Imagine someone has asked you to describe what an area was like hundreds, thousands, perhaps tens of thousands of years ago. They're interested in the ecology, the climate, and so forth. What could you use to accomplish this? If there are lakes and marshes, you might start by taking sediment cores and looking for pollen, which tell you what kinds of plants were present, and how the flora changed over time. If you're on the coast, you might look for deposits of shells, which are useful for describing the conditions of the water (salinity, temperature, how energetic the setting was) and for stable isotope analyses. If there's a lot of ancient wood, you might get into tree-ring data. In dry protected places of western North America, there is another type of paleoecological indicator: middens constructed by packrats.
|Good try, guys, but not quite; needs more juniper and less fabric.|
Packrats, also known as woodrats and sometimes trade rats, belong to the genus Neotoma. They are brown or gray and, as the name suggests, rat-like in appearance, although the tail is sometimes quite furry. Packrats gather materials such as plant fragments, bones, and dung to build dens in sheltered areas like caves and crevices. Their less-common name "trade rat" comes from their habit of dropping what they are carrying if they find something they like better (such as shiny things). There are numerous species spread across North America, some of which have stronger den-building behaviors than others, and some of which apparently prefer certain types of den-building materials. Packrats have unwittingly left us excellent records of the plants, animals, and ecology of the areas around their dens. The secret ingredient, and I warn you that if you are eating, you may perhaps want to pause for the next few paragraphs, is rat pee.
|"Who, me?" A bushy-tailed woodrat (Neotoma cinerea) at Wind Cave, posing for a National Park Service photo.|
Fossil middens, "resembling blocks of asphalt with the consistency and mass of an unfired adobe brick" (Spaulding et al. 1990), represent only part of an active den. Packrats will typically use a portion of their den as an outhouse, and unused and discarded plant fragments that accumulate in these areas become saturated in rat urine. In dry climates, the urine crystallizes into a substance called "amberat" which has a number of useful properties. Most obviously, it binds the midden materials. It is self-sealing to an extent: it rehydrates under humid conditions and becomes sticky, so dust and dirt become trapped on the outside surfaces and prevent moisture from penetrating more deeply. Saturating plant material with amberat is comparable to packing it in salt, protecting it from decay. Amberat also appears to prevent lichen growth and insect feeding. Finally, it affixes material to shelter walls and ceilings, so it is not uncommon to find "hanging" middens where underlying materials have eroded, providing one way to examine changes in cliff retreat (Spaulding et al. 1990). Packrats are solitary animals (Vaughan 1990), and individual middens are thought to represent a few years of activity. However, choice settings are reused over and over again, so large fossil middens spanning thousands of years can be produced. There can be millimeter-scale layers in a midden deposit, which may represent "melted" remains of individual middens (Spaulding et al. 1990). The foraging range of a packrat has a radius of about 100 to 160 ft (30 to 50 m) (Vaughan 1990), so their middens are local records. This is where a long-term midden deposit is quite useful: as conditions change and different plants become more or less abundant, enter an area, or disappear from it, the changes will be recorded by generations of rats.
|Rat-tested, rat-approved (a photo from Nevada borrowed from Wikipedia, originally from a no-longer accessible USGS page).|
Packrat middens are common, and we have reports of them being encountered (although not necessarily understood) since the mid-1800s, one of the most entertaining and stomach-turning being a party of '49ers running across some in the Papoose Lake area of Nevada. They encountered an odd glossy material resembling candy in niches in a cliff, some of them were feeling peckish... Nobody died, fortunately. True scientific investigation (as opposed to spitballing about their origin) did not begin until the 1960s, Wells and Jorgensen (1964) being the key early reference. So, what can we learn from this stuff?
Packrat middens are excellent sources of pollen, plant fragments, body fossils of arthropods (insects, spiders, millipedes, etc.) and (typically) small vertebrates, and fossil rodent droppings (coprolites). They span at least 40,000 to 50,000 years and are particularly well-represented in the deserts from western Texas to eastern California, areas that are lacking in other sources of paleoecological fossils like lakes and marshes. By far the most popular aspect of fossil middens has been their plant fossils. Researchers reconstruct changes in plant assemblages over time, which can be used as a proxy for climate. Many other topics have been studied, though. To borrow from a National Park Service inventory from a couple of years ago with a familiar lead author (Tweet et al. 2012):
"Studies following the distribution of a particular plant taxon through time are common (Lanner, 1974; Wells and Hunziker, 1976; Spaulding and Van Devender, 1977, 1980; Van Devender and Hawksworth, 1986; Van Devender et al., 1990; Lanner and Van Devender, 1998; Hunter et al., 2001). Various categories of fossils outside of plant macrofossils have also been studied, including pollen (Thompson, 1985; Davis and Anderson, 1987), arthropods (Hall et al., 1989, 1990; Elias and Van Devender, 1990, 1992; Elias et al., 1992), small vertebrates (Mead et al., 1983, 2003; Van Devender and Bradley, 1990; Van Devender et al., 1991), and rodent coprolites, which have been used to study rodent size and response to climate (Smith and Betancourt, 1998). At least one taxon has been named from midden fossils (the rabbitbrush Chrysothamnus pulchelloides, from a midden at Chaco Culture National Historical Park; Anderson, 1980). Middens have been used to study the rate of erosion in the Grand Canyon (Cole and Mayer, 1982), the extinction of ground sloths (Phillips, 1984), the disappearance of people from Chaco Canyon (Betancourt and Van Devender, 1981; Betancourt, 1990) and to provide points of comparison for resource management at Capitol Reef National Park (Cole and Henderson, 1993; Cole et al., 1997; Cole and Murray, 1999) and Glen Canyon National Recreation Area (Fisher et al., 2006, 2009). Middens have also been incorporated into cultural resource studies (Emslie et al., 1995)."Not bad for heaps of plant matter and other things cemented by rat pee.
|Distribution of midden sites from the old USGS-NOAA midden database (no longer functioning).|
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