how bats see in the dark

The Mechanism

*Donald Redfield Griffin* (born Southampton, New York, 3 August 1915; died Lexington, Massachusetts, 7 November 2003, aged 88) was an American zoologist whose 1938 collaboration with the Harvard physicist *George Washington Pierce* — and with his fellow undergraduate *Robert Galambos* — established that microchiropteran bats navigate and locate prey in flight by emitting brief ultrasonic chirps and listening for the echoes. The phenomenon Griffin named *echolocation* in 1944 was the resolution of a problem 145 years old. In *1793* the Italian Catholic priest and University of Pavia natural philosopher *Lazzaro Spallanzani* (1729-1799) — already famous for his work disproving spontaneous generation and for the first artificial inseminations of frogs and dogs — became interested in nocturnal animal navigation after watching a barn owl in his study fly normally under candlelight, then crash into a wall when the candle was extinguished. Spallanzani caught three live bats from the cathedral belltower in Pavia, covered their eyes with discs of opaque *birdlime* (the sticky bird-trapping resin used by Italian fowlers), and released them in his closed study. The bats flew perfectly normally — circling the room, avoiding the walls, the candle, the furniture, and Spallanzani himself, with the same precision as sighted bats. In a more drastic follow-up Spallanzani surgically removed the eyeballs of a single bat and released it into a darkened underground passageway. The blinded bat *navigated the subterranean passageway perfectly*, locating and entering its small crevice roost on the first try. The Swiss physiologist *Louis Jurine* in 1794 added the complementary experiment: plug the bats' ears with wax or turpentine, and they crash into walls. Spallanzani and Jurine published in the *Memorie della Società Italiana delle Scienze* (1798); their joint conclusion was that the bat's *ear* is the organ of nocturnal navigation. The conclusion was correct but the mechanism was unknown. The dominant alternative theory was that of the French anatomist *Georges Cuvier*, who held in his *Leçons d'anatomie comparée* (1800) that bats navigated by an *exquisite sense of touch* in the air currents around the wing membranes (Spallanzani's blinded bats supposedly steered by feeling tiny pressure differentials against the cathedral wall as they approached it). Cuvier's theory was the textbook explanation for 140 years; Spallanzani-Jurine's ear-based explanation was relegated to a footnote. The resolution began in *1937*. Griffin was a 22-year-old Harvard senior, a student of the ornithologist *William Hovgaard* and the comparative anatomist *Glover Morrill Allen*, with a side interest in animal navigation and a long-standing personal interest in bats. *George Washington Pierce* (1872-1956), the chair of the Harvard Department of Physics and inventor of the *Pierce oscillator* circuit, had spent the early 1930s developing a portable *ultrasonic detector* — a microphone-and-vacuum-tube apparatus that could pick up acoustic frequencies above the upper limit of human hearing (about 20 kHz) and shift them down into the audible range. Pierce had used it to record the songs of insects. Griffin had heard of the device; in the late autumn of *1937* he walked from the Biology Department to Pierce's Physics Department laboratory in Lyman Hall carrying a bag of *little brown bats* (*Myotis lucifugus*) he had collected in a cave in western Massachusetts the previous weekend. Pierce released a bat in his laboratory in front of the microphone. The chart recorder erupted into a dense pattern of brief sharp clicks — ultrasonic pulses with frequencies of approximately 30-50 kHz and durations of 1-5 milliseconds, repeated several times a second, with the click-rate accelerating dramatically when the bat changed direction or approached a wall. The bat was *singing* in a register no human had ever heard. Griffin and Pierce published the joint observation in *1938* — Pierce, G.W. & Griffin, D.R., "Experimental determination of supersonic notes emitted by bats," *Journal of Mammalogy* 19(4): 454-455 (November 1938) — a single page reporting the existence and frequency range of the bat clicks but, in Griffin's later self-description, *absurdly cautious* about whether the clicks had anything to do with navigation. They reported the existence of the chirps and stopped. The fuller experiment came in 1938-39. Griffin teamed with *Robert Galambos* (1914-2010), a fellow Harvard graduate student and an expert in auditory physiology, and the pair designed a series of experiments at the Harvard Biological Laboratories and at the *Edmund Niles Huyck Preserve* in Rensselaerville, New York, in which bats were released into a darkened room strung with thin wires hung from the ceiling. With ears intact, the bats avoided every wire. With ears plugged with collodion, they crashed into wires. With mouths covered (so they could not emit chirps), they crashed. With ears uncovered and mouths uncovered but the room flooded with continuous high-amplitude ultrasonic noise (which masked the echoes), they crashed. Griffin and Galambos published the experimental design and the results in two papers in 1941: Griffin, D.R., "Echolocation by blind men, bats, and radar," *Science* 100: 589-590 (15 December 1944); and Griffin, D.R. & Galambos, R., "The sensory basis of obstacle avoidance by flying bats," *Journal of Experimental Zoology* 86(3): 481-506 (April 1941); and Galambos, R. & Griffin, D.R., "Obstacle avoidance by flying bats: the cries of bats," *Journal of Experimental Zoology* 89(3): 475-490 (1942). In *1944* Griffin coined the word *echolocation* — by analogy with the *radio detection and ranging* (radar) systems whose existence had just been declassified in the war effort, and which operated on the same active-emitter / passive-listener principle that the bats had been using for fifty million years. Cuvier had been wrong about the wing membranes. Spallanzani and Jurine had been right about the ear. The bats had not been feeling their way around the wall; they had been *talking to the wall and listening to it answer*. Griffin spent the rest of his career on the comparative biology of animal cognition: the navigation of homing pigeons (the 1955 demonstration that homing pigeons use solar bearings); the discovery in 1953-58 that *toothed whales and dolphins* (Odontocetes) also use echolocation; and, in his later years, the controversial argument — advanced in his 1976 book *The Question of Animal Awareness* — that nonhuman animals have subjective conscious experience, which he called *cognitive ethology* and which is now standard in modern animal-behaviour research. Galambos went on to do important work in mammalian auditory neurophysiology and to live to 95. As of 2026 there are about *1,400 known species of bats*, comprising roughly *one-fifth of all known mammal species*; over a thousand of those species are microchiropterans that navigate, hunt, and communicate using ultrasonic echolocation in the 9-200 kHz frequency band. Every one of them was solving the problem the way Pierce's vacuum-tube circuit revealed in the Harvard physics lab in 1937, fifty million years before Spallanzani put birdlime on the eyes of three bats from a cathedral belltower in Pavia.