The Blind Bats That Flew Perfectly in the Dark

What happened?

Lazzaro Spallanzani was 64 years old and already one of the most accomplished naturalists in Europe when bats caught his attention. A Catholic priest, professor, and physiologist at the University of Pavia, he had already helped dismantle the theory of spontaneous generation and would later be the first to describe tardigrades, the microscopic “water bears.” His bat experiments began, almost by accident, with an owl. Spallanzani noticed that a tame barn owl kept in his room flew confidently near a lit candle but crashed into the wall the moment the flame went out. That contrast — a night bird helpless in true darkness — made him wonder how bats managed the same conditions so effortlessly.

He caught three wild bats near his hometown of Scandiano and released them in a darkened room with his brother and cousins watching. According to Spallanzani’s own account, dated August 20, 1793, the bats “continued to fly around as before and never struck against obstacles,” proving that the room, though it looked pitch black to the humans present, wasn’t functioning as darkness for the bats. His next step was more direct: he covered the eyes of two bats with an opaque paste made from birdlime, a sticky substance normally used to trap small birds. Both bats flew normally. Then he went further, surgically removing a bat’s eyeballs entirely. The blinded bat flew “with the speed and sureness of an uninjured bat,” navigating underground passages and landing precisely on walls and ceilings.

Word of Spallanzani’s results reached Louis Jurine, a Swiss physician and naturalist in Geneva, who tried a different variable: instead of blinding bats, he plugged their ears with wax. Those bats, with vision intact, collided helplessly with obstacles, as if they’d suddenly gone blind. When Spallanzani heard about Jurine’s results, he was skeptical — his own first attempt at plugging ears hadn’t produced the same effect, and besides, he could hear no sound coming from the bats at all, so how could hearing be the missing sense? He repeated the experiment with more thorough ear plugs, made from turpentine, wax, and pomatum, and eventually with more invasive procedures including damaging the bats’ inner ears. The results were consistent: bats that couldn’t hear crashed into things. Bats that couldn’t see did not.

By 1794, Spallanzani had published his findings as letters, first to the naturalist Anton Maria Vassalli and then more widely, under the title Lettere sopra il Sospetto di un Nuovo Senso nei Pipistrelli — “Letters on the Suspicion of a New Sense in Bats.” He had done as much to explain bat navigation in a year or two, historian Donald Griffin later wrote, as anyone else would manage in the following 140 years. And then, almost as quickly, the scientific world mostly moved on and mislaid the answer.

Why was it strange?

Spallanzani had assembled overwhelming evidence that bats depend on hearing, not sight, to navigate in darkness — and he still couldn’t quite believe it, because he had one piece of the puzzle missing. Bats, as far as any human in that cellar could tell, were completely silent. Spallanzani himself wrote that he suspected the ear might be responding to “some new organ or sense which we do not have” — he was reaching for an explanation without vision or conventional hearing, because the sound that made the whole system work was too high-pitched for a human ear to register. He had proven that hearing mattered without being able to hear the thing the bats were listening to. It’s a rare case in the history of science where the experiments were essentially correct decades before anyone could explain why.

The idea also ran headlong into influential skepticism. The naturalist Georges Cuvier, one of the most powerful scientific voices in Europe, dismissed the ear-based explanation and argued instead that bats navigated using an acute sense of touch spread across their wings. Cuvier’s authority carried enough weight that the “touch” theory dominated textbooks for much of the 19th century, even though nobody had produced experimental evidence for it to match Spallanzani and Jurine’s work.

What did scientists learn?

The full answer had to wait for instruments Spallanzani never had. In the 1930s, the physicist G.W. Pierce built a device capable of detecting ultrasonic sound — frequencies well above the range of human hearing. In 1938, Harvard undergraduate Donald Griffin brought a cage of bats to Pierce’s lab and discovered that the “silent” bats were in fact shrieking constantly, just at pitches no human ear could catch. Working with physiologist Robert Galambos, Griffin showed that bats deafened or muzzled lost the ability to avoid obstacles, while bats that could both call and hear navigated with precision. Griffin coined a name for the process: echolocation. Bats were emitting sound and reading the echoes that bounced back, building a picture of their surroundings out of reflected noise, sensing not just where objects were but their distance, size, and texture. Spallanzani had correctly identified which sense mattered nearly a century and a half before anyone could explain the mechanism behind it.

How does it affect us today?

Echolocation research fed directly into the development of sonar and modern biosonar engineering, and bat-inspired systems are still studied for navigation aids, obstacle-detection devices for the visually impaired, and drone sensing technology. More broadly, the story is a case study historians of science point to when discussing how an authoritative wrong answer — Cuvier’s touch theory — can suppress a correct one for generations simply through force of reputation. Spallanzani’s meticulous method, testing one variable at a time across dozens of trials, is still recognizable as the basic structure of a well-designed experiment.

Fun fact

Spallanzani’s curiosity extended well past bats: in 1777 he was also the first to name Tardigrada, the phylum of tardigrades — the nearly indestructible “water bears” that would go on to become one of this site’s other favorite strange animals.

Sources

• Lazzaro Spallanzani, Wikipedia — biographical detail and direct quotations from Spallanzani’s 1793 letters
• Discovering Sonar in Bats, American Association for the Advancement of Science — overview of Spallanzani, Jurine, Cuvier’s rival theory, and Griffin and Galambos’s 1938 discovery
• Lazaro Spallanzani Discovers Echolocation or Biosonar, History of Information — publication history of Spallanzani’s 1794 letters, quoting Donald Griffin’s Listening in the Dark (1958)
• Galambos, Robert. “The Avoidance of Obstacles by Flying Bats: Spallanzani’s Ideas (1794) and Later Theories.” Isis 34, no. 2 (1942): 132–140.
• Griffin, Donald R. “Echolocation by Blind Men, Bats and Radar.” Science 100, no. 2609 (1944): 589–590.