513.

Argentinean Swamp Rat

Scapteromys aquaticus

French: Akodon des marais / German: Argentinien-Sumpflandratte / Spanish: Rata de ciénaga de Argentina

Taxonomy. Scapteromys aquaticus Thomas, 1920, Isla Ella, Delta del Parana, Buenos Aires, Argentina.

This species is monotypic.

Distribution. Parana and Uruguay river basins from S Paraguay to CE Argentina, extending marginally into S Brazil (Rio Grande do Sul State) and W Uruguay along E bank of Uruguay River.

Descriptive notes. Head-body 126-252 mm, tail 108-147 mm, ear 20-24 mm, hindfoot 35-43 mm; weight 64-155 g. The Argentinean Swamp Rat is moderately large, with tail equal to or slightly longer than head-body length. Pelage is thick, soft, and glossy; upperparts of head and body are somewhat gray to dark brown and underside of head, chest, and belly are more or less grayish white, washed with buff to ocherous. Tail is well-developed but not thick, covered by stiff but not hispid dark brown pelage; ventral keel hairs are usually long; and terminal tuft is present although composed of a handful of long hairs. Ears are small and rounded, covered by hairs on both surfaces. Forefeet are well-developed but not markedly enlarged, second to fifth digits are strong, and claws are slender and of equal length. Hindfeet are strong and elongate, their length (including claws) is ¢.24% of head—body length, and claws are slender but powerful. There are narrow bands of webbing between first phalanges of three middle digits and along inner side of each outer digit. The Argentinean Swamp Rat is slightly smaller than the Uruguay Swamp Rat (S. tumidus). Chromosomal complement is 2n = 32, FN = 40.

Habitat. Flooded places and coastal wetlands associated to the Paraguay-Parana-Plata and Uruguay river systems, including those covered by moderate to dense riparian or gallery forests. Understory cover is an important aspect of habitats preferred by the Argentinean Swamp Rat, abundant in floodplains thickly covered with pampas grass ( Cortaderia, Poaceae) and the sedge Scirpus giganteus ( Cyperaceae).

Food and Feeding. The Argentinean Swamp Rat apparently prefers to eat oligochaete worms rather than beetles. It detects prey by smell and then captures it by rooting and digging. After an earthworm is exposed, the Argentinean Swamp Rat seizes and immobilizes it by gashing the worm’s body with its incisors. The worm is then bitten into segments, chewed thoroughly, and noisily and quickly swallowed. Bait preferences displayed by the Argentinean Swamp Rat suggest a tendency to omnivory.

Breeding. Gravid female Argentinean Swamp Rats were recorded in November and January, but none had full term fetuses. In late January, young individuals were recorded with eyes still closed, and females with two well-developed fetuses were captured. These findings suggest that reproductive period is late spring to summer. Litters have 2—4 young. A lactating female was found nesting on the ground hidden only by thick vegetation.

Activity patterns. The Argentinean Swamp Rat is crepuscular and nocturnal. Although usually typified as semi-aquatic sigmodontines, Argentinean Swamp Rats are mostly captured in terrestrial habitats and not directly from water bodies or daily flooded areas. It dives readily and swims easily and swiftly; while swimming, only its head and anterior portion of back are exposed. It propels itself by horizontal undulations of its tail, rowing movements of hindfeet, and short paddling motions of forefeet. Fur remains nearly completely dry during immersion. Fossorial proclivities of the Argentinean Swamp Rat are not remarkable. Although its claws are long and stout, they are primarily adapted for securing food. The Argentinean Swamp Rat is a good climber. Its scansorial ability combined with swimming skill permit it to live on floodplains where tall plants provide a haven from floods. Long claws and flexible tail are particularly useful in climbing. A captive individual leaped up and hooked its claws into mesh walls of its cage. The Argentinean Swamp Rat keeps tail and hindlegs on the ground, even when grooming or feeding.

Movements, Home range and Social organization. Density variation was limited despite seasonal reproduction, with major intensity during spring and summer. Known individuals persisted in a sampling area for more than 20 months, which suggests minimal longevity of c.2 years. Density, residency, and reproduction were higher in microhabitats with extensive vegetative cover at least 0-50 m high. A variety of vocalizations were recorded, including sharp cry by nestling young and sonorous adult call. Suckling young emit very high pitched “chi-chi’ or “chio-chio.” Adult vocalization is lower pitched. Two captive adults uttered a slow “ro-ro-ro,” which became a shrill “creccrec” when an individual was excited or harassed; it can rise to a higher and sustained “cree... ec’ if an individual was cornered or in pain. The Argentinean Swamp Rat has a highly developed sense of hearing and can perceive sounds of extremely high wave lengths. It is easily startled and seeks safety by plunging into the nearest water. It is an attractive animal in captivity. It seems to tame easily but remains shy. It does not hesitate to bite when seized and is cannibalistic, at least in captivity.

Status and Conservation. Classified as Least Concern on The IUCN Red List.

Bibliography. Bianchini & Delupi (1993), Bonvicino et al. (2013), Brum-Zorrilla et al. (1986), Contreras (1966), Cueto, Cagnoni & Piantanida (1995), D'Elia & Pardinas (2004, 2015, 2016b), Hershkovitz (1966a), Massoia (1961), Massoia & Fornes (1964a, 1965b), Thomas (1920b).