Pack rats build complex nests of twigs, called "middens", often incorporating cactus. Nests are often built in small caves, but frequently also in the attics and walls of houses. Some Neotoma species, such as the White-throated Woodrat (N. albigula), use the base of a prickly pear or cholla cactus as the site for their home, utilizing the cactus' spines for protection from predators. Others, like the Desert Woodrat (N. lepida) will appropriate the burrows of ground squirrels or kangaroo rats and fortify the entrance with sticks and bits of spiny cactus stems fallen from Jumping and Teddy-bear Chollas.
In houses, pack rats are active nocturnally, searching for food and nest material. A peculiar characteristic is that if they find something they want, they will drop what they are currently carrying, for example a piece of cactus, and "trade" it for the new item. They are particularly fond of shiny objects, leading to tales of rats swapping jewelry for a stone.
Historically, houses in or near ghost towns were typically infested with pack rats.
Some species of pack rats were called "prairie flounders" by settlers. This might have occurred because the eyes of pack rats are set somewhat higher in the head than other rodents.
The term pack rat is also used in English as slang to refer to a person who collects miscellaneous items and has trouble getting rid of them (a compulsive hoarder) and more recently the term Digital Pack Rat has been used to describe the same problem with digital files.
A pack rat midden is the nest of a pack rat. Due to a number of factors, pack rat middens may preserve the materials incorporated into it up to 40,000 years. The middens may thus be analyzed to reconstruct the environment around the midden when it was built, and comparisons between middens allow a record of vegetative and climate change to be built. Examinations and comparisons of pack rat middens have largely supplanted pollen records as a method of study in the regions where they are available.
Pack rats are known for their characteristic searching of materials to bring back to their nests creating an ever expanding collection known as a "midden" for its messiness. In natural environments, the middens are normally built out of sticks in rock crevices or caves for protection from predators. In the absence of crevices or caves, the middens are often built under trees or bushes. The packrats will also use plant fragments, animal dung and small rocks in building the nest. The vast majority of the materials will be from a radius of several dozen yards of the nest. The packrat urinates in the midden during the time it lives there; the sugar and other substances in the urine crystallize as it dries out, cementing the midden together. After a few decades, the packrat will abandon the midden and move on to start a new nest.
The unsuspected resilience of the middens is due to three factors. The crystallized urine dramatically slows the decay of the materials in the midden. The dry climate of the American Southwest further slows the decay, and middens that are protected from the elements under rock overhangs or in caves survive even longer.
Zoologists examine the remains of animals in middens to get a sense of the fauna in the neighborhood of the midden, while paleobotanists can reconstruct the vegetation that grew nearby. Because middens are abandoned after a short period of time, they are uncontaminated "time capsules" of several decades of natural life, centuries and millennia after they occurred. The analysis of middens was key in understanding the fauna around Pueblo Bonito, and thus helping to explain its history.