Scientists drill ‘poop core’ revealing 4,000 years of bat diet and environment

An undisturbed pile of ancient guano provides a unique medium for examining natural history, writes Harry Cockburn

Study is the first time scientists have examined ancient bat guano in this way (Getty )

You may have heard of scientists studying ice cores and sediment cores as a means of learning more about our planet’s natural history, but a team of Canadian and US scientists are probing something altogether more icky: a deep pile of ancient bat guano.

Describing it as a “treasure trove of bat poop”, the scientists are examining the hillock of waste which has been found deep in a Jamaican cave where it has remained relatively undisturbed and has been slowly growing for thousands of years.

The guano has been deposited in sequential layers by generations of at least five species over a period of around 4,300 years, and is around two metres deep.

The team said that just as ice cores and sediment cores can reveal insights into the natural world, the new “poop core” contains information about changes in climate and how the bats’ food sources shifted over the millennia.

“We study natural archives and reconstruct natural histories, mostly from lake sediments. This is the first time scientists have interpreted past bat diets, to our knowledge,” said Jules Blais of the University of Ottawa and an author of the study.

Dr Blais and his colleagues said they applied the same techniques used for lake sediments to the large guano deposit located in Home Away from Home Cave, in Jamaica.

The team extracted a vertical core extending from the top of the pile to the oldest deposits at the bottom and took it to the lab for biochemical analysis.

About 5,000 bats from five species currently use the cave as daytime shelter, the researchers said.

“Like we see worldwide in lake sediments, the guano deposit was recording history in clear layers. It wasn’t all mixed up,” Dr Blais said.

“It’s a huge, continuous deposit, with radiocarbon dates going back 4,300 years in the oldest bottom layers.”

The team looked at biochemical markers of diet called sterols, a family of sturdy chemicals made by plant and animal cells that are part of the food bats and other animals eat.

Cholesterol, for example, is a well-known sterol made by animals. Plants make their own distinctive sterols. These sterol markers pass through the digestive system into excrement and can be preserved for thousands of years.

“As a piece of work showing what you can do with poo, this study breaks new ground,” said Michael Bird, a researcher in environmental change in the tropics at James Cook University in Queensland, Australia, who was not involved in the new study.

“They really extended the toolkit that can be used on guano deposits around the world.”

Like sediment and ice core records, the guano core extracted from the Jamaican cave recorded the chemical signatures of human activities like nuclear testing and leaded gasoline combustion, which, along with radiocarbon dating, helped the researchers to correlate the history seen in the guano with other events in Earth’s climate history.

Bats pollinate plants, suppress insects and spread seeds while foraging for food. Shifts in bat diet or species representation in response to climate can have reverberating effects on ecosystems and agricultural systems, the scientists said.

“We inferred from our results that past climate has had an effect on the bats. Given the current changes in climate, we expect to see changes in how bats interact with the environment,” said Lauren Gallant, a researcher at the University of Ottawa and an author of the new study.

“That could have consequences for ecosystems.”

The research team, which included bat biologists and a local caving expert, also followed living bats in Belize, tracking their food consumption and elimination to gain a baseline for the kinds of sterols that pass through to the guano when bats consume prey from different food groups.

Plant sterols spiked compared to animal sterols about 1,000 years ago during the Medieval Warm Period (900-1,300 CE), the new study found, a time when cores of lakebed sediments in Central America suggest the climate in the Americas was unusually dry.

A similar spike occurred 3,000 years ago, at a time known as the Minoan Warm Period (1350 BCE).

“Drier conditions tend to be bad for insects,” Dr Blais said. “We surmised that fruit diets were favored during dry periods.”

The study also found changes in the carbon composition of the guano that likely reflect the arrival of sugarcane in Jamaica in the fifteenth century.

“It’s remarkable they can find biochemical markers that still contain information 4,000 years later,” Professor Bird said.

“In the tropics, everything breaks down fast.”

The researchers said their approach demonstrated how their techniques could be used to glean ecological information from guano deposits around the world, even those only a few hundred years old.

“Quite often there are no lakes around, and the guano provides a good option for information about the past. It also contains biological information that lakes don’t.” Professor Bird said.

“There’s a lot more work to do and a lot more caves out there.”

The research is published in the Journal of Geophysical Research: Biogeosciences.