With the highest number of chromosomes known for a mammal, the plains viscacha rat is a genetic marvel (3). Yet the relatively unexceptional appearance of this rodent goes to show that eccentric genetics are not necessarily outwardly expressed. In most respects it resembles other species within the octodontidae, being a medium-sized rat with buff yellow fur over its back and head, and white fur on its throat and belly (2). Its tail is fairly long and is covered in lengthy hairs that are reddish brown towards the tip (2)(4). Like other rodents adapted to open, arid environments, it has inflated tympanic bulla, that give it the appearance of having a large head (2)(4)(5), and are thought to amplify sound to allow early detection of predators (6).
Although the plains viscacha rat is normally considered to be nocturnal, it is also sometimes active during the day (2)(5)(8). It is a solitary animal that lives in complex burrows, excavated out of large, soft soil mounds. These mounds often comprise numerous holes that access an intricate network of tunnels connected to food chambers. With a strictly herbivorous diet, it specialises on the leaves of the halophytic plants that are prevalent on the salt basins and sand dunes it inhabits. In adaptation to the high concentrations of salt found on the leaves of salt bush (Atriplex sp.), the plains viscacha rat has developed bundles of rigid hairs on each side of its mouth which it uses to remove and discard the salt-laden outer leaf layer (2)(5). Other adaptations it exhibits to feeding on plants with a high salt content include chisel-shaped lower incisors that are also employed in scraping away unwanted salt, and kidneys capable of processing high salt loads (5).
As a habitat specialist occurring in low densities in fragmented populations over a very restricted range, the plains viscacha rat is extremely vulnerable to habitat loss. Given that oil and gas industries in Argentina are putting increasing pressure on the remaining patches of suitable habitat, this rodent is currently listed as Near Threatened on the IUCN Red List (1).
There are no conservation measures in place for the plains viscacha rat but it is protected in the Biosphere Reserve of Nacunan (1) and the Ischigualasto Reserve in San Juan (8). Ongoing research indicates that to conserve genetic diversity in the plains viscacha rat, it may be important to ensure the protection of some of the more isolated populations (9).
Contreras, L.C., Torres-Mura, J.C. and Spotorno, A.E. (1990) The largest known chromosome number for a mammal in a South American desert rodent. Experientia, 46: 506 - 508.
Nowak, R.M. (1999) Walker's Mammals of the World. Johns Hopkins University Press, Baltimore, Maryland.
Ojeda, R.A., Borghi, C.E., Diaz, G.B., Giannoni, S.M., Mares, M.A. and Braun, J.K. (1999) Evolutionary convergence of the highly adapted desert rodent Tympanoctomys barrerae (Octodontidae). Journal of Arid Environments, 41: 443 - 452.
Squarcia, S.M., Sidorkewicj, N.S. and Casanave, E.B. (2007) The hypertrophy of the tympanic bulla in three species of dasypodids (Mammalia, Xenarthra) from Argentina. International Journal of Morphology, 25(3): 597 - 602.
Ojeda, A.A., Gallardo, M.H., Mondaca, F. and Ojeda, R.A. (2007) Nuevos registros de Tympanoctomys barrerae (Rodentia, Octodontidae). Mastozoologia Neotropical, 14: 267 - 270.
Ojeda, A.A. (2009) Pers. comm.
Ojeda, A.A. (2009) Phylogeography of the South American rodent Tympanoctomys barrerae (Rodentia, Octodontidae) along its geographic range. Fourth Biennial Meeting of the International Biogeography Society, 8-12 January 2009, Merida, Yucatan, Mexico.
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