The secret lives of the UK’s rarest bats could be revealed thanks to a new sensor that can record ultrasonic calls in dense woodland areas that have previously been difficult to reach. For decades, acoustic monitoring has been done by bat detectors carried by experts reluctant to stray into woodland thickets during night-time walks. Now a new £60 sensor that can be left in woodlands for up to a fortnight, called AudioMoth, is giving researchers a richer insight into the lives of these elusive mammals after a successful pilot study. Target species include pug-like barbastelle bats that roost in crevices in deciduous woodlands in southern England and Wales. There are believed to be as few as 5,000 left in the UK. Lesser horseshoe bats are one of Britain’s smallest mammals – around the size of a plum – and there are believed to be 50,000 in the UK. There is too little information to make population estimates for Leisler’s bats, one of the most understudied mammals in the country. “For a long time we didn’t know much about the interior part of woodlands,” said Carol Williams, director of conservation at the Bat Conservation Trust (BCT). “Our knowledge was more from the rides, tracks and woodland edges because people could relatively safely walk through there with a bat detector.” Previous static sensors cost £1,000, too much for most conservationists, but now hundreds of the new devices can be put out at once in harder-to-reach parts of woodland while it is safe for volunteers to go in. In 2019, a pilot study led by Forestry England and the BCT detected 7 million potential bat calls in 60 different monitoring locations across 16 forests. Algorithms developed by researchers from University College London and the University of Edinburgh sifted through this data and identified 1.7 million bat calls from eight species, including barbastelle, lesser horseshoe and Leisler’s bats. “This 2019 dataset was the biggest we’ve ever collected,” said Dr Katherine Boughey, head of science and monitoring at the BCT. “The revelation for us has been that we’ve been able to detect these rare woodland bat species and generate a huge amount of data that would have be impossible to do before.” The next step is to analyse the data. Listening to these calls will help conservationists understand where and at what time bats are feeding and mating. If hotspots for rare bats can be identified, conservationists hope they can make a better case for protecting these habitats against road, rail and housing developments. “We desperately need to know more about these species … They’re rare but they’re also secretive – that sounds like they’re purposely hiding away and of course they’re not, they’re leading their lives as they’re designed to do – but we haven’t been able to have a window into that life quite as easily,” said Williams. Bats are also important indicator species which means they help us understand more about wildlife we’re not as aware of, such as insects. A better understanding of bat ecology will help conservationists understand how biodiversity is faring more generally in woodland areas.