparative Skull Gallery

FAMILY MURIDAE - Old World Murids
Two introduced genera with an almost worldwide distribution as a result of their close association with mankind.
Mus "Old World Mice" - Small. Tail naked with covering of fine hairs. Upper incisors notched. Hind feet narrow with shortened outer digits. Skull is lightweight and flattened.Length of first upper molar greater than combined lengths of Molars 2 and 3.
Rattus "Old World Rats" - Large. Incisors not notched. Length of first upper molar less than combined lengths of Molars 2 and 3.
Click here to view the Muridae Comparative Skull Gallery

REFERENCES
Diaz MM & Barquez RM 2002 - Los Mamíferos de Jujuy, Argentina - LOLA
Emmons LH & Feer F 1999 - Mamíferos de los Bosques Húmedos de América Tropical - FAN Bolivia
Nowak R ed 1991 - Walker´s Mammals of the World 5th Ed Volume 2 - Johns Hopkins
Redford K 1992 - Mammals of the Neotropics Vol 2: The Southern Cone Chile, Argentina, Uruguay and Paraguay - University of Chicago Press.
Weksler M, Percequillo AR, Voss RS 2006 - Ten New Genera of Oryzomyine Rodents (Cricetidae: Sigmodontinae) - American Museum Novitates.