Ancient Bat Guano Reveals Thousands of Years of Human Impact on the Environment
Like sediment cores, ice samples and tree rings, bat excrement can be used to study the climate of the past
Deep in the forests of northwestern Jamaica, a secluded cave has sheltered an unabridged account of the environment since the early Bronze Age. The cave’s inhabitants live in near-total darkness, swarming out to feed at night through a mist of their own urine and retreating back inside to roost. The colony of five thousand or so bats then add to the archived climate record much as their ancestors did before them: by swooping down from the walls and defecating on the cave floor.
“People might think of guano as just a big pile of crap,” says Jules Blais, an environmental toxicologist from the University of Ottawa. But buried in that pile are the secrets of the past.
Guano, a sticky brown paste and a staple in many tropical caves, is a festering compilation of a colony’s droppings, remnants of nearby plants, fruits and insects, as well as the odd fallen bat. Guano piles can reveal exactly what the bats were eating as well as details about the environment the bats were exposed to. Conditions in the soil, water and atmosphere are consumed, processed and left—via the bats’ digestive system—in accumulating layers on the floor, like pages in an ever-expanding book. After years of accumulation, paleoclimatologists can read the details of that record to recreate the environmental conditions of the past.
Despite its usefulness as an environmental indicator—joining the ranks of sediment cores, ice samples and tree rings—ancient guano is hard to find. With its high levels of nitrogen, guano from bats and birds has been harvested through the ages as a natural fertilizer. Wars have even been fought over the stuff ...
Blais was part of a team lead by Lauren Gallant, a PhD student at the University of Ottawa, which analyzed a 129-centimeter-long (4.2 feet) guano core extracted (with some difficulty and climbing equipment) from the Jamaican cave. The research team wanted to see if they could detect traces of human activity. Radiocarbon dating put the base of the core at around 4,300 years old, long before the first humans arrived on the island. Gallant’s team then looked for shifts in a range of metals and isotopes—chemical elements with varying numbers of neutrons in their atoms’ nuclei—that could indicate human influence. Their study, published this month in the journal Paleogeography, Paleoclimatology, Paleoecology, presents a strong case that such anthropogenic signals can be identified in cores of guano. ...
