People have always been amazed at the ability of bats to hunt and catch prey in complete darkness. Bats are able to navigate complex environments with precision and capture prey with superior accuracy. These feats are possible through a process known as ‘echolocation’. During flight, bats emit calls in rapid succession and then process the returning echoes to understand the location of objects in the environment. Even though echolocation is a powerful tool for bats, there are some limitations. For example, they have to contend with potential masking effects of additional echolocation calls produced by other foraging bats. Therefore, it seems doubtful that bats would stress their vocal system by producing additional vocal calls that are not directly connected with echolocation. However, a recent study published this past month by researchers at the University of Maryland demonstrated that big brown bats (Eptesicus fuscus) produce additional social calls on top of their echolocation calls during flight to repeal competing foraging bats.
Figure 1. A bat using echolocation to determine the location of a fly
(http://faculty.clayton.edu/sburnett/Bat-Visits/How-do-bats-navigate)
The use of vocalizations for social communication in big brown bats is not fully understood. One particular social call know as a frequency-modulated bout (FMB) is composed of three to four ultrasonic pulses that are longer in duration and lower in frequency compared to echolocation calls. The researchers in this study wanted to determine the function of these FMBs by examining the circumstances where these calls are emitted as well as their effect on other foraging bats. 1106 flight trials in a darkened flight room with specialized recording equipment for ultrasonic calls were completed in various social contexts. Trials were conducted with flying males and females alone, another bat of the same sex, or another bat of the opposite sex. The movements of the bats following FMBs were measured relative to other foraging bats as well as relative to the location of a prey item in the flight room. From these trials, only 186 FMBs were detected.
Figure 2. A big brown bat
(http://www.liveandlocalenc.com/bats-barracks/)
The researchers noticed that FMBs were only produced by males and in the presence of a second bat in the flight room. Moreover, when the FMBs were produced they appeared to immediately change the behavior of the other bat. The researchers saw that within 500 ms following a FMB, the distance between the two bats increased by an average of half a meter. Also, if a bat was close to a prey item when the other bat emitted a FMB, the bat would immediately move away from the prey item. Individuals that emitted more frequent FMBs had about twice as much prey capture success compared to those that didn’t emit FMBs. These results show that FMBs repelled other foraging bats from both caller and prey. Therefore, the researchers concluded that the emission of FMBs increased the caller’s foraging success.
The researchers reveal a new level of functionality in one of the most complex vocal systems studied. While the study increased our understanding of social communication in the field of sensory ecology, it also raised new questions. For example, how does the sensory system integrate different aspects associated with foraging such as echolocation, emitting social calls and responding to social calls? Moreover, the results provide evidence that FMBs are able to modulate the behaviors of foraging bats, but they do not explain how these vocalizations induce changes in behavior. Although, this question will be the focus of many future studies, the current study provides valuable insight into understanding how vocalizations mediate social interactions among foraging animals. If you want to learn more about how social calls predict foraging success in big brown bats check out the paper in the April 2014 issue of the Journal of Current Biology.
References:
G.S. Wright, C. Chiu, W. Xian, G.S. Wilkinson, C.F. Moss (2014). Social calls predict foraging success in big brown bats Curr. Biol., 24, 885–889
