4,300-year-old Bat Poo Pile May Tell Tale of Climatic Changes

Tall tell tells tale of bats in Jamaica, which started eating more fruit instead of bugs during the Minoan and medieval warm periods

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The Jamaican fruit-eating bat, which contributed to the poo tell in Home Away from Home Cave
The Jamaican fruit-eating bat, which contributed to the poo tell in Home Away from Home CaveCredit: Sherri and Brock Fenton

How are we going to adapt to climate change? That remains to be seen, but meanwhile a groundbreaking analysis of a 4,300-year-old accumulation of bat guano in a cave in Jamaica shows how they may have coped throughout that time.

The analysis was reported in the Journal of Geophysical Research: Biogeosciences in April. The report starts by extolling the virtues of the aviating animal – no horror of the night but an ecological mainstay by eating the insects we loathe; pollinating; and dispersing seeds, explain Lauren Gallant of the University of Ottawa and colleagues.

We all know there are “fruit bats” that eat fruit and “entomophagous bats” that dine on insects, plus a few outliers like fishing bats and the vampire bat.

But how is one to study change in bat diet over time? The answer lies in sampling guano in a remote cave, in Jamaica, that has been building up for about 4,300 years. By now, the poo pile is about 2 meters (6.5 feet) deep, bless them.

Crucially to the study, as the bat poo was largely left undisturbed all this time, it’s layered by age, just like an archaeological tell unmolested by amateurs, looters and so on. Also, thankfully for posterity and its inhabitants, this cave is not one of Jamaica’s tourism hot spots – neither tourists nor locals ply the place. Nor do archaeologists, despite signs of ancient human presence.

How does one study this largely undisturbed pile of bat guano? One starts by making sure one knows how to detect a diet of fruit, bugs or blood by analyzing fresh bat guano.

Then one samples of pile by coring, a method more associated with oil-drilling or arctic science circles, using a donut-shaped drill bit to extract samples. This process creates a cylinder-shaped core of pristine substance – rock, ice, ancient bat feces – to rise up inside the pipe above it. Coring accrued bat bowel movements hadn’t been done before.

A fruit bat swooping down for an apple.Credit: Yossi Yuval

What did Gallant and colleagues discover in the deep cavern dubbed “Home Away from Home Cave,” but actually the home that is home to about 5,000 bats of five species, bless them.

Their analysis revealed two periods of heavy fruit-eating relative to insect-eating from about 3,000 to 2,500 years ago (known as the “Minoan warm period”, which was a whopping 4 degrees Celsius warmer than the average today) and from the year 700 to 1900 – coinciding in part with the famous Medieval Warm Period aka the Medieval Climatic Anomaly, from 900-1300.

Enter the Minoan warm period

What can we learn from this? Maybe nothing. One possibility is that, for reasons unknown, the mix of bats inside the cave fluctuated, with fruit bats gaining the ascendancy during those two periods. Or there is the possibility that the bat roost as a whole was adapting to change in the environment.

Separate research has shown that those intervals of intense fructivorousness in the Jamaican cave were relatively hot and arid in the region, sub-ideal for the insects.

“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,” Gallant said, adding: “That could have consequences for ecosystems.”

Indeed. In this era of accelerating climate change, any indicator of adaptation bears study. Minoan or medieval warm periods may have affected Jamaican insect populations; and now, in our current era of anthropogenic planetary heating, our friends the bugs are being slammed.

A couple of common fruit bats hanging out.Credit: Itai Belson / Weizmann Institute of Science

“Climate change impacts on insects have the potential to be considerable, even when compared with changes in land use,” a team wrote in PNAS earlier this year. Insects may have a 450-million-year history of adapting, but then humans and their pesticides became involved, leading to concerns of “insectageddon.” The dimensions of the decline of insects are unclear, but the trend is unarguable and examples are legion.

And what will the bat do then? Does the study indicate that insect-eating bats might move to “our” orchards? Well, that depends. Not to denigrate from the importance of the bee, but fruit bats are crucial to pollinating some of humankind’s favorite crops – including bananas, guavas, peaches and mangoes. Raise a toast to the bat with tequila – made of the agave, which also depends on bat services for pollination. But insects are crucial too, and they did it first.

Incidentally, the researchers note that their analysis also detected changes in the composition of bat bowel movements in the 15th century, possibly reflecting the start of sugarcane cultivation on the island.

By the way, the cave wasn’t always the exclusive fief of bats. Explorers describing the cave in 2005 also found petroglyphs engraved on the walls, which seem – they say – to be similar to prehistoric Taino petroglyphs elsewhere in the region. The images are clearly old as they were patinated, though how old remains to be seen. Settlers reached the region at least 6,000 years ago.