Tympanoctomys barrerae

1.

Red Viscacha Rat

Tympanoctomys barrerae

French: Octodon de Barrera / German: Rote Viscacharatte / Spanish: Rata vizcacha colorada

Other common names: Plains Viscacha Rat, Red Viscacha-rat

Taxonomy. Octomys barrerae Lawrence, 1941,

“La Paz, Mendoza Province, Argentina.”

This species is monotypic.

Distribution. Patchily in arid regions of W & C Argentina, known only from 13 localities in San Juan, Mendoza, La Pampa, and Neuquén provinces (between 29° S and 38° 8S).

Descriptive notes. Head-body 125-160 mm, tail 93-147 mm, ear 13-19 mm, hindfoot 30-36 mm; weight 52-104 g. Males tend to be larger than females. The Red Viscacha Rat has a relatively large head; cranial width is greater than cranial length due to extremely developed auditory bullae that are more than 45% of greatest length of skull. Dorsal pelage is reddish brown; ventral pelage is white. Tail is long (82% ofits head-body length) and terminates in a black brush. Whitish hairs cover forefeet and hindfeet. It has a specialized oral bundle of rigid hairs behind upper incisors. Kidneys are specialized with an elongated renal papilla able to concentrate urine at a maximum of 7080 mOsm/liter, similar to other desert rodents that forage on halophytic vegetation. The Red Viscacha Rat has a 2n = 102, FN = 200 karyotype, the largest in both diploid and fundamental number among mammals. Karyotype is composed of completely biarmed chromosomes that include 36 pairs of metacentric to submetacentric chromosomes, 14 pairs of subtelocentric autosomes, and a XY sex chromosome system. Sperm morphology is peculiar with a paddle-like sperm head, and sperm lengths are the largest reported for rodents. The Red Viscacha Rat was proposed to be the first tetraploid mammal known, but other studies presented evidence that its genomeis diploid rather than polyploid.

Habitat. Salt basins and sand dunes in the lowland habitats of Monte and Patagonian deserts at elevations of 300-1400 m. The Red Viscacha Rat lives in complex burrow systems built in soft soil associated with sand dunes and salty environments. Each burrow system has several entry holes that connect with internal tunnels and galleries. Orientation of holes changes seasonally; in winter, holes are mostly oriented toward sunlight, and galleries are constructed so that they receive direct sunlight in winter and indirect sunlight in summer.

Food and Feeding. The Red Viscacha Rat has a strictly and highly specialized herbivorous diet and feeds mainly on halophytic vegetation (e.g. Atriplex, Allenrolfea, Heterostachys, and Suaeda, all Amaranthaceae). Low proportions of species of true grasses (Poaceae) are detected in diets and are eaten in lower proportions than available. The Red Viscacha Rat scrapes surfaces of Atriplex leaves with lower incisors and oral bundle of rigid hairs behind upper incisors before consumption, which helps remove and discard salt excess on leaves and reduces salt ingestion.

Breeding. On average, captive female Red Viscacha Rats have four litters per year. Neonates have closed eyes and weigh 4 g at birth. At five days of age, they weighed 8 g and opened their eyes after six days. Juveniles start to consume plants at c.10 days old, although weaning is not complete. Juveniles scrape Atriplex leaves as their mothers do.

Activity patterns. The Red Viscacha Rat is normally considered to be nocturnal, but it is sometimes active during the day.

Movements, Home range and Social organization. The Red Viscacha Rat is a grounddwelling solitary species and occurs at low densities. One individual lives in each burrow system. Anecdotal reports indicate that a male and a female occupy the same burrow system, probably during mating; in another case, two males were trapped in the same burrow. Isolated, patchy populations of Red Viscacha Rats are, in some cases, separated by hundreds of kilometers.

Status and Conservation. Classified as Near Threatened on The IUCN Red List. Several attributes of the Red Viscacha Rat (e.g. patchy distribution, habitat and trophic specialization, and low densities) increase its vulnerability. Change from Vulnerable to Near Threatened since the last assessment reflected a better understanding of its biology within its distribution. In particular, occurrence and area of occupancy (new population records) were extended. Recent studies have detected genetic distinctiveness in some populations that could have important implications for conservation and management of the Red Viscacha Rat.

Bibliography. Berman (2003), Diaz & Ojeda (1999), Diaz et al. (2000), Gallardo, Bickham et al. (1999), Gallardo, Gonzalez & Cebrian (2006), Gallardo, Kausel et al. (2004), Gallardo, Sauthier et al. (2009), Giannoni et al. (2000), Grzimek (2004), Lawrence (1941), Mares, Braun & Channell (1997), Mares, Ojeda et al. (1997), Ojeda, A.A. (2010), Ojeda, A.A. et al. (2007), Ojeda, R.A. & Bidau (2013j), Ojeda, R.A. & Diaz (1997), Ojeda, R.A., Gonnet et al. (1996), Ojeda, R.A., Roig et al. (1989), Reca et al. (1996), Sauthier et al. (2009), Svartman et al. (2005), Torres et al. (2003), Torres-Mura et al. (1989), Verzi et al. (2002), Woods & Kilpatrick (2005), Yepes (1942).