The Isthmus of Panama has not always been a true land bridge. For many millions of years before its formation, the two continents of the Americas were quite separate and distinct. When the connection first came to be, around 3 million BC in the Late Neogene (Pliocene epoch), terrestrial and freshwater animals migrated from north to south, while other terrestrial and freshwater animals spread into the opposite direction into North America. After this event (the Great American Interchange), one of the few South American mammals to have flourished in North America was the opossum.
Now, in 5 million AD, another South American mammal has made the journey--the rattleback. The rattleback evolved from the paca (Cuniculus), a large ground-dwelling rodent (comprising of three species) found in a wide variety of habitats in South America and a small portion of North America. South American rattlebacks were highly successful on the dry grasslands that were spreading across the Amazon Basin. So much so, in fact, that they were able to migrate northwards into the North American Desert. There, the rattleback evolved into yet another species, one adapted for the cold desert environment.
At 40 pounds (18.1 kilograms) in weight and 44.875 inches (114 centimeters) long including its tail, which measures 11 inches (28 centimeters), the desert rattleback is a larger animal than its grassland cousin. It has evolved a larger body in response to the cold climate. Big animals have less surface area relative to their body mass, making them more efficient at keeping in the heat. However, despite the desert rattleback's larger body size, its nose, ears and lips are much smaller than those of the grassland rattleback. In such a cold climate, the smaller an animal's extremities, the less susceptible they are to frostbite.
The hairs on the backs of both species of rattleback have evolved into hard, interlocking scales, although the desert rattleback's scales are smaller than those of the grassland rattleback. Because there are fewer big predators in the desert, the scales need not be as strong as those of their grassland relatives. Heat insulation, on the other hand, is vital in the cold desert, and larger air pockets within the scales provides excellent protection against the elements. The desert rattleback's face is covered in thick hair to shield its eyes and nostrils from the piercing, windborne sand.
Food and water are scarce in the North American Desert, so when a desert rattleback comes across food, it gluts itself. What nourishment is not immediately used will be stored away as fat reserves in its tail, to see the animal through times of famine, in much the same way that camels stored fat in their humps. The rattleback's kidneys are also very efficient. It cannot afford to waste water, so its urine is highly concentrated. In fact, the desert rattleback hardly needs to drink at all. It obtains nearly all its moisture from the food it eats.
Unlike its omnivorous southern cousin, this rattleback is a herbivore, subsisting on the tubers of desert turnips. Its acute sense of smell can detect plants from the surface and it uses broad, clawed paws to dig them out.
Although the desert rattleback spends much of its time burrowing below the desert surface, it is not a subterranean creature. It foes not dig tunnels and pits as other, more specialized desert animals do. During a violent sandstorm, however, it sometimes works its way into the soft sand and dust with a kind of a swimming action, shuffling down with its broad feet. Then it lets the displaced sand spill back, partially burying itself.
Sometimes desert rattlebacks will find a hole in the ground with sand being kicked out of it by spinks, and rattlebacks know that wherever there are spinks, there are bound to be tubers. Often, the rattlebacks' movements are followed by deathgleaners (who occasionally prey on their young) in the hope that it leads them to an easier and more abundant source of food. They wait for the rattlebacks to unwittingly dig up smaller animals like spinks, which the deathgleaners will try to catch for themselves.
Deathgleaners may occasionally prey on young desert rattlebacks, but adults are too tough for them. A mother rattleback will protect her baby from the huge bats by rattling her scales fiercely, and this is what makes the deathgleaners retreat, not willing to risk tearing their fragile wings on her sharp scales.