Sonar Jamming

Some moths may use high frequency acoustic signals to warn bats of their potential toxicity and thus deter depredation.

Acoustic warning signals emitted by tiger moths (Barsine cuneonotatus) were found in laboratory experiments to deter bat predation. That behavior has now been shown to occur in nature and is used as a defense mechanism. Field research conducted at Wake Forest University on free-flying bats in their natural habitats shows that tiger moths of two species (Pygarctia roseicapitis, the Red-headed Pygarctia Moth and Cisthene martini, the Lichen Moth) produce ultrasonic signals to warn bats they don’t taste good (a form of sonar jamming?). Biology graduate student Nick Dowdy and colleagues documented this behavior – technically referred to as acoustic aposematism (“ah-peh-sem-a-tism”), a sound-based antipredator warning – that was previously proven in the laboratory by Wake Forest Professor Dr. Bill Conner and his former graduate student Jesse Barber (now a professor at Idaho State University).

Many birds, reptiles, amphibians, and even plants use visual aposematic signals such as highly contrasting color patterns or specific colors (red is a common theme) to advertise their toxicity. Many bat species prey heavily on moths and although most bats see quite well, they don’t rely solely on vision at night. Instead, they rely more on sound. Therefore it makes sense that the moths, which are primarily nocturnal, could or would develop acoustic signals to deter the bats. “The signals are, in essence, a warning to the bats that the moth is unpalatable and potentially harmful if ingested . . .” said grad student Dowdy.

And the story gets better! Dowdy says that most moths enact evasive dives and spiraling flight when a bat is about to capture them, to which he attributes an energy cost. However, he continues, “We’ve found that this is only sometimes true in tiger moths and different species appear to use these [evasive] behaviors at different rates.” From this he conjectures that certain species may have evolved to rely on their warning sounds instead of the evasive maneuvers that are common to most eared moths. The fact that some of these moths do NOT use evasive maneuvers suggests that their ultrasonic emissions may function as a sonar jamming function that would frustrate sonar-based hunting patterns of otherwise predatory bats.

Copyright© 2016, Pacific Coast Conservation Alliance/Central Coast Bat Survey

What Bats Say

So what exactly DO bats talk about anyway?

Researchers using a machine-learning algorithm investigated vocal signals of Egyptian Fruit Bats (Rousettus aegyptiacus) in their roost. Their findings suggest not only to they communicate with each other but seem to communicate directly with other members of the roost.

The Egyptian Fruit Bat is a highly social mammal that roosts in crowded, tightly-packed colonies. Many animals communicate vocally, but often non-specifically, but researchers at Tel Aviv University recently discovered that when Egyptian Fruit Bats communicate, they aren’t just making high pitched squeals when they gather together in their roosts. Instead, they often have something specific that they want to communicate.

Here is a summary of the article (published online, 22 December 2016) in the journal “Scientific Reports”: Neuroecologist Yossi Yovel and his colleagues recorded sound and video of a group of 22 Egyptian Fruit Bats over a period of 75 days. Using a modified machine-learning algorithm originally designed for recognizing human voices, they fed 15,000 bat calls into the software and then analyzed the corresponding time-stamped video to see if they could match the calls to certain activities.

They found that the bat calls were not random but instead, they were able to classify 60% of the calls into four categories:
1. Arguments about food,
2. Disputes about position within the sleeping cluster,
3. Responses to male bats making unwanted mating advances, and
4. Bat to bat warnings or scolds when one bat gets too close to another.

Even more specific, the researchers found that a bat would make slightly different versions of his/her calls when speaking to different individuals within the group. Only a few species – including humans and dolphins – are known to address individuals rather than making broad communication sounds.

Where is this research headed? Yovel and his team want to investigate whether bats are born knowing this “language” or if they learn it over time while living in their colonies. They also want to find out if the bats use similar communication modes outside/away from the roost using battery-powered recording devices that they will attach to a small sample of bats, which will be released into the wild.

Citation: Prat, Y., M. Taub & Y. Yovel. 2016. Everyday bat vocalizations contain information about emitter, addressee, context, and behavior. Scientific Reports 6, Article number: 39419.