Our Window on Nature

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Uninvited Guest

Filed under: Mammals — Lowell Christie -- September 11, 2014 @ 2:40 pm

Wood RatWe think we’ve finally trapped the right pack rat – the one who’s taken up residence atop our gray-water tank after building its granary in the engine compartment. Other than the fact that it has chewed through several spark plug cables, the problem is the critter’s nocturnal habits.

The trouble starts when, along about 2:00 a.m., “PR” scratches and scrapes out bits and pieces of insulation with which to line his nest. Our Doberman, Sassy, mutters under her breath in protest, and then Lowell growls at the dog. Hoping to ignore all three, Kaye pulls her pillow over her head.

Trap it, you say. That’s easily suggested, but not so easily accomplished. We bought a live trap and armed it with a gourmet spread of peanut butter and birdseed, but with the abundance of seeds scattered about the desert at the time, our offering held no attraction.

Even when we laboriously remove all of the cholla and palo verde pods from the engine compartment, PR simply gathers more. One night when Lowell was especially desperate for sleep, he got up and played the water hose, full force, into the nest area. The result was nothing but an irate rat scratching more industriously than before.

A local resident suggested that we try placing moth crystals in the nest and the granary. PR didn’t seem to mind the strong odor wafting through every nook and crack in the vehicle, but we certainly did.

Finally, one night, hunger or curiosity gained the upper hand, and just before bedtime we heard the sharp crack of a trap closing. Well, hello there, fellow. Don’t you look cute peering out through the bars. Now can we get some rest?

Although it caused us some sleepless hours, this pack rat, or wood rat as it is more properly called, was simply doing what came naturally. Gentle but wide-awake creatures, wood rats prefer to “ratnap” during the day and do their work at night. Unfortunately, their work is gathering and storing food and adding (endlessly) to the comfort of their nests.

Wood rats aren’t even true rats, although they bear a superficial resemblance to that critter of cities and slums, the Norway rat. Unlike city rats, wood rats have big black eyes; rather large, platter-shaped ears; and a coat of soft fur that covers their entire body. (The tail of a Norway rat is scaly rather than hairy).

One more significant difference: city rats are brazen little devils, frequently abroad in the daylight, and known to scamper right across your feet; wood rats are timid, retiring creatures. You know of their presence if they move into or under your coach, but you’re not likely to see them.

Another indication that pack rats are around is the presence of one of their conspicuous houses. The wood rat is a compulsive nest-builder. In the Sonoran desert, where friend wood rat moved in on us like an out-of-work brother-in-law, typical pack rat nests are built mainly of downed sticks and cactus joints, with enough pieces of plastic wrap, aluminum foil, and horse manure thrown in to make their houses look more like rubbish heaps than animal habitations.

How do small creatures with velvetsoft feet and equally tender mouths go about gathering and carrying cacti? Very carefully, say the experts. Somehow, wood rats manage to scramble up the spines with nary a stabbing, and then to carry off a cactus joint after nipping through enough of the spines to give them a grip on the object without it reciprocating in kind. It takes a heap of cholla to build a nest big enough to keep out predators.

In areas where wood rats don’t have the protection provided by nests covered with cactus spines, their houses are oftentimes even bigger. In our area, white-throated wood rat nests are seldom more than a couple of feet high, with a base a yard in diameter. Edmund Jaeger, writing of California’s Mojave Desert, told of finding a brown-footed wood rat nest that was five or six feet broad at the base and equally as high.

Many of these huge nest heaps are centuries old, as additional layers of material are added by each successive occupant. As time passes, the accumulation of plant matter mats down into a thick, tarlike deposit made up of nesting material and food debris, cemented together by urine and fecal matter.

The really record-setting pack rat middens are those found in rocky caves in the desert. Dr. Tom Van Devender, an expert on pack rat middens, has found pack rat nests dating back more than 45,000 years. The significance of these nests isn’t so much what they teach us about wood rats, but what the buried seeds, pollen, and other recognizable plant materials reveal about past geological and climatological conditions.

Kaye accompanied Van Devender on one of his “midden trips.” After a steep scramble up to an almost inaccessible cave, the scientist broke off brown chunks of history, then wrapped and labeled them as to where they were located in the midden and packed them down for further study. Back at his van, he “cleaned” the packets of foreign debris, pointing out places where, thousands of years ago, wood rats had stored juniper leaves and pine needles. Rat middens such as this one prove that this arid desert region was once covered with grasslands and even with coniferous forests – evidence that the climate then was far wetter than it is today.

Clearly, wood rats have been collecting and saving things for a long time, but it was their light-fingered habit of thievery that caused early cowboys and miners to give them the name of trade rat. It’s an exaggeration to say that the creatures exhibit aesthetic taste, but they do seem to have an affinity for shiny objects. And, because they rarely travel without carrying something in their mouths, when a trade rat encounters something that strikes its fancy, it drops whatever it has (trades, as it were), in order to carry away a watch, a sock, or even a set of dentures. An animal may have to carry this new possession a good distance before reaching its nest; so, once stolen by a pack rat, your valuables are unlikely to be found – unless the little thief finds something on the way home that it likes even better.

A wood rat’s nest holds more than foodstuff and ornaments collected from human owners. These huts are veritable wildlife hostels. Mice, lizards, snakes, rabbits, and an assortment of spiders, ticks, and insects move in uninvited. Some feed on stored food or on each other; others enjoy the same protection desired by their wood rat host. If hungry coyotes, foxes, bobcats, and badgers can’t get into the cave or past the barrier of cactus spines, both the wood rat and its neighbors will survive.


For More Information:
Packrat Middens: The Last 40,000 Years of Biotic Change (Garland Studies in Entrepreneurship)

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