Discriminating predation attempt outcomes during natural foraging using the post-buzz pause in Japanese large-footed bat Myotis macrodactylus

Background

Sensing the world through sound is a well-known ‘bat thing’, and their use of echolocation has been a research topic for decades¹. Echolocating bats usually emit a call (pulse), wait for that to bounce back from objects (echo), listen to those echoes and use them for navigation, prey capture, and obstacle avoidance, amongst other things. A call emitted during foraging usually comprises of three phases– the search, approach and terminal phases – depending on their acoustic features. After approaching a prey, bats rapidly emit a series of repetitive calls to close in and capture prey that becomes visible in a spectrogram during their ‘terminal phase’ as a feeding or terminal buzz. Despite being a good indicator of prey capture attempts this unique acoustic feature provides little information on capture success. In this preprint, the authors have studied call features of the terminal buzz in the Japanese large-footed bat Myotis macrodactylus and provide a reliable acoustic indicator specific for capture success in echolocating bats. Building upon previous research from other bat species^2,3, the authors have recorded foraging activity in wild bats using synchronized video and audio recordings. This synchronization helped them to tease apart acoustic differences between successful and unsuccessful captures. They show that the time between a terminal buzz and the following search phase call, the post-buzz pause (Figure), was longest during successful captures and suggest that this could serve as an acoustic indicator of foraging success in echolocating bats.

Key findings

• From a natural foraging site, the authors collected 220 minutes of synchronized video and audio recordings spanning six nights. First, they identified 137 capture attempts from their videos and showed that bats kept hold of their prey in 51.8% of these attempts, dropped prey in 29.2% and failed to capture in 19% of the attacks.
• Using the acoustic recordings that corresponded to the attacks filmed in videos, the authors identified 135 terminal buzzes. From the spectrograms of these buzzes, they measured 87 post-buzz pauses – the time between the last sound in the terminal buzz and the start of the next search pulse (Figure).
• Modelling the post-buzz pause as a function of attack outcome along with weather and temperature measurements, they showed that changes in post-buzz pause was best explained by the attack outcome i.e., capture success, drop or failure.
• They also showed that the post-buzz pause was shortest during failed captures, intermediate during drops and longest during successful captures.
• In addition, the authors described some additional behaviors from their video recordings, such as bats attacking inedible objects like tree branches, attempting to capture a prey item that was initially failed to be captured and dropping objects after holding them for a long time

Figure (from the preprint) (A) Spectrogram of an echolocation call sequence from Myotis macrodactylus during prey capture. The terminal buzz is emitted just before attacking the prey. The time between the attack and the next search pulse is called the post-buzz pause. (B) Example search-phase echolocation call from M. macrodactylus. The fundamental frequency of this frequency-modulated (FM) pulse drops from about 90 to 40 kHz, with a peak frequency of 50 kHz.

The authors highlight and discuss the reliability of post-buzz pauses in measuring predation success in echolocation bats as it varies among the different capture outcomes (success, drop and failure). Since M. macrodactylus dropped prey during 30% of their attempts, the authors point out that these bats might be weak in discriminating prey before capture but rather select prey after capturing, thus dropping those items that were rejected. Next, on citing a laboratory-based study in another species⁴, the authors recognize the role of prey size on the post-buzz pause and point out the importance of follow-up studies on this topic in the future.  Finally, the authors shortly discuss a field experiment where they only recorded post-buzz pauses without videos in the same location. They point out that the time of bat visits, foraging attempts (terminal buzzes) and success (long post-buzz pause) varied within a night at different time points. As such, they highlight the utility of a measure such as the post-buzz pause in investigations of temporal aspects of foraging behavior in naturally foraging bats.

Why I chose this preprint

Echolocating bats that emit frequency-modulated (FM) calls have the unique acoustic feature of a terminal buzz to indicate foraging attempts. This is a fairly common and easily noticeable feature in an ultrasonic spectrogram recorded in any natural foraging site. However, an acoustic indicator that teases apart successful and unsuccessful attempts has not been identified. The current study has addressed this issue and describes a reliable acoustic indicator that could be used as a proxy for foraging success in the Japanese large-footed bat Myotis macrodactylus. While spectrograms are useful in visualizing the acoustics of foraging, they do not help much in distinguishing a successful capture from a failure. The authors in this study have achieved this by visualizing foraging attempts using videos and synchronizing them with corresponding audio recordings. This combination of tools not only provides us a new proxy for foraging success in wild bats but has also opened up new ways of studying foraging ecology in echolocating bats.

Questions to the authors

1. Do these bats capture prey in flight in addition to trawling on the water surfaces? If so, do you think the post-buzz pauses will be influenced by that?
2. Were there any differences in the post-buzz pauses within the same individual? Since there were calls recorded from the same bat, I am curious to know if the time of handling prey varied among individuals
3. If post-buzz pauses are influenced by prey-size, do you think different bat species handling the same sized (or type of) prey could have similar post-buzz pause times?
4. Can other acoustic indicators such as average inter-pulse intervals, lengths of terminal buzz I and II be measured and validated using your methodology? If so, can there be more than one indicator in the future that could be used in combination to measure capture success in bats?

References

1. Fenton, M. B. The foraging behaviour and ecology of animal-eating bats. Can. J. Zool. 68, 411–422 (1990).
2. Britton, A. R. & Jones, G. Echolocation behaviour and prey-capture success in foraging bats: laboratory and field experiments on Myotis daubentonii. J Exp Biol 202, 1793–1801 (1999).
3. Acharya, L. & Fenton, M. B. Echolocation behaviour of vespertilionid bats (Lasiurus cinereus and Lasiurus borealis) attacking airborne targets including arctiid moths. Can. J. Zool. 70, 1292–1298 (1992).
4. Surlykke, A., Futtrup, V. & Tougaard, J. Prey-capture success revealed by echolocation signals in pipistrelle bats (Pipistrellus pygmaeus). Journal of Experimental Biology 206, 93–104 (2003).

Tags: acoustics, bats, foraging, predation

doi: https://doi.org/10.1242/prelights.30556

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