Foraging bats can warn each other to stay away from their dinners
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.
A five-person team from the University of Maryland, including Professor Cynthia Moss (Psychology/ISR) and two of her alumnae, 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 team was led by Biology Research Associate Genevieve Spanjer Wright, a University of Maryland alumna (2012 Ph.D. Biology) advised by Moss and Professor Gerald Wilkinson (Biology), who is also one of the researchers here. Psychology/ISR Postdoctoral Researcher Chen Chiu, the manager of the University of Maryland Auditory Neuroethology Laboratory, is also part of the research team. She is an alumna (2008 Ph.D. NACS) who also was advised by Cynthia Moss. Auditory Neuroethology Laboratory Research Assistant Wei Xian is also on the team.
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. Future investigations will explore the potentially sophisticated nature and function of bat social calls.
Movies and figures
Movie S1: Animation showing an increase in interbat distance following FMB emission as two males compete for prey (Movie S1 is related to Figure 3.) In this movie, the two bats are represented by different 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 bottom graphs show characteristics of the bats’ calls. The first graph represents the time between vocalizations, the second is the duration of each call, the third is the frequency (pitch) of the call. The FMB calls are shown in green.
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.
Movie S2: 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 S3: 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: 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: Bat flight configurations before and after FMB emission. 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: 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.)
Published March 27, 2014