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Tiny tracking tags help decode how echolocating bats navigate

BIG BROWN BAT (Eptesicus fuscus), IN FLIGHT AT NIGHT, ROGUE RIVER NATIONAL FOREST, OREGON

  • Although navigation in echolocating bats has been studied for a long time, questions remain on how bats differentiate among echoes from different objects.
  • Researchers designed a small, lightweight tag that can capture movement and sound information in three dimensions to create a map of a bat’s sensory environment.
  • The data helped researchers pinpoint the movements of bats during flight and while catching prey, as well as how echoes from various objects differ.
  • One-third of bat species are threatened with extinction or lack basic ecological data, so such information can help scientists and wildlife managers understand bats’ foraging behavior and develop better measures for their conservation.

While we fumble and crash without any light, bats zipping away in the dark, effortlessly skirting obstacles and pouncing on prey, seem magical. To better understand this ability of bats to “see” in the dark, researchers fixed tags that can simultaneously measure both bats’ movements and all the sounds they send and hear, creating a map of the sensory inputs in a bat’s environment and how it reacts to them. Such information can help us better understand how bats navigate and feed in the wild, something previously difficult to study.

More than half of all bat species use sound to “see”, emitting sounds and listening to the echoes to go about their daily lives. Their brains process the returning echoes and help them determine how far away objects are, how big they are, if they are moving, and how fast and in which direction the objects are moving. This echolocating ability of bats has long been studied, but questions remain, such as how can bats pick out the echoes they are interested in, for example a juicy meal or a predator swooping down, versus all the other echoes they perceive.

A big brown bat (Eptesicus fuscus) in flight in Oregon, US, looks either hungry or sinister but possibly is just busy echolocating, a process by which bats emit high-frequency calls and use information about the return echo to navigate and detect objects around them at night. Image by Angell Williams, CC  2.0.

One way to measure events of interest is to use the appropriate sensors, just like your smartphone does to allow you to talk, count the number of steps you walk, capture images, and much more in a palm-sized device.

Sensors have been used for studying animal behavior too, such as tracking animals’ movements and capturing sounds. However, when it comes to small creatures like bats, sensors have to be tiny and almost weightless so they don’t hinder their natural movements. Such small and lightweight sensor technology is not commonly available, so there have not been many instances of the use of multiple sensors for studying bats.

Now, a team of researchers has used a small tag capable of measuring movement and sound, yet light enough for a small bat to carry it around without being weighed down, to study how bats fly and hunt.

“The original motivation behind developing these tags, was due to the success similar tags have had for understanding how marine mammals use sound for communication and to find prey in the deep sea,” Laura Stidsholt of Aarhus University in Denmark and the lead author of the study told Mongabay. Similar movement and prey echo recording tags were used to understand the foraging behavior of porpoises in the wild.

The tag, developed by co-author Mark Johnson of the University of St. Andrews, Scotland, comprises a printed circuit board that has an accelerometer to measure movement, a battery, and an ultrasonic microphone and weighs about 2.6 grams (0.09 ounces). Data can be continuously recorded for up to five hours on one charge of the battery.

Mapping three dimensions of sound and movement

The bat research team performed two types of experiments in the lab, one to understand how bats approach a target and another to understand how they catch prey.

A European noctule bat (Nyctalus noctula), found throughout Europe, Asia, and North Africa. It uses loud echolocation, fast flight and deep dives from above the tree canopy to catch moths, crickets and beetles. Image by Ján Svetlík, CC-BY-NC-ND 2.0.

For the first experiment, they glued the tag onto the back of a European noctule (Nyctalus noctula) that was trained to land on a nylon ball sitting on a pole in a flight room. The tag recorded the movements of the bat and the echoes from the ball and these data were used to construct a flight path.

To understand how bats catch prey, researchers placed tags on four big brown bats (Eptesicus fuscus), trained to catch a worm hanging from the ceiling of the flight room.

Using the sensor data, the researchers created a map of the types of calls made by the bat, echoes received by it from different objects in the room, including the walls, and its flight path. As a bat approached the nylon ball, the frequency of its calls changed. The tag could also differentiate among echoes from the various types of objects; echoes from the walls were more diffuse than those from the prey, information that bats may use to distinguish between food and obstacles.

The flight path of a tagged European noctule bat as it approaches a nylon target sphere (black) based on the time of arrival to microphones behind the target of the calls emitted by the bat. The tag measures the bat’s movements in three dimensions, its acoustic output, and the actively generated echo. The calls, color‐coded according to their energy, require less energy as the bat approaches the target. The “buzz” calls emitted roughly 1 meter (3 feet) before landing (black line) could not be extracted from the array recordings. Image is Figure 3a from Stidsholt et. al. (2019) .

The data obtained from the tag also allowed the researchers to map the fine maneuvers of the bat when approaching and capturing prey. The bat captured the prey in a powerful stroke, immediately rotating its body and turning back. When combined with the simultaneously captured sound data, the researchers were able to relate the range of echo sources to how the bats flew.

Understanding foraging behavior

“For bats, that would mean that these types of tags potentially could be able to address on an individual basis how bats use echoes to guide their navigation and feeding behavior outside of laboratory settings,” Stidsholt said.

The researchers’ next step is to use the tags in the wild. To this end, they have released greater mouse-eared bats (Myotis myotis) carrying the tags into the wild, capturing the tagged bats about a week later. Using the data recorded by the tags, the team hopes to understand how the bats navigate through the different echoes to capture prey.

A male greater mouse-eared bat (Myotis myotis) roosting under a bridge. These European bats also roost in caves, mines, and cellars, and they forage for insects, spiders, and larvae in deciduous woodlands and meadows. Image by Miss Mhisi, Wikimedia Commons CC 4.0.

Although so far, the tags have been used only to understand how bats feed and navigate, in the future, they could also be used to study communication between bats, or understand behaviors in birds. Such data provide us with information on the ecosystems in which bats live and thrive.

With the dramatic changes human activity has induced across these ecosystems, data on bats’ foraging behavior and how they use sound to find resources in their environment may prove crucial to helping us conserve them, as nearly one-third of bat species lack basic ecological data or are threatened with extinction.

Citation

Stidsholt, L., Johnson, M., Beedholm, K., Jakobsen, L., Kugler, K., Brinkløv, S., … & Madsen, P. T. (2019). A 2.6‐g sound and movement tag for studying the acoustic scene and kinematics of echolocating bats. Methods in Ecology and Evolution, 10(1), 48-58.

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