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I attended a talk that glossed over some biology as it was talking about a certain protein. The speaker mentioned humans can hear up to, often less than 20kHz frequencies, whereas bats can hear up to 100kHz. Presumably this is to do with differences in cochlear anatomy.

What about the inner ear of bats is different form ours, and how does this allow them to hear such high frequencies?

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Short answer
Echolocating bats have relatively large sensory epithelia in their inner ear, that may correlate with their high upper frequency limit of up to 200 kHz. The basilar membrane is thinner and stiffer, possibly allowing it to decode higher frequencies.

Background
In terms of the place theory of hearing, the cochlea acts as a frequency transformer, where high frequencies are encoded basally, and low frequencies apically. The cells responsive to sound, the hair cells, are located along the basilar membrane (BM). Going from base (high frequency) to apex (low frequency), the BM becomes wider and less stiff, making it more susceptible to encode low frequencies apically. Note that sound is encoded as a travelling wave, where the point of resonance of a particular frequency corresponds with the characteristic frequency of the hair cells (Fig. 2).

cochlea
Fig. 1. Rolled out cochlea. Source: New York University. Travelling wave
Fig. 2. Travelling wave and resonance frequencies along the basilar membrane. Source: University of Minnesota.

A study in Microchiroptera species revealed that the basal part of the basilar membrane was relatively narrow and stiff, which increases the resonance frequency of the basal part of the BM and hence may allow it to encode higher frequencies in these bats (Pye, 1966).

Because the high frequencies are encoded basally, it would be tempting to speculate the basal region would be enlarged in echo locating bats. Although this is true in some species, it may have more to do with frequency sensitivity than the upper limit of hearing, simply because a larger surface of the BM allows for more exquisite resolution of the BM. Although there is a tendency for larger BM lengths in echolocating bats, it is not consistent (Davies et al., 2013).

The human BM is 33.5 mm, while the BM of the bat with the highest frequency limit known (200 kHz, Hipposideros bicolor) is only 9.13 mm. However, compared to that of acoustically unspecialized mammals of a similar body size, e.g. Sorex araneus (BM length 4.4 mm) and Mus musculus (BM 7.0 mm) it is relatively long. However, as said, other bats with lower upper-frequency limits have longer BM lengths, e.g., Pteronotus parnellii (13.2 mm) and Rhinolophus ferrumequinum (16.1 mm). Indeed, the BM length has been correlated both with an extension in frequency range as well as enhanced frequency resolution (Dannhof & Bruns, 1991).

References
- Dannhof & Bruns, Hear Res (1991); 53: 253-68
- Davies et al., Front Zool (2013); 10: 2
- Pye, J Morph (1966), 118: 495-510

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  • $\begingroup$ i really don't see how this is an answer. Any other differences except length of BM? cellular anatomy? Different properties of fluid in the cochlea? I believe there is more to it than page from Encyclopedia Britannica $\endgroup$ – aaaaa says reinstate Monica Jul 5 '15 at 23:10
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    $\begingroup$ @aaaaaa, I have added material. Thanks for the suggestion. But what page of Britannica? I haven't used it here? There are three credible references listed. $\endgroup$ – AliceD Jul 6 '15 at 0:42
  • $\begingroup$ What's also important is basilar membrane thickness, since that is part of what dictates the resonance at various frequencies (as I understand it, thicker = stiffer = higher frequencies). $\endgroup$ – Chelonian Jul 6 '15 at 1:04
  • $\begingroup$ @AliceD If being thicker at the basal end promotes lower-frequency hearing, than why are the higher frequencies transduced at the basal end? I know width matters, too, but it seems thickness results in stiffness, which--per given load?--should resonate with higher frequencies. E.g: "Part of the stiffness change is due to the increasing width of the membrane and part to its decreasing thickness." (link: 147.162.36.50/cochlea/cochleapages/theory/bm/bm.htm) $\endgroup$ – Chelonian Jul 6 '15 at 1:56
  • $\begingroup$ @AliceD Right--my comment was intended to suggest that perhaps these bats have thicker basilar membranes than humans do. But yes, there are other structural factors (and active tuning) which would also be at play. $\endgroup$ – Chelonian Jul 6 '15 at 2:09

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