Foraging bats can warn each other to stay away from their dinners

COLLEGE PARK, Md., March 27, 2014 /PRNewswire-USNewswire/ -- Look up into the sky at dusk this spring and you're likely to see small groups of bats flitting here and there, gobbling up their insect meals in an intricately choreographed aerial dance. It's well known that echolocation calls keep the bats from hitting trees and each other. But now scientists have learned some bats emit another call: one that tells their comrades to "back off" from bugs they've claimed for themselves.

Led by Biology Research Associate Genevieve Spanjer Wright, a five-person team from the University of Maryland found that male big brown bats can produce a special sound, called a "frequency-modulated bout" (FMB), that tells other bats with whom they are foraging to keep away from their prey. The Maryland researchers are the first to report this ultrasonic social call produced exclusively by flying, foraging male big brown bats.

The study, appearing in the March 31 issue of the Cell Press journal Current Biology, shows how important and useful vocal social communication is for a nocturnal animal foraging with others of its species in a fast-paced setting.

The researchers first got curious after examining audio recordings from two bats flying and foraging together. They noticed calls that seemed different from typical echolocation calls. To find out more, they used careful analysis of the video and audio recordings of bats' flight paths and calls as male and female big brown bats flew alone and in pairs foraging for tethered mealworms. This led them to discover the FMB.

The FMB is an ultrasonic "social call" that uniquely identifies the bat emitting it. It is a sequence of three to four sounds that are longer in duration and lower in frequency than the typical echolocation pulses big brown bats use to navigate, often followed by several short, buzz-like calls.

The researchers found the FMB causes competing bats to alter their flight behavior and increases the success of the caller in snagging the insect for himself. After hearing the FMB, other bats increased their distance from the caller, changed their flight configurations, and moved further away from the prey. In addition, says Wright, "when two males flew together in a trial, it was not uncommon for each bat to emit FMBs. What we found was that the bat emitting the greatest number of FMBs during the time leading up to prey capture was more likely to capture the mealworm than the bat emitting fewer FMBs."

While some animals that forage in groups are known to emit calls to attract others towards food sources, the FMB is used to repel, not attract, other bats.

"Despite decades of study, many things about common bat behaviors such as foraging remain mysterious," says Wright. "We were able to study a social call that is likely occurring thousands of times a night all over North America during the summer months, yet had not been described or studied before now."

The researchers discovered only male big brown bats emitted the FMB, possibly to advertise their dominance or claim their territory. Female bats did not emit FMBs. One possibility why is that females form close associations with their roost mates and may forage near individuals familiar to them, while males often roost alone or in small bachelor colonies and may be more unfamiliar with those with whom they forage.

In addition to Wright, the research team includes Postdoctoral Researcher Chen Chiu, Research Assistant Wei Xian, Professor Gerald Wilkinson, Department of Biology; and Professor Cynthia Moss, Department of Psychology and the Institute for Systems Research. Future investigations will explore the potentially sophisticated nature and function of bat social calls.

Funding

This research was funded by National Institutes of Health (NIH) grants R01-MH056366 and R01-EB004750, and by NIH National Institute of Deafness and Communicative Disorders grant DC-00046.

Additional information

Current Biology article: http://www.cell.com/current-biology/abstract/S0960-9822%2814%2900259-0

Auditory Neuroethology Lab: www.batlab.umd.edu
Neuroscience and Cognitive Science Program: www.nacs.umd.edu
Department of Biology: www.biology.umd.edu
Department of Psychology: www.psychology.umd.edu
Institute for Systems Research: www.isr.umd.edu

Available movies and figures

Movie 1. http://ter.ps/MovieS1: Animation showing an increase in interbat distance following FMB emission as two males compete for prey. (Movie 1 is related to Figure 3). In this movie, the two bats are represented by the colors red and blue. The bats' movements and vocalizations have been slowed by a factor of 10. The top graph in the movie traces the bats' movements around the flight room. Each circle on the bats' paths indicates a vocalization. The FMB vocalizations are indicated in green. The prey item is a tethered mealworm in the center of the room, represented by a yellow circle. The red bat emits FMBs around 00:07, 00:24, and 00:59 and initiates a feeding buzz and attack on the prey item (yellow circle) around 01:06. Notice the distance between the two bats is greater after the red bat emits the FMBs. The bottom three graphs show the time interval, duration and pitch of the bats' calls; again, the FMB calls are in green.

Movie 2. http://ter.ps/MovieS2: Animation showing a change in flight behavior by one bat after the other emits an FMB as two males compete for prey. In this trial, the blue bat emits FMBs around 00:11 and 00:34. Prior to the first FMB, the red bat is approaching a tethered mealworm and initiates a feeding run but appears to abort the attack after hearing the blue bat's FMB. The blue bat captures the prey near the end of the trial.

Movie 3. http://ter.ps/MovieS3: Animation demonstrating that the bat emitting the greatest number of FMBs attacks the prey as two males fly together. In this trial, the red bat emits FMBs around 00:23 and 00:42, whereas the blue bat emits FMBs around 00:01, 00:09, and 00:27 before initiating a feeding buzz and attack on the prey item around 00:55. Note that in this trial and in general, the bat emitting the greatest number of FMBs attacks the prey item.

Figure 1. http://ter.ps/Fig1: Spectrograms of FMBs and echolocation calls. Examples of FMBs recorded in a flight room from two male bats (top two panels) and in the field (third panel) from a third bat. Regular echolocation calls recorded in the flight room (bottom panel) are shown for comparison. Note that, compared with the echolocation pulses, FMBs last longer, have a shorter amount of time between vocalizations, and have lower pitches. The x axis indicates ms, and the y axis indicates kHz.

Figure 2. http://ter.ps/Fig3: Bat flight configurations before and after FMB emission. Average flight patterns of bats 500 ms before and 500 ms after the start of an FMB. "Trailing" and "leading" are in reference to the bat emitting a given FMB. Flight patterns differed significantly before versus after call emission.

Figure 3. http://ter.ps/Fig4: Interbat distances before and after FMB emission. Mean interbat distance during the 500 ms before versus the 500 ms after FMBs were emitted. Bats flew significantly farther apart after FMB emission (See also Movie S1.)

About the University of Maryland

The University of Maryland ranks among the top 20 public research universities in the nation, and is the closest in proximity to the nation's capital. As the state's flagship university, UMD educates the most talented students from Maryland and beyond. For more information, visit www.umd.edu.

SOURCE University of Maryland