ArticlePDF Available
45
ISSN: 1911-4745
Comparative Cognition
& Behavior Reviews
DOI:10.3819/CCBR.2020.150007 Volume 15, 2020
A Comparison of Hearing and Auditory Functioning
Between Dogs and Humans
A. L. A. Barber
School of Life Sciences
and School of Psychology
University of Lincoln
A. Wilkinson
and F. Montealegre-Z
School of Life Sciences
University of Lincoln
Given the range of tasks that requires dogs and humans to work effectively together, it is important
for us to appreciate the similarities and differences in hearing ability across the two species, as well
as the limits of our knowledge of this comparative information. Humans often assume that dogs
hearing abilities are similar to their own and try to communicate with them verbally as they do with
other humans. In the rst part of this review, we compare the auditory system of the two species in
relation to their ability to function generally as a sound amplication and detection system before
considering the specic capacities of the system in the second part. We then examine the factors that
disturb hearing function before reviewing a range of potentially problematic behavioral responses
that are closely associated with the functioning of the auditory system. Finally, we consider important
aspects of comparative auditory perception and related cognitive processes. A major observation
of this review is how little research has been done in investigating the auditory capabilities of the
dog. There may be signicant mismatches between what we expect dogs (and perhaps specic types
of dog, given historic functional breed selection) can hear versus what they can actually hear. This
has signicant implications for what should be considered if we wish to select specic dogs for work
associated with particular hearing abilities and to protect and maintain their hearing throughout
life. Only with a more complete understanding of the dogs’ hearing ability compared with our own
can we more fully appreciate perceptual and associated cognitive differences between the species
alongside behavioral differences that might occur when we are exposed to a given soundscape.
Keywords: hearing, auditory functioning, dog, human
V. F. Ratcliffe
Defence Science
and Technology Laboratory,
Sevenoaks, UK
K. Guo
School of Psychology
University of Lincoln
D. S. Mills
School of Life Sciences
University of Lincoln
46
COMPARATIVE COGNITION & BEHAVIOR REVIEWS
Barber et al.
Introduction
It is widely believed that dogs have better hearing than
humans and that they may be able to hear sounds that are
up to 4 times farther away than humans can (e.g., 90m in
humans and 400m in dogs; Audicus, 2015; Cole, 2010).
However, hearing involves not just detection but also the
resolution of sounds and, from a functional perspective,
the recognition of certain compositions as meaningful in
some way. For effective auditory communication between
species, such as humans and dogs, each must appreciate
at some level what sounds the other can detect and the
informational content of different sound qualities. Some
of this may be based on general physical properties of
the sound, such as the relationship between body size
and vocalization frequencies, whereas other levels may
be more species specic. To this end, researchers have
found that humans and dogs do indeed have some
reciprocal appreciation of the qualities of each other’s
vocalizations (Pongrácz, Molnár, & Miklósi, 2006; Yong
& Ruffman, 2014). This result is perhaps not surprising
given the history of the two species and the range of
challenges involving dogs and humans working together
effectively. Nonetheless, it is clear that compared with
humans, dogs have a different understanding of word
meanings (Braem & Mills, 2010; Fukuzawa, Mills, &
Cooper, 2005b; Markman & Abelev, 2004; Mills, 2005;
Ramos & Ades, 2012; van der Zee, Zulch, & Mills, 2012).
Even when this is appreciated, humans often assume that
dogs’ hearing abilities are similar to their own, but there
is surprisingly little research to support this. Further,
given the enormous morphological and functional
diversity of dogs, it would perhaps be surprising if there
was not considerable variability in their hearing ability.
Where direct evidence of hearing ability is limited,
it is possible to make working assumptions based on
our understanding of the functional anatomy of sound
detection and perception until more direct evidence
becomes available. Therefore, in this review we not only
compare what is known about the hearing ability of dogs
and humans but highlight important considerations
based on such fundamental principles.
Part 1: General Comparative Anatomy
and Functioning of the Auditory System
as a Sound Amplied in Dogs and Humans
The auditory systems of dogs and humans share
the same basic plan and physical structures with sound
waves collected in the outer ear and amplied via the
middle ear before being transduced into electrical signals
by the inner ear. The main body of this review assumes
that the reader has a general working knowledge of the
nature of sound and structures making up the auditory
system in terrestrial mammals (though details of this
are provided in the Supplementary Information), and
so we focus here on comparing the specic details in
humans and dogs and their functional consequences.
Available data on the physical characteristics of the
component structures are summarized in Table 1.
Needless to say, there is greater variation among dogs
given their adult size and morphological variability,
so comparisons with humans need to be carefully
appraised to determine to what extent they apply at the
level of dog versus some more specic morphological
feature within the species. As a general observation,
there appears to be a widespread lack of empirical data
quantifying the variability that occurs in dogs and its
correlates. For example, as discussed in the next section,
surprisingly little attention appears to have been given to
characterizing the obvious heterogeneity of the external
pinnae among dogs, which plays an obviously important
role in the general ability of an individual to detect
sound waves (sensitivity to sound). The auditory system
is tuned to amplify some frequencies better than others;
for example, as a result of anatomic constraints, some
wavelengths may be caught by the pinna or collected
by the external auditory meatus (EAM) with variable
levels of efciency; sensitivity is further related to the
characteristics of the basilar membrane and the auditory
pathway, which are tuned to enhance some frequencies
more than others. Variability between humans and dogs
Author Note: A. L. A. Barber, University of Lincoln, School
of Psychology and School of Life Sciences, Brayford Pool,
Lincoln, Lincolnshire, LN6 7TS, UK.
Correspondence concerning this article should be addressed to
A.L.A. Barber at abarber@lincoln.ac.uk.
Acknowledgments: We thank Katie Mitchell and Naomi Westgate
for administrative support, Charlie Woodrow for artistic help
with Figures 2 and 3, and Amy West for proofreading, as well as
the members of the Animal, Behaviour, Cognition and Welfare
Research Group and the Perception, Action and Cognition
Research Group at the University of Lincoln for inspiring
and constructive discussions on the topic. This research was
supported by the Defence Science and Technology Laboratory
and the European Union’s Horizon 2020 research and innovation
programme under the Marie Skłodowska-Curie grant agreement
No. [793559].
47
comparison of hearing and auditory functioning between dogs and humans
VOLUME 15, 2020
will likely reect, to a large extent, the differing but
also shared ecology of the two species over the whole
of their evolutionary history (phylogeny) and within
dogs on breed function, where this has been maintained
(Fadeletal., 2016). The extent to which domestication
has caused changes to the auditory system akin to those
noted for the visual system (see Burda, 1985; Burda
& Branis, 1988; McGreevy, Grassi, & Harman, 2004)
remains unknown. Nonetheless, the occurrence of
consistent differences within subpopulations of dogs
would suggest potential selection in favor of or against
these traits. This has important implications for the
potential future breeding of individuals to perform
better in different soundscapes.
Pinna
At its simplest, the pinna acts as a sound funnel,
which passively but selectively collects and amplies
certain wavelengths (Fletcher, 1992). The pinna plays a
substantial role in the localization of sound which, in the
horizontal plane, has been reported to be more accurate
than the vertical plane (Heffner & Heffner, 1998). The
pinna is especially important for the attenuation of high
frequencies from behind and thus reduces front–back
confusion in the sound localization (Keidel & Neff, 1975).
In humans, very little variation in pinnae size
has been observed, though males have slightly bigger
pinna than females (Bruckeretal., 2003; Itoetal., 2001;
Salvinellietal., 1991). In contrast, pinna shape and size
vary immensely in dogs; this is based on not only body
and head size but also breed-specic characteristics: A
Pomeranian might have an ear size of a few centimeters,
whereas in the Guinness Book of World Records some
dogs have ear sizes between 34.3 and 34.9 cm (Guinness
World Records Ltd., 2015; for human data, see Table1).
Three main forms of pinna are described in the dog:
(a)erect (e.g., huskies, German shepherds), (b)semi-erect
(e.g., pugs, greyhounds), and (c)dropped (e.g., beagles,
poodles).
Within these main types (see Figure1), there are
several subcategories (e.g., rose, button, candle ame,
cocked, V-shaped, Filbert shaped, folded, hooded, bat
ears, round tipped, cropped). Surprisingly, there appears
to be no systematic evaluation of amplication effects
of different pinna shapes in dogs. However, it can be
assumed that dogs with large erect ears are especially
good at localizing distant noises (Nummela, 2008; Strain,
2011). Both the exibility of its constituent cartilage and
the presence of hair on the reective surface of the pinna
affect its acoustic properties. The ability to control the
orientation of their pinnae is thought to improve hearing
sensitivity by as much as 28dB, particularly at higher
frequencies (Phillips, Calford, Pettigrew, Aitkin, &
Semple, 1982; Strain, 2011). It has been proposed that
dogs with erect upright pinnae amplify both high- and
low-frequency sounds (Strain, 2011); this would appear
to increase their hearing range compared with dogs
with other ear shapes, which are anecdotally reported
to experience greater limitation in the amplication of
sounds (Denzer, 2018). How any of these compare with
the human pinna shape remains unknown.
External Auditory Meatus
Sound waves collected by the pinnae are transmitted
along the EAM to the tympanum (eardrum; Figure2).
Physical properties of the EAM for example, its
length, width, and surface characteristics determine
which wavelengths are attenuated and transmitted and
thus the peak frequency of sound sensitivity. Given the
morphological variation in the EAM of different breeds
and even individual dogs (e.g., because of the presence
of hair), it is likely that dogs may vary considerably in
the sounds they are most able to detect at this level. In
humans, consistent variation in the EAM based on sex
Figure 1. Examples of dog ear shapes: erect, semi-erect, and dropped, from left to right.
48
COMPARATIVE COGNITION & BEHAVIOR REVIEWS
Barber et al.
Tab le 1. Comparison of Physical Features of the Auditory System of Dogs and Humans.
Part of
the Ear
Auditory
Structure Measure Species Specications
Data
(M ± SD
Where Available) Method Reference
Outer ear Pinna Length Human N = 123, ages 18– 65 y
Male n = 34
63 mm
Male ear 6.5% bigger
Unknown Brucker et al., 2003
Human N = 280, ages 18– 60 y
Male n = 80
Female n = 60
63.5 ± 12 mm
Male: 63.0 ± 12.9 mm
Female: 59.0 ± 11.2 mm
Postmor tem Salvinelli et al., 1991
Human N = 1,958, ages 0 –94 y Vital samples Ito et al., 2001
Ages 10–14 y Male: 62 mm
Female: 59 mm
Ages > 60 y Male: > 72 mm
Female: > 66 mm
Dog Unknown Unknown
External
auditor y
meatus
Length Human N = 280, ages 18– 60 y
Male n = 80
Female n = 60
23.5 ± 2.5 mm
Male: 25.2 ± 2.6 mm
Female: 22.5 ± 2.3 mm
Postmor tem Salvinelli et al., 1991
Human 25–31 mm Unknown Pensak & Choo, 2015
Dog N = 28, di breed, age
(2–13 y), and sex
53 ± 10 mm
Range = 30 –70 mm
Postmor tem Huang, Lit tle, &
McNeil, 2009
Dog 22–57 mm Unknown Harvey, Harari, &
Delauche, 2001
Diameter Human N = 280, ages 18– 60 y
Male n = 80
Female n = 60
Max: 9.3 ± 1.5 mm
Min: 4.8 ± 0.5 mm
Max: 9.7 ± 1.1 mm
Min: 5.1 ± 0.7 mm
Max: 8.5 ± 0.7 mm
Min: 4.4 ± 0.3 mm
Postmor tem Salvinelli et al., 1991
Human 6–9 mm Unknown Pensak & Choo, 2015
Dog N = 28, di breed, age
(2–13 y), and sex
Max: 58 ± 15 mm
Max range: 21–79 mm
Min: 7 ± 2 mm
Min range: 3 –10 mm
Unknown Huang et al., 2009
Dog Max: 21–79 mm Unknown Harvey et al., 2001
Middle ear Tympanic
membrane
Diameter Human N = 280, Ages 18– 60 y
Male n = 80
Female n = 60
9.4 ± 1.5 mm
9.7 ± 1.8 mm
9.2 ± 1.2 mm
Unknown Salvinelli et al., 1991
Area Human 90 mm² Unknown Pensak & Choo, 2015
Human 68.3 mm² Unknown Hemilä, Nummela,
& Reuter, 1995
Dog Small dog: 4.3 kg
Large dog: 45.5 kg
30 mm²
63 mm²
Unknown Hener, 1983
63.3 mm² Unknown Hemilä et al., 1995
Tympanic
cavity
Length Human Unknown Unknown
Dog Tympanic cavity proper:
< 10 mm
Ventral cavity: 15 mm
Unknown Harvey et al., 2001
Dog 14. 2–22.6 mm Unknown Wysocki, 2006
Volume Human N = 51, di sex
(male n = 19, female
n = 25), and age (age
range = 19– 69 y)
M = 5.2 ± 3.1 ml
Range = 0.6 –13.4 ml
CT scan Ahn et al., 2008
49
comparison of hearing and auditory functioning between dogs and humans
VOLUME 15, 2020
(continues)
Part of
the Ear
Auditory
Structure Measure Species Specications
Data
(M ± SD
Where Available) Method Reference
Middle ear
(continued)
Tympanic
cavity
(continued)
Volume
(continued)
Human N = 91, di sex (male
n = 45, female n = 46),
and age (M = 48.1 y)
0.49 ± 0.0436 c Cavalieri
principle
Kürkçüoğlu et al., 2010
Human N = 55 M = 6.5 cm³
Range = 2–22 cm³
Postmor tem,
acoustic method,
X-ray, measure
includes mastoid
cells
Molvær, Vallersnes,
& Kringlebotn, 1978
Human Male right ear: 0.52 cm³
Male left ear: 0.55 cm³
Female right ear: 0.45 cm³
Female left ear: 0.49 cm³
CT cavaliere
method
Kavakli et al., 2004
Dog Tympanic cavity proper:
2.5 cm3Unknown Harvey et al., 2001
Dog 1.5 ml Unknown Cole, 2009
Dog N = 8, di breed,
age, mesocephalic
1.85 ± 0.15 ml CT, po st mo rtem Defalque, Rosenstein,
& Rosser, 2005
Dog N = 8, di breed,
age, mesocephalic
2 ± 0.2 ml Water-lling
method,
postmortem
Defalque et al., 2005
Dog N = 18, di breed, age
(M = 4.8 y), and sex
(male n = 10, female
n = 8), M weight = 20.5 kg,
mesocephalic
1.5 ± 0.8 ml CT, vital
samples
Defalque et al., 2005
Malleus Human N = 50, male ears 7.8 m m Postmor tem Sodhi et al., 2017
Human N = 870 7.7 2 m m Summary of
average length
of 14 studies
Sodhi et al., 2017
Human N = 92 8.1 6 m m Postmortem Heron, 1923
Dog 10 mm Unknown Harvey et al., 2001
Incus Human N = 50, male ears 6.45 mm Postmor tem Sodhi et al., 2017
Human N = 578 6.07 mm Summary of
average length
of 10 studies
Sodhi et al., 2017
Human N = 94 5.2 mm Unknown Heron, 1923
Dog 4 mm Unknown Harvey et al., 2001
Stapes Human N = 50, male ears 3.4 mm Postmortem Sodhi et al., 2017
Human N = 734 3.21 mm Summary of
average length
of 12 studies
Sodhi et al., 2017
Human N = 31 3.45 mm Unknown Heron, 1923
Dog 2 mm Unknown Harvey et al., 2001
Auditory tube Diameter Human 3 mm Unknown
Dog 1.5 mm Unknown Cole, 2009
Length Human 35 mm Unknown Pensak & Choo, 2015
Human N = 90, adults 42.9 mm CT scan Takasaki et al., 2007
Dog 15–20 m m Unknown Harvey et al., 2001
Dog 10–15 mm Unknown Berghes et al., 2010
50
COMPARATIVE COGNITION & BEHAVIOR REVIEWS
Barber et al.
differences have been described (Table1), but it is not
known whether this also applies to dogs. In general, the
EAMs of dogs are relatively longer, wider in diameter,
more mobile, and more cone-like than those of humans,
which may improve the amplification of and thus
sensitivity to sound and specic wavelengths depending
on the actual shape in a given individual (Harvey &
Ter Haar, 2017). However, there is a lack of systematic
investigation of the practical consequences of these
physical effects in dogs.
For those with hair within the EAM, the number of
hairs tends to decrease toward the tympanum, although
some dog breeds may have a large amount of hair in
the EAM (hirsute ears), such as cocker spaniels and
poodles (Cole, 2009), and it is likely that this results in
greater attenuation of sound waves in these instances.
This may make certain breeds less sensitive to sound,
and it is probably not a coincidence that some of those
breeds, typically with hairy EAM (e.g., cocker and
springer spaniel), also have dropped ears and so may
Middle ear
(continued)
Oval window Area Human 2.98 mm² Unknown Hemilä et al., 1995
Dog 1.9 6 mm ² Unknown Hemilä et al., 1995
Inner ear Vestibule Diameter Human 5 mm Unknown Harvey et al., 2001
Human 4 mm Unknown Pensak & Choo, 2015
Dog 3 mm Unknown Harvey et al., 2001
Cochlea Length Human 33.5 mm Unknown Manoussaki et al.,
2008
Human 32 mm Unknown Pensak & Choo, 2015
Human 35 mm Unknown Fay & Popper, 1994
Dog Unknown Unknown
Height Human 5 mm Unknown Pensak & Choo, 2015
Dog 7 mm Unknown Ha rvey & Ter Haar, 2017
Dog 5.8 5 –7. 4 m m Unknown Wysocki, 2006
Tur ns Human 2.75 Unknown Pensak & Choo, 2015
Human 2.5 Unknown Wolfe et al., 2010
Human 2.5 Unknown Manoussaki et al.,
2008
Dog 3.25 Unknown Har vey et al., 2001
Helicotrema Area Human 0.25 m Unknown Lit tle r, 1965
Dog Unknown Unknown
Outer hair
cells
Number Human 11,000 Unknown Wolfe et al., 2010
12,000–15,000 Unknown Pensak & Choo, 2015
Dog 10,5 00 Unknown Dukes & Reece,
2004
Inner hair
cells
Number Human 3,500 Unknown Wolfe et al., 2010
Human 3,000–3,500 Unknown Pensak & Choo, 2015
Dog 2,500 Unknown Dukes & Reece, 2004
Round
window
Area Human 0.2–1.26 mm² Unknown Jain et al., 2019
Dog Unknown Unknown
Note: For a pictorial overview of the anatomical features of a dogs and humans ear, see Figure 2. CT = computerized tomography; di = dierent;
NA = not applicable; y = years.
Tab le 1. Comparison of Physical Features of the Auditory System of Dogs and Humans. (Continued)
Part of
the Ear
Auditory
Structure Measure Species Specications
Data
(M ± SD
Where Available) Method Reference
51
comparison of hearing and auditory functioning between dogs and humans
VOLUME 15, 2020
be less disturbed by loud sounds such as gunshots.
Further indirect evidence comes from the widespread
observation that when dogs are exposed to loud noises,
they will typically fold back their ears so they are more
dropped (Blackshawetal., 1990). It seems reasonable to
suppose that this limits both incoming frequency range
and volume.
A notable difference between humans and dogs is
the proportion of the EAM being covered by cartilage
or temporal bone (30% in humans vs. up to 98% in
dogs; Huangetal., 2009), though this varies with breed
(Harvey & Ter Haar, 2017). Especially the differences in
cartilage coverage may reect may reect differences in
the mobility of the ears between species but comes at a
potential cost in terms of the efciency of sound wave
conveyance along the EAM.
Tympanic Membrane and Ossicular Chain
The tympanic membrane (TM) both absorbs and
shunts acoustic energy to prevent the reection of sounds
within the ear, so it plays an important role in auditory
sensitivity (Bergevin & Olson, 2014). In both humans and
dogs, the TM is a rounded, cone-shaped structure with
reports of consistent differences associated with body
size (Table1; Harveyetal., 2001; Heffner & Heffner,
1983; Hemiläetal., 1995; Salvinellietal., 1991). Unlike
humans, sex differences in the size of the TM of dogs
have not been reported. Once again, variability in size is
much greater among dogs than humans, but the size of
the TM does not relate to the hearing frequency range or
the absolute threshold of hearing in dogs (Heffner, 1983)
and so may be of little practical consequence.
The three ossicles (Figure 3), which act as levers
across the air-lled space of the middle ear, are much
more important in this regard. Transducing sound
waves from the outer ear to a liquid pressure wave in
the inner ear show evidence of inter- and intraspecic
variation. The broad shape of the ossicles in humans and
dogs resemble each other (Berghesetal., 2010), but the
malleus of the dog is relatively bigger (Table1), and its
consequentially greater leverage increases magnication
toward the incus so that more energy can be transduced
effectively. The malleus-to-incus ratio (lever ratio) is 3:1
in dogs but only 1:3 in humans (El-Mofty & El-Serafy,
1967; Harvey et al., 2001), and this increases dogs’
potential sensitivity to sound (Strain, 2011; Wendell Todd
& Creighton, 2013). However, there are also reports that
although the ossicular chain amplies pressure waves
by a factor of 20 in dogs, it is increased by a factor of 22
in humans (Pensak & Choo, 2015; Strain, 2011), which
Figure 2. Scheme of a dog ear (left) and human ear (right). Not to scale.
52
COMPARATIVE COGNITION & BEHAVIOR REVIEWS
Barber et al.
appears to be at odds with the anatomical conclusions.
This inconsistency might relate to experimental
measures being taken at noncomparable frequencies and
highlights the problem of making simple generalizations
about sensitivity to sound, without reference to specic
frequencies an issue that we discuss further later on.
Cochlea
Ossicular chain vibrations are transmitted to the
cochlea via the oval window. In the cochlea, these
vibrations travel in the form of a tsunami wave (commonly
known as a traveling wave), which increases in amplitude
as it propagates within the cochlear uid and basilar
membrane. The cochlear has several adaptations that
affect the detection and decomposition of different
frequencies (i.e., the pitch of a sound) of the traveling wave
within it (Robles & Ruggero, 2001).
First, the basilar membrane is thicker, narrower, and
stiffer at the base compared with the apex (Figure4).
Thus, high-frequency energy in the traveling sound
wave displaces only regions close to the cochlear base,
whereas lower frequency waves can travel farther along
the cochlear spiral, achieving maximal amplitude near
the apex (Manoussaki et al., 2008). The subsequent
differences in the deection of basal hair cells within the
cochlea along different lengths determine the information
available on sound pitch at this level (place coding). The
basilar membrane of dogs is generally stiffer in the basal
regions compared to that of humans (Fay & Popper,
1994), and this difference may be the cause for their ability
to hear higher frequencies; however, the extent to which
the stiffness of the basilar membrane (disregarding other
anatomical differences) affects the variability in high-
frequency hearing between species remains unknown.
Second, afferent nerve bers re when the hair cells
are in an upward movement, and the frequency with which
they re may also aid hearing specic frequencies (phase
locking). For example, when the nerve will re 100 times
per second, this would indicate that the original sound
wave contains a 100 Hz component. However, this is
consistent only for certain sound frequencies. For humans,
this mechanism has been described to be consistent only
for frequencies up to 1000Hz; synchronization of action
potentials is lost above this frequency (Fettiplace, 2002)
because the maximum rate of action potential ring of the
auditory nerve is exceeded. Of interest, some perceptual
biases can be measured in humans below 1000Hz, in
spite of perfect synchronization between action potentials
and sound cycles. Humans seem to show a “perceptual
magnet” effect centered around the noteA. This means
that the perception of a tone (G#, 414Hz) is distorted by
its neighboring tone (A,440Hz), and this error seems to
be associated with the use of A as the universal tuning
frequency (Athosetal., 2007). There do not appear to
be any reports of phase locking in dogs, but it seems
reasonable to assume that a similar process occurs for
some frequency ranges.
The value of these two processes (place coding and
phase locking) is evident from our poorer perception
of sound near our hearing thresholds, where one of the
aforementioned mechanisms is engaged. In general,
place coding is more accurate in the basal region and
Figure 3. Structures of the middle and inner ear. Not to scale.
53
comparison of hearing and auditory functioning between dogs and humans
VOLUME 15, 2020
phase locking more accurate around the more apical
regions where it can occur (Fay & Popper, 1994).
The presentation of signals to the brain from the
inner ear depends on the activation of hair cells, and
so the number and the location of these are important.
Stimulation of the inner hair cells (IHC) provides
the primary information within the auditory nerve,
whereas the outer hair cells (OHC) make up a feedback
system that actively and selectively amplies parts of
the traveling wave (the cochlear amplier), resulting in
improved detection of sound and frequency resolution.
This process results in measurable low-intensity sound
coming from the ear, referred to as otoacoustic emissions
(OAE); these are the product of the back projection
of sound via the OHC and signal normal cochlear
functioning (see Supplementary Information for further
details on this phenomenon). In humans, females have
been reported to produce signicantly greater amplitude
and more OAE (McFadden, 1998; Sax, 2010); this may,
at least in part, explain a lower tolerance to background
noises and higher sensitivity, especially to high
frequencies. There do not seem to be comparable data for
dogs, but a similar sex effect has been reported in rhesus
monkeys and sheep (McFadden, Pasanen, Raper, Lange,
& Wallen, 2006; McFadden, Pasanen, Valero, Roberts,
& Lee, 2009); there is also growing evidence that female
dogs may be at higher risk of developing noise-related
fears (e.g., Storengen & Lingaas, 2015), which could
provide indirect evidence of similar processes and issues
in this species as occurs in humans. We suggest that it
would be useful to establish whether the production of
OAE can be used to predict the risk of noise sensitivities
and the potential suitability of individual dogs for work
in noisy environments.
Humans have been reported to have, on average,
between 14,500 and 19,000 hair cells (11,000–15,000OHC
and 3,0003,500IHC; Wolfeetal., 2010; Pensak & Choo,
2015); in contrast, dogs appear to have fewer, with around
13,000 hair cells (10,500OHC and 2,500IHC; Dukes &
Reece, 2004). These differences may appear surprising,
given the longer cochlea in dogs, but may not be related
to the hearing range; instead it may reect a demand to
be able to resolve different frequencies more accurately
by humans, given our use of language in communication.
At 1000Hz, humans can detect changes of 3 Hz, whereas
dogs have been reported to discriminate changes of only
8–10Hz (Fay & Popper, 1994; Sinnott & Brown, 1993).
However, it should be noted that 1000Hz is not near
the frequency best heard by dogs, which is 8000Hz (for
further reading, see below) but 4000Hz in humans. It
would be useful to evaluate sensitivity thresholds in the
full range or “range of best hearing” for dogs compared
with humans.
The extent to which a cell is displaced is a function
of the amplitude of the wave and reects the loudness on
the sound. Human hair cells have been reported to be
sensitive to deections of only 1nm, and the hair cells
can react to differences as little as 10µs. As a result,
humans may be sensitive to volume changes of less
than 1decibel or 1Hz (Wolfeetal., 2010). Comparable,
physiological data, which might indicate the equivalent
thresholds in dogs, appear to be absent.
Figure 4. Schema of an uncoiled cochlea with basilar membrane.
54
COMPARATIVE COGNITION & BEHAVIOR REVIEWS
Barber et al.
Part 2: Specic Hearing Capacities
in Dogs and Humans
Threshold of Hearing
Hearing threshold, and thus to some extent hearing
range, depends on factors beyond the amplication
processes of the ear discussed in the rst part of this
review. It is widely assumed that dogs’ sensitivity to
sound is greater than humans’, which has been shown to
be around 20dB higher at frequencies of 4000–8000Hz
(Lipman & Grassi, 1942). However, at lower frequencies,
the sensitivities of dogs and humans do not differ
substantially. Thus, one must recognize that the absolute
threshold of hearing varies with pitch. Pitch detection
generally differs with size, with smaller individuals
generally able to perceive higher frequencies ( Heffner,
1983). This may relate to the interaural distance that
is determined not only by the size of an individual but
also the specic skull morphology, which is highly
variable in dogs. To allow comparison, the threshold
of hearing is reported to be normally 20µPa or 0dB
(sound pressure level [SPL]) at 1000Hz for humans and
0dB (SPL) at between 1000 Hz and 16000 Hz for dogs,
depending on their size (Heffner, 1983). Thus, there may
be some overlap between dogs and humans regarding
the thresholds, but where exactly the similarities lie will
depend on the size of the dogs and sound frequencyused.
It is generally believed that a sound needs to last for
at least 100–200ms to be detected reliably by either dogs
or humans (Baru, 1971; Poulsen, 1981). Longer signals
are more readily perceived because of differences in the
temporal integration of the signal. For example, dogs
may perceive a sound of a given frequency presented for
only 1ms at 28dB but require the more typical 100ms
when presented at 0dB (Fay & Popper, 1994). How this
perception is inuenced by the nature of sound (see,
e.g., supplementaryFigure1) and by differences in the
audible frequency range, both between dogs and human
and between dog breeds of different sizes and physical
appearance, still needs scientic evaluation.
Frequency Range Detection
and Sensitivity Within It
The relationship between the ability to detect sound
waves of different frequencies and size obviously impacts
on the frequency range that animals of different sizes
may be able to perceive. Nonetheless, other factors such
as the form and shape of auditory structures, cochlear
length, and the stiffness of the basal area of the basilar
membrane (Fay & Popper 1994) are perhaps of more
importance in this regard. Sensitivity within the audible
range may also be driven, at least in part, by size, its
relationship with the pitch of certain vocalizations,
and the importance of conspecific size detection
(Bowling et al., 2017). The ability to generate lower
pitch sounds within a given vocalization can serve as an
honest signal of size and thus possibly resource holding
potential in any future contest. Indeed, in dogs it has
been found that the pitch of growls may be used to infer
the size of another individual (Faragóetal., 2010). Thus
it is expected that humans and dogs will differ not only
in the range of pitches they can detect but also in their
perceptual sensitivity to specic frequencies within it.
Some of this sensitivity may occur through differential
amplication or reduction of sensory signals within
auditorystructures.
The length of the cochlear basilar membrane is
widely thought to be critical in determining the extent
of the hearing frequency range, although an earlier
report by West (1985) claims that the cochlear length is
not correlated with frequency range detection. Longer
basilar membranes, such as those that occur in dogs
relative to humans (Table1), are believed to be able to
detect a wider range of frequencies (Fay & Popper 1994;
Heffner & Heffner, 1998, 2008), and this is consistent
with the data summarized in Table2. An audiogram of
the hearing frequency range of an animal should include
the absolute upper and lower thresholds for frequencies
and their sensitivity throughout their hearing range.
Audiograms are commonly recorded at a sound pressure
level of 60dB (SPL) but, depending on the study question,
deviations from this do occur (Table2). Typically, the
human hearing range is reported to be between 20 and
20000Hz and that of dogs between 65 and 45000Hz,
but as the amplitude (loudness) increases, so the hearing
range widens. Accordingly, the detectable hearing
frequency range can be increased by having larger ears,
but the relationship is not absolute. Indeed, even though
some dogs have substantially larger pinnae than humans,
it seems that humans generally outperform dogs in their
sensitivity to lower frequencies.
There is a clearly established physical relationship
between the size of an animal and the frequencies it is
able to generate for vocal communication ( Bowlingetal.,
2017; Titze, Riede, & Mau, 2016), and this is often
reected in their hearing range (Heffner & Heffner,
2008). Although Heffner (1983) argued that hearing
range in dogs may depend on species-typical size rather
than individual differences, this appears to be based
on a single behavioral study, comparing four dogs of
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Table 2. Hearing Frequency Ranges of Dogs and Humans for Different Intensity Levels
Species Amplitude Frequency Range Publication
Human 60 dB (SPL) 29 –1900 0 Hz West, 1985
Human 60 dB (SPL) 31–1760 0 Hz H. E. Hener, 1998
Human 30 dB (SPL) 110–16000 Hz West, 1985
Human 30 dB (SPL) 130–16000 Hz Nadol, as cited in Fay & Popper, 1994
Human 10 dB (SPL) 250– 8100 Hz H. E. Hener & Hener, 2007
Human Unknown 64–23000 Hz Strain, 2011
Human Unknown 16–20000 Hz Alberti, 2001
Dog 60 dB (SPL) 62–45000 Hz H. E. Hener, 1976
Dog 60 dB (SPL) 67–45000 Hz H. E. Hener, 1983
Dog 60 dB (SPL) 64–44000 Hz West, 1985
Dog 60 dB (SPL) 67–44000 Hz H. E. Hener, 1998
Dog 30 dB (SPL) 200–36000 Hz West, 1985
Dog 10 dB (SPL) 1800–22000 Hz H. E. Hener & Hener, 2007
Dog Unknown 67–45000 Hz Strain, 2011
Note: SPL = sound pressure level.
Figure 5. Relationship between functional head size (interaural distance) of dogs and highest frequency heard (from McMahon, 2015).
Stimuli were presented at 70 dB (sound pressure level; measured at a distance of 50 cm from the speaker, using calibrated equipment)
covering the frequency range of 0–70 kHz.
56
COMPARATIVE COGNITION & BEHAVIOR REVIEWS
Barber et al.
different size and different pinna characteristics. By
contrast, a more recent unpublished study (McMahon,
2015), supervised by some of the authors, suggests that
this biological relationship is maintained between dogs
of substantially different sizes (see Figure5).
It is likely, within the ranges of hearing relating
to dogs and human, that the lower limit of detection
of frequencies may actually be more dependent on
properties of the TM and tympanic cavity (TC; Fay
& Popper, 1994; Heffner & Heffner, 2003), which are
larger in humans than dogs (Table1). A larger TC is
more capable of amplifying low-frequency components
and compensating for associated pressure changes. This
is important because high intra-TC pressure is assumed
to increase the pressure on the TM, which in turn will
be more impeded in its vibratory abilities (Packer, 1987).
The size of the TM is also important, with larger TM
more readily able to transduce lower frequencies.
Dogs can hear and potentially respond to frequencies
that humans cannot perceive, which has the potential
to cause confusion for a handler who may be unaware
of this. For example, many dogs are probably able to
perceive the ultrasonic vocalization produced by mice
or some insects (e.g., fundamental frequency of mice is
40000Hz, and dogs can hear up to 60000Hz; Arriaga
& Jarvis, 2013; McMahon, 2015; Peterson, Heaton, &
Wruble, 1969), which is outside the hearing range of a
human. Further, there are many articial sources that
transmit high-frequency sounds that are inaudible to
humans but that can most likely be heard by dogs (for
further reading, see Part4).
Extension of hearing into higher frequency ranges
can be assisted by several adaptations of the auditory
system. Specic folds of the pinna cartilage might
improve sensitivity to high frequencies by selectively
collecting and amplifying certain high-frequency sounds
(Heffner & Heffner, 2008). Such folds are more evident in
the outer ear of humans compared with dogs but may be
more important in sound localization than recognition
(see following section). The nature of structures within
the middle and inner ear may be particularly important
for increasing high-frequency hearing. Within the middle
ear, the TM-to-oval-window ratio and lever ratio of the
ossicular chain (Puria & Steele, 2010) favors dogs over
humans. Within the inner ear, a particularly narrow and
thick (i.e., stiffer) basal membrane within the cochlea
(Fay & Popper, 1994), as well as shorter OHC (Vater &
Kössl, 2011), will enable better high-frequency hearing
abilities. Adult humans do not generally perceive sounds
above approximately 20000 Hz, whereas dogs are
generally believed to hear up to 45000Hz. However, some
authors have found that dogs can hear up to 60000Hz
(McMahon, 2015). Dogs, in comparison to humans, also
hear better by approximately 15dB at 10000Hz and up
to 20dB at 16000Hz (Dworkin, Katzman, Hutchison, &
McCabe, 1940). Strong systematic differences are believed
to exist in the upper frequency limit between closely
related species; this may relate to preferred prey and their
typical vocalization frequencies (dog ~60000Hz, coyotes
~80000Hz, wolves ~80000Hz, red fox ~65000Hz, cat
~100000Hz; Petersonetal., 1969). (It should be noted that
the methods used by Petersonetal., 1969, are outdated
and require validation but are included here to show that
species-specic differences may be expected, at least to a
certain degree.) Some dog breeds have been historically
selected for nding and killing small prey (e.g., Jack
Russell terriers), whereas others have been bred to do the
same for larger ground dwelling species (e.g., dachshund).
Therefore it might be expected that their respective
hearing ranges would reect this, but the nature and
extent of variability in hearing frequency range between
dog breeds based on their original function is unknown.
Even in the absence of breed-specic variability data in
dogs, such clear anatomical correlates with function and
historical selection of breeds provide a strong argument
for genetic differentiation between breeds and individuals
in hearing frequency range, and the potential opportunity
to select for this.
Every species (and potentially every breed of dog)
can be expected to have a range of best-perceived
frequencies, which are the frequencies for which the
ear is most sensitive and hence detectable at very low
amplitudes. In humans the best-perceived frequencies
are between 128 and 4000Hz (Fay & Popper, 1994). The
human TM and ossicles have been reported to transmit
sounds best for the frequency ranges between 800–1600
and 5003000Hz, respectively, and therefore enhance
sensitivity to sounds between 500 and 3000Hz, which
are the important frequencies in human speech (Pensak
& Choo, 2015). The frequency with the highest sensitivity
(i.e., lowest detectable amplitude) in humans is 4000Hz
with a sensitivity of −10dB (SPL; H.E. Heffner, 1998).
For dogs, in general, it is believed that they are most
sensitive in the frequency range of 200–15000 Hz
(depending on size) and the best perceived frequency
with a sensitivity of −1dB (SPL) is 8000 Hz (Beaver,
1999; Heffner & Heffner, 1998), but it is not known to
what extent there might be breed variation in this regard.
In addition, it is not known which structures of the ear
might transduce particular frequencies best in the dog.
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It would also be valuable to determine if the perception
and preference of frequencies in dogs has changed
because of selective pressure. Certain breeds living or
working in close audible contact with humans may have
been selected for increased sensitivity toward human
vocal communication, requiring perception of a range
of frequencies that is well below a frequency range to
which they have been most sensitive (Riede & Fitch 1999;
Traunmüller & Eriksson 1994).
The importance of higher frequency sounds to some
animals deserves special consideration. Reasons for this
may exist, but the importance of crying by young depen-
dents (Daga & Panditrao, 2011; Solomon, Luschei, &
Liu, 1995) and distress vocalizations (Pongrácz etal.,
2006), which typically involve higher pitch sounds, may
also be selected for. Selective amplication of these
sounds may be important because these higher frequen-
cies are attenuated easily and therefore rapidly reduce
with distance. It is therefore not surprising that research
shows that female humans are more sensitive to higher
pitch sounds after giving birth and during specic phases
of the oestrous cycle (McFadden, 1998), a relationship
that suggests an important role for hormonal factors
in temporal differences in sensitivity. Whether similar
changes occur in dogs has not been evaluated, but its
potential occurrence should be noted by those working
with dogs in tasks potentially dependent on their hearing
acuity. The effects of neutering also remains unknown. It
should also be recognized that higher frequency sounds
are more generally thought to be more salient, causing
more attentiveness and alertness than lower frequency
sounds. Indeed, the cries of a human baby will cause
distress to a dog, even though the vocalization is not of
direct evolutionary importance (Huber, Barber, Faragó,
Müller, & Huber, 2017; Yong & Ruffman, 2014).
Sound Localization and Distance Detection
Research suggests that the most important role of
sound localization is its role in the orientation of the
eyes toward a sound source (Heffner, 1998); however,
some dogs can be observed to hunt blind (e.g., diving
on prey through thick snow). For sound localization,
high-frequency components of the sound source are
indispensable, with localization becoming inaccurate
and even impossible, when the high frequencies of
a sound are ltered out (Heffner & Heffner, 2008).
Humans have been reported to primarily (though not
exclusively) use frequencies above 4000Hz for localizing
sound sources (Heffner & Heffner, 2008), but there is no
comparable work for dogs.
To localize sounds in the environment, it has been
suggested that two aspects are of importance; these
relate to interaural differences: binaural temporal
differences and binaural spectral differences (Fay &
Popper, 1994; Heffner & Heffner, 2008; Keidel & Neff,
1975; Sterbing-D’Angelo, 2010; see Figure 6). Binaural
temporal differences occur because of an offset in the
time of arrival of a sound at each ear depending on its
source relative to each ear (Heffner, 1997). There are also
slight differences in the intensity, and therefore spectral
composition of the sound received by the two ears, as
frequencies (and especially higher frequencies) attenuate
the longer they travel and may be modied by reection
or shadowing from the head or pinna (Heffner, 1997).
This explains the functional value of the folds within the
pinnae, which particularly affect higher frequency sound
waves. In humans the temporal interaural difference for
sound is 900µs on average, whereas in dogs the interaural
difference has been reported to be 450µs (Heffner &
Heffner, 2003), and breed differences can be expected
because of differences in skull size. However, interaural
distance alone is not, in itself, predictive of the accuracy
of sound localization in dogs (Heffner & Heffner, 2008;
Heffner, 1997). Nonetheless, it would be interesting to
establish whether sound localization is generally better
in smaller dogs, because of their potentially increased use
of higher frequencies.
The specic structure of the cartilage of the pinnae has
an important function for directional hearing ( Heffner &
Figure 6. Visualization of interaural time and spectral differences.
58
COMPARATIVE COGNITION & BEHAVIOR REVIEWS
Barber et al.
Heffner, 1992), and any distortion of the pinna (surgically,
e.g., by cropping or even by piercings in humans) may
require readaptation of the auditory system. In humans,
temporary attening of the pinnae has been shown to
reduce the effective localization of sounds in the vertical
plane (Keidel & Neff, 1975). Although empirical evidence
is lacking, it seems reasonable to argue that ear cropping
may not only impair hearing by reducing the effective
funnel size of the ear but also reduce the ability of a dog
to localize a sound. Accordingly, it is recommended that
working dogs, who may depend on good quality hearing,
should not have their ears cropped.
Humans have been reported to have a sound
localization accuracy of 1°–2° (Heffner & Heffner, 2003;
Pujol, 2010), whereas studies on dogs report a wider range
of values, from 4°–8° in laboratory studies ( Heffner &
Heffner, 2003) to approximately 20° in the eld, with
the sound source being 300m away (for review, see Fay
& Popper 1994). This relatively poor performance may
simply reect the fact that the primary function of sound
localization at such a distance will be to direct visual
orientation toward a source. Species with a narrow eld
of view and high dependence on visual acuity, such as
humans, may therefore benet more from better sound
localization acuity than those who may not need to
locate a sound source so precisely (Heffner & Heffner,
2003). However, it is worth noting that current studies on
dogs have used only breeds with a moderate visual streak
rather than ones with a focal spot comparable with that
seen in humans, which may be associated with greater
visual dependence (dolichocephalic vs. mesocephalic vs.
brachycephalic; McGreevyetal., 2004; Peichl, 1992). It
might be that brachycephalic breeds, with their more
pronounced area centralis, also have higher sound
localization acuity compared with other breeds, such as
dolichocephalic sight hounds.
Hearing can also provide an animal with important
cues about the distance of a source from the subject via
its frequency composition and amplitude. In general, it is
easier to detect broadband sounds than pure tones, and
several factors interfere with the ability to estimate the
distance of an audible sound, including the presence of
environmental structures (hard, soft ground, barriers),
which will differentially attenuate various sound
frequencies (Wiley & Richards, 1978). It is known that
wolves can hear another wolf howling with a frequency
range of 300–1800Hz for 14 s, from up to 6 miles
(~10km) away in a forest and up to 10miles (~16km) in
a at country (Mech & Boitani, 2003), but this relates
more to their auditory threshold than a specic distance
estimate beyond knowing that the other wolf is within
this range. In general, the amplitude of a sound source
drops by 6dB when the distance of the sound source
is doubled (Holt, Schusterman, Kastak, & Southall,
2005; Klump & Shalter, 1984; Kolarik, Moore, Zahorik,
Cirstea, & Pardhan, 2016; Wiley & Richards, 1978), and
so it may be possible to estimate the distance of a sound
source only if the sound source has an expected volume.
Indeed, there is little evidence for accurate distance
perception in nonecholocating mammals (Moore &
King, 1999).
Part 3: Disturbances of the Auditory System
Disturbances to the auditory system can be evoked
by noises of high amplitude and can be temporary
or permanent, partial or complete. High-frequency
hearing is most frequently affected, and noise damage
is typically most extensive to frequencies above those
involved in the exposure (Ryan, Kujawa, Hammill, Le
Prell, & Kil, 2016). Severity and recovery depend on
factors such as stimulus type (impact or continuous),
exposure and resting times, and sound characteristics
(temporal characteristics, intensity, range, etc.) but
also on individual preconditions (sex, age, health
status, preexposure, etc.; Ryanetal., 2016). Long-term
desensitization is commonly caused by modulations
of the auditory nervous system, whereas short-term
attenuation of sounds is most often caused by reversible
changes of the connective and muscular tissue of the
middle ear. It is possible for an exposure that damages
hearing to be neither annoying nor painful for
example, prolonged exposure to loud music or
infrasound (Harding, Bohne, Lee, & Salt, 2007).
Hearing loss in humans has been studied extensively,
and guidelines for noise exposure have been formulated.
The Occupational Safety and Health Administration
states that on a daily basis 40 million individuals in
the United States are exposed to hazardous noise levels
that could cause permanent hearing damage, and in
the United Kingdom approximately 20% of the total
population suffers from hearing loss (Lynch & Kil, 2005;
NHS England, 2014; World Health Organization, 2013).
Commonly, the pain threshold for humans is dened as
120–130dB (SPL), which is a noise comparable to an
amplied speaker at a heavy metal concert. However,
depending on the frequency composition of the noise,
thresholds can be higher or lower (including infra- and
ultrasounds; Lawton, 2001; Leventhal, 2003). Every 3dB
increase of amplitude is accompanied by a doubling
59
comparison of hearing and auditory functioning between dogs and humans
VOLUME 15, 2020
of sound energy, and it has been recommended that
every 5dB increase in sound exposure requires a 50%
reduction in exposure time (Lynch & Kil, 2005). Further,
it is recommended that no one should be exposed to
noises greater than 140 dB, even for short periods.
However, a shot with a rie reaches around 150dB
and can therefore, without proper hearing protection,
result in noise-induced hearing loss (NIHL; Lynch &
Kil 2005). This is especially important to consider for
both humans and dogs in military and police services,
as well as hunters.
Surprisingly little research has investigated
disturbances to hearing in dogs. In reality, dogs are often
exposed to noises such as trafc, sirens, construction
sites, music, children, or daily household noises, and
working dogs may be exposed to exceptionally noisy
environments (e.g., kennels, gunshots, transport), which
may result in disturbances to the auditory system. In
a comparison of several kennel environments, research
has shown that kennels can have a continuous noise
level above 100dB (Scheifele, Martin, Clark, Kemper,
& Wells, 2012) with peaks of around 120dB (Coppola,
Enns, & Grandin, 2006). In humans, such noise levels
require hearing protection to prevent NIHL, if exposure
lasts more than 1hr (Table3; Ryanetal., 2016). Indeed,
an assessment of 14 dogs that were exposed to kennel
noises for 6 months revealed that nine of the 14dogs
suffered a threshold shift of greater than 10dB. It could
not be determined whether this threshold shift was of
a temporary or permanent nature, and the range of
affected frequencies was not identied. The authors
were also unable to identify actual threshold shifts for
the dogs, as the dogs were living in kennels and the
authors could not evaluate unaffected preexposure
levels ( Scheifeleetal., 2012). Because of the missing
values for preexposure levels, they assumed that the level
of threshold shift in their study was underestimated.
For this reason, further studies are urgently needed
to identify the nature and severity of NIHL in dogs
as a result of environmental noise from kennels. This
research should take into consideration both dogs that
are kennelled for prolonged periods (e.g., in shelters
and working dogs) and those kennelled for only a few
daytime hours (day care). Design and management
recommendations have been formulated for kennels;
these stress that a mean sound level of 45dB should be
the norm for animal housing, following standards for
human dwellings (Coppolaetal., 2006; Hewison, Wright,
Table 3. Exemplary Sound Pressure Levels (at 1 m) and Recommended Exposure Levels
dB (SPL) Duration Sound Source
> 14 0 < 1 min Firearms, (turbo) jet engines, rockets, bomb and grenades
130 > 1 min Jackhammers, magnetic resonance imaging (peak values)
120 (threshold of pain) > 5 min Amplied speaker (e.g., concerts), symphonic orchestra, heavy thunder
110 > 15 min Heavy engines (e.g., diesel truck engine, bulldozer)
100 > 1 hr Chainsaw, car at highway speed, magnetic resonance imaging
90 > 4 hr Motorcycle, lawn mower, air compressor, subway
85 > 8 hr Plane cabin, heavy city trac, kindergarten break room
80 Alarm clock, dishwasher, singing
70 Vacuum cleaner, toilet ushing, car cabin
60 Conversational speech, quiet oce environment
50 Average home
40 Quiet library
30 Whispered conservation
20 Quiet bedroom
10 Rustling leaves
0Hearing threshold (human)
Note. Adapted from Engineering Toolbox (2018); Lynch and Kil (2005); Sengpiel (2017); and Venn, McBrearty, McKeegan, and Penderis (2014).
60
COMPARATIVE COGNITION & BEHAVIOR REVIEWS
Barber et al.
Zulch, & Ellis, 2014). However, such standards are not
yet regularly implemented or legislatively supported.
Another example of a noisy environment to which
dogs may be exposed is the process of magnetic
resonance imaging (MRI). It has been suggested that the
MRI environment can have peak values of 120–130dB
(SPL; Venn et al., 2014). Humans are normally not
exposed to MRI noise without hearing protection. This
may not be common practice in veterinary work, as the
animals are normally anaesthetized; however, it has
been reported that the exposure of dogs to MRI noise
can result in a threshold shift of up to 5dB (SPL) for
frequencies between 1000 and 7000Hz (Vennetal., 2014).
This includes the frequency range of most human speech
(<3000Hz), and so dog–human communication may
be affected following MRI assessment. Unfortunately,
there are no studies assessing the inuence of MRI
noise on the high-frequency range above 7000Hz, and
the duration of any effect has not been established.
Although the need for hearing protection for dogs in
an MRI environment has been stressed (Baker, 2013;
Vennetal., 2014) and should be legislatively mandated
under the animal welfare regulations of those nations
that highlight the need to avoid unnecessary suffering.
Last but anecdotally, it has been reported that military
working dogs can experience a threshold shift up to
50 dB after transportation in helicopters (S.Scheifele,
personal communication, November30, 2018). Hence,
noise during the transportation of dogs not only
in helicopters or airplanes but also, for example, in
trailers should be taken into consideration as a factor
inuencing dog welfare. Generally, in the absence of
specic research to the contrary, dogs should be given
at least the same level of protection as people in noisy
conditions, with a view to trying to prevent problems
including complete hearing loss in the longer term.
There are anecdotal reports suggesting that, as in
humans, loud noises can be painful to dogs, resulting
in whining, barking, howling, or aversive responses, but
no guidelines or thresholds have been formulated. Some
research has been done on animal repellents providing
evidence that a frequency sweep of 17000Hz to 5000Hz
to 55000Hz with an intensity of 120dB is aversive to dogs
(Blackshawetal., 1990). It is likely that this might be pain-
ful and potentially harmful, but further research is needed.
However, as amplication processes of the ear appear to
be stronger in dogs than humans (see Part1 of this article),
it should be noted that thresholds and guidelines made for
prevention of auditory disturbances in humans will most
likely be higher than the thresholds for dogs.
Mechanisms within the auditory system prevent
disturbance. First, the auditory tube is involved in the
equalization of pressure between the middle ear and the
throat to maintain proper tension for optimal vibration
of the membranes of the TM, oval and round window
(Strain, 2011). In humans, the upper half of the auditory
tube opens with every third or fourth swallow (Alberti,
2001; Pensak & Choo, 2015). Malfunctioning of this
process can lead to effusion from the TC (Kent, Glass,
de Lahunta, Platt, & Haley, 2013). The auditory tube
is longer and wider in humans than dogs (~38mm vs.
15mm; Table1), but functional differences have not been
reported. Second, in humans, specialized cells the
mastoid cells can be found above the tympanic cavity
(Figure1; Berghesetal., 2010). These help to compensate
for pressure changes (Alberti, 2001). Such a reservoir does
not appear to exist in dogs, and so it should be assumed
that they are not able to so easily compensate for air
pressure changes, which could be especially important
to consider when dogs are worked at altitude or taken
on an airplane. Third, to prevent injuries of the inner ear
by, for example, high-energy sounds, the ossicles are able
to limit the sound amplication for sounds above about
80dB (SPL; Pujol, 2010) through a reex triggering small
muscles, which contract and stiffen the ossicular chain,
limiting their leverage action. However, this reex has
limitations: It reduces predominantly the transmission of
low-frequency sounds (i.e., below 2000Hz) and it takes up
to 50–100ms to occur and is therefore ineffective for short
pulse noises (e.g., reworks; Pujol, 2010; Strain, 2011).
Disturbance to the auditory system can also arise
from rupture of the TM from strong pressure changes.
The rupture of the TM leads to a conductive hearing
loss, as sounds cannot be transduced effectively via
the ossicular chain anymore. A fast-rising shock, such
as the abrupt change in pressure associated with an
explosion, can rupture at least 50% of the TM in humans
at a force of 194dB (SPL) and 192dB (SPL) for dogs
(Richmond, Fletcher, Yelverton, & Phillips, 1989). For a
static pressure exposure, the average threshold is slightly
higher at 198dB (SPL) for humans and 194dB (SPL) for
dogs (Richmondetal., 1989). Therefore, values do not
differ substantially between species despite anatomical
differences in the TM (see Part1 in this article). Still,
there can be individual variation in the threshold at
which the eardrum ruptures. Note that a rupture in the
reported studies was described as a loss of at least 50%
of the TM, and smaller ruptures can be experienced at
much lower pressure levels. Existing data emphasize that
dogs’ TM is as sensitive to pressure changes as humans’.
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It is therefore advisable to check hearing function in
a dog after it has been exposed to any event that has
the potential to cause damage to the TM of a human.
NIHL can have a signicant impact on an individual’s
everyday performance, and it is thought that this may be
as true for dogs as it is for people. In humans, hearing
loss has been associated with a decline in an individual’s
professional and social functioning, possibly resulting
in cognitive decline, decreased physical activity, poorer
health conditions, and depression (Ryan et al., 2016;
Scheifeleetal., 2012). Hearing acuity is also important for
dogs, and hearing loss in this species has been correlated
with stress-related (e.g., startle) and aggressive behaviors;
dogs with a disturbance of the auditory system are also at
greater risk of being involved in accidents and are harder
to train (Baker, 2013; Strain, 2011; Vennetal., 2014).
Temporary Threshold Shift
A temporary threshold shift (TTS) is an acute
change in the hearing ability that recovers over time.
Several criteria for dening a TTS have been formulated
(for a review, see Ryanetal., 2016), but a standard de-
nition is a minimum 10dB change in hearing threshold
across the frequency range of 2000–4000Hz, which can
recover within minutes, hours, or weeks (but at most
30days) depending on severity of exposure. It is impor-
tant to appreciate that recent research stresses that even
though the auditory system has been reported to recover
after a TTS, repeated exposure can lead to permanent
functional changes in the auditory system (Kujawa &
Liberman, 2006, 2009). Ryan et al. (2016) postulated
that acute threshold shifts up 50dB after a single noise
exposure are likely to recover. No distinct values have
been formulated for dogs, but it can be assumed that the
affected frequency range after an NIHL varies because
of differences in the audible hearing range. As dogs hear
better in higher frequency range and higher frequencies
are most often affected by loud noises, in general NIHL
could be more easily induced, and possibly more severe,
in dogs compared with humans, although empirical data
are missing.
A TTS can be perceived as dull hearing or tinnitus
in humans. It is caused by fatigue of the ear structures
(especially muscles of the ossicular chain) but also
changes in metabolism along the whole auditory
pathway (such as glutamate accumulation in the
basilar membrane or auditory brainstem; for a review,
see Pensak & Choo, 2015; Ryanet al., 2016). Some
studies have demonstrated that otoprotective agents
such as magnesium or glutamate antagonists can be
administered to reduce the extent of a TTS, but the
necessity of very high doses and individual differences
complicate reliable predictions regarding efciency (for a
review, see Kujawa & Liberman, 2009; Ryanetal., 2016).
There do not appear to be any studies into the use of
otoprotective drugs on dogs.
Research on threshold shifts in dogs does not
normally differentiate between temporary and
permanent damage, as long-term measures are usually
missing. For this reason, and given that TTS can cause
long-term consequences, the existing literature is
summarized in the next section on permanent threshold
shifts, even though it has not been demonstrated that
this is necessarily the case. However, the lack of data in
this area underlines the necessity for further research
on disturbances of the auditory system in dogs, to tease
apart possible short- versus long-term dangers.
Permanent Threshold Shift
A permanent threshold shift (PTS) is a change in the
hearing sensitivity upon exposure to a noise that does
not recover to preexposure level. A threshold shift of
greater than 50dB from baseline has been reported to
most likely be unrecoverable (Ryanetal., 2016). Typi-
cally, sustained noises cause more severe PTS than single
blasts (Fausti, Wilmington, Gallun, Myers, & Henry,
2009). As with TTS, there are several denitions of PTS
(for a review, see Ryanetal., 2016), but the following
threshold-based denitions can be taken as a guideline
for hearing loss: slight hearing loss is 16–24dB deviation
from baseline, mild is 25–40dB, moderate is 41–55dB,
moderately severe is 5670 dB, severe is 71–90 dB,
and profound is more than 91dB (Ryanetal., 2016).
However, these denitions are for acute measurements
of NIHL after exposure to a noise source and do not
take pre exposure hearing abilities into account; only a
threshold is measured, and the actual level of hearing
loss can vary with the individual. Threshold levels and
guidelines for dogs have not been formulated to date.
PTS is of a sensorineural nature and therefore
impacts cochlear hair cells, nerve cells, and structures
of the auditory pathway. The OHC are most sensitive to
damage, and this results in decreased cochlear sensitivity
and selectivity from reduced cochlear amplication.
The biochemical mechanisms for how this damage
occurs are not known with certainty, but excitotoxicity
due to antioxidants and glutamate has been reported
to play an important role in cell damage and apoptosis
(for a review, see Kujawa & Liberman, 2009; Puel,
Ruel, Gervais d’Aldin, & Pujol, 1998; Ryan et al.,
62
COMPARATIVE COGNITION & BEHAVIOR REVIEWS
Barber et al.
2016). Among humans, enhancement of frequencies
at the highest sensitivity (4000 Hz) is common, and
PTS is prevalent around this frequency, in the range of
4000–6000Hz (Ryanetal., 2016). Therefore, it can be
assumed, in the absence of data to the contrary, that a
common frequency range for damage related to sounds
in dogs is also at their highest sensitivity (>8000Hz).
Anecdotally, it is the opinion of one of the authors
(DSM) from his clinical behavior work, that loss of high-
frequency hearing may also result in a general increase
in sound-related problems, and this may be due in part
to a reduced capacity for sound localizability, increasing
the fear associated with loud noises.
Upon experiencing an acoustic trauma, immediate
symptoms in humans are otalgia (ear pain), tinnitus,
aural fullness, dizziness, noise sensitivity, or distorted
hearing, which in the longer term can lead to
sensorineural hearing loss accompanied by peripheral
hearing loss or central auditory processing decits. It is
important to note that certain medications used for the
treatment of injuries can be ototoxic and therefore may
exacerbate hearing impairments following blast trauma
(Faustietal., 2009; Wilson & Mills, 2005). Symptoms
of acoustic trauma are reported to frequently overlap
with posttraumatic stress disorders (Faustietal., 2009).
Hence (working) dogs exposed to acoustic blast events
might experience similar physical effects to humans
because of their biological similarity.
Deafness
Besides partial permanent threshold shifts,
connected to partial deafness, an individual can also
experience total deafness, which can be unilateral (i.e.,
just one ear) or bilateral. There are several causes of
deafness and, besides exposure to high-intensity noises,
it can be congenital or caused by disease. Some dog
breeds are especially at risk of congenital deafness
(dalmatian, bull terrier, English setter, English cocker
spaniel, Australian cattle dog, Norwegian dunkerhound,
and dappled dachshund) with an estimated prevalence
of up to 30% in the United States (Kemper, Scheifele,
& Clark, 2013; Strain, 1999). However, in European
countries prevalence rates for deafness for some of these
breeds, especially dalmatians, are lower because of the
prohibition of blue eyes in the breed standard, which
is linked to the piebald gene and the risk of deafness
(Juraschko, Meyer-Lindenberg, Nolte, & Distl, 2003;
Strain, 2011). Deafness in dogs is positively associated
with the Merle gene, which is also associated with ocular
defects and therefore multisensory restrictions. It is
advisable to avoid breeding lines that have been reported
to be susceptible to deafness. Identication of congenital
bilateral and especially unilateral deafness in puppies
can be challenging because of the timing of development
of auditory functioning, such as the opening of the
EAM, and early behavioral indications within the litter
may not be conspicuous. However, deaf dog puppies may
be prone to excessive vocalization and startle reexes
(including snapping or biting), and play with conspecics
can be more aggressive because of the lack of auditory
feedback (Kemperetal., 2013; Strain, 2011). Still, deaf
animals can quickly compensate for their auditory
decits and are trainable, when using appropriate cues.
Brainstem auditory evoked potentials testing is therefore
recommended for objective identication (Strain, 2011).
Strain, 2011 reported anecdotally that during
ontogeny, deaf dogs can develop anxious or aggressive
behavioral patterns, but provided no data to support
this. Later onset deafness can be caused by acoustic
or physical trauma, ototoxicity, or otitis. Management
of dogs with later onset deafness is normally easier,
as they already have basic training and will often use
visual cues as well as audible ones for human-directed
actions. Still, animals with hearing difculties are, like
humans, at higher risk of accidents and more likely to
get lost. Management of deaf dogs has been substantially
reviewed by Strain (2011) and more recently by Becker
(2017).
Presbycusis is an age-related form of hearing loss
that arises due to the loss of IHC and spiral ganglion
nerves, atrophy of the organ of Corti and vessels of the
cochlear duct, and an age-dependent thickening of the
basilar membrane. Such changes are generally more
prominent at the base of the cochlea compared with the
apex, resulting in a greater loss of high-frequency hearing
(Shimada, Ebisu, Morita, Takeuchi, & Umemura, 1998),
which gradually affects the entire frequency range (Ter
Haar, 2011a). Mechanisms have been reported to be
similar in dogs and humans but can vary substantially
on an individual basis (Adler & Hart, 1992; Pensak &
Choo, 2015; Shimadaetal., 1998; Ter Haar, de Groot,
Haagen, van Sluijs, & Smoorenburg, 2009). In both
dogs and humans, presbycusis has been described
as a cumulative effect of heredity, disease, noise, and
ototoxic agents superimposed on the aging process (Ter
Haar, 2011a). Presbycusis is the most common form of
hearing loss in dogs, and the onset of presbycusis has
been reported to be around 8–10years of age (in mixed
breeds of comparable body weight; Ter Haar, 2011a; Ter
Haar, Venker-van Haagen, van den Brom, van Sluijs,
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comparison of hearing and auditory functioning between dogs and humans
VOLUME 15, 2020
& Smoorenburg, 2008), whereas early onset in humans
occurs at older than 40 years of age but typically older
than 60 (Arvin, Prepageran, & Raman, 2013; Ter Haar,
2011a). Sex differences have been reported, with males
suffering from an earlier onset of presbycusis than
females (McFadden, 1998; Pearsonetal., 1995). It has
been reported that individual lifestyle and exposure
to environmental noises are important predictors for
presbycusis in humans (Goycooleaetal., 1986), and it is
also likely that such predictions hold true for dogs, with
dogs with high levels of exposure to loud environments
experiencing presbycusis earlier. Presbycusis has also
been reported to impact binaural localization, which
subsequently causes problems in selective hearing and
attention, especially in environments with background
noise because of the inability to separate several auditory
streams (Alberti, 2001; Faustietal., 2009).
Other disturbances of the auditory system, such
as tinnitus, diseases, and lesions, which are of less
relevance to this comparative review, are discussed in
the supplementary information.
Part 4: Potentially Problematic Behavior
Responses Associated with the Auditory System
Acoustic Startle Response and Noise Reactivity
The acoustic startle is a reexive response (latency
6–8 ms; Lee, Lopez, Meloni, & Davis, 1996) to a sudden
noise, causing a physiological chain reaction that results
in increased arousal and attention, hypervigilance, and
behavioral preparation for a potential ght-or-ight
response. Learning and training can be used to modify
and attenuate the acoustic startle reex (Berg & Davis,
1985; Leeetal., 1996), for example, through the use of
prepulse inhibition or habituation (Valsamis & Schmid,
2011). The use of prepulse inhibition in training may be
a future area of value to explore for the conditioning
of individuals expected to work in environments with
sudden noises, where a signicant startle could inhibit
functionality. In mice, pulses of 120dB can provoke a
startle response, but prepulses of varying interstimulus
intervals of about 70–80dB can reduce the intensity of
this (Valsamis & Schmid, 2011). Repeated controlled
exposure to prepulses and pulses can subsequently lead
to long-term habituation (Valsamis & Schmid, 2011), but
there are no data on the practical use of this in dogs,
despite its potential clinical utility (Lindsay, 2013).
Further, the failure of an individual to show prepulse
inhibition could be indicative of decits in sensorimotor
processing and a variety of other disorders (Lindsay,
2013), and so a prepulse inhibition test could form a useful
part of the selection process for working dogs, who need
an attenuated reaction to noise. A more practical solution
in some circumstances might be the use of pressure
vests, which are reported to attenuate the response to
sudden sounds and potentially have a calming effect in
such situations (King, Bufngton, Smith, & Grandin,
2014). These vests apply pressure to the torso, potentially
increasing vagal tone by encouraging diaphragmatic
breathing. Recovery times after exposure to noise may
also be reduced by these pieces of clothing (see Buckley,
2018, for a review of their use and efcacy in dogs).
Noise reactivity can be problematic for both humans
and dogs; indeed, between 40% and 50% of pet dog
owners report that their dog is “scared” of some sort
of noise (Beaver, 1999; Blackwell, Bradshaw, & Casey,
2013). However, the extent to which dogs affected with
this problem are overly sensitive to sound, in terms
of stimulus perception rather than simply reactive to
them—perhaps because of learned associations—
remains unknown. Noise reactivity takes many forms
and can arise through various processes in both humans
and dogs (Riccomini, 2011), ranging from traumatic
exposure and stress-induced dishabituation to potential
social transmission. A review of the development of this
problem and its management is beyond the scope of
this article, but see Riccomini (2011) and Sherman and
Mills (2008). Repeated or continuous exposure to noise
can also result in health and welfare problems in both
humans (Shepherd, Welch, Dirks, & Mathews, 2010) and
dogs (Mills, Karagiannis, & Zulch, 2014).
Noise sensitivity/reactivity may be specic to a
particular sound or a broader behavioral trait relating
to sound more generally and is quite a stable predisposi-
tion in humans (Zimmer & Ellermeier, 1999). Females,
whether human or dog, may be more noise sensitive
than males (McFadden, 1998; Roche, Siervogel, Himes,
& Johnson, 1978; Rogers, Harkrider, Burcheld, &
Nabelek, 2003; Storengen & Lingaas, 2015). Further,
there is evidence that noise sensitivity increases (i.e.,
tolerance for loud sounds decreases; Fucci, McColl, &
Petrosino, 1998) with age or due to disturbances of the
auditory systems (e.g., threshold shift), although reports
in dogs are anecdotal.
Hyperacusis is a frequent auditory disorder in
humans and dened as a heightened aural response;
sounds of normal volume are perceived as too loud or
even painful. This can concern everyday environmental
sounds and range from a strong dislike of sounds
(mysophobia) to a fear of these sounds (phonophobia;
64
COMPARATIVE COGNITION & BEHAVIOR REVIEWS
Barber et al.
Sheldrake, Diehl, & Schaette, 2015). Hyperacusis can
greatly impact quality of life and ability to function,
as loud environments will tend to be avoided. The
occurrence of hyperacusis may be associated with
tinnitus and a variety of psychological disorders
including bipolar disorders, obsessive compulsive
disorder or posttraumatic stress disorder (Faustietal.,
2009), and there is some evidence to suggest similar
associations may occur in the dog (Drobny & Miller,
2016). Although it has been suggested that the hearing
ability of most sound reactive dogs is probably normal
(Scheifele, Sonstrom, Dunham, & Overall, 2016), the
same could be said of individuals who are “jumpy”
when hearing noises, so it seems reasonable to suggest
that hyperacusis may occur in dogs. Given the potential
signicance of this condition for both the working
potential and welfare of dogs, this subject deserves
specic research attention. Recently, evidence has begun
to emerge of a relationship between the occurrence of
sound sensitivity and musculoskeletal pain in dogs
(Lopes Fagundes, Hewison, McPeake, Zulch, & Mills,
2018). Certain evidence suggests that noise sensitivity
may be related to the strength of cerebral lateralization
in dogs. Those without a paw preference have been
shown to be more reactive toward noises compared
with dogs without a paw preference (Branson & Rogers,
2006). However, the extent to which a simple test of
laterality may be a useful screening test for at-risk
individuals remains unknown.
Responses to Ultrasound
Ultrasound refers to frequencies above the upper
limit of the human hearing range 20000Hz (children
up to 30000 Hz; Ueda, Ashihara, & Takahashi, 2016).
In addition to ultrasounds originating from natural
sources (e.g., some animal’s communication; for further
reading, see Part2) everyday devices produce ultrasound
(e.g., motion detectors, audio systems, humidiers,
televisions, and telephones; Wohlfahrt, Waniek, Myrzik,
Meyer, & Schegner, 2017). Ultrasound diagnostic
imaging is also used in medical as well as military
settings for reconnaissance purposes, which function
in the ultrasonic range above 20000 Hz (Ter Haar,
2011b; Watson & Gorski, 2011). Even though there is,
to date, no evidence that medical diagnostic ultrasound
causes harm to humans (including the developing
fetus), concerns have been expressed about the potential
long-term impact on hearing (Marmor, Hilerio, &
Hahn, 1979; Ter Haar, 2011b). It is well established that
ultrasound can have thermal and mechanical (acoustic
cavitation) effects on tissue indicative of the high-energy
content of these sound waves (Ter Haar, 2011b). In
humans, exposure to ultrasound has been reported to
be accompanied by annoyance, disorientation, tinnitus,
headache, fatigue, nausea, and high arousal, producing
unpleasant subjective effects (Bunker, 1997; Lawton,
2001). Airborne ultrasounds with an intensity up to
120dB have been judged to be nonhazardous, as they
do not cause temporary hearing loss (Lawton, 2001),
although very high-frequency sounds have the capacity
to cause cell damage and death (Marmoret al., 1979).
There are strict guidelines concerning the exposure of
humans to ultrasound (Lawton, 2001; Ter Haar, 2011b).
By contrast, dogs can hear ultrasounds up to 45000 Hz.
Ultrasound is also often used in dog whistles, with the
advantage that humans are not disturbed by the whistle.
Further, ultrasound is often used as an animal repellent,
and high-intensity ultrasound can provoke aversive
responses in many species, including dogs, and may
be perceived as painful if very loud (Blackshawetal.,
1990). In an experiment on dogs, 120 dB ultrasonic
sweeps were shown to effectively expel dogs from tested
areas ( Blackshawetal., 1990). It was postulated that
the effectiveness of such devices depends not only on
their frequency range but also on the wave amplitudes
alongside individual features in the dog, with reactions
ranging from no reaction to surprise and curiosity to ear
pricking and aversion (Blackshawetal., 1990; Edgar,
Appleby, & Jones, 2007). However, ultrasounds of 120dB
are described as nonhazardous in humans, and although
there may be variation in their effect depending on the
exact frequency of the ultrasound, this disparity with the
response of dogs needs to be carefully considered when
reviewing the environment in which human–dog teams
are required to work.
Responses to Infrasound
Infrasound, dened as frequencies below 20 Hz,
can travel long distances and is not easily attenuated
by environmental obstacles; it is even able to
penetrate buildings. Besides the many natural sources
of infrasound, including storms, breaking waves,
earthquakes, and volcanic eruptions there are also
articial sources, including heavy engines, windmill
power plants, and ventilation systems (Leventhal,
2003). For humans, background infrasound can have
signicant impacts on welfare, in terms of both loss of
sleep and reduced wakefulness, which can reduce task
performance to a level similar to that associated with
alcohol intake (Leventhal, 2003). Body organs resonate
65
comparison of hearing and auditory functioning between dogs and humans
VOLUME 15, 2020
within the low-frequency range, and common complaints
relating to being within environment emitting certain
types of infrasound include the subjective feeling of
vibrations; nausea; disorientation; and, potentially with
higher intensities, organ damage or death (Bunker, 1997;
Leventhal, 2003). There is much anecdotal evidence
that dogs, like many other species, are able to predict
natural disasters such as earthquakes or storms (Liso,
Fidani, & Viotto, 2014), and they are used as part of
an early warning system in some countries (Woith,
Petersen, Hainzl, & Dahm, 2018). However, the way that
they might be able to sense these changes has not been
scientically investigated. Studies on the consequences
of urban infrasound on dog behavior and well-being are
also missing. This is likely to be important, given the
potentially cumulative effect of prolonged exposure on
human mental health and well-being.
Part 5: Auditory Perception
and Related Cognitive Processes
Auditory perception is dependent on (a) the
appropriate transduction of sound waves to electrical
signals, (b) the filtering of certain sounds (e.g.,
background noise), and (c) the identification and
interpretation of sound (Carreiro, 2009). We now
consider how this relates to cognition and performance.
Dogs hear and respond to a different frequency range
than humans, so it is important for handlers to be aware
that dogs may react to sounds that humans cannot hear.
Studies on discrimination, generalization, and reversal
learning in dogs indicate that unlike some primates,
they are very sensitive to auditory stimuli (Kowalska
& Zieliński, 1980), but the extent to which this might
a compensation for differences in visual acuity is
unknown. However, in a practical context, some
evidence suggests that when a dog makes an error in an
obedience class, refocusing attention using auditory cues
may be of more value than using corrections (Lynge &
Ladewig, 2005).
High-frequency sounds are generally considered
more salient by many species (Alberti, 2001; McDermott,
2012; Yong & Ruffman, 2014), but sounds that provide
information about the sound source are also prioritized
(Heffner, 1998). Natural sounds may also be more
likely to catch attention, as they provide the recipient
with information about their surroundings and so are
generally more relevant for survival (e.g., the rattle of a
rattlesnake; Alberti, 2001). Humans are generally good
at differentiating and evaluating the content of dog vocal
communication based on only auditory cues (Chen &
Spence, 2010; Molnár, Pongrácz, Dóka, & Miklósi, 2006;
Molnár, Pongrácz, & Miklósi, 2010). Like humans, dogs
are reported to be able to distinguish natural from articial
sounds (Heffner, 1998) and to categorize novel sounds into
previously learned categories (Heffner, 1998). Playback
experiments with dogs also provide evidence that dogs
not only can detect the semantic content of conspecic
barks but also remember individual characteristics
(Bálint, Faragó, Dóka, Miklósi, & Pongrácz, 2013;
Faragóetal., 2010; Molnár, 2007). Further, they are also
able to detect the difference in the non- semantic content
of human speech ( Albuquerque et al., 2016) and can
discriminate emotionally relevant information of human
communication based on auditory cues (Huberetal.,
2017; Yong & Ruffman, 2014). Dogs (like humans; see
Pisoni & Luce, 1987) discriminate human spoken words
on the basis of their phonetic composition (Fukuzawa,
Mills, & Cooper, 2005a) and can normalize spoken sounds
across different individual speakers (Root-Gutteridge,
Ratcliffe, Korzeniowska, & Reby, 2019). However, it has
also been postulated that dog performance to cues is best
under natural conditions, in which auditory and visual
stimuli are combined suggesting that (possibly learned)
nonverbal features moderate responsiveness to auditory
cues (Fukuzawa, Mills, & Cooper, 2005b). However,
some evidence suggests that dogs may be capable of some
syntactic understanding (Ramos & Ades, 2012). For a
wider review of dogs’ understanding of words, see Mills
(2005).
The proximity of a sound source may affect the level
of attention given to it, and dogs may learn an auditory
go/no-go procedure faster if the required response is
spatially close to the signaling sound source (Dobrzecka,
Szwejkowska, & Konorski, 1966). This has some impli-
cations for command-based training, which should
probably begin in close proximity to the dog, even for
commands that will later be given at a distance.
Preference for a sound may be based on many
factors, such as its amplitude, frequency range, and
composition, but also the experience of the individual.
Certainly, individuals prefer sounds that are not pain-
ful (in humans <120dB) or distracting for them, and
plenty of studies show that loud sounds can be aversive
to animals (Ballantyne, 2018; Blackshawetal., 1990;
Heffner & Heffner, 1998; Job, 1999; Landsberg, Mougeot,
Kelly, & Milgram, 2015). However, whether a sound is
perceived as aversive may depend on other factors, such
as frequency composition (McDermott, 2012) as well as
learned associations (Lopes Fagundesetal., 2018) and
66
COMPARATIVE COGNITION & BEHAVIOR REVIEWS
Barber et al.
more general previous experience. For example, an indi-
vidual who has not been reared in a noisy environment
will most probably prefer a quiet environment over a
noisy one, and vice versa. Thus, both familiarity and
habituation may play an important role in auditory pref-
erences (Heffner & Heffner, 1998). In humans, frequen-
cies between 2000Hz and 4000Hz, which is in the range
of the best perceived frequencies, have the highest poten-
tial for annoyance (McDermott, 2012). If this effect also
applies to dogs, then the frequencies around 8000Hz
may be the most problematic ones, but further research
is needed to determine this.
In general, individuals prefer sounds of ecological
relevance (i.e., they prefer natural sounds over articial
and prefer sounds that are species specic and familiar;
Heffner & Heffner, 1998; Snowdon, Teie, & Savage,
2015; for a review, see McDermott, 2012). In humans,
McDermott (2012) postulated that natural sounds,
such as ocean waves or rainfall, are rated as pleasant;
it is thought that this is due to their low-frequency
components and slow temporal modulations (McDermott
2012). Further, infants prefer their mother’s voice over a
stranger’s and the native language of the mother over
a foreign one (Barker & Newman, 2004). Adults prefer
frequencies that are within the fundamental frequency
of human speech (i.e., approximately 200Hz; Huber,
Stathopoulos, Curione, Ash, & Johnson, 1999; Ratcliffe,
2015). Generally, humans prefer harmonic sounds
over dissonance, which has the capacity to distress an
individual (McDermott, 2012). In music, the composition
of fundamental frequency and harmonics can modulate
physiological reactions (synchronization of heart rate
to the beat rate of the music) and even emotional states
(Det & Fakultet, 2017; Khalfa, Isabelle, Jean-Pierre, &
Manon, 2002; Paquette, Peretz, & Belin, 2013; Wang
& Huang, 2014). As a consequence, music can have an
intrinsic capacity to calm or excite an individual.
Less is known about auditory preferences in dogs.
They can discriminate familiar from unfamiliar sounds
(Pongrácz, Szabó, Anna, András, & Ádám, 2014;
Quervel-Chaumette, Faerber, Faragó, Mashall-Pescini,
& Range, 2016), and have been reported to recognize
their handlers by their voice (Coutellier, 2006). They
can match the voice of a human to age categories
(Ratcliffe, 2015), but whether dogs prefer familiar over
novel sounds, or harmonic sounds over dissonances,
has not been investigated. It has been postulated
that the same features of music (i.e., low-frequency
components and slow temporal modulations) may have
similar physiological effects, albeit with species-specic
adaptations regarding frequency ranges (calculated
on basis of species-typical fundamental frequency
during communication) or tempo (calculated on basis
of species-typical heart rate; Snowdonetal., 2015), that
is, species-specic music. There is some research on the
perception of music and species-specic music in dogs
(Leeds & Wagner, 2008), where it was found that soft
rock, reggae, and classical music may have positive
effects, whereas heavy metal had negative effects on
dogs. Surprisingly, species-specic music appears to have
no effect on dog behavior (Bowman, Dowell, & Evans,
2017; Bowman, Scottish, Dowell, & Evans, 2015; Kogan,
Schoenfeld-Tacher, & Simon, 2012; Wells, Graham, &
Hepper, 2002), unlike the preference shown by cats and
monkeys (Snowdon & Teie, 2010; Snowdonetal., 2015).
In contrast, it has been claimed that the calming effect
of audiobooks exceeds that of music for dogs (Brayley &
Montrose, 2016), although Wellsetal. (2002) previously
argued that classical music may outcompete speech.
These differences in the dog might reect adaptation
to the human environment or simply a familiarity
effect. Nonetheless it seems reasonable to suggest that
appropriate auditory enrichment can increase dog
welfare and inappropriate stimulation reduce it.
Infant-directed speech and dog-directed speech
(“motherese” and “dogerese”) have the capacity to
attract the attention of the individual; infant-directed
speech, characterized by high and variable pitch as well
as a slower tempo and clearer articulation, is preferred
by human infants over adult-directed speech (Xu, Burn-
ham, Kitamura, & Vollmer-Conna, 2013). Similar results
have been obtained for dogs (Ben-Aderet, Gallego-
Abenza, Reby, & Mathevon, 2017). However, even
though dog-directed speech is used for dogs of all ages,
only puppies seem to prefer this form of communication
(Ben-Aderetetal., 2017; Benjamin & Slocombe, 2018).
It has been suggested that dogs can be trained faster
to perform a passive action such as sit or stay when using
a long note with descending fundamental frequency,
whereas active actions such as approaching the trainer
are more likely followed if a sequence of rapidly short
notes with rising frequency is used (McConnell, 1990;
McConnell & Baylis, 1985). It is thought that this might
relate to some inherent bias between action and sound
such that acoustic structure can bias the response of the
dog. Whether this is due to an auditory preference or a
general correlation with certain emotional states has not
been disentangled.
In humans, Cohen, Horowitz, and Wolfe (2009)
postulated that auditory memory is inferior to visual
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comparison of hearing and auditory functioning between dogs and humans
VOLUME 15, 2020
memory, that is, we are more likely to remember a scene
based exclusively on visual information than one based
solely on its auditory associations. No comparable
experiments have been conducted with dogs, but in a
learning paradigm, it was suggested that a cat is more
likely to associate an auditory cue with an aversive
stimulus than a visual one, as an auditory cue is more
salient (Jane, Masterton, & Diamond, 1965). It is not
clear whether visual or auditory cues are more salient
to dogs, although visual cues may overshadow verbal
ones during command learning (Skyrme & Mills, 2010).
Although long-term auditory memory in dogs has been
investigated (e.g., Kowalska, 1997), little is known about
their short-term auditory retention. It has been stated
that dogs can localize sounds depending on the memory
of an auditory stimulus (Heffner, 1978) and that they
learn auditory tasks relatively easily compared with
monkeys (Kowalska & Zieliński, 1980). Still, in a delayed
matching-to-sample test using auditory cues, it has been
shown that task performance decreased gradually with
an increase in delay between presentation and response;
at a delay of 30s, only about 70% of responses were
correct, and this fell to around 60% with a delay of 90s.
Dogs’ short-term auditory memory therefore appears to
be more limited under experimental conditions, but the
extent to which this applies to the “real world” is unclear.
However, various studies have shown that context can
play an important role in what appears to be learned,
when training verbal–action associations (Braem &
Mills, 2010; Fukuzawa et al., 2005b). It is perhaps
surprising that more research has not been undertaken
in this area, given the importance of verbal commands
in the control of dog behavior.
Part 6: Ontogenetic and Age-Related Changes
in Hearing
At birth, the level of development of the auditory
system of humans and dogs differs. For human fetuses
the onset of hearing has been estimated to be 27 to
28 weeks of gestation (Litovsky, 2015); by 35weeks,
fetuses can discriminate 250 and 500 Hz tones, but
sensitivity to sounds of a frequency of 4000Hz (the
frequency with highest sensitivity in humans) is reached
only by around 6months of age. Frequency and intensity
discrimination, as well as selective attention, mature
between 3 and 6months of age but do not reach adultlike
performance (at 4000Hz) before 12months. Localization
of sound is evident within hours after birth, but the full
maturation of the orienting responses can take several
years (Litovsky, 2015). By contrast, at birth, dogs’ ear
channels are closed, and they are considered to be largely
deaf. The ear channels open at about 12–14days of age;
from this time point an acoustic startle response can
be observed (Breazile, 1978). After opening of the ear,
hearing matures rapidly and reaches adult sensitivity
by about Day20 (Mech & Boitani, 2003). Dogs respond
to frequencies of 250–750Hz at about Day13–16, but
sensitivity to dogs’ “best-heard” frequencies of 8000 Hz
is not reached until after Day20–22 (Mech & Boitani,
2003). Orientation toward sound sources is observed
from Day18 to 25 (Beaver, 1982), with full maturity of
the auditory system reached at Week6 to 8 (Mech &
Boitani, 2003; Plonek, Nicpoń, Kubiak, & Wrzosek,
2017; Wilson & Mills, 2005). For both dogs and humans,
all auditory structures naturally need to grow with the
individual, which can be expected to cause changes
in the hearing amplication processes, the frequency
ranges detected, and possibly sensitivity within them.
This is potentially important to appreciate with respect
to the start of the early training of working dogs, which
may begin before some dog breeds (especially large
dogs) have their adult size (18months of age for some
large breeds; Royal Canin, 2018) and so their perceptual
responses to auditory cues may change during training
as they mature.
In adult humans, body and head size are relatively
consistent and the pinna has an average length of 55mm
when mature at 12–13years of age. However, the pinna
continues to increase in size until death and can reach
up to 70–75mm by the age of 80 (Bruckeretal., 2003;
Itoetal., 2001). It seems unlikely that this is some form of
compensation for functional loss of hearing with aging,
as much of the change may relate to the earlobe; also
unlike humans, dogs’ pinnae (and those of many other
species) do not appear to show a similar size change
throughout their life.
Conclusions
This review highlights extensive gaps in our
knowledge concerning the hearing ability of dogs, how
sound might impact the performance and the level of
variation that might exist between individuals and
breeds. Priority areas for future research should be
those that are minimally invasive and can be broadly
divided into (a)aspects that may be relatively easy to
model and (b)aspects that may be possible to assess
using behavioral observation. For example, it would
be relatively straightforward to model some of the
68
COMPARATIVE COGNITION & BEHAVIOR REVIEWS
Barber et al.
known physical differences between dogs and humans,
such as the effects of differently shaped EAMs (ear
ap and auditory canal length and dimension) on the
amplication of sound across the potential hearing
range spectrum of dogs, or to model the mechanical
effect on force transmission of the differently shaped
ear ossicles using three-dimensional models. This has
the potential to transform our basic understanding of
the hearing in dogs and the factors that might contribute
to its variability between individuals. Priorities that
might be assessed behaviorally include fundamental
investigations of preferred sounds and sound qualities
that attract attention, as well as the features of sound
(amplitude, frequency, and composition) that can lead
to avoidance. From a practical perspective, exposure
to noisy environments, such as during transport and
kennelling, may result in temporary or even permanent
hearing loss. Therefore, there is a need to monitor dogs’
hearing functionality across the spectrum on a regular
basis. This process should include an assessment across
the ultrasonic waveband, as it seems that this is not
widely researched in the literature. For this reason, more
applied behavioral work should consider the impact of
different soundscapes on behavior and performance as
well as the development of strategies to help dogs cope
better in challenging environments, in terms of both the
prevention of damage and maintaining performance.
Factors related to sound interference include the effects
of noise and input from other sensory modalities on a
dog’s performance (both positive and negative) and the
dog’s response to the cues used to control their behavior
in the eld (visual and verbal commands). Finally,
it is essential that, in the absence of so much basic
information on the hearing ability of dogs compared
with humans, there is further investigation into the
factors that might affect hearing loss in this species.
This should be investigated alongside research into
practical solutions to minimize the risk of hearing loss
or the development of sound reactivity in dogs. In the
short term, it should be assumed that dogs are at least
as sensitive to hearing damage as humans in equivalent
settings. In the longer term, it is clear that we need a
much more comprehensive understanding of dog hearing
so we can identify the source of potential issues and
develop an evidence-based approach to prevention and
management for both working and pet dogs.
Conicts of Interest
No conicts of interest are declared by the authors.
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ISSN: 1911-4745
Comparative Cognition
& Behavior Reviews
DOI:10.3819/CCBR.2020.150007 Volume 15, 2020
The Nature of Sound and Hearing
Sound refers to air vibration, a wave of pressure.
The transduction of sound to a sensory signal depends
on the energy transmitted from the sound wave to the
sensor, and this process involves the sound wave crossing
through a number of structures and materials, which
affect the properties of the pressure wave. The speed of
sound in air (at 20 °C) is 340m/s, but it is about 4times
faster in water at 1500 m/s (as the particles of a uid are
closer, they more readily transmit vibrations).
Sound waves ow through different media (e.g., water
vs. air) not only at different speeds but also in different
ways; this determines both the level of transmission
of sound waves at material boundaries and the degree
of sound reection at these points (Rossing, Wheeler,
& Moore, 2002). These differences in wave ow are
described in terms of their specic acoustic impedance
(Pa ×s m or rayl) or acoustic impedance (Pa ×s or
rayl m2). Thus, there is a change in the sound wave
at each material interface between its source and the
sensory cells, which are responsible for detecting it.
The movement of sound waves may be thought to
be relatively slow compared with the speed of light
(~300millionm/s), but from a biological perspective,
this is still fast enough to allow the rapid exchange of
information between individuals, compared with the
potential association with some other sensory modalities
(e.g., smell from chemical diffusion); an advantage of
auditory signals over visual signals is that they are not
blocked to the same extent by physical barriers and
obstacles in the environment (Rossinget al., 2002).
Accordingly, audible signals have often evolved for use
in situations, where the rapid exchange of information
is required, but there may be environmental constraints
on the effectiveness of visual signals for example,
the presence of cover in the environment, or when
individuals are separated.
The subjective perception of the loudness of a sound is
a function of the amplitude of the sound wave, which is
expressed in terms of the intensity of the sound (see also
Supplementary Figure1). The amplitude of the sound
wave is commonly measured on a logarithmic scale
(measured in decibels [dB]) relative to a reference value
(typically a standard level of air pressure of 20 micro-
Pascals [µPa]) as a measure of the sound pressure level
(SPL). Thus, in the dB (SPL) scale, 0dB is equivalent
to 20µPa (or micro-Newton/m² [µN/m2]), a level near
the human hearing threshold in air, whereas 10 dB
equates a 100-fold increase in pressure. In humans, a
sound at 100Hz and 60dB (SPL) is perceived as quieter
than a sound of 1000 Hz at 60 dB (SPL). This is due
to the perception of loudness being a psychological
phenomenon that depends on the temporal integration
of the signal (Vater & Kössl, 2011). It is unknown whether
this is also the case in dogs, but it seems likely.
The pitch of a sound is the subjective perception of
the frequency of the sound wave. This is commonly
described in terms of the linear frequency of the sound
wave — that is, the number of oscillations of the wave per
unit time, typically a second, as Hertz (Hz). Frequencies
below the hearing range of the human are commonly
referred as infrasound (<20Hz) and those above it
as ultrasound (>20000Hz). It is generally assumed
that the ability to hear high frequencies declines as
low-frequency hearing improves (Heffner, 1983; Packer,
1987). High- and low-frequency hearing are therefore
competing abilities, and it has been reported that
in mammals, low-frequency hearing is restricted to
prevent low-frequency components from interfering
with the analysis of high-frequency components, which