The history of the Andes might well be written in llama poop. Researchers have found that in a small, dried-up lake in highland Peru, mites that ate these creatures’ feces closely track major historical events through their population growth, including the rise and fall of the Incan Empire. In certain kinds of environments, this new method of peering back in time might be more accurate than another common one: using dung-dwelling fungal spores to track environmental conditions in the past.
The ancient lake in question, called Marcacocha, is now a wetland high in the Andes, near the Incan city of Ollantaytambo. But before it disappeared about 200 years ago, it was a small pool surrounded by grassland and a popular stop for Incan llama caravans. Thousands of llamas carrying trade goods such as salt and coca leaves marched through the basin, drank from the lake, and defecated en masse. That dung washed into the lake, where it was eaten by oribatid mites, a half-millimeter-long spider relative that lived in the lake.
The more llamas that passed through Marcacocha, the more poop the mites had to eat, and the larger their populations could grow. When the mites died, they sank into the lake mud, preserved where Alex Chepstow-Lusty, a paleoecologist at the University of Sussex in Brighton, U.K., found them in a sediment core centuries later.
When Chepstow-Lusty counted the number of mites in each layer of the core, he found that their population boomed when the Incan Empire dominated the Andes from 1438 C.E. to 1533 C.E. But after the Spanish arrived, the number of mites in the core plummeted. That’s because so many of the Indigenous people and their animals died during and after the conquest of the empire, Chepstow-Lusty says. Although the mite population rose again once European cows and pigs moved in and started to poop around the lake, it dropped off around 1720 C.E., when a smallpox epidemic swept through the area.
Intrigued by the mite record, Chepstow-Lusty decided to see what another poop-eating microorganism could tell him. The spores of a fungus called Sporormiella live on herbivore dung and are often used to track past populations of large plant eaters, including ice age giants like mastodons and mammoths. An abrupt drop-off in Sporormiella spores is often interpreted as a sign of when those animals went extinct.
Chepstow-Lusty saw the Sporormiella population rise and fall in the Marcacocha core. But those cycles didn’t track with the mite population or the known historical events that led to llama die-offs. Rather, the spores boomed during dry periods, when the lake got smaller and the llamas were able to poop closer to its center (the eventual source of the sediment core) and shrank when the lake was bigger, the team reports today in The Journal of Archaeological Science. For certain kinds of small, shallow lakes like Marcacocha, therefore, the Sporormiella record might offer misleading information about past herbivore populations.
Mark Bush, a paleoecologist at the Florida Institute of Technology in Melbourne, agrees that the environment of Marcacocha doesn’t lend itself to Sporormiella studies. Although the mites “provide an interesting alternative,” he says, there haven’t been enough studies in other places testing the relationship between the numbers of mites and the size of herbivore populations to be sure the mites are truly an accurate proxy.
Chepstow-Lusty hopes other researchers will start to tally up oribatid mites in their sediment cores, in hopes of figuring out when and where they may offer accurate information beyond Marcacocha. “You never know what you’re going to find in your lake muds,” he says. All microorganisms—especially the poop-eating ones—deserve a closer look.