Which Bat Is That? The Smell Will Tell

This study aims to identify VOC, or chemical biomarkers, that can indicate if a bat is infected with the Pd pathogen or is healthy. Photo courtesy of U.S. Fish and Wildlife Service, via Flickr.

For the first time, people can distinguish one bat species from another by smell alone.

Scientists from the USDA Forest Service and Arkansas State University found that a new, portable electronic nose (e-nose) device is capable of distinguishing between bat species by their smells.

This study is part of a larger effort to help bats survive White-Nose Syndrome (WNS). Since the disease first appeared in North American bats in 2006, it has killed well over 6 million bats.

Certain bat species are vulnerable to this fungal disease in their hibernation state, or torpor, when bodily functions like metabolism largely shut down. “The vast majority of their immune system also shuts down, so they can’t effectively defend against it,” says USDA Forest Service research pathologist Dan Wilson.

Bats eat mosquitoes and biting flies as well as many other agricultural and forest pests that annually cause millions of dollars in damage to crops and trees in the U.S. The sooner WNS is diagnosed in bats by early-detection methods, the better their chances of recovery through early control treatments.

Wilson and colleagues tested a new method for distinguishing between bat species. The researchers will later test its capabilities for diagnosing WNS in individual bats. The results were published in a special issue of the journal Biosensors about noninvasive early disease diagnosis.

In Phase 1 of their research, the team learned that the WNS-causing fungus, Pseudogymnoascus destructans (Pd), releases volatile organic compounds (VOCs) that differ from those of similar fungi common to caves and bat skin surfaces.

In Phase 2, Anna Doty tested a handheld Cyranose 320 (C-320) e-nose that is lightweight and small enough to carry in the field. Study results indicated that healthy bats of nine species, sampled outdoors in summer, each produced a distinct aroma signature pattern.

“Like any organism, different species have their own unique smells, created by their different metabolic pathways,” says Wilson. In fact, each individual animal also has its own specific smell pattern. “It’s like a human-tracking hound dog (used by law enforcement) that can smell the difference between individual humans.”

This is the first use of an electronic nose to study bats. It’s the first study to assess and record the whole-body VOC composition of bats as a single, species-specific smellprint signature. This study phase resulted in a library of species signatures for normal, healthy bat emissions.

Wilson teamed up with WNS researchers at Arkansas State University who knew the locations of bat caves in northern Arkansas and had permission to conduct research in certain caves—including many on private property.

The researchers also collected air samples from bats in Louisiana and Arkansas, not far from Wilson’s lab at the Center for Bottomland Hardwoods Research in Mississippi.

The team had to create a new device to collect bat air samples. “We built a small sampling chamber that would accommodate live bats so that we could collect an air sample without suffocating them,” Wilson says. While the device removes a sample of air from around the bat for testing, it simultaneously brings in purified air so the bat can breathe.

The researchers built a bat VOC chamber to collect samples to test with the C-320 e-nose. Photo by Dan Wilson, USFS.

They collected bat-air samples on site and tested these using the portable C-320, and again in the lab with an analytical e-nose combined with gas chromatography.

“The C-320 e-nose has 32 sensors,” says Wilson. “You put all those sensor outputs together and you get a pattern like a bar graph. The relative position and magnitude of sensor output patterns were unique for each bat species.”

With Wilson’s e-nose, land managers could determine the species of bats on managed land. “It could help wildlife management because you need to know what species you’re managing,” Wilson says. “A lot of smaller bat species look so similar that you can’t easily tell them apart, especially in larger caves where bats are not close due to being very high up on cave ceilings.”

This portable e-nose technology is already available commercially, but the software libraries of bat signatures would need to be added to the e-nose before it could be used for field applications.

In the meantime, the article contains sufficient information to instruct researchers or land managers how to build their own database of smellprints for any wildlife species. The article also provides details on how to build a sampling chamber for obtaining air samples safely from bats or other animals (alternatively, bat air samples in caves may be collected directly and noninvasively from torpid bats without capture or physical contact).

In Phase 3, the researchers will acquire smellprints of Pd-infected bats to include in their database. They expect that bats infected with WNS will smell substantially different from healthy bats. They’ll also smell different from the fungus by itself, studied in Phase 1. The result will make an e-nose that is capable of WNS diagnosis in the field in real time.

“Ultimately, we want to be able to detect this disease noninvasively in bats in very early stages of the disease before WNS symptoms appear,” says Wilson, “because then you can treat it early and get a much better prognosis or likelihood that the individual will recover from the disease.”

Read the full text of the article. Read Wilson’s summary of the special journal issue.

For more information, email Dan Wilson at alphus.wilson@usda.gov.

Access the latest publications by SRS scientists.