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Rodents on the Range: The Treasure House

One winter day, I was surveying different burrows and den sites on Chico Basin Ranch looking primarily for active swift fox dens. However, other burrows were relevant, as these animals (mostly rodents) are the primary prey of the swift fox. I usually find fox dens on the outskirts of areas with a high density of rodent burrows. As I zigzagged across the prairie, I could identify various animal burrows by the size and shape of the entrance. Also, were the entrances open or closed? Were the burrows found in clusters like a colony? Were there fresh signs of activity like tracks or scat, and so on. I found the burrows and tracks of several rodents such as prairie dogs, ground squirrels, pocket gophers, voles, and mice. I also found burrows for a rabbit warren, a badger den, and a couple of old swift fox dens. As I walked, I pondered the sheer abundance and diversity of wildlife that lives on the prairie, often remaining unseen.

After a while, weaving among the shrubs and cacti, I came upon a very strange structure, something I had not seen before, at least not of this size. Whereas most burrows I find were simple penetrations into the ground with some loose dirt around the lip, this structure was constructed above the surrounding surface, formed from dirt built up into a mound underneath and around the base of a cholla cactus. Presumably the dirt was brought up from subterranean chambers. It measured about six feet in length, at least three feet wide, and about 20 inches high. The dirt mound had multiple entrances each about three inches in diameter. Buried in with the dirt and strewn over the surface were segments of old cholla covered in spines. Perhaps these contributed structural integrity to the house walls, but likely the sharp needles protected the mound by discouraging predators like badgers, swift foxes, and coyotes from digging out the nest. Also piled up on top of the mound on several sides were rocks, some shiny pieces of metal, and stacks of dried cow pies.

A large woodrat (aka pack rat) house found on the prairie of Chico Basin Ranch.
A large woodrat (aka pack rat) house found on the prairie of Chico Basin Ranch.

I wondered: what animal built this curious house? What animal would collect such odd paraphernalia as shiny rocks and cow dung? It had to be an animal that was big enough to carry and stack these objects along with the pieces of cactus.

I had a theory. I recalled reading about pack rats in the desert. Pack rats are compulsive collectors. They will find some precious object that attracts their fancy, especially shiny objects, whereupon they may decide to drop whatever they happened to be carrying and exchange it for the more valuable object. They have been known to carry off lost jewelry, bottle caps, and car keys.

I did further research before drawing a conclusion, but I am fairly confident now that the animal that built this interesting house is likely a woodrat, also known as a pack rat. On the Chico, this was likely a Southern plains woodrat or possibly a white-throated woodrat. I can’t be sure without actually seeing the current tenant, and they are nocturnal.

I’ve learned that woodrats build multi-chambered houses out of a variety of material. These houses have areas called middens made up of a variety of collected material. (In some references, the whole house is referred to as a midden.) The nest for sleeping and birthing young is in one chamber of the structure lined with soft grasses.

Woodrats eat green shoots, seeds from grasses and shrubs, green flesh of cactus, as well as the yellow fruit of the cholla cactus and red fruit of prickly pear. If you break open cholla or prickly pear fruit, you will find lots of seeds that are undoubtedly high in nutrition. Some rooms in the midden serve as granaries for storing various seeds. Still other chambers in a woodrat midden contain collections of material as well as deposits of rat fecal pellets and urine. As a woodrat urinates on parts of the midden, the material becomes coated. As the urine dries, these materials are encased in what is called “amberat,” a protective coating somewhat similar to resin-based amber, which can help prevent decay over centuries under dry conditions.

Some very large pack rat middens have been found in desert caves or under rock outcroppings, dating back as far as 50,000 years. These middens form valuable time capsules. Paleo-botanists and zoologists will examine the material in these middens and will carbon date the organic matter built up in the layers to determine a timescale. These layers are a record of the vegetation and climate in the immediate vicinity reaching from prehistoric times down to the present. Pollen and seeds can also help identify plants that were growing during past centuries reflecting the climate and amount of precipitation.

Woodrats weigh approximately 16 ounces, plus or minus six ounces or so. Their size varies quite a bit with the prevailing ambient air temperature. Fecal pellet size corresponds to the mammal’s body size. By measuring the pellet sizes in the layers of a midden, zoologists have estimated changes in body size. In turn, a formula called Bergman’s Rule is used to then estimate the average ambient air temperature of the prevailing climate.

While the large pack rat midden I found on Chico is probably not centuries old, the age is not certain. It would be interesting to excavate it and find out just how old it is, and learn what tiny rat-treasures may be hidden within its layers.

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