A scientist used chalk in a box to show that bats use sunsets to migrate

When it comes to migration science, birds rule. Although many mammals — antelopes, whales, bats — migrate, too, scientists know far less about how those animals do it. But a new device, invented by animal navigation researcher Oliver Lindecke, could open a new way to test how far-ranging bats find their way.

Lindecke, of Leibniz Institute for Zoo and Wildlife Research in Germany, has been studying bat migration since 2011. He started with analyzing different forms of hydrogen atoms in wild bats to infer where they had flown from. But figuring out how the bats knew where to go was trickier.

Lindecke needed a field setup that let him test what possible cues from nature helped bats navigate across vast distances. The first step was studying in which direction the bats first take flight. Such experiments on birds typically involve confining the animals in small, enclosed spaces. But that doesn’t work for bats, which tend to fall asleep in such spaces.

“My challenge was to build a box that bats won’t sleep in, but will show me how they take off,” Lindecke says.

So he invented what he calls the circular release box: a flat-bottom, funnel-shaped container topped by a wider lid. To escape, the bat crawls up the wall and takes off from the edge. Bat tracks in a layer of chalk (Lindecke says he was inspired by a snow-covered Berlin street) indicate where the bat took off.

In August 2017, Lindecke captured 54 soprano pipistrelle bats (Pipistrellus pygmaeus) in a large, 50-meter-wide trap at the Pape Ornithological Research Station in Latvia as the animals were migrating along the coast of the Baltic Sea toward Central Europe. Experiments with the new device showed that the adult bats flew straight in the direction in which they took off, Lindecke and colleagues report online March 1 in the Journal of Zoology.

In other tests, Lindecke found that bats used the setting sun to orient their night flights. Some bats were given a view of a sunset, while others were shown a mirror reflection of the same sunset. The bats were then moved inland to a new site and released from Lindecke’s box. The bats that watched the natural sunset flew west — back to their migratory route along the coast — while those that watched the mirrored version flew east, Lindecke and colleagues report online April 4 in Current Biology. Things got unpredictable when the team tested juvenile bats, which took off in random directions. That finding suggested that the younger bats need to learn navigation skills from their elders.

The release box, which is inexpensive and easy to build, could inspire more studies about how bats negotiate migration routes, Lindecke says. Some of his colleagues have already dubbed the device a “Lindecke funnel” — playing off the cone-shaped Emlen funnel widely used in bird navigation studies.

“I am not one to call it that,” Lindecke says, a little sheepishly. “It’s a stupidly simple design.”