Potent Packrat Leavings Tempted Starving 49ers

By JOE EATON Special to the Planet
Friday December 12, 2003

Christmas, 1849: not a joyous time for a party of gold seekers led by a Vermonter named William Lewis Manly. An alleged shortcut from Salt Lake City to California has left them stranded in Death Valley. They’ve slaughtered their draft oxen and dismantled their wagons for firewood. They’re hungry enough to try anything, however unpromising. 

Up a canyon, in niches set in a high cliff, Manly’s group finds what he describes in his journal as “balls of a glistening substance looking something like varigated [sic] candy stuck together” the size of small pumpkins. Concluding it “evidently food of some sort,” they break off pieces and share them around. Manly records the taste as “sweet but sickish.” Those who sample it have a touch of nausea afterward, but no lasting ill effects. 

Manly believed they had discovered an Indian food cache, and was apprehensive about the consequences of raiding it. The pioneers apparently thought the “glistening substance” was some kind of Native American rock candy. But it had been left not by the local Paiutes, but by packrats. 

Although some might consider the packrat my totem animal, I’ve only seen a couple: a trophy brought home by one of my late cats, another treed by a friend’s dog. But they’re common enough in the right places, and it’s easy to find evidence of their presence. They’re more properly called woodrats, although only a few of the 21 species (10 in North America) live in forests; most inhabit the Southwestern deserts. In the Bay Area, the dusky-footed woodrat is widespread in woodland and oak scrub. It’s smaller than a Norway rat, from which it’s also distinguishable by its furry tail. 

Packrats are secretive, but their homes are hard to miss. Called “houses,” their dwelling are massive stick constructions, conical in shape. Biologist Elden Vestal, who studied the dusky-footed woodrat in Strawberry Canyon in the 1930s, measured over 300 houses: They averaged 4 feet in height (maximum 6 feet 7 inches) and 5 feet in diameter (maximum 7.5 feet). Rat houses often harbor smaller creatures: mice, voles, shrews, alligator lizards, tree frogs, three species of salamander. A typical house has sunporches, internal passageways, a nest chamber deep inside, and storage space for food. The exterior may be decorated with objects that catch the rat’s fancy. Vestal found one house adorned with a great horned owl’s skull, another with two golf balls. 

These are short-lived creatures, but their houses may be occupied by generation after generation. A good home is never empty for long. Solitary except during the mating season, neighbors communicate by rattling their tails to signal potential danger. 

Dusky-footed woodrats will take a wide variety of plant matter; field work at UC’s Hastings Reservation in the Carmel Valley tallied 73 species. But a few plants, mainly live oak and California laurel, are staples. The rats eat both acorns, which they stash in impressive quantities (one house contained 20 pounds), and oak leaves. Other mammals, including lab rats, are unable to handle the toxic tannins in oak foliage. Woodrats, though, thrive on it. Peter Atsatt and Trudy Ingram, biologists at UC Irvine, found that captives maintained their weight on a strict oakleaf diet, and theorized that the rats rely on specialized bacteria in their digestive tracts to break down the tannins. 

Anything the rat brings home but doesn’t eat goes into the midden, an accumulating trash heap just outside the house and often downslope. The midden also serves as a latrine. And it’s this behavioral trait, shared by most packrat species, that’s made these rodents a valuable resource for the study of climate change. 

What the Manly party unwittingly ate was dried packrat urine. Climatologists call it “amberat,” which has a nice ring. In our area, with its winter rains, dusky-footed woodrats don’t produce amberat. But their desert relatives do, and the substance is a potent preservative. 

Just how potent was discovered by botanist Philip Wells and zoologist Clive Jorgensen in 1961. Near Frenchman Flat in the Nevada Test Site, the scientists broke off a chunk of packrat midden and found it full of juniper. There was no juniper anywhere in sight. Wells and Jorgensen sampled eight more middens, all containing juniper, one with the skull of a marmot, a resident of alpine tundra. Carbon 14 testing showed the skull was 12,700 years old, and the juniper bits ranged from 7,800 to at least 40,000 years in age, as far back as the dating method was effective. The middens had evidently been used by successive generations of rats, stretching back into the Pleistocene Era, and the juniper and marmot remains dated from a time when southern Nevada was cooler and wetter. 

When they published their findings in Science, Wells and Jorgensen suggested further study of packrat middens as a secondary source of evidence in climate studies, along with pollen analysis. It’s become a lot more than that. Several university labs are now devoted to middenology. Teasing the datable bits out of the viscous rat urine is tedious work. But the result is a detailed, site-specific history—site-specific because the rats are homebodies—of local vegetation, and thus of local climate. And it’s possible to measure how rapidly plant life changed when the great ice sheets melted, or during later cycles of drought, like the one that may have driven the Anasazi from their cities. 

Our local woodrats may not have made a major contribution to science, but desert rats have left a record enabling us to reconstruct ancient climates and project the likely impact of changes to come. Not bad for a rodent. 

And yes, the Manly party made it out alive. Manly and a companion scouted an escape route and came back for the others. When they reached the pass leading out of their encampment, they doffed their hats and shouted “Goodbye, Death Valley!” to the silent desert. And the name stuck.