Plains viscacha rat
|Plains viscacha rat|
(B. Lawrence, 1941)
The plains viscacha rat is a moderately-sized rat, with a large head, long tail, and short ears. Adults measure about 13 cm (5.1 in) in total length, with a 15 cm (5.9 in) tail, and weigh an average of 90 g (3.2 oz), with males being slightly larger than females. The rat has buff-yellow fur with white underparts, fading to dark brown at the tip of the tail.
Distribution and habitat
The plains viscacha rat is endemic to central western Argentina, where it has a fragmented range in Mendoza Province and western La Pampa. Its natural habitat is desert scrubland, dunes and salt flats, between 300 and 1,400 m (980 and 4,590 ft). There are no recognised subspecies.
The species is threatened by destruction of its fragmented and restricted habitat.
Biology and behaviour
Plains viscacha rats are solitary, and nocturnal. They construct complex burrow systems within large artificial mounds. Typical mounds are 13.6 by 8.7 m (45 by 29 ft) across, and 1.25 m (4 ft 1 in) in height, and have an average of 23 burrow entrances. Within the mound, the burrow system has up to three levels and contains numerous chambers and dead-end tunnels.
The rats are herbivorous, feeding primarily on halophytic vegetation, such as Atriplex and Suaeda, although they will occasionally eat other plants such as grass. The rats scrape salt from the leaves of Atriplex saltbushes with their teeth and bristles around their mouths before eating them. Although this reduces their salt intake, they still produce highly concentrated urine to help maintain their water balance.
The young are born blind, and weighing about 4 g (0.14 oz). Their eyes open at about six days, and they begin to take solid food at ten days.
This species of rodent is unusual because it is tetraploid (4x = 2n = 102). Scientists think that this species may have arisen by hybridization and chromosome doubling from an ancestor (very possibly closely related to the mountain vizcacha rat, Octomys mimax, chromosome count 2x = 2n = 56). The doubling of its chromosome number was presumably by errors in mitosis or meiosis within the animal's reproductive organs. The animal's spermatozoa are roughly twice normal size by virtue of having twice as many sets of chromosomes.
The closest living relation of T. barrerae is Octomys mimax, the Andean viscacha-rat of the same family, whose 2n = 56. It was therefore surmised that an Octomys-like ancestor originally produced tetraploid (i.e., 2n = 4x = 112) offspring that were, by virtue of their doubled chromosomes, reproductively isolated from their parents, after loss of some chromosomes.
The golden vizcacha rat (Pipanacoctomys aureus) is also tetraploid and has 4x = 2n = 92 chromosomes. It is a sister-species to Tympanoctomys barrerae (4x = 102). In both cases the animals evolved from animals related to the diploid mountain vizcacha rat, Octomys mimax (2x = 2n = 56) as a result of doubling and then loss of some chromosomes.
New data point to a hybrid nature of the T. barrerae karyotype, suggesting a hybridization event in the origin of this species.
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