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Changing scene. Based on findings from pack rat middens, this artist's conception shows what Hueco Tanks may have looked like some 12,000 to 14,000 years ago (far left), some 9,300 years ago (middle) and as it appears today (right). Colorado pinyon needles found in the oldest middens imply that Hueco Tanks was cooler and wetter 14,000 years ago. By 9,000 years ago, the climate had become more arid and the pinyons disappeared, leaving a juniper-oak woodland. Today Hueco Tanks is dominated by xeric grasslands and desert scrubs. However, Arizona oak and juniper still remain in localized, sheltered sites. Images courtesy of Texas Parks and Wildlife Department.
photo of a Pinyon and juniper stand near Guadalupe Mountains
Pinyon and juniper, such as this stand near Guadalupe Mountains National Park, grew in the Hueco Bolson in the distant past. Between 12,000 and 10,800 years ago, a shift in vegetation occurred; pinyon began to disappear and grassland communities emerged. Photo by Susan Dial. Enlarge image
Map of average annual precipitation.
Map of average annual precipitation. Although less than 14 inches of rain falls in an average year today in the Hueco Bolson, the area was cooler and more moist in the Late Pleistocene. Enlarge image
photo of a portion of a 13,000-year-old pack rat midden
This portion of a 13,000-year-old pack rat midden was hydrated, screened, and dried so that its contents could be sorted and examined. A small sample yielded, from left to right, pinyon needles, rat pellets, prickly pear and cholla needles and seeds, hackberry seeds, and juniper needles. Photo courtesy Texas Parks and Wildlife Department. Enlarge image
photo of common snowberry (Symphoricarpos sp.)
Common snowberry (Symphoricarpos sp.) grew in the area when earlier conditions were favorable for woodland species, based on evidence found in pack rat middens from Hueco Tanks. Photo by J. S. Peterson, United States Department of Agriculture. Enlarge image
photo of a packrat midden
Pack rat midden from the Hueco Mountains on display at the El Paso Museum of Archeology. Plant materials from this specimen indicate it is only several thousand years old. Photo by Susan Dial. Enlarge image
photo of Hueco Tanks
Dark, resinous creosote bushes grow amid grass and scrub around Hueco Tanks. Drought tolerant when mature, these evergreen desert plants began to appear in the landscape around 4000 years ago, as the modern, warmer and more arid climatic regime was established. Photo by Susan Dial. Enlarge image
photo of thorny scrub and grasses
Thorny scrub, including low cacti, sotol, and mesquite, grow among grasses at Hueco Tanks. Photo by Susan Dial. Enlarge image


The paleoclimatic sequence of the Hueco Bolson, and Hueco Tanks in particular, has been approximated through pack rat midden analyses. The plant fossils preserved in middens from Hueco Tanks contain a history of the park's vegetation during the past 14,000 years. This history is further elucidated when compared to the regional vegetation history developed through many methods of paleoecological analysis. These include studies of cave deposits, archeological sites containing vertebrate remains, and pollen analysis of ancient sediments. These studies help to reconstruct ancient environments and climates.

The basis for any pack rat midden analysis is an understanding of the contemporary flora and fauna of a site. At Hueco Tanks a survey was conducted of the plant communities now occurring in the park, and a detailed modern species list was developed (see Natural Setting for examples). Such information is used to interpret the significance of ancient plant remains preserved in the middens. Portions of at least six pack rat middens from Hueco Tanks have been analyzed. Radiocarbon analysis of the oldest one yielded a date of approximately 13,500 years before the present. A younger midden was dated to about 9,380 years ago, while several other undated middens are estimated to date from as long ago as 11,000 years to as recently as 800 to 900 years ago. Perhaps two middens are modern.

Analysis of the oldest midden at Hueco Tanks revealed the presence of 29 different species of plants, 5 of which do not grow in the park today. The most significant extralocal species (not occurring at the site today) was Colorado pinyon. Today this pine is widely scattered throughout the Southwest and is known in Texas from higher elevations in the Guadalupe Mountains, the Sierra Diablo, and the Delaware Mountains. Other midden records from greater than 34,000 to about 11,000 years ago in Trans-Pecos Texas also reveal the presence of Colorado pinyon.

The conclusion a paleoecologist makes, based solely on this information, is that 12,000 to 14,000 years ago this pinyon was more widespread. The questions then arise as to which factors, present several thousand years ago but not present today, provided a favorable environment in the Hueco Tanks region for Colorado pinyon pine. Several lines of evidence lead to the conclusion that the climate in this portion of the Southwest and northern Mexico was cooler, and there was probably greater rainfall and lower evaporation rates. The environment must have been sufficiently different at that time to have met the ecological requirements of Colorado pinyon.

Also recovered from our oldest midden were teeth of a vole. Although the exact species is not known at present, it may be the Mexican vole which is found today at high elevations in the Sacramento Mountains of New Mexico and in the Guadalupe Mountains of Texas. The Mexican vole prefers montane grasslands in ponderosa and mixed-coniferous forests. In favorable wet years, the vole may descend into the pinyon-juniper woodlands. It could have been able to survive at Hueco Tanks 14,000 years ago when conditions there favored the development of a pinyon-juniper woodland.

Analysis of remains preserved in the 9,300-year-old midden from Hueco Tanks revealed that vegetation at the site had changed from that which was present approximately 4,000 years earlier as recorded in our oldest midden. Only 13 species were recovered from this midden, with only one ecologically important extralocal species recovered —Toumey oak. Today this species occurs in favorable habitats in woodlands above the Sonoran Desert in Arizona and adjacent Mexico and is known to occur at one site in the Franklin Mountains approximately 25 miles west of Hueco Tanks State Historical Park. Toumey oak usually grows in mesic, igneous areas at elevations of approximately 5,000 feet in association with some of the same plants which occur at the tanks today.

Significantly, Colorado pinyon pine was absent from the midden. Apparently, by 9,300 radiocarbon years ago, Colorado pinyon already was locally extinct at Hueco Tanks State Historical Park. Evidence from other middens in the region indicates that approximately 11,000 years ago environmental conditions were modified to the point that Colorado pinyon no longer was able to survive at low elevations in the northern Chihuahuan Desert.

When the pack rat midden data from Hueco Tanks is integrated with other paleoecological data from the region, a picture of the past climate and environment of the park and its surroundings emerges. Since the Late Pleistocene, the trend in the Trans-Pecos in general has been toward higher temperatures and decreasing precipitation. Prior to 12,000 years ago the northern Chihuahuan desert was covered by pinyon-juniper-oak woodland and temperatures were milder than they are today, with cooler summers, warmer winters, and greater rainfall, most falling in the winter.

The period between 12,000 and 10,800 years ago marked the beginning of a vegetation shift. Summer temperatures began to rise, a monsoon season started to develop, and more mesic species such as Colorado pinyon disappeared. By 9,400 years ago, summer temperatures had increased further and precipitation had shifted to a dominant summer monsoonal pattern. A juniper-oak woodland had developed, which persists to the present at Hueco Tanks, due to the greater moisture retention of its igneous-derived soils. Between 11,000 and 8,000 years ago a major shift toward warmer and drier conditions took place.

By 8,000 years ago the woodland had retreated up the mountain slopes and was replaced by grassland, while xeric species, such as prickly pear and honey mesquite, appeared in the area. Relict populations of juniper-oak woodland exist today at Hueco Tanks, due to the greater moisture retention of its igneous-derived soils, and at high elevations in the surrounding mountains. The modern climatic regime was established by about 4,000 years ago, with fewer winter freezes and adequate summer rainfall, punctuated by more frequent droughts. At this time, ocotillo, lechuguilla, and creosotebush appeared, and the transition to an essentially modern desertscrub community at Hueco Tanks was complete by around 3,600 years ago.

Other indicators provide a glimpse of past environments. Pictographs from Hueco Tanks include depictions of what appear to be bighorn sheep. Although bighorn sheep remains have been recovered from a cave in the nearby Hueco Mountains, these animals are not present in the park today. Bone recovered from a small Formative village at Hueco Tanks dating to circa 900 years ago included bison, another species extirpated from the region.

This section draws largely from the article "Pack Rats: Unwitting Helpers of Archeologists" by David H. Riskind and Thomas R. Van Devender first published in Texas Parks and Wildlife magazine, March 1979 (Volume 37, No. 3, pp. 6-9).

The white-throated wood rat, (Neotoma albigula,) probably is the species of pack rat responsible for collecting bits of plants later used by scientists to construct a picture of the vegetation changes at Hueco Tanks.


Pack rats have gained notoriety for their remarkable habit of collecting samples of anything movable within their territory. Such items, usually vegetation, are brought to the den and incorporated into their houses. However, pack rats also gather rocks, animal bones, insect parts or anything small and interesting. For archeologists, the trash-collecting habits of these rodents have proved to be an unlikely boon: pack rat middens can provide a treasure trove of clues about past environments.

The pack rat's gathering and hoarding nature, and the fact that these rodents thrive in arid, often rocky environments, have made them and their remains a valuable scientific tool. Ancient pack rats of the dry deserts of North America have provided modern-day scientists with a figurative window to the past.

Pack rats of several thousand years ago, like their modern counterparts, gathered quantities of plant materials from their immediate environs. Based on modern behavioral studies, we know that pack rats ordinarily do not venture more than 300 feet or so from the protection of their houses. On their gathering forays pack rats select such things as twigs, leaves, seeds, fruits and cactus spines, some of which they use to enlarge or fortify their houses or for food.

Significantly, pack rats not only collect excellent samples of local vegetation, but some species also deposit their material in dry caves or rocky shelters where it is preserved. Based on modern analytical techniques, we know that some pack rat accumulations, or middens, have been preserved for more than 54,000 years. Similarly deposited materials would not have been preserved in areas of high humidity and rainfall, such as Central or East Texas.

Pack rat houses usually are built around rocks, shrubs, cacti or within caves, rock ledges, and fissures. Their houses often are well constructed, containing internal passages, specialized living or food chambers, and sometimes levels. Houses, or nests, protect the rat from predators, as well as extreme heat or cold. Near the entrances are waste piles, called middens, where fecal pellets and unwanted materials accumulate and are consolidated through trampling and the addition of urine.

The copious addition of rat fecal pellets and urine, in addition to plant materials, can cause these pack middens to build up rapidly. Interestingly, it is thought that the viscous urine, which may impregnate and consolidate the middens due to its chemical composition, also serves to preserve them from fungal attack and subsequent decay. For a midden to be preserved, however, it must have been deposited in a dry site in an arid environment free from rapid erosion.

Although pack rats are solitary except during times of breeding and rearing of young, some middens may attain remarkable size and represent the work of several generations. There are records of midden accumulations five feet in diameter and up to three feet thick.

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