Brunoy (Essonne), report
The setting sun coats the tops of the oaks with orange shards. A smell of damp moss envelops the cooling forest. At 8 p.m., only a few joggers still venture into the sandy paths of the forest of Sénart (Essonne), in the south-east of Île-de-France. Humans are hiding. The nocturnal concert of bats can begin.
Headlamp screwed onto the skull, Alexandre Butin, specialist in flying mammals at the National Forestry Office (NFB), sets up his equipment on the shores of a pond: a camping chair, a tablet, an audio headset, and above all a microphone, capable of picking up and “ translate » ultrasound emitted by bats at frequencies inaudible to the human ear .
A long night of work awaits the forester. With three of his colleagues, the forty-year-old with an easy smile and a mischievous gaze carries out an inventory of the local populations. The objective: to identify the places in the forest where the bats have settled, in order to better preserve them. In the most frequented places, they can for example decide to keep more cracked trees, where bats like to take refuge, or even set up “ islets » untouched by humans.
- The forester Alexandre Butin and his team before leaving to listen to the bats. ©Mathieu Genon / Reporterre
In late summer, bats congregate at sites of “ swarming »sort of gigantic nightclubs dedicated to the search for sexual partners. “ Bats have very free morals », smiles Alexandre Butin. The period is conducive to recording social cries. In one night, the forester must make ten “ listening points » : once the sun has set, he records for ten minutes the soundscape of ten corners of the forest. Traces of phosphorescent paint on the trees allow him to find his bearings. “ If we’re doing well, we’ll be home at 1:30 in the morning. » His recordings are then slowed down (which makes it possible to reduce the frequency of the sounds emitted by the bats) and analyzed using software. “ It allows us to understand their language, to know where they parade, where they hunt… »
- their scope of study. ©Mathieu Genon / Reporterre
“ That’s a pipistrelle »
The ultrasounds tracked by Alexandre Butin and his colleagues are used by bats to “ echolocate ». The principle is simple: thanks to their vocal cords, they emit sounds which are “ bang » against the walls of the surrounding elements before returning to their ears. Bats are able, by analyzing this echo, to create a three-dimensional mental image of their environment. Despite the darkness, hunting becomes easy: they can determine the size, shape and speed of their prey without seeing it.
- Ultrasound is used by bats to “ echolocate “. ©Mathieu Genon / Reporterre
Each species has its own acoustic signature. Learning to recognize them takes years of practice. “ Me, I’m not a musician at all, confides Alexandre Butin. I had more difficulty at the start than my colleagues who have a musical ear. » Ten years after its debut, it manages to identify most species live. Eyes riveted to the sound waves scrolling across the screen of his tablet, the forester is reminiscent of Chanteraide, the hero of the film The song of the wolfable to detect a submarine thanks to its overdeveloped sense of hearing. “ We also have our ears of bronze, silver and gold »he says.
- In the dark, you can’t see any bats, yet “ there, there are at least four species “, according to the forester. ©Mathieu Genon / Reporterre
In the early evening, the bats are shy. Only the chirping of crickets, grasshoppers and mosquitoes interrupts the silence of the forest. The night envelops us little by little. We can only make out the outline of the trees, as if drawn in Indian ink against the sky. The enclosure connected to the ultrasonic detector begins to broadcast coppery crackles, evoking in turn maracas, a fingernail tapping a table, or the crash of hooves against the bitumen.
“ That’s a pipistrelletranslates Alexandre Butin. It tinkles like a set of keys being shaken. » Evocative sounds “ a drop of water exploding » suddenly reach our ears: a Common Noctule (Nyctalus noctula) just passed. The exercise has something magical about it: in the dark, you can’t see the slightest winged hand. And yet: “ There, there are at least four different species around us »whispers the forester.
- A pipistrelle. Wikimedia Commons/CC BY–HER 2.0/Gilles San Martin
High risk of disappearance
This inventory work and the protective measures it enables are crucial. Of the 35 species of bats listed in mainland France (about 170 throughout France), 20 face a high risk of extinction. Between 2006 and 2019, common Serotine (Eptesicus serotinus)which is distinguished by its back strewn with golden hairs, lost 30 % of its workforce ; Nathusius’ Pipistrelle (Pipistrellus nathusii)46 %. The Common Noctule, recognizable by its rounded ears and reddish coat, is in the worst shape: 88 % of them have disappeared in just thirteen years, alert researchers from the Vigie-Chiro collective.
- Some ultrasound sounds like a “ set of keys that we shake “. ©Mathieu Genon / Reporterre
The causes of this spectacular decline are multiple. Urban expansion, intensive agriculture and the disappearance of hedgerows are reducing the amount of habitat available for bats, explains Kévin Barré, researcher at the Center for Ecology and Conservation Sciences of the National Museum of Natural History. Added to this are predation by cats, light pollution, climate change, which alters the ranges of bats, as well as pesticides, which accumulate in the food chain and destroy the insects on which they feed. feed.
- Their work is essential: 20 species out of the 35 listed in mainland France have a high risk of extinction. ©Mathieu Genon / Reporterre
Road traffic is also responsible for many deaths. When they are not crushed to death under the wheels of cars, bats can collide with the blades of wind turbines, sometimes located in migration corridors. Changes in air pressure near these devices can also cause “ barotrauma », often fatal tissue damage to these small organisms. It is still difficult to estimate the losses linked to wind farms, points out Kévin Barré. “ But it is very likely that we underestimate the real mortality, because the corpses disappear very quickly. »
- Tonight, the bats are discreet. ©Mathieu Genon / Reporterre
Certain species tend to be attracted to wind turbines, increasing the risk of collision. Others, on the contrary, seek to avoid them, which further reduces their habitat. “ The pressures on bats are so diverse and numerous that it becomes very difficult to find solutions », laments the researcher. However, they all have a common denominator, he insists: our economic model. “ The reasons why we practice intensive agriculture, why we have so many roads and why we need to install so many wind turbines are the same: our constant search for profits. » Without a change of model, he thinks, “ it will be difficult to slow the decline of bats ».
Their progressive disappearance has consequences “ serious » on the functioning of ecosystems. Bats do farmers a favor by devouring insect pests. They are also valuable allies for foresters. “ They predate cockchafers, leaf-cutting caterpillars and processionary caterpillars, which tend to increase due to climate change », explains Alexandre Butin. Bats are also fond of mosquitoes, potential vectors of disease: they can devour up to 1,000 a night. Flying mammals are “ sentinel species » : “ If they disappear, the rest of the ecosystem will follow »concludes the forester with a serious air.
- Foresters hope to prevent their disappearance at their own level. ©Mathieu Genon / Reporterre
Thanks to his review work, he hopes to prevent such a catastrophe on his own scale. Especially since there is still much to discover about these winged creatures. Among their biological prowess: delayed ovulation. After mating, in autumn, females can keep sperm in their bodies throughout the hibernation period. They only trigger fertilization in the spring.
Of exceptional longevity for their small size – despite its 30 grams, Myotis myotis, the Greater mouse-eared mouse, can live up to 38 years — bats also have an extraordinary immune system. Their resistance to infection fascinates doctors. Scientific research would probably not be the only ones to suffer from the loss of flying mammals, we say to ourselves while watching Alexandre Butin spin between the trees towards his next listening point. Without bats, starry nights would be much less poetic.
A night to meet bats
The French Society for the Study and Protection of Mammals has organized each year, for more than twenty years, a “ International Bat Night ». The next edition takes place this weekend, August 27 and 28. Night outings to listen to bats, workshops, conferences, exhibitions and film screenings (free and open to all) are planned throughout France. A map of activities is available on the event website. To your ultrasonic detectors !