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Pack rats
Last updated on Wednesday, January 01, 2003
One day, Molly discovered an odd burrow at a friend's house: a den at the base of a cholla cactus, surrounded by a "fence" of cactus joints and assorted debris.
I suspect from her description that the mystery burrow belongs to a pack rat, one of a kind of large rodents more closely related to mice than true rats. Named for their acquisitive nature, pack rats scavenge objects such as mesquite branches, spiny cactus pads, cowpies, newspaper, cloth, and aluminum cans and tote these treasures home to construct their nest, a conspicuous mound at the base of a tree or shrub, in a rock crevice, or in a building.
Pack rats are particularly attracted to shiny objects - watches, jewelry, coins, and, of course, aluminum cans. One nest even sported an upper denture plate! An average den of a desert-dwelling pack rat contains about 20 cubic feet of material - enough to fill a trash bag‹and may reach 4.5 feet high by 2 feet wide.
Building with trash is an elegant adaptation to the desert environment. Where construction materials are scarce, pack rats take advantage of what is easily available.
Further, the untidy-looking pile of debris provides excellent insulation, keeping the interior cool in summer and retaining the pack rat's body heat in winter. Inside one desert nest, the temperature never rose above 88 degrees F, despite searing ground-level temperatures of 167 degrees outside the nest. Pack rats also armor their homes by placing cactus pieces, spiny branches, broken glass, and other pieces of sharp-edged trash atop the nest and along the paths leading to it.
Choice pack rat nests may be occupied for hundreds or even thousands of years by successive generations of the nocturnal, plant-eating rodents. Even when abandoned, the nests and adjacent "trash middens" created when the occupants cleaned house persist in the arid environment, partly fossilized by varnish-like coats of pack rat urine. Such nests provide a window into the past: paleontologists and plant ecologists "read" the layers to learn about climates and plant communities dating back as far back as 25,000 years.
Molly's friends had never seen the occupant of the unusual burrow. Since pack rats are nocturnal, the best way to see one is to watch from a nearby vantage point at night with a flashlight. High-pitched, mouselike squeaks and a thumping gait give away the rodent's presence. (Pack rats are usually solitary except when raising young.)
Their large size (adults reach a foot to a foot-and-a-half long including their long, hairy tail) and their strikingly bicolored fur (dark above, light below) are unmistakable. Although pack rats' noisy, nocturnal habits and acquisitive nature make them nuisances when they take up residence inside houses and other buildings, these characteristic Southwest residents make fascinating outdoor neighbors.
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