So you guano the secrets of bat poop?

04.24.24

By Alyson Brokaw

Peer down into the roughly 100 foot crescent crack that makes up the entrance to Bracken Cave, you might be able to make out the rocky slope that leads into the darkness of the cave. Past the rocks, the ground gives ways to soft, dusty dirt – only it isn’t just dirt. It’s the decomposed poop of the over 15 million bats that call the cave home each summer. Bat poop (along with bird poop) is also called guano, a word originating from the Andean indigenous language Quechua and refers to any form of animal dung used as fertilizer. The bats in Bracken Cave deposit an estimated 50 tons of guano each year, resulting in dung heaps that can reach 60 feet deep. And even if all that all sounds like a load of crap, bat poop may still hold a few surprises. 

The floor of Bracken Cave. Photo by Andy Moore.

Is bat poop dangerous? 

Bat guano can have a lot of benefits, but it is also important to consider safety. In areas with large amounts of bat or bird guano (such as inside a bat cave or a chicken coop), there can be risks to human health. One of the biggest risks is histoplasmosis, a disease caused by a microscopic fungus called Histoplasma capsulatum. The fungus is found naturally in soils, especially in areas of accumulated bat and bird droppings. When these poop-rich soils are disturbed, fungal spores become airborne where they can be inhaled, infecting the lungs of people and pets. Those who are most at risk include farmers, workers digging or doing demolition, cavers and those who are immunocompromised. Protective equipment like fine-particulate respirators and gloves help reduce the chance of exposure. That said, a few pellets of bat poop here or there do not pose a major health risk (just don’t taste it!).

Guano under microscope. By Babita Garung.

Trying to figure out if bats are responsible for some scattered small poops that you discovered in your attic or along the edges of your house? Squish it up a bit (make sure to wear gloves and wash hands afterwards) and hold it up to the light. If it appears dull, then it likely came from a mouse or other small rodent. But if it glitters and sparkles like a ‘Twilight’ vampire, then it’s bat poop! That’s because bats are not able to digest the shiny, hard outer shells of insects, resulting in shimmery poop. 

When guano happens, make compost

The bits of insect shells found in guano are rich in elements like nitrogen, phosphorus, and potassium, all important for plant growth. Nitrogen promotes the growth of green stems and leaves, while phosphorus supports roots and potassium helps with producing flowers and fruit. The recommended way of applying guano to gardens is by mixing the fertilizer with water to form a guano “tea” that can then be applied to plants. Guano-based fertilizers are naturally organic, but make sure you check that any purchased bat guano is organic-certified (look for OMRI and NOP labels). 

Hardwicke’s Woolly Bat (Kerivoula hardwickii) at a pitcher of Nepenthes baramensis. Ch’ien Lee/Minden Pictures

Beyond gardens, nightly bat flights help distribute these important fertilizing elements throughout the landscape. Thousands to millions of bats pooping while they fly across forests and fields results in a “salt-shaker effect” of free fertilizer. Bat colonies also help fertilize their roost trees and some pitcher plants in southwest Asia have evolved mutually beneficial relationships with bats, providing a safe place for bats to roost in exchange for a bat poop boost. 

A delicate balance

Around sunset, bats living in caves around the world will begin their exodus into the skies in search of food. But many cave-dwelling critters will never leave the relative safety of their cave world. Many of these troglobites, including mites, beetles, isopods, and flies rely on guano as a source of food. In turn, these animals form the foundation of a guano food pyramid, eaten by predators like whip scorpions, spiders, and scorpions. Some vertebrates also rely on the environment created by guano, including some salamanders and frogs. Protecting bats at their caves also means protecting and conserving these fragile cave ecosystems. 

Scooping poop for science

One animal’s waste is another’s treasure! Bat scientists have been using bat poop to uncover what bats are eating. For a long time, this meant delicately dissecting poop pellets, sorting through half-digested sparkly bits and playing “guess the insect” from just a fragment of leg or wing. Now scientists use DNA barcodes, which are very short sequences of DNA that vary predictably between different species, thus allowing identification of those little sparkly bits. In addition to meaning fewer hours squinting through a microscope, DNA techniques also mean scientists can start to identify the different fruits, flowers, and even other vertebrates that bats eat. 

MALLORY DAVIES COLLECTS BAT GUANO. VIDEO BY SHAWN THOMAS.

Remember the dung heaps in Bracken Cave? In some caves, mounds of bat poop have been accumulating for hundreds, even thousands of years. Similar to the way scientists count tree rings or examine deep ocean soil cores to uncover information about the deep past, ancient bat poop can help provide information about ancient environments. The abundance of isotopes of different elements can give useful information about long-ago climates. In the case of insect-eating bat poop, nitrogen and hydrogen isotopes give clues about patterns of rainfall in ancient history, from as far back as 12,000 years ago! 

Who knew bat poop could be so poop-ular?!

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