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Research paper
Inner ear morphological correlates of ultrasonic hearing in frogs
q
Victoria S. Arch
a
,
b
, Dwayne D. Simmons
c
, Patricia M. Quiñones
d
, Albert S. Feng
e
, Jianping Jiang
f
,
Bryan L. Stuart
g
, Jun-Xian Shen
h
, Chris Blair
c
, Peter M. Narins
b
,
c
,
*
a
Abbott Vascular Inc., 3200 Lakeside Drive, Santa Clara, CA 95054 2807, USA
b
Department of Ecology and Evolutionary Biology, University of California, Los Angeles, Los Angeles, CA 90095, USA
c
Department of Integrative Biology and Physiology, University of California, 621 Charles E. Young Drive S, Los Angeles, Los Angeles, CA 90095, USA
d
Department of Neurobiology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, CA 90095, USA
e
Department of Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
f
Chengdu Institute of Biology, Chinese Academy of Sciences, Sichuan 610041, China
g
North Carolina Museum of Natural Sciences, Raleigh, NC 27601, USA
h
State Key Laboratory of Brain and Cognitive Science, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China
article info
Article history:
Received 28 May 2011
Received in revised form
9 November 2011
Accepted 10 November 2011
Available online 25 November 2011
abstract
Three species of anuran amphibians (Odorrana tormota, Odorrana livida and Huia cavitympanum)have
recently been found to detect ultrasounds. We employed immunohistochemistry and confocal micros-
copy to examine several morphometrics of the inner ear of these ultrasonically sensitive species. We
compared morphological data collected from the ultrasound-detecting species with data from Rana
pipiens, a frog with a typical anuran upper cut-off frequency of w3 kHz. In addition, we examined the
ears of two species of Lao torrent frogs, Odorrana chloronota and Amolops daorum, that live in an acoustic
environment approximating those of ultrasonically sensitive frogs. Our results suggest that the three
ultrasound-detecting species have converged on small-scale functional modifications of the basilar
papilla (BP), the high-frequency hearing organ in the frog inner ear. These modifications include: 1.
reduced BP chamber volume, 2. reduced tectorial membrane mass, 3. reduced hair bundle length, and 4.
reduced hair cell soma length. While none of these factors on its own could account for the US sensitivity
of the inner ears of these species, the combination of these factors appears to extend their hearing
bandwidth, and facilitate high-frequency/ultrasound detection. These modifications are also seen in the
ears of O. chloronota, suggesting that this species is a candidate for high-frequency hearing sensitivity.
These data form the foundation for future functional work probing the physiological bases of ultrasound
detection by a non-mammalian ear.
Ó2011 Elsevier B.V. All rights reserved.
1. Introduction
Among vertebrates, mammals are considered high-frequency
hearing specialists (Heffner and Heffner, 2007). The majority of
mammalian species hears well into the ultrasonic range (i.e.,
>20 kHz) while other vertebrates possess comparatively limited
sensitivity (Heffner and Heffner, 1998; Fettiplace and Fuchs, 1999;
Dooling et al., 2000, but see Mann et al., 2001). Anuran amphib-
ians (frogs and toads) are among the taxa that have been consid-
ered to possess restricted high-frequency hearing ability, with an
upper detection limit of 5e8 kHz (Loftus-Hills and Johnstone, 1970).
However, this assumption has been challenged by the recent
discovery of three frog species that detect ultrasound: Odorrana
tormota,Odorrana livida and Huia cavitympanum (upper hearing
limits of 34, 22 and 38 kHz, respectively). Two of these species,
O. tormota and Huia cavitympanum, have been demonstrated to
Abbreviations: AEP, auditory evoked potentials; AP, amphibian papilla; BL,
bundle length; BP, basilar papilla; ESA, epithelium surface area; HCC, hair cell
count; BP REA, basilar papilla recess entrance area; RT, room temperature; SL, soma
length; SVL, snout-vent length; TM, tectorial membrane.
q
Support information: Financial support was provided by a National Science
Foundation Doctoral Dissertation Improvement Grant (no. 0806207) to VSA, grants
from the National Institute on Deafness and Other Communication Disorders to
DDS (no. DC004086) and PMN (no. DC00222), the Paul S. Veneklasen Research
Foundation to PMN, the National Science Foundation (CRCNS-0422073) to ASF, and
the National Natural Science Foundation of China to JPJ (no. 31071906) and JXS (no.
30730029).
*Corresponding author. Department of Integrative Biology and Physiology,
University of California, 621 Charles E. Young Drive S, Los Angeles, Los Angeles, CA
90095, USA. Tel.: þ1 310 825 0265; fax: þ1 310 206 3987.
E-mail address: pnarins@ucla.edu (P.M. Narins).
Contents lists available at SciVerse ScienceDirect
Hearing Research
journal homepage: www.elsevier.com/locate/heares
0378-5955/$ esee front matter Ó2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.heares.2011.11.006
Hearing Research 283 (2012) 70e79
Author's personal copy
communicate ultrasonically (Feng et al., 2006; Arch et al., 2009).
The Odorrana and Huia genera are from distinct evolutionary
lineages (Stuart, 2008) suggesting that the frogs converged on the
ability to detect extraordinarily high frequencies. All three species
are torrent frogs; they inhabit rapid-flowing hill or mountain
streams, and call alongside rushing water that produces an abun-
dance of broadband, predominately low-frequency, background
noise (Feng et al., 2002; Narins et al., 2004; Arch et al., 2008). The
convergence of O. tormota and H. cavitympanum on ultrasonic
communication may have resulted from parallel selection pressure
to place acoustic signals within relatively noise-free windows of
their environments’ambient spectra (Narins et al., 2004; Arch et al.,
2008).
The inner ear mechanisms subserving exceptional high-
frequency hearing in frogs are unknown. In mammals, the advent
of high-frequency sensitivity is attributed to key morphological
innovations within the ear, including the extension of a flexible,
mechanically tuned membrane on which the sensory receptors sit
(i.e., the basilar membrane), and the specialization of inner ear
supporting and sensory cell types (Fettiplace and Fuchs, 1999).
These features are not present in frogs.
The anuran inner ear is unique among vertebrates in its
possession of two dedicated auditory endorgans, the amphibian
papilla (AP) and the basilar papilla (BP) (Wever, 1973; Baird, 1974;
Capranica, 1976; Lombard and Bolt, 1979; Lewis et al., 1992). Each
lies within its own chamber and is sensitive to a distinct band of
frequencies. The AP responds to low and middle frequencies and is
tonotopically organized, with low-frequency-sensitive hair cells
located rostrally and mid-frequency cells located caudally (Lewis
et al., 1982a, 1982b). The BP is a simpler organ that acts as
a mechanical resonator, responding to a restricted, higher-
frequency band (Feng et al., 1975; Lewis et al., 1982a, 1982b;
Megela and Capranica, 1982; Wilczynski and Capranica, 1984;
Ronken, 1990; van Dijk and Manley, 2001; Meenderink et al.,
2005). Mechanotransduction in the ear is performed by hair cells,
which are common to all vertebrates. However, unlike most
amniote hair cells, those of the auditory epithelia in anurans are
firmly attached to the walls of the organ chambers (Lewis et al.,
1982a). As a result, acoustic stimuli are not filtered by graded
mechanical properties of an underlying basilar membrane prior to
hair cell transduction. Hence, extrinsic tuning of the stimulus
preceding transduction is restricted to the motion of an overlying
tectorial structure in which the hair cell ciliary bundles are
embedded (Shofner and Feng, 1983; Hillery and Narins, 1984).
Additional tuning in the frog ear depends on intrinsic properties of
the hair cells themselves, including ciliary mechanical coupling and
ion channel kinetics (Pitchford and Ashmore,1987; Simmons et al.,
1994; Smotherman and Narins,1999a, b).
Across vertebrate taxa, intrinsic hair cell tuning is related to
morphological properties of the hair cell soma and bundle. Soma
lengths of mammalian outer hair cells (Bohne and Carr, 1985;
Fettiplace and Fuchs, 1999), goldfish saccular hair cells (Sugihara
and Furukawa, 1989) and frog AP hair cells (Simmons et al., 1994)
decrease systematically with increasing frequency sensitivity along
the auditory organ’stonotopic axis. Experiments with isolated frog
AP hair cells demonstrate that whole-cell capacitances vary
predictably with soma length, providing additional evidence that
the hair cell’s resonant frequency is inversely related to its length
(Smotherman and Narins, 1999a, b). Bundle heights also negatively
correlate with the frequency of maximal hair cell sensitivity in the
mammalian cochlea (Lim, 1980; Fettiplace and Fuchs, 1999), and
chick (Tilney and Saunders, 1983) and lizard basilar papillae
(Mulroy, 1974; Turner et al., 1981).
To gain a greater understanding of the extrinsic and intrinsic
mechanisms subserving high-frequency hearing in the frog inner
ear, we used immunohistochemistry and confocal microscopy to
examine the auditory organ morphology of O. tormota, O. livida and
H. cavitympanum. We investigated features of the auditory papillae
such as papillar surface area and number of hair cells, as well as
hair-cell-specific morphometrics, including soma and bundle
lengths. This investigation allowed us to test the hypothesis that
the ultrasonically-sensitive frogs’inner ears had undergone a major
reorganization to enable ultrasound reception, and to examine
smaller-scale functional modifications that may play key roles in
high-frequency detection. We compared the inner ear features of
O. tormota, O. livida and H. cavitympanum with one another, and
with those of the leopard frog (Rana pipiens), which has a typical
anuran hearing range (upper cut-off frequency of w3 kHz). We
additionally examined the inner ears of two sympatric species of
torrent frogs in Laos eOdorrana chloronota and Amolops daorum -
which call in an environment with an ambient noise spectrum that
is very similar to those of the ultrasound-sensitive species. As yet,
there are no behavioral data indicating whether these frogs hear or
communicate using ultrasound. Our comparative analysis provides
a context within which to evaluate the species’auditory apparatus
and form hypotheses about their auditory sensitivity.
2. Materials and methods
2.1. Specimen collection
We collected sexually mature O. tormota males by hand from
their natural calling sites along the banks of the Tau Hua Creek,
Anhui, China (30
06
0
N, 118
10
0
E), from 9 to 11 May, 2010. We ob-
tained O. livida males in the same manner from their calling sites in
Hongyuan Gou, Sichuan, China (28
38
0
N, 106
18
0
E), from 30 May e
5 June, 2010. We collected both species between approximately
190 0e2230 h.
We found H. cavitympanum males along the banks of the Nyipa
River in Gunung Mulu National Park, Sarawak, Malaysia (04
03
0
N;
114
51
0
E). We collected the frogs as part of a separate research
project and preserved their ear tissues after the project’s comple-
tion. Additional details are presented in Arch et al. (2009).
We collected males of O. chloronota and A. daorum in the vicinity
of the Tad Loi Waterfall in the Phou Louey National Protected Area,
Viengthong District, Huaphahn Province, Laos (20.23
N 103.21
E),
on 13e18 March, 2009. We found the frogs between 1900 and
2230 h at approximately 1200 m elevation. After sacrificing the
frogs (see below), we removed their heads and preserved the
bodies as voucher specimens at the Field Museum of Natural
History (O. chloronota: BLS12978, BLS13047, BLS13048, BLS13049,
BLS13095; A. daorum: BLS13044, BLS13058, BLS13059).
Rana pipiens were purchased from a commercial supplier and
housed in UCLA vivarium facilities.
To sacrifice the frogs, we first deeply anaesthetized them by
liberally applying topical anesthetic (R. pipiens,O. tormota,O. livida;
Benzocaine, 7.5%; Del Pharmaceuticals, Inc., Uniondale, NY), or by
immersing them in a solution of tricaine methanesulphonate [0.3%
solution for H. cavitympanum; an effective but unknown concen-
tration (prepared upon collection in the field) for O. chloronota and
A. daorum; MS-222; Sigma, Saint Louis, MO], followed by swift
decapitation.
All animal care adhered to the ABS Guidelines for the use of
animals in research and was approved by the UCLA Animal
Research Committee (Protocol # 094-086-51).
2.2. Tissue preparation
We removed the lower jaw and opened the otic capsules
ventrally, via the roof of the mouth, to expose the inner ear
V.S. Arch et al. / Hearing Research 283 (2012) 70e79 71
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membranous labyrinth. We then made a small opening in the
labyrinth using fine forceps and dripped a freshly prepared solution
of 4% paraformaldehyde (Ted Pella, Redding, CA) in phosphate
buffer with frog-specific osmolarity (e.g., frog-specific phosphate
buffered saline; FPBS) into the inner ears to insure that the sensory
epithelia came into immediate contact with the fixative. Following
this procedure, we immersed the entire head in the fixative solu-
tion and gently agitated it at room temperature (RT) for 2 h. We
then rinsed the tissues in FPBS and, when possible, stored them at
4
C. We kept the heads collected in the field in China at ambient
temperature until there was access to refrigeration (w2e7 days).
We stored the H. cavitympanum tissues in fixative at 4
C for
approximately 4 months, and then in FPBS for an additional ca.6
months at 4
C before using them in the present study.
2.3. Immunohistochemistry
We removed the inner ears from the chemically fixed heads and
dissected them down to the auditory organs while maintaining
immersion in FPBS. We kept the surgically isolated BPs intact since
the hair cell array is unobscured in the intact form. To expose the AP
sensory epithelium, we cut off the ventral wall of the organ with
microdissection scissors and removed the tectorial membrane
using fine forceps. We placed the organs in a well plate and bathed
them in 1% Triton X-100 in FPBS for 30 min to enhance their
permeability; the tissues were gently agitated through this period
and during the subsequent steps. We rinsed the organs 4 15 min
in FPBS and soaked them for 1 h in a blocking solution to minimize
non-specific antigen binding. The blocking solution consisted of 5%
normal donkey serum (NDS) and 0.05% bovine serum albumin
(BSA; Sigma) in a low-calcium buffer (BSA-block), and was used for
all succeeding dilutions. We immediately followed blocking with
an overnight incubation at RT in monoclonal rabbit antibody
against Myosin VI (1:500; Proteus Biosciences 25-6791; Ramona,
CA) to label hair cell somata. The next morning, we rinsed the
tissues [4 15 min in 0.1% Tween in FPBS (TwFPBS)] and incubated
them for 2 h in the secondary antibody Alexa Fluor 594 donkey
anti-rabbit IgG (1:200; Invitrogen A21207, Carlsbad, CA). We then
rinsed the tissues again (4 15 min in TwFPBS) and incubated them
for another 2 h in a cocktail containing phalloidin conjugated to
Alexa-Fluor 488 (1:100; Invitrogen A12379) and the nucleic acid
stain, DAPI (1:1000; Thermo Fisher Scientific 46190, Waltham, MA).
Phalloidin binds to filamentous actin which forms of the core of
stereocilia, therefore it selectively labels hair cell bundles. After
afinal rinse (4 15 min in FPBS), we whole-mounted the tissues on
glass coverslips using Mount-Quick aqueous mounting medium
(Thermo Fisher Scientific). Basilar papillae were mounted directly
between two coverslips. Amphibian papillae were mounted using
a Secure-Seal spacer (Invitrogen) between two coverslips to ensure
that the 3-dimensional structure of the organ remained intact.
After drying for 24 h, we attached the mounted tissues to Super-
frost Plus slides (Thermo Fisher Scientific) using double-sided tape;
this process allowed the tissues to be rotated and/or turned over if
necessary for better imaging results.
2.4. Confocal microscopy and data analysis
Labeled organs were imaged with a confocal microscope (Zeiss
LSM 5) attached to an upright microscope (Zeiss, AxioImager) using
Zen Software (Carl Zeiss MicroImaging Inc., Thornwood, NY). This
confocal microscope is equipped with single-photon (Argon (488,
514 nm), HeNe (543 nm) and Red Diode (633 nm)) lasers. The same
acquisition parameters were used for all control and experimental
scans. The epithelia were first scanned in their entirety using a low-
powered (10) dry objective (Fig. 1A). Subsequently, we took
a single high-powered image with a 63water immersion Plan
Achromat objective from the BP and three high-powered images
from each AP. We captured the AP 63images from the organ’s
rostral end, middle (approximately at the position of the tectorial
curtain; Lewis and Li, 1975; Lewis, 1976) and caudal extension. We
verified the locations of the AP high-powered scans by taking
a subsequent 10imageusing a single laser line; photobleachingof
the fluorescent labels during the high-magnification scans allowed
us to determine their positions precisely (Fig. 1B).
Fig. 1. A: Scanning confocal image stack (projected onto a 2D plane) of the amphibian papilla of O. chloronota. The image was captured with a 10objective. Hair cell somata appear
red (530 nm laser line), and hair bundles appear green (480 nm laser line). B: The same amphibian papilla after high-powered (63) scans have been taken from the rostral (R),
middle (M) and caudal (C) regions of the epithelium. Photobleaching causes the epithelium to appear dim in the regions of the high-powered scans. The tissue is being illuminated
by the 633 nm laser line. L, lateral. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
V.S. Arch et al. / Hearing Research 283 (2012) 70e7972
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Image processing was performed with Volocity Visualization
(Improvision, PerkineElmer, Coventry, England) software, a 3D-
image analysis program. All measurements were all taken by the
same person, who was unfamiliar with the hypothesis of the
experiment. We collected data from the confocal image stacks
using the measurement module of Volocity. Our measurement data
from 10images included epithelium surface area (ESA) and hair
cell count (HCC). ESA was determined by Volocity from a line drawn
manually around the perimeter of the epithelium. We counted the
hair cells by using the “point”tool to label each cell manually
(Fig. 2). As a proxy for the size of the BP organ, we measured the
basilar papillar recess entrance area (REA; see Discussion;Fig. 2).
The data we collected from the 63images included hair cell soma
length (SL) and bundle length (BL). We measured soma length from
the mid-point of the bundle base to the soma base, through the
center of the nucleus. The hair cell nuclei were not labeled by the
antibody against Myosin VI so they appeared as voids in the fluo-
rescently labeled cell bodies. We measured bundle length from the
mid-point of the bundle base to the tip of the tallest stereocilium.
We measured all cells and bundles from which measurements
could be taken unambiguously. Due to the orientation of the whole-
mounted tissues, we were able to collect AP SLs predominately
along the periphery of the organ. Our measurements of hair cell SLs
were restricted to cells in the peripheral regions of the rostral,
middle and caudal portions of the AP where the full soma was
visible/measurable, whereas BLs measurements were taken from
cells throughout these regions of the frog’sAP.
We exported the resulting morphometric data to Excel for
sorting and analysis. We pooled data from the left and right ear of
a single frog since we did not consider these data to be indepen-
dent. We used both SPSS (SPSS Inc., Chicago, IL.) and the R computer
package (2004) to test for differences among the species’means for
each measured parameter using univariate ANOVA, and performed
pair-wise comparisons with Tukey’s post-hoc test.
3. Results
We did not correct our morphological measurements for
shrinkage caused by chemical fixation of the tissues. The inner ear is
comprised of a complex mixture of tissue types including
Fig. 2. Scanning confocal image stacks (projected onto a 2D plane) of the A: Rana pipiens and C: Huia cavitympanum basilar papillae. Images were captured with a 10objective.
Hair cell somata appear red (530 nm laser line), and hair bundles appear green (480 nm laser line). B: and D: Schematics illustrating the measurement data collected from the
basilar papillae. Black lines: area of recess entrance (red lines indicate recess perimeter); Blue: epithelium surface boundary; Black points: individual hair cells. Arrows in D indicate
approximate points of pronounced narrowing of the H. cavitympanum papillar recess. (For interpretation of the references to colour in this figure legend, the reader is referred to the
web version of this article.)
V.S. Arch et al. / Hearing Research 283 (2012) 70e79 73
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cartilaginous, epithelial, neural and gelatinous structures and the
amount of shrinkage can be expected to differ amongst the various
tissue types. However, gross inner-ear morphology was consistent
among the frogs we examined; thus, there is no evidence for
interspecific differences in histology in the tissues of interest. As
a result, we assume that differential shrinkage between species was
minimal, and its effect on relative dimensions insufficient to influ-
ence the overall conclusions drawn from the data. We found
significant differences in the means of all morphological variables
across all species studied (the results of the ANOVAS including F-
and P-values are provided in Table 1, Supplemental Materials). More
specifically, we were interested in which species showed significant
differences among the suite of morphological variables tested.
3.1. Amphibian papilla
We measured hair cell SLs (from the periphery) and BLs from the
rostral, middle and caudal regions of the frog’s AP. These sampling
locations span the presumed tonotopic axis of the organ (Lewis
et al., 1982a, 1982b; Simmons et al., 1994). In accord with the
results of Simmons et al. (1994), SL decreased from the organ’s
rostral (low-frequency sensitive region) to caudal (mid-frequency
sensitive region) end in all six species (Fig. 3AeC). We found the
same trend in our BL measurements (Fig. 3DeF) in all but one
species; in R. pipiens the BL values increased slightly from the
rostral to middle AP (Fig. 3D, E). These results suggest that, other
than R. pipiens, inverse relationships exist between both the hair
cells’SL and BL, and the frequency to which the cells are tuned.
Soma length values followed the same overall trend in all three
AP regions. R. pipiens and A. daorum had the longest SL, and
H. cavitympanum and O. chloronota had intermediate values. O.
tormota and O. livida SL were the smallest (Fig. 3AeC). Statistically
significant groupings varied slightly in each AP region, however. For
the rostral AP, the SL values of R. pipiens and A. daorum were
significantly greater than that of O. livida, and the SL of A. daorum
was greater than O. tormota. The SL values of H. cavitympanum and
O. chloronota were intermediate and did not differ significantly
from any other species (Fig. 3A). In the middle AP, the SL values of
H. cavitympanum and O. chloronota were smaller than those of
R. pipiens and A. daorum, and were greater than those of O. tormota
and O. livida (Fig. 3B), but these differences were not statistically
significant. For the caudal AP, the SL value of H. cavitympanum
only differed significantly from that of O. tormota, which had
a significantly smaller SL value than all species except O. livida. The
SL value of O. chloronota was intermediate, and did not differ
significantly from any other species (Fig. 3C).
Bundle length only differed significantly between O. livida and
O. chloronota in the rostral AP (Fig.3D). However, in the middle AP, the
differences between thespecies’BL valuesbecame more pronounced:
R. pipiensand A. daorum had significantlygreater BL values thanthose
of all three ultrasonically sensitive frogs, and O. chloronota was
intermediate (Fig. 3D). In the caudal region, R. pipiens and A. daorum
BLs were greater than that of all other species (Fig. 3E, F).
The AP ESA and HCC scaled approximately with the average
body size of the species (r
2
¼0.83 and 0.74, respectively; Fig. 4D, E).
3.2. Basilar papilla
Unlike the AP, the ESA and HCC of the BP scaled much less
allometrically with the species’average body size as reflected in the
lower correlation coefficients (r
2
¼0.28, P¼0.277; and 0.45,
P¼0.148, respectively; Fig. 4B, C). The same was true for BP REA,
our proxy for BP organ size (r
2
¼0.58, P¼0.078; Fig. 4A). Rana
pipiens had values for these metrics that were significantly greater
than all other species (P<0.05; Fig. 5AeC). The trends in the data
suggest that A. daorum also had larger values for these metrics than
the ultrasonically sensitive species and O. chloronota, although the
small sample size for A. daorum precluded some of these differ-
ences from being statistically significant. The three ultrasound-
detecting frogs had values for these metrics that were statistically
indistinguishable from one another and from O. chloronota
(Fig. 5AeC). These data indicate that H. cavitympanum,O. tormota,
O. livida and O. chloronota have significantly smaller BP organs and
sensory epithelia than those of R. pipiens and A. daorum (Pvalues for
all sixteen comparisons <0.0001).
Basilar papilla SL and BL values from the ultrasonically sensitive
frogs and O. chloronota were not significantly different and their SL
values were significantly smaller than those of R. pipiens and
A. daorum. The same pattern was seen in the BL values with the
exception that the BL of O. chloronota was not statistically different
from that of A. daorum (Fig. 5D, E).
4. Discussion
This study comprises a quantitative investigation of the auditory
organs in the inner ears of H. cavitympanum,O. tormota and
Fig. 3. Soma and bundle lengths from the rostral (A and D), middle (B and E) and caudal (C and F) regions of the species’APs. Numbers indicate sample sizes. Letters denote
statistically significant differences in pairwise comparisons using Tukey’s post-hoc analysis with
a
¼0.05. If a pair of species shares a common letter, they are not significantly
different in that trait.
V.S. Arch et al. / Hearing Research 283 (2012) 70e7974
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O. livida, the first amphibians found to detect ultrasonic frequencies
(Feng et al., 2006; Arch et al., 2009). To date, these are the only frogs
known to detect frequencies above a previously postulated upper
sensitivity limit of w5e8 kHz for the taxon (Loftus-Hills and
Johnstone, 1970). Therefore, we hypothesized that the inner ears
of these species have undergone a radical reorganization of their
inner ears to facilitate their extended sensitivity. Our observations,
however, clearly indicate that this is not the case; the layout and
gross structural features of the species’auditory organs are
consistent with those of frogs that lack high-frequency sensitivity.
These observations suggest an alternate hypothesis: that a mosaic
of smaller modifications within frogs’inner ears might facilitate
high-frequency sensitivity. We have enumerated these modifica-
tions which include: 1. reduced BP chamber volume, 2. reduced
tectorial membrane mass, 3. reduced hair bundle length, and 4.
reduced hair cell soma length. While none of these factors on its
own could account for the US sensitivity of the inner ears of these
species, the combination of these factors appears to extend their
hearing bandwidth, and facilitate high-frequency/ultrasound
detection.
4.1. Comparison between ultrasound-sensitive and control inner
ears
Our results reveal interesting morphological differences
between the auditory epithelia of the ultrasound-sensitive species
and those of R. pipiens. The frequency ranges to which the
ultrasound-detecting frogs’auditory organs are sensitive are
currently unknown. Therefore, interpreting the significance of our
morphological data necessitates formulating preliminary hypoth-
eses about the distribution of frequency sensitivity between these
organs. In R. pipiens, the AP transduces frequencies from ca.
100e1250 Hz and the BP is tuned to w2 kHz (Feng and Shofner,
1981; Ronken, 1990). Thus, R. pipiens auditory sensitivity spans
ca. 1900 Hz. Although none of the ultrasonic frogs’frequency
sensitivity has been measured electrophysiologically below 1 kHz,
tympanic membrane vibration data from the species indicate that
their eardrums vibrate in response to frequencies as low as 200 Hz
(Gridi-Papp et al., 2008; Arch et al., 2009; Gridi-Papp, unpublished
data). These results suggest that the low-frequency detection
ability of these species is consistent with other frogs tested to date
Fig. 4. A: Recess entrance area (REA) of the basilar papilla; epithelium surface area (ESA) of the B: basilar papilla and D: amphibianpapilla and hair cell count (HCC) of the (C) basilar
papilla and (E) amphibian papilla plotted against the average body size of the species. Solid lines indicate the best fit regressions. Data points represent means and standard errors.
Fig. 5. Morphometric data from the basilar papilla. A: Recess entrance area (REA); B: Epithelium surface area (ESA); C: Hair cell count (HCC); D: Hair cell soma length; E: Hair cell
bundle length. Numbers indicate sample sizes. Letters denote statistically significant differences in pairwise comparisons using Tukey’s post-hoc analysis with
a
¼0.05. If a pair of
species shares a common letter, they are not significantly different in that trait.
V.S. Arch et al. / Hearing Research 283 (2012) 70e79 75
Author's personal copy
(ca.100e200 Hz) (Feng et al., 1975; Lewis et al., 1982a; Narins and
Hillery, 1983; Hillery and Narins, 1987). Under this assumption,
H. cavitympanum,O. tormota and O. livida auditory sensitivity spans
ca. 37,800 Hz, 33,800 Hz and 21,800 Hz respectively, and therefore
the species display a substantially broader sensitivity bandwidth
than that of R. pipiens. Across frog species, the BP is consistently
tuned to a species-specific band of frequencies (Wilczynski and
Capranica, 1984; Ronken, 1990). Auditory-evoked potentials (AEP)
recorded from the central auditory system of H. cavitympanum and
O. tormota show a high-frequency sensitivity peak at ca. 25 kHz
(Arch et al., 2009) and ca. 20 kHz (Feng et al., 2006), respectively.
Since the BP is the high-frequency detection organ of anurans,
these data suggest that BP is likely responsible for transducing
ultrasounds. A high-frequency peak is less apparent in the AEP
recordings from O. livida (Feng et al., 2006) but given that
O. tormota and O. livida are congeneric, and the species display
a remarkable convergence in auditory organ morphometrics (see
below), we hypothesize that the O. livida BP is similarly responsible
for high-frequency/ultrasound transduction. Lower-frequency
sensitivity peaks at ca. 8 and 12 kHz in the AEP audiogram of
H. cavitympanum (Arch et al., 2009), and a sensitivity plateau below
ca. 10 kHz in O. tormota (Feng et al., 2006) are presumed to originate
from the AP. Based on this hypothesized distribution of frequency
sensitivity between the H. cavitympanum and O. tormota auditory
organs, we suggest that the ultrasonic frogs’BPs are sensitive to
substantially higher frequencies than that of R. pipiens and that
their APs are sensitive to a wider bandwidth of frequencies (i.e.,
H. cavitympanum:w200 Hz - 12,000 Hz; O. tormota:w200 Hz -
10,000 Hz; R. pipiens:w100 H ze1250 Hz). Direct tests of these
inferences, and a more detailed examination of the frequency
sensitivity of the O. livida peripheral and central auditory system,
are planned in future studies.
4.2. The basilar papilla
The sensory epithelium of the frog BP is located at the base of
a tubular outpocket of the sacculus, an inner-ear endorgan
primarily responsible for detecting substrate vibrations (Narins,
1990; Yu et al., 1991). A tectorial membrane (TM) spans the
lumen of the papillar recess, connecting to the stereociliary bundles
of the sensory hair cells that are embedded in the recess’s carti-
laginous wall (Wever, 1985). The tuning curves of BP nerve fibers
within a particular animal have nearly identical shapes and char-
acteristic frequencies (Ronken, 1990; van Dijk and Meenderink,
2006). Therefore, the BP is believed to act as a single tuned,
mechanical resonator (Capranica and Moffat, 1977; Ronken, 1990;
van Dijk and Manley, 2001; Meenderink et al., 2005).
Mechanical tuning of the BP presumably results from a combi-
nation of anatomical features, including the large-scale fluid
dynamics of the inner ear and the frequency tuning of the contact
membrane (Purgue and Narins, 2000a, 2000b). These features
influence the movement of the TM which interacts directly with the
hair cells. Recently, Schoffelen et al. (2009) optically measured the
mechanical response of the BP TM in R. pipiens and found that it is
tuned to 2 kHz, corresponding closely to the characteristic
frequencies of the species’BP nerve fibers. These data suggest that
TM movement may be primarily responsible for the frequency
selectivity of the organ (Schoffelen et al., 2009).
Tectorial membrane tuning is influenced strongly by its inter-
action with the lumen boundary. In fact, it has been suggested that
mechanical coupling between the TM and stereociliary bundles
acts as a local resonance system (Zwislocki, 1980a, 1980b; Lewis
and Leverenz, 1983) with a resonance frequency that is directly
proportional to the resonator’s stiffness (for additional detail, see
Shofner and Feng, 1984). Our hypothesis that the ultrasonically
sensitive frogs’BPs transduce ultrasounds suggests that they have
a substantially higher resonance frequency, and thus greater stiff-
ness, than the R. pipiens BP. A key factor in the stiffness of the
system is the stereocilia, which are rigid structures (Flock, 1977,
1982; Strelioff and Flock, 1982). Bundle stiffness is inversely
correlated with stereociliary height (Flock, 1982; Strelioff and Flock,
1982; Authier and Manley, 1995) and directly correlated with
stereocilia number (Authier and Manley, 1995; Fettiplace and
Fuchs, 1999). The latter two factors typically covary: a reduction
in bundle height is accompanied by an increase in the number of
stereocilia per bundle (Tilney and Saunders, 1983; Hackney et al.,
1993; Köppl and Authier, 1995). Our data indicate that the ultra-
sonic frogs’BP hair bundles are, on average, w1.3
m
m(w30%)
shorter than those of R. pipiens (means: BL
Hc
¼3.31
m
m,
BL
Rp
¼4.75
m
m, Fig. 5E). These shorter stereocilia will be compar-
atively stiffer, increasing the system’s resonant frequency. As
a comparison, the mean bundle lengths for basal (high-frequency)
inner hair cells from several mammals sensitive to ultrasound, and
the upper frequency limit of their hearing are: chinchilla:
BL ¼1.75
m
m, upper limit: 25 kHz; rat: BL ¼2.3
m
m, upper limit:
59 kHz; bat: 2.5
m
m, upper limit: 100 kHz (Echteler et al., 1994).
From our confocal images, we were unable to determine the
number of stereocilia per bundle; future studies employing elec-
tron microscopy will be necessary for this assessment.
Along with increased stereociliary stiffness, a decrease in the
resonator mass of the ultrasonic frogs’BPs could also play a signif-
icant role in increasing their resonance frequencies (for additional
detail, see Shofner and Feng, 1984). The mass term affecting the
local resonance frequency of the coupled TM and sensory epithe-
lium is primarily determined by the mass loading of the TM on the
stereocilia (Zwislocki, 1980a, 1980b). We did not measure TM mass
in this study; however, our data indicate that the ultrasonic frogs’
BP REAs, a proxy for the organ’s size, are approximately 18%
(H. cavitympanum), 24% (O. tormota) and 27% (O. livida) that of
R. pipiens (Fig. 5A). The average body size [snout-vent length (SVL)]
of the ultrasound-detecting species is 71% (H. cavitympanum), 51%
(O. tormota) and 76% (O. livida) that of the R. pipiens used in our
study [H. cavitympanum avg. SVL ¼45 mm (Yang, 1991); O. tormota
avg. SVL ¼32.5 mm (Fei, 1999); O. livida avg. SVL ¼48 mm (Fei,
1999); R. pipiens avg. SVL ¼63.5 mm (Arch, personal obs.)] hence
the considerable size difference between the species’BPs is not
solely due to allometric scaling (Fig. 4A). In a study of developing
auditory organs in the frog, smaller tectorial membrane volume,
and thus mass, has been shown to be correlated with decreased BP
chamber volume (Shofner and Feng, 1984). This suggests that the
substantially reduced size of the ultrasound-sensitive frogs’BPs
will correlate with a significantly less-massive TM. Furthermore,
qualitative observations of our image stacks indicate that the
H. cavitympanum, O. tormota and O. livida BP chambers narrow
adjacent to the sensory surface (e.g. Fig. 2 D). This narrowing results
in a smaller area over which the TM must stretch to cover the
sensory epithelium. In sum, we conclude that a possible explana-
tion for the substantially reduced size of the ultrasound-sensitive
frogs’BPs is that it permits a markedly smaller TM; this smaller,
and therefore lighter TM decreases the mass loading of the BP
resonance system, increasing the organ’sresonance frequency.
Our SL data from the BP suggest another way that the
ultrasound-detecting ear is specialized to transduce high frequen-
cies. The average BP SL of ultrasonically sensitive frogs is w3e4
m
m
shorter than in R. pipiens (Fig. 5D). A negative relationship between
hair cell SL and frequency sensitivity has been demonstrated across
vertebrate classes, including the frog AP (Simmons et al., 1994), the
chick basilar papilla (Fuchs et al., 1988) and the mammalian cochlea
(Wada,1923; Iurato, 1967; Bohne and Carr, 1985). The time constant
of the hair cell membrane largely determines how quickly the
V.S. Arch et al. / Hearing Research 283 (2012) 70e7976
Author's personal copy
membrane can charge and discharge, and thus defines the
maximum frequencies the cell can encode. Whole-cell capacitances
of frog AP and saccular hair cells have been shown to correlate
directly with cell SL (Smotherman and Narins, 1998, 1999b).
Assuming this relationship holds in the BP and that variation in
membrane resistance is low (Smotherman and Narins, 1999a), the
shorter hair cells of H. cavitympanum, O. tormota and O. livida are
predicted to have a shorter membrane time constant and thus
faster dynamics than those of R. pipiens, consistent with their role
in transducing higher frequencies.
Taken together, the highly convergent morphological data
collected from the BPs of the frogs that detect ultrasounds suggest
that this organ appears specialized for transducing high-frequency
sounds. Shorter hair bundle length and smaller organ size imply
that the TM-sensory epithelium coupling in these frogs is
substantially stiffer than in R. pipiens. Additional potential contri-
butions to BP tuning of larger scale factors like inner-ear fluid
dynamics and contact membrane will be of interest in future
studies.
4.3. The amphibian papilla
The AP is the larger and more complex of the two inner-ear
organs in frogs (Geisler et al., 1964; Wever, 1973) and lies in
a medial extension of the sacculus. Our data indicate that the tested
species’AP ESA and HCC scale approximately allometrically with
average body size (Fig. 4D, E). This is in contrast to the BP, which has
significantly smaller ESA and HCC values in ultrasound-sensitive
frogs relative to body size (Fig. 4B, C). Thus, if our hypothesis of
AP frequency sensitivity bandwidth is correct, the broader band-
width of the ultrasound-detecting frogs’APs appears to be inde-
pendent of epithelium size or cell number. Ourdata also indicate no
significant difference in BL between the species in the rostral AP
(Fig. 3A). Soma length in this region only differs significantly
between R. pipiens and O. livida. These data suggest that mechanical
tuning related to hair cell structural composition is comparable
between H. cavitympanum,O. tormota and R. pipiens in this AP
region. This result agrees with our inference that the frogs have
similar low-frequency hearing limits, although a thorough test of
this prediction requires electrophysiological exploration of
H. cavitympanum and O. tormota low-frequency hearing. In the
middle portion of the AP the ultrasound-sensitive frogs’SL and BL
are significantly shorter than in R. pipiens (Fig. 3B). A similar pattern
is seen in the BL data from the caudal AP; however there is no
difference between H. cavitympanum and R. pipiens SL in this region
(Fig. 3C). As in the BP, shorter hair cells in the middle region of the
ultrasound-detecting frogs’AP are expected to have a smaller
capacitance and thus shorter membrane time constant, allowing
the encoding of higher frequencies. This conclusion is in agreement
with our expectation that these frogs’AP transduction range covers
a wider high-frequency bandwidth than that of R. pipiens.
As in the basilar papilla, the absence of a flexible membrane
underlying the AP sensory epithelium restricts extrinsic hair cell
tuning to the coupling between the hair bundles and the overlying
TM. If we again consider this coupled system as a resonator (Lewis
and Leverenz, 1983; Zwislocki, 1980a, 1980b) with stiffness, and
thus resonant frequency,inversely related to bundle length (Shofner
and Feng, 1981), our BL data indicate an increased resonant
frequency of the ultrasound-detecting frogs’AP relative to R. pipiens
as we move away from the rostral patch along the tonotopic axis
(Fig. 5D, E). This provides additional support for our hypothesis that
these frogs’APs transduce an extended range of frequencies that
includes substantially higher frequencies compared to R. pipiens.
Further research related to the electrical properties of the AP
hair cells may afford added insight into their frequency responses
along the organ’s tonotopic axis. While BP tuning is hypothesized to
be mechanical in origin, there is electrical tuning in low-frequency
AP hair cells (Pitchford and Ashmore, 1987; Smotherman and
Narins, 1999b, 2000). Furthermore, ion channel kinetics have
been demonstrated to change by an order of magnitude along the
tonotopic axis of the R. pipiens AP (Smotherman and Narins, 1999b,
2000). Electrical tuning properties and ion channel compositions
that differ between species’AP hair cells may complement
mechanical tuning mechanisms to differentiate further the
frequency response properties of the ultrasound-sensitive frogs
and R. pipiens APs.
4.4. Lao torrent frogs
Currently, very little is known about the life histories of
O. chloronota and A. daorum, including their acoustic communica-
tion systems. However, we hypothesized that they are candidates
for the use of high-frequency/ultrasonic communication because
they are found in sympatry alongside rushing montane streams
that produce high-level, predominately low-frequency ambient
noise that closely matches the noise in the environments of
H. cavitympanum,O. tormota and O. livida (Arch and Narins,
personal obs.; Feng et al., 2006; Arch et al., 2008; Arch and Narins,
2008).
We found that the morphometrics of the O. chloronota auditory
organs are remarkably convergent with those of the ultrasonically
sensitive frogs, and are statistically indistinguishable from
H. cavitympanum in every morphological feature measured (Figs. 3
and 5). As a result, we suggest that this species is a strong candidate
for high-frequency/ultrasonic hearing. By contrast, the statistical
results for A. daorum were generally intermediate between the
ultrasound-detecting species and R. pipiens.The power of these
tests, however, was significantly reduced by our small sample size.
Based on the trends in the data, it appears that the auditory
morphology of A. daorum is different from the ultrasonically
sensitive frogs in every metric for which there was a group differ-
ence among the species (Figs. 3 and 5). Interestingly, A. daorum SL
and BL were comparable to those of R. pipiens in both auditory
organs, despite the fact that A. daorum and R. pipiens are the
smallest and largest of the species examined, respectively [SVL:
36 mm (Bain et al., 2003) versus 63.5 mm (Arch, personal obs.)].
These results imply that SL and BL values are decoupled from body
size. This conclusion is consistent with our hypothesis that these
are key morphological features influencing high-frequency sensi-
tivity; to date, ultrasonic hearing ability does not appear to relate
directly with body size [e.g., H. cavitympanum avg. SVL ¼45 mm
(Yang, 1991); O. tormota avg. SVL ¼34 mm (Fei,1999); O. livida avg.
SVL ¼48 mm (Fei, 1999)]. Based on our data from A. daorum,we
conclude that this species is unlikely to detect ultrasound. Future
experiments employing electrophysiological recording from the
auditory midbrain and/or VIIIth nerve of the Lao species will help
determine whether auditory morphology can be used to predict the
extent of high-frequency hearing.
5. Conclusion
Our comparison of the morphological features of the
H. cavitympanum,O. tormota,O. livida and R. pipiens inner ears is
afirst step toward understanding the structural and physiological
mechanisms that enable high-frequency detection in frogs. A
striking inner-ear morphological convergenceoccurs among species
from two distantly related genera. From these data, we postulate
that relatively small-scale adjustments in morphological features
related to the mass and stiffness of the resonance systems within the
auditory organs play key roles in facilitating high-frequency
V.S. Arch et al. / Hearing Research 283 (2012) 70e79 77
Author's personal copy
transduction. Accordingly, we propose that the acquisition of
ultrasonic hearing evolved through gradual and convergent modi-
fications of the frog ear, rather than through a major reorganization
of the transduction apparatus. Subsequent experimental work will
be necessary to determine causal relationships between our
morphological observations and high-frequency transduction
ability. Exploring the frequency sensitivityof O. chloronota provides
an excellent opportunity to begin this work, since the species’ear
morphology is remarkably convergent with that of the demon-
strated ultrasound-sensitive species. These ongoing investigations
will continue to elucidate the peripheral mechanisms enabling
a non-mammalian vertebrate to transduce extraordinarily high
frequencies.
Acknowledgments
We are grateful to Dr. Larry Hoffman (UCLA, Department of Head
and Neck Surgery) for providing advice, resources and time during
the initial stages of this research project. We thank Aubrey Hawkes
for substantial assistance with methodological development and
Dr. Stephen Arch for helpful comments on the manuscript.
Appendix. Supplementary data
Supplementary data related to this article can be found online at
doi:10.1016/j.heares.2011.11.006.
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