As discussed in Why is polyploidy lethal for some organisms while for others is not?, polyploidy is normally lethal in mammals.
However, two species of Octodontidae (South American rodents), are tetraploid due to a recent doubling of all chromosomes:
- Tympanoctomys barrerae: 4x = 102
- Pipanacoctomys aureus: 4x = 92 (apparently some chromosomes were lost after the polyploidy event)
See the paper discussing T. barrerae, the first of these tetraploid rodents discovered.
According to the Wikipedia article, both are believed descended from the same species, Octomys mimax (or possibly now-extinct close relatives thereof), which has 2x = 2n = 56 chromosomes, half those of T. barrerae.
What was special about Octomys that allows it to survive polyploidy, unlike most mammals?
The sperm head of Tympanoctomys is is by far the largest naturally occurring in mammals, and its size is causally related to the double genome size it has to accomodate.