It was described as the first known tetraploid (4x = 2n) mammal, thought to 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). Some later studies have cast doubt on its tetraploid nature, while others have reasserted it. The doubling of its chromosome number was presumably by errors in mitosis or meiosis within the animal's reproductive organs. A comparison of the chromosomes of the plains viscacha rat and the mountain viscacha rat suggested that the chromosomes of the plains viscacha rat increased relatively rapidly (in evolutionary terms) due to a diverse set of highly repetitive elements. The animal's spermatozoa are roughly twice normal size, thought to be by virtue of having twice as many sets of chromosomes.