The Southern Free-tailed Bat

Authored by Janelle Montgomery, CottonInfo Regional Extension Officer. 

Dr Roger Coles from QUT was recently in the Moree district recording the calls of an important beneficial organism of cotton – the insectivorous (insect-eating) bat.

Insectivorous bats play an important role on cotton farms, controlling insect numbers (including a range of pest species like heliothis moths and beetles) by eating 50 to 70 per cent of their body weight in insects each night. However, due to their small size, nocturnal behaviour and cryptic roosting habits, these valuable mammals are rarely seen by growers, so their value to the farm is easily unrecognised.

The bats are relatively long lived for a small mouse-sized mammal, from 5 to 10 years in nature. Reproduction is slow with most species giving birth to a single young once a year. Many bat species live in tree hollows (or under bark) in the remnant vegetation. Some can be found in buildings such as sheds, also under bridges or in culverts. Insectivorous bats will live in a wide variety of tree hollows (where they roost during the day), including dead trees, at least until they fall over. Dead broken limbs on river red gums are popular, and it depends on the size of bat, the time of year (for breeding) and they have to compete with birds and possums as well. Importantly, it takes years for trees to develop hollows, in fact some trees don't develop hollows until they're well over 100 years old, so maintaining remnant vegetation is crucial.

“We know from research – in cotton and elsewhere – that insectivorous bats play a significant role in natural pest control in cotton landscapes. However, if we want to keep these important natural pest controllers, we need to maintain and even improve their remnant habitats,” Leah MacKinnon, CRDC PhD, University of Sydney (2007)

All insectivorous bats use ultrasonic ‘echolocation’ for navigation when flying, including insect prey location. Echolocation is a biological sonar system, and in this case, bats can determine the distance and location of objects using high pitched sound. When they are flying and hunting in the air, they produce rapid sequences of very brief sound pulses through their mouth (in some species the nose is used). Then they listen for any reflected sound returning as echoes. The ability to use echolocation allows insectivorous bats to move around in pitch darkness and behave like miniature aerial acrobats. They navigate, hunt, catch insects on the wing, identify friends and enemies, and avoid obstacles – very clever stuff!

It’s only been through pretty innovative equipment that we can detect and record these echolocation calls from flying bats, as well as being able to translate the sounds into something that we can hear and also see on a computer screen. The technology has advanced so much that we can now survey for insectivorous bats based on their echolocation sounds in flight rather than necessarily having to capture them for identification.

Dr Coles (pictured) is working as part of the Cotton Landcare Tech-Innovations 2021 project* that is investigating the biodiversity of insect eating bats (and birds) which live in the remnant vegetation on farms adjoining cotton fields and finding those species which hunt for insects over growing cotton.  He is looking at the number of different bat species there are (the biodiversity), along with how common each species is, all by recording and analysing their echolocation signals.   

To make this approach work, there are differences (sound signatures) that can be recognised between the echolocation calls used by the 16 or so bat species that live in and around the cotton growing areas in New South Wales and Queensland. However in some cases bats have very similar signals so it can be difficult distinguishing the different species. To try to solve this problem, part of the study that Dr Coles is involved with, is to develop a more effective technique to identify bat species by the sounds they use for echolocation. This is being done using computer-based AI (artificial intelligence).

To be more specific, the team of researchers at QUT is working on an advanced machine learning technique known as a neural network. The idea is to use computers to automatically recognise the bat signals to tell what species is present. They do this by ‘training’ (and testing) the computer with the reference recordings (known bat species signals) that we already have as a call library. New recordings of ‘unidentified flying objects’ made at night on farms are then loaded into the computer and it sorts through them ‘intelligently’ to tell you which echolocating bat species you are likely to have. Eventually it is planned to make this process accessible to farmers on their properties as it happens during each night. So the QUT team is also developing a small stand alone device (called an embedded microprocessor) which will do everything in the field: detect bats flying around, identify which species and send the information to the farmer via the internet!

The Cotton Landcare Tech Innovations 2021 project is funded by the Cotton Research and Development Corporation, with support from the Australian Government’s National Landcare Program Smart Farming Partnership round 1 initiative.