Summary

Ghost bat population numbers are declining and the species has now been declared a threatened species. We know next to nothing about the ghost bat and this makes it very difficult to protect them. Using new technological advances we are deciphering the ghost bat's social communication to uncover necessary information on social structure, behaviour and movements. The result will be more targeted threatened species management plans and the development of an innovative low disturbance monitoring methods that uses social calls to indicate important colony events such as reproduction and disturbance. Your pledges will buy sound recording backpacks that will be worn by ghost bats and will allow us to collect important demographic information about the producers of the calls and their vocal reactions to a variety of aural and visual stimuli allowing us to determine the function of each social call.

Background

Eerie high-pitched calls emanate across the northern Australian night. The source is the ghost bat (Macroderma gigas), a large carnivorous ‘micro’ bat that lives in caves during the day and forages in tropical woodlands at night. It is among the most distinctive of Australian bats, but we know very little about this iconic and threatened mammal. What we do know tells us that the ghost bat is a highly unique species with a particularly unusual social lifestyle. Where else do you find top-level predators living together in such high numbers in a confined space!? 

Ghost bats communicate vocally with calls in the audible and ultrasonic range (listen here), but it is yet unknown what the specific purposes of the calls are. If their calls can be deciphered, we can unlock a wealth of information about ghost bats, including on their social structure and movements between roosts, which is important for informing successful management plans.

It has historically been difficult to study bat social vocalisations, but fortunately, recent advances in acoustic technology and analytical techniques mean that we can now study such vocalisations in great detail. 

If this crowdfunding campaign is successful, we will use your pledges to obtain cutting-edge miniaturised acoustic recording units (tiny microphones carried by the bats), which will open our ears to the complex vocal interactions that appear critical to the survival and reproductive success of this unique social mammal. 

Why do we want to know? 

Sadly, this year the ghost bat was upgraded to a vulnerable listing under the EPBC Act due to population declines across much of its range. While roost disturbance and mining are implicated in the declines at some locations, in other locations the reasons for falling numbers is entirely unclear. Information on social organisation is key to understanding the environmental threats faced by ghost bats as it affects gene flow, population growth rates and dispersal; and thus is necessary for assessing the species’ vulnerability to environmental change.

Little is known about social interactions in bats and particularly the roles that vocal communication plays in this group. Research on animal communication to date has concentrated on birds, cetaceans, and humans; and thus, our study will add a valuable facet to the study of the evolution of vocal communication and sociality more generally.

Our work has the added advantage in that once the ghost bat’s vocal repertoire has been deciphered, colonies can be monitored over long periods with passive sound recorders that cause little or no disturbance to the bats. This is of utmost importance as the disturbance of roosts by humans has been identified as one of the main threatening processes affecting ghost bat populations. We hope that our monitoring methods can soon be expanded beyond ghost bats to a range of vocal animals with calls in both the ultrasonic and human audible range.

Plate 1 The ghost bat (Macroderma gigas) Photo Credit: Nicola Hanrahan

To understand ghost bat vocal communication, we are already recording continuously at known cave roosts across the Northern Territory using passive sound recorders. We are using this extensive collection of recordings to characterise the ghost bats’ vocal repertoire and to assess how calls vary within and between isolated colonies throughout the year. While this method is incredibly useful for studying calls at a colony scale, it does not allow us to know who exactly is vocalising, but this more detailed information is required for determining the functions of the different calls.

This is where your pledges come into play!

Wouldn’t it be great if we could have ghost bats in a group setting with minimal human interference but still be able to determine which individuals are calling? Devices such as the Neurologger (Evolocus LLC) lets us do exactly that! The Neurologger was originally developed for looking at brain activity in birds and rodents. Application of the device is expanding, for example Anisimov et al.  2014 (more here) used the Neurologger to investigate vocal interactions in zebra finches allowing vocalisations to be separated irrespective of the number of individuals calling simultaneously.

Figure 1 (a) The Neurologger 2A device shown fitted on a zebra finch (b) the output expected from 

the Neurologger 2A showing the male calling and the female listening. Figure from Anisimov et al 2014

For our study colony, we propose to use captive ghost bats already held at a wildlife park as this will allow us to manipulate group dynamics and easily retrieve recorded data from the devices. Each unit is fitted to one ghost bat using a modified collar or ‘batpack’ for up to a week. The microphone is positioned towards the bat’s head and records each individual bat’s vocalisations, while the accelerometer records the body vibration caused by the call enabling the caller to be identified (Figure 1). We know that this attachment method will work because other bat species have already been fitted with similar-sized devices for the purposes of tracking or physiological measurements with no adverse effect.

Bats fitted with the recorder backpack will be recorded while communicating naturally in a group. This allows us to determine if there are any age or sex biases within their call repertoire. Following this, we will record the behavioural response of the ghost bats to the playback of conspecific calls. The reaction of each of the bats to these stimuli will be recorded, allowing the function of the call to be worked out and subsequent playback experiments to be designed. Hetero-specific stimuli such as models and calls of predator and prey will also be used to provide further understanding of behavioural and vocal reactions to other species.

Plate 2 PhD student Nicola Hanrahan with a captured ghost bat in the field.
Bat was released shortly after. Photo credit: Damien Stanioch

The Team

This project brings together a team dedicated to studying animal behaviour and bioacoustics. Nicola Hanrahan is a PhD student at the Hawkesbury Institute for the Environment at Western Sydney University with a keen interest in using innovative, low disturbance methods to increase our knowledge of the social organisation of threatened bats. Nicola is working with experts in the forefront of this field; Dr. Justin Welbergen, Dr. Christopher Turbill from the Hawkesbury Institute for the Environment, Dr. Anastasia Dalziell from the Cornell Bioacoustics Research Program, and Dr. Kyle Armstrong from the University of Adelaide.

Plate 3 Clockwise from top left: Dr. Justin Welbergen, Dr. Christopher Turbill, Dr Kyle Armstrong and Dr. Anastasia Dalziell

Our funding target is $6,000 and if raised, will go towards the purchase of three Neurologger devices and the usb cable required to download the data recorded. The units are reusable, so once pruchased can be used for further studies on ghost bats or other vocal animals. 

The funds will also be used to purchase and deliver our exciting range of pledge rewards and to cover Pozible and credit card fees.

One of the main challenges when studying communication is first identifying the range of sounds a species makes. This step has already been completed by recording a huge amount of vocalisations at wild ghost bats roosts across the Northern Territory over an eight-month period. We have analysed the recorded calls and determined the range of distinct vocalisations that make up the ghost bats repertoire.

Determining the function of calls is challenging but we have overcome this by monitoring both captive and wild individuals in their roosts using video surveillance. This allows us to observe the behaviour that is linked to each of the vocalisations letting us form clear hypotheses on the function of the calls.

Despite the ghost bat's large size in comparison to other echolocating bats, they are difficult to detect in the wild. However, in this project we capitalise on recent technological advances that make studying ghost bats much easier. Both ghost bat echolocation and social calls are very difficult to detect and record using traditional bat monitoring methods eg. using zero crossing detectors. This is due to the low amplitude of ghost bat echolocation calls and the complexity of their social calls. To overcome this problem, we are recording in full spectrum format, which allows us to capture the vocalisations in fine detail.