IN 1849, a group of hungry gold prospectors were crossing the Nevada desert when they noticed strange glistening balls of a gummy substance littering the top of a cliff. The men had not eaten a proper meal for days, so they ate the sweet‑tasting spheres. In a few minutes, all were violently sick, though each managed to survive the ordeal.
Now this faintly distasteful story has one interesting feature ‑ it is the first recorded interaction of man and pack rat midden. Today, pack rat middens, as we call these shiny spheres, are widely known among scientists because of their power to shed light on our remote history ‑ from the dawn of modern humans, 40,000 years ago, right up to relatively recent events, including the fall of the Byzantine empire.
It all sounds very peculiar, so let us take a look at the creature responsible: the pack rat, or the wood rat, or Neotoma, of which there are 22 different species. Pack rats can be pale buff colour, grey or reddish brown, with white undersides and feet. They measure from between nine and 19 inches in length. They are found throughout North and Central America in a range of habitats, from deserts to forests and mountains. They sound fairly ordinary creatures. However, the pack rat has one particular habit that makes it quite distinctive ‑ its nest‑building habits.
Scientists have discovered that pack rats protect themselves by building dens out of any debris coming their way—twigs, fragments of food, pieces of plant and waste products of other animals. Pack rats live in these dens which fill up with their excrement and become saturated with their urine.
This latter, fairly disgusting addition crystallises and cements the den into a bricklike consistency. It then becomes a midden. The miners' reaction to these gummy balls is therefore not surprising. However, their scientific importance was only recently appreciated, when researchers discovered, in Nevada again; an old pack rat midden filled with fragments of juniper, a plant that not grown there for thousands of years.
So they dated the midden and found that it was 9,300 years old! Yet inside it were the perfectly preserved remains of the plants and animals that had thrived in that area many millennia ago. For thousands of years, the pack rat has been creating perfect time capsules of past life on Earth. Essentially, the pack rat collects detritus for several months from land in a radius of about 50 metres round its den. This material is then preserved in crystallised urine, creating tiny archaeological treasure troves.
Studies of these have already produced one learned tome – Pack Rat Middens, edited by Julio Betancourt, Thomas Van Devender and Paul Martin (University of Arizona Press) – which covers an enormous range of midden investigations: from the reasons why mainland America’s big mammals became extinct 11,000 years ago to alterations in the intensity of cosmic ray bombardments over the past 20,000 years. In addition, reports on middens made by other small mammals are included in the book.
Most intriguing are those chapters that concentrate on the very specific uses of middens, such as the study of the fate of the ancient Byzantine city of Petra, in Jordan. Anyone who has seen the Steven Speilberg and George Lucas film, Indiana Jones and the Last Crusade will recall the vast, magnificent facades of Petra among which Harrison Ford and Sean Connery search for the Holy Grail.
Petra was once a thriving Neolithic village before it became one of the richest cities in the Roman, and later the Byzantine, empires. Then it was abandoned and was not rediscovered until 1812. The question that archaeologists have puzzled over is this: what caused the collapse of Petra?
The solution, decided three researchers from Arizona—Patricia Fall, Steven Falconer and Cynthia Lindquist—might lie with middens. However, as there are no pack rats in the Middle East, they had to search for another, similar den‑making animal, and selected the rock hyrax, of the Procavia family.
The rock hyrax ‑ a rabbit-sized herbivore ‑ creates middens similar to those of pack rats, and the contents of these fossilised dens were studied by the Arizona scientists for three historical periods: AD300, 500, and 900. These dates roughly correspond to three crucial moments in Petra's history: its Roman heyday, the flowering of its Byzantine phase -and its post-imperial collapse.
Each midden from these epochs yielded more than 100 plant species and studies of these painted an all too clear picture of the fate of Petra. In the early samples, pollen found in middens came largely from the oak and pistachio trees that once formed vast forests throughout the Mediterranean. But by AD500, these had died out and most pollen came from shrubs, herbs and grasses. By AD900, even these plants had virtually disappeared.
The culprit? Mankind, of course. Two thousand years ago, primitive farmers began clearing land and started cutting down trees for firewood and house construction. With the arrival of the Romans, deforestation increased dramatically.
Then the Romans left and Byzantine Empire took over. It too collapsed and the inhabitants of Petra were left to struggle alone in a devastated landscape. They turned to intensive grazing, letting their goats loose to eat their way through every shrub, herb and piece of grass. In the end, only sand dunes were left.
In short, Petra flourished because the city was built in prime, verdant real estate. It collapsed because its inhabitants utterly destroyed the countryside around it and eventually brought themselves to the point of starvation. The shining desert round Petra is man made.
It is a depressing but important story, given our present fears about the environment. The people of Petra were fairly unsophisticated by today's standards but still managed to wreck their local ecology in spectacular fashion. The power of twentieth‑century science is potentially much more destructive.
We clearly have many lessons to learn ‑ and some of these are coming from the very humblest of Earth's creatures.