by Barbara French
What do tacos, beer, and Mexican free-tailed bats have in common? Believe it or not, it is the characteristic odor of taco shells. That odor drifting through the air from a bat roost is sometimes mistaken for the aroma of freshly prepared tacos.
As a bat rehabilitator for many years, I have cared for thousands of bats and I often describe the bats I care for in terms of their distinctive odors. Evening bats (Nycticeius humeralis) smell like burnt oranges and reproductively active free-tailed bats like fungus. Even individuals of the same species seem to have distinct odors.
I can often detect bacterial infections in the bats I care for by smell alone. This isn’t really surprising. Before the advent of modern technology, physicians often relied on their sense of smell when diagnosing illness.
Bat researchers have shown that bats use smell not only to detect food, but to recognize one another and to attract mates. Smell appears to be important to both insect- and fruit-eating bats.
But back to the Mexican free-tailed bats (Tadarida brasiliensis). The compound responsible for the odor is called 2-aminoacetophenone. It is present in products made from corn masa, such as corn tortillas, and it is an important flavoring in beer, some wines, teas, and other food products. It is also emitted by the queen honey bee and the larch sawfly.
The real nitty gritty on odors comes from Larry Nielsen, David Eaton, and Donald Wright of Microanalytics in Round Rock, Texas. They designed an elaborate system called AromaTrax™, to connect odors to the chemical compounds that cause them.
The odor compounds from a particular item are absorbed onto a thin, polymer film and released into a gas chromatograph for detection at a “sniff port” that identifies which compound is producing each odor. Various manufacturers have contracted with Microanalytics to determine what is producing aromas that are associated with their products, either to eliminate or accentuate certain smells. It is also used to develop “smell friendly” products.
The chemists at Microanalytics noticed the familiar taco-shell aroma while driving past a large bat roost in Round Rock. Intrigued, they contacted BCI and eventually lowered one of their pencil-sized instruments into the ventilating shaft of Bracken Cave, home to as many as 20 million Mexican free-tailed bats near San Antonio, Texas. Back in the lab, they identified dozens of compounds among the cave smells, including that most characteristic odor of the Mexican free-tails: 2-aminoacetophenone.
Using the same instrument, we collected samples from captive bats. The characteristic odor was present in roosts and urine samples from my captive free-tailed bats — but not from the other bat species, including a hoary bat (Lasiurus cinereus), a northern yellow bat (Lasiurus intermedius), an evening bat, and a cave myotis (Myotis velifer).
The AromaTrax™ technology is likely to prove a powerful tool for bat researchers in the future. It might be used to identify species of bats roosting in inaccessible sites or species that use a roost seasonally but are not currently present. It might even be used to develop attractants to entice bats to artificial roosts.
Barbara French is BCI’s Science Officer and the Bat Rehabilitator for Wildlife Rescue in Austin, Texas.
