Stand near the mouth of most any cave favored by Mexican free-tailed bats, take a deep breath, and sniff the air. Those with a discerning nose may notice that the pungent wave of odors includes a pleasantly familiar smell: the aroma of fresh corn tortillas. In fact, a primary odor of both bat roosts and tortillas is produced by the same chemical compound.
Bats seem to use their sense of smell for many critical tasks that we are only now starting to unravel. Mother free-tailed bats apparently use smell to help identify their offspring in crowded nursery roosts, to recognize one another, and to attract mates.
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 used their sense of smell to diagnose illness in people.
As for the Mexican free-tailed bats (Tadarida brasiliensis), the compound responsible for the odor is called 2-aminoacetophenone. It is present in products made from masa, the corn flour used in tortillas and many other foods and is an important flavoring in beer, some wines, teas, and other food products. 2-aminoacetophenone is also emitted by the queen honeybee.
The chemical secrets of 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.
Odor compounds from any particular item are absorbed onto a thin, polymer film. Then they are released into a gas chromatograph for detection at a “sniff port” that identifies which compounds are producing each odor. A number of manufacturers in an assortment of industries have contracted with Microanalytics to determine the source of aromas associated with their products, either to eliminate offensive odors or to accentuate pleasant ones.
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, 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. The bats in my care engage in a variety of scent-related behavior that might be illuminated through such research.
Another bat rehabber, Amanda Lollar, and I have watched many a Mexican free-tail male dribble urine and rub the gular gland on his throat against cage surfaces and roosting pouches. By vigorously rubbing the top of his head in this sticky mixture, a bat marks both himself and his tiny territory and clearly signals the presence of an amorous male.
Male sac-winged bats (Saccopteryx bilineata) undertake a similar kind of “perfume blending” by mixing urine and other secretions in special sacs on their wings. These aromatic sacs open and close as they hover in front of females, fanning their scent toward the objects of their affection.
I've also noticed that free-tailed females that have just given birth have a very distinctive odor. A new mother typically rubs her glandular face on her pup, and apparently uses these chemical cues and special vocalizations to locate and identify her own infant among thousands or millions of others that often are jammed together in caves.
AromaTrax and other smell-related technologies may help clarify such behavior and identify other ways in which bats use odors. The technology might permit us 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.
We have so much yet to learn about how bats interact with odors that it's hard to predict where new knowledge might lead us and what uses we might find for it. We have barely scratched (and sniffed) the surface.