Do you hear what I hear? Hearing sensitivity of the common vampire bat

Yep, that’s a corny Christmas reference in the title… A recent paper compared the hearing sensitivity of common vampires with other bats. We humans can hear from about 20 Hz to just under 20,000 Hz (20 kHz). Here, listen:

Vampires like other bats have excellent high frequency hearing. Anything above about 17 kHz starts to sound like weirdly uncomfortable silence to us, but  a vampire bat can hear well up to 113 kHz. They need this ultrasonic hearing of course to analyze the echoes of their biosonar calls. As you would expect, vampire bats have excellent hearing in the range in which they echolocate (about 70 kHz).

The echolocation ability of vampire bats is comparable with other bats. If you strung wires throughout a room without light, vampires would be able to avoid wires as thin as 0.5 mm, albeit they might need several trials of practice before they can maneuver adeptly around 0.25 mm wires (source). This performance is on par with larger bats, but exceeded by smaller aerial insectivores*.

What’s particularly unique about vampires, though, is not their high frequency hearing, but rather their low frequency hearing. The authors of this study confirmed that- as far as we know- common vampires can hear the lowest frequencies of any bat, including the big Paleotropical fruit bats that don’t echolocate at all. At 60 dB, a common vampire bat can hear sounds as low as 716 Hz. Now, to us that doesn’t sound very low. See (er, hear):

The vast majority of bats could not hear that tone. In fact, no other echolocating bat can hear well below 1,500 Hz. Next, consider what 250 Hz sounds like to us humans:

For a bat, that is infrasonic, way too low even for a vampire bat. They can’t hear that sound even when it is broadcast at 80 dB SPL (think really loud vacuum cleaner). That means that when I’m holding a struggling vampire bat and talking to it in a quiet, soothing, and re-assuring voice, they hear none of that. Oh well.

Still, a vampire’s hearing limit is more than an octave lower than other Neotropical leaf-nosed bats. In fact, common vampires have better low-frequency hearing than the Virginia opposum. While bats can’t hear bass, they could feel vibrations of intense low sounds with their bodies.

Vampire bat sensitivity for low frequencies seems to be a special adaptation. One piece of evidence for this is the fact that the hearing sensitivity of the common vampire at decreasing frequencies looks somewhat normal until it hits 5 kHz. Below that point, the sensitivity suddenly jumps up, beyond other bats. So, interestingly, vampires can hear 4 kHz better than 5 kHz. Then with even lower frequencies, the bat continues to lose sensitivity… until 2.4 kHz where, again, the sensitivity of the vampire ear jumps up again to another high point at about 2 kHz. These strange peaks indicate that vampire bats might have some kind of yet-undescribed mechanism that gives them improved low-frequency hearing.

Why do vampire bats have such good low frequency hearing? Check out this paper entitled “Classification of human breathing sounds by the common vampire bat, Desmodus rotundus“

To summarize, in the authors’ words:

The current behavioral study shows that the common vampire bat, Desmodus rotundus, is very sensitive to breathing sounds. In the three-alternative, forced-choice setup, it spontaneously associates unknown breathing sounds with the subject who emitted them. This exceptional performance is underlined by the inability of human listeners to match the vampire bats’ accomplishment under the most difficult experimental condition where the sounds had been recorded under physical strain. Numerical simulations show that while the human listeners relied on breathing-frequency information, the vampire bats appeared to recruit different acoustic parameters and to choose amongst these parameters depending on which provided the highest discriminative power. On the basis of these findings, it is suggested that vampire bats can memorize and classify complex acoustic features of prey-generated breathing sounds to facilitate the identification of prey animals that they have successfully fed on before.

Most bat ears are tuned to hear their prey, and in this case, vampire bats prey on large animals that make low pitch breathing sounds. To us, breathing might seem too low pitch, but  it actually produces faint sounds at high frequencies:

Not only can the bats learn what their prey sound like, but they can even learn particular host individuals. This might be why a vampire bat will sometimes bite the same individual in a group night after night regardless of where they are sleeping amongst others. Perhaps the bats can identify their favorite host by their breathing pattern alone.

In general, bat ears are not only tuned for hunting and echolocation, but also for communication. Like other bat species, vampires have a number of social calls. All bats locate  and recognize their pups  using isolation calls**. Adult vampires also produce contact calls that sweep from  about 36 kHz down to 19 kHz, with most of the sonic energy at 20-25 kHz. Vampire pup isolation calls are even lower, perhaps so that they travel farther at lower intensities. The frequency that vampire bats hear best at is 20 kHz, right beyond our limit, and the same pitch as the isolation calls of their pups:

 

Footnotes:

* Rhinolophus ferremequinum can detect wires as thin as 8/100th of a mm.

**These low frequency pup calls were first described in detail by Ed Gould, whom I’ve had the pleasure of getting to know in person because he volunteers at Our House center for youth. He now crafts amazing pottery and keeps himself quite happily busy. Ed’s student Dennis Turner did some of the earlier foundational field studies on vampire bats in the early 70s.