THE ANATOMICAL RECORD PART A 281A:1123–1137 (2004)
Primate Auditory Diversity and Its
Influence on Hearing Performance
1
MARK N. COLEMAN1* AND CALLUM F. ROSS2
Interdepartmental Doctoral Program in Anthropological Sciences, Stony Brook
University, Stony Brook, New York
2
Department of Organismal Biology and Anatomy, University of Chicago,
Chicago, Illinois
ABSTRACT
The auditory region contains numerous structures that have proven useful for phylogenetic classification at various taxonomic levels. However, little work has been done in
primates relating differences in morphology to variations in hearing performance. This study
documents anatomical and physiological distinctions within primates and begins to address
the functional and evolutionary consequences of these and other auditory features. The
dimensions of the outer ear (pinna) were measured in cadaveric specimens representing
nearly every primate family and used to calculate a shape ratio (height/width). It was found
that nonanthropoids have a significantly higher ratio than anthropoids, although the actual
height was not found to differ. This indicates that most nonanthropoids have ears that are
tall and narrow, whereas monkeys and apes are characterized by ears with more equal height
and width dimensions. Eardrum area, stapedial footplate area, and ossicular lever arm
lengths were measured in dried specimens to calculate an impedance transformer ratio. A
distinction was found between anthropoids and strepsirrhines, with the latter group having
a transformer ratio indicative of a higher percentage of acoustic energy transmission through
the middle ear. Audiogram data were gathered from the literature to analyze hearing
sensitivity and it was found that platyrrhines illustrate more low-frequency sensitivity than
like-sized lorisoids. The effects of intraspecific variation on the audiogram results were also
examined and were found to produce similar results as the analysis using species mean
threshold values. Lastly, correlations between morphological and audiogram variables were
examined. Several measures of hearing sensitivity were found to be correlated with pinna
shape but correlations with middle ear transmission properties were weaker. In addition to
using traditional statistical techniques, phylogenetic corrective methods were applied to
address the problem of statistical nonindependence of the data and the results of both
analyses are compared. These findings are discussed with respect to how sensory adaptations
and phylogenetic history may be related to the current radiation of primates.
©
2004 Wiley-Liss, Inc.
Key words: primate hearing; outer ears; middle ears; audiogram; phylogeny
Primates express numerous modifications on the fundamental mammalian middle ear morphology and these differences have proven useful for phylogenetic classification
at various taxonomic levels (MacPhee and Cartmill, 1986).
Among extant strepsirrhines, most Malagasy primates
are characterized by a free-floating tympanic ring surrounded by a single relatively large tympanic cavity while
lorisoids have a tympanic ring that is fused to the lateral
wall of a smaller tympanic cavity. Lorises appear to maintain a substantial effective cavity volume by having additional pneumatic spaces off of the epitympanic recess.
Extant haplorhines are similar to lorises in possessing
epitympanic sinuses (except tarsiers) (MacPhee and Cartmill, 1986) and a fused tympanic ring but are further
©
2004 WILEY-LISS, INC.
distinguished by the presence of a diverticulum off the
eustachian tube called the anterior accessory cavity. In
Presented at the Symposium on Primate Sensory Systems at
the 2004 American Association of Physical Anthropologists
(AAPA) Meeting in Tampa, Florida.
*Correspondence to: Mark N. Coleman, Academic Tower A,
T-8069, Stony Brook University, Stony Brook, NY 11794. Fax:
631-444-3947. E-mail: mcoleman@ic.sunysb.edu
Received 20 May 2004; Accepted 1 July 2004
DOI 10.1002/ar.a.20118
Published online 6 October 2004 in Wiley InterScience
(www.interscience.wiley.com).
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COLEMAN AND ROSS
Despite the well-documented abundance of diversity in
the outer and middle ears of primates, the functional
implications of these differences are not well understood
(Cartmill, 1975; Fleagle, 1999). A better understanding of
how diversity in ear structure affects hearing performance
will shed considerable light on the sensory adaptations
that led to the radiation of extant primates and their fossil
relatives. The goals of this study are to test for taxonomic
differences in primate middle and outer ear morphology
and estimate their functional characteristics using models
based on acoustical theory to investigate how well these
model predictions relate to actual hearing performance.
Similar lines of investigation have been applied to other
vertebrate groups (e.g., rodents and felids), but primates
have received only limited attention in this regard, with
the few relevant studies narrowly focused on only a few
taxa (Packer and Sarmiento, 1984; Masali et al., 1992;
Hemilä et al., 1995; Moggi-Cecchi and Collard, 2002).
Theoretical Background
Fig. 1. Basic pinna morphology scaled to approximately the same
size for various primates representing each family/subfamily. Note the
elongated auricular lamina characteristic of most prosimians (first two
rows minus Callithrix) that results in a relatively tall and narrow pinna
compared with most anthropoids (last two rows plus Callithrix).
tarsiers, this cavity is nontrabeculated, while in all other
haplorhines, it is filled with trabeculae.
The outer ears of primates also show considerable diversity in morphology and mobility that often follows phylogenetic patterns. Ceboids, lemuroids, and most lorisoids
have an ear canal that is almost entirely cartilaginous
while in cercopithecoids, hominoids, and tarsioids the canal is composed mostly of bone formed by a tubular expansion of the ectotympanic. Primate pinnae are highly
variable in overall shape as illustrated by Figure 1, but
can generally be characterized by one of two major patterns. Most prosimians have a superiorly elongated auricular lamina (helix and antihelix) that is commonly ovate
or conical in shape while anthropoids typically show a
more subquadrate outline with an involuted helix (Hershkovitz, 1977). This results in prosimians having what
appear to be tall and narrow pinnae compared with those
of monkeys and apes that are more equal in width and
height dimensions. In addition, the majority of prosimians
have larger pinnae than those of all anthropoids, with
aye-ayes having the largest ears and orangutans having
the smallest ears relative to head size (Schultz, 1973).
Prosimians also show better developed auricular musculature (Huber, 1930; Lightoller, 1934; Schultz, 1969; Burrows and Smith, 2003) and generally more mobile pinnae
than anthropoids (Hill, 1955; Hershkovitz, 1977).
Several models have been developed to describe the
functional characteristics of the outer ear, but one simple
approach relates the shape of the outer ear to its frequency selectivity and directionality. Outer ears with long
and narrow dimensions should show a decrease in lowfrequency reception while ears that are wider or larger
will be sensitive to low- as well as high-frequency sounds
and will provide more directional cues at low frequencies
(Rosowski, 1994). Although intuitively it might seem advantageous for all animals to maximize sensitivity to as
wide a range of frequencies as possible, specialization
might entail trade-offs. For example, a species that relies
on high-frequency reception for survival might find it advantageous to avoid reception of low-frequency sounds
that could mask more important high-frequency sounds.
The main role of the middle ear is to help overcome the
impedance mismatch that results from the higher acoustic
impedance of perilymph in the inner ear compared to that
of air (⬃ 134:1) (Zwislocki, 1965). The two primary mechanisms that have been proposed to assist in this task are
the ossicular lever arm ratio (the lever action that results
from the uneven lengths of the manubrium of the malleus
and the long process of the incus) and the areal convergence ratio (the increase in pressure that results from the
larger surface area of the tympanic membrane relative to
the stapedial footplate; Fig. 2).
Using these two ratios, it is possible to calculate the
impedance transformer ratio (ITR) of the middle ear,
given by the formula
ITR ⫽ (As/(2/3 Ad))*(Li/Lm)2,
where As is the surface area of the stapedial footplate, Ad
is the area of the tympanic membrane, Li is the lever arm
of the incus, and Lm is the lever arm of the malleus. The
ITR is often considered an ideal transformer ratio because
it ignores the intrinsic impedance of the components of the
auditory system itself (Dallos, 1973), and several researchers have found a lack of association between the
ITR or its component ratios (e.g., areal convergence ratio)
and measures of auditory sensitivity (Lay, 1972;
Rosowski, 1994). However, it provides a useful starting
point for analyzing middle ear function and continues to
be used by investigators as a proxy for estimating middle
AUDITORY DIVERSITY IN PRIMATES
1125
MATERIALS AND METHODS
Fig. 2. Schematic representation of the areal convergence and ossicular lever arm ratios used to calculate the impedance transformer
ratio. Ad represents the area of the tympanic membrane, As the surface
area of the stapedial footplate, Lm the lever arm of the malleus, and Li the
lever arm of the incus. Methods used for obtaining the measurements
are described in text.
ear performance (Webster and Webster, 1975; Hunt and
Korth, 1980; Masali et al., 1992).
Using the ITR, it is possible to estimate the theoretical
maximum percentage of acoustic transmission (T) at peak
performance through the middle ear using the formula
To investigate the functional consequences of differences in primate ear morphology, outer ear shape, middle
ear impedance matching performance, and audiograms
were assayed in 49 genera of primates. These taxa fall into
two monophyletic suborders, Strepsirrhini (lemurs, lorises, and galagos) and Haplorhini (tarsiers and anthropoids). The terms “Prosimii” and “prosimians” are used to
refer to a paraphyletic grouping of strepsirrhines and
tarsiers, i.e., nonanthropoid primates. Complete data sets
could not be gathered for all genera; audiogram data were
only available for 10 genera, outer ear data were available
for 36 genera, and middle ear data were available for 34
genera. Morphometric data were gathered from specimens
in the collections at the American Museum of Natural
History, Field Museum of Natural History, and National
Museum of Natural History. Whenever a structure from
both ears of a single specimen was measurable, the average value of both ears was used. Morphometric data were
pooled at the generic level due to the difficulty in obtaining
adequate sample sizes for various species.
In addition to the morphometric measures on auditory
structures described below, 17 cranial measurements (Table 1) were taken on the dried skulls in order to calculate
a geometric mean of skull size for use in size evaluation.
Geometric mean (GM) equals the Nth root of the product of
N variables ((GM ⫽ N 冑 [CM1 ⫻ CM2 ⫻ . . . ⫻ CMN]). The
general combination of cranial measurements used to calculate GM has been found to produce consistent analytical
results while maximizing statistical power (Coleman,
2003).
Outer Ear
T ⫽ 4r / (r ⫹ 1)
2
where
r ⫽ Z1 / Z2,
the ratio of the specific acoustical impedance at the eardrum (Z1) to the characteristic specific acoustical impedance of air (Z2 ⫽ 41.5 dynes sec/cm3). Z1 is determined by
multiplying the specific acoustical impedance of the inner
ear by the ITR.
The actual hearing performance that results from the
combined effects of the outer, middle, and inner ears can
be evaluated through behavioral testing in several ways,
including determining absolute auditory thresholds, localization acuity, and amplitude, temporal, or frequency difference limens (smallest detectable differences). Absolute
auditory thresholds were examined in this study since
they are considered the most fundamental evaluation of
audition (Stebbins, 1975) and provide basic measures of
hearing such as high- and low-frequency sensitivity, frequency of greatest sensitivity, and overall range of audible
frequencies. Threshold values are graphically represented
as bivariate plots called audiograms with frequency measured in hertz (Hz) displayed on the abscissa and amplitude measured in decibels (dB) displayed on the ordinate.
Audiograms are available in the literature for 17 species of
primates, including four strepsirrhines, three platyrrhines, eight cercopithecines, and two hominoids (Fig. 3).
To calculate a shape index (Psi) for the outer ear, measurements of the maximum height and width of the pinnae were taken to the nearest millimeter using the traditional landmarks shown in Figure 4: Psi ⫽ pinna height /
pinna width.
Since many primates do not have a true ear lobe, subaurale was determined as the most inferior aspect of the
outer ear as it extends laterally from the side of the head.
Due to the concern that drying and storage of museum
skins may distort the size and shape of the outer ear,
pinna dimensions were taken only on cadaveric specimens
(204 ⫹ 2 anesthetized animals). The cadavers are housed
at the National Museum of Natural History and were
fixed in 10% buffered formalin and are stored in 70%
ethanol.
Middle Ear
To calculate the ITR, estimates were obtained of the
surface areas of the tympanic membrane and stapedial
footplate and the lengths of the malleolar and incudal
lever arms. The tympanic membrane is often deteriorated
in most dried specimens, so the area was estimated by
making a mold that preserves the impression of the tympanic ring (or the rarely preserved membrane) and then
taking measurements on the impression. The mold was
made by injecting polyvinylsiloxane into the lateral aspect
of the middle ear cavity. The mold was then removed and
sectioned under a microscope along the line of attachment
of the tympanic membrane. The modified molds were then
digitally photographed at a distance at least 12 times the
maximum dimension to be measured to minimize the ef-
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COLEMAN AND ROSS
Fig. 3. Audiograms for 17 species of primates representing every
primate superfamily except Tarsioidea. Data taken from the original
literature: Aotus trivirgatus (Beecher, 1974a); Callithrix jacchus (Seiden,
1957); Cercopithecus mitis (Brown and Waser, 1984); Cercopithecus
neglectus and Chlorocebus aethiops (Owren et al., 1988); Galago
senegalensis (Heffner et al., 1969); Lemur catta (Gillette et al., 1973);
Macaca fuscata and Homo sapiens (Jackson et al., 1999); Macaca
mulatta (Pfingst et al., 1978); Macaca nemestrina and Macaca fascicularis (Stebbins et al., 1966); Nycticebus coucang and Perodicticus potto
(Heffner and Masterson, 1970); Pan troglodytes (Kojima, 1990); Papio
cynocephalus (Hienz et al., 1982); Saimiri sciureus (Beecher, 1974b).
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AUDITORY DIVERSITY IN PRIMATES
TABLE 1. Measurements taken on dried skulls used to calculate the geometric mean of skull size
Measurement
Skull length
Skull width
Facial width
Orbital height
Orbital depth
Basicranial length
Interaural distance
Neurocranial length
Neurocranial width
Neurocranial height
Palate width
Palate length
Ramus height
Symphyseal height
Symphyseal width
Corpus height
Corpus width
Osteometric landmarks
Region/apparatus
Prosthion-inion
Bi-zygion
Bi-ectoconchion
Orbitale inferiorus-orbitale superiorus
Orbitale inferiorus-anterior optic canal
Prosthion-basion
Bi-ectotympanic
Nasion-inion
Bi-euryon
Basion-vertex
Bi-ecotomolare
Prosthion-staphylion
Gonion-condyle of ascending ramus
Infradentale-gnathion
Pogonion-posterior symphyseal border
Superior-inferior border of corpus at M2
Mandibular corpus width at M2
Full skull
Full skull
Facial/visual
Facial/visual
Facial/visual
Basicranial
Basicranial/auditory
Neural
Neural
Neural
Palate/masticatory
Palate/masticatory
Mandible/masticatory
Mandible/masticatory
Mandible/masticatory
Mandible/masticatory
Mandible/masticatory
Fig. 4. Measurements taken on cadaveric pinnae to calculate a
height/width shape index. Drawing of Papio hamadryas outer ear.
fects of parallax (Spencer and Spencer, 1985). Care was
taken to place the plane of the tympanic ring impression
parallel to the lens of the camera. Digital images were
imported into Sigma Scan Pro 5.0 image measurement
software, calibrated, and the surface area was calculated
by tracing the perimeter (excluding the pars flaccida portion). The procedure is preferred over traditional methods
that calculate area based on two perpendicular axes because it permits deviations from strictly round outlines to
be incorporated into the estimate, allows the measurement of tympanic rings that are not fully visible externally, and creates a permanent mold that can be repeatedly measured. However, because this technique cannot
be applied to taxa with a substantial bony ear canal,
tympanic membrane estimates were not obtained for catarrhine or tarsier specimens.
The surface area of the stapedial footplate was measured by taking a digital image of the footplate and measuring the image using the same protocols outlined for
tympanic membrane molds. To increase the number of
specimens for which areal convergence ratios could be
calculated, the oval window was measured as a proxy for
the stapedial footplate whenever possible. Preliminary investigation on specimens for which both measurements
could be obtained shows a tight correlation and isometric
relationship between these two measures (r2 ⫽ 0.961;
slope ⫽ 0.996 ⫾ 0.079). Replication experiments were
carried out to evaluate the precision of these digital measurement techniques and incorporated the potential variation associated with lens-to-object angle, parallax, calibration, and measurement error. Nonsignificant
differences were found between two different measurement experiments of the same specimens for both tympanic membrane area (paired t-test, n ⫽ 54; P ⫽ 0.623)
and stapedial footplate area (paired t-test, n ⫽ 18; P ⫽
0.538).
The lengths of the malleolar and incudal lever arms
were measured using similar digital imaging and measurement techniques as described above. The ossicles
were positioned so that the axis of rotation and the lever
arms were parallel to surface of the lens. The lengths were
determined by first drawing a line representing the axis of
rotation from the short process of the incus through the
long process of the malleus and then drawing perpendicular lines from this axis to the tips of the manubrium and
long process of the incus. Although the malleus-incus complex rarely remains intact in the loose ossicles found in
museum collections, the complex saddle-shaped articular
surfaces of these bones should permit a good reconstruction of the orientations of the malleus and incus in their
natural positions relative to each other. This supposition
was verified in the following manner. First, articulated
malleus-incus pairs from specimens from the Stony Brook
Comparative Anatomical Museum were measured. Next,
the pairs were separated and then rearticulated, with the
aid of a microscope, on a piece of clay, then rearticulated
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COLEMAN AND ROSS
ossicles were measured again. It was found that the articulated and rearticulated measurements did not significantly differ (paired t-test, n ⫽ 24; P ⫽ 0.885).
To calculate the theoretical percentage of acoustic
transmission through the middle ear (T), one needs an
estimate of the specific acoustic impedance of the cochlea
in addition to the ITR. The specific acoustic impedance of
the cochlea is primarily determined by “the viscous drag of
the cochlear fluids against the walls of the scalae” (Webster and Webster, 1975) and different animals will show
some variation in cochlear impedance due to differences in
the dimensions of the scalae. However, the specific acoustic impedance of the cochlea has not been determined for
any nonhuman primates and only a few other mammalian
species. Previous studies on great apes (Masali et al.,
1992), dermopterans (Hunt and Korth, 1980), and heteromyid rodents (Webster and Webster, 1975) have used a
value of 5,600 dynes sec/cm3 as an approximation for
mammalian cochleae in general based on an estimate of
cochlear impedance derived for humans (Zwislocki, 1965).
However, subsequent measurements of the specific acoustic impedance of the inner ear suggest a value of 11,200
dynes sec/cm3 for humans (Zwislocki 1975), twice the previous estimate. In this study, a value of 5,600 dynes sec/
cm3 will be used to calculate the percentage of transmission (T1) in order to compare it to the results from previous
studies and using 11,200 dynes sec/cm3 to derive a more
accurate estimate of the theoretical percentage of acoustic
transmission through the middle ear (T2).
Fig. 5. Audiometric variables measured on each audiogram. These
include frequency and threshold of the primary peak (lowest point on the
audiogram), low-frequency cutoff (lowest audible frequency at 40 dB),
high-frequency cutoff (highest audible frequency at 60 dB), total audible
range (measured in octaves at 40 dB), total area of the audible field (area
encompassed within threshold values below 60 dB), low area (area
below 1,000 Hz), middle area (area between 1,000 and 8,000 Hz), and
high area (area above 8,000 Hz). Additional measurement information
available in text.
Audiograms
Audiograms were analyzed by measuring nine audiometric variables similar to those used in previous studies
(Masterson et al., 1969; Rosowski and Graybeal, 1991):
frequency and threshold of the primary peak, low-frequency cutoff at 40 dB, high-frequency cutoff at 60 dB,
total range in octaves at 40 dB, total area of the audible
field below 60 dB, low area, middle area, and high area
(Fig. 5). The cutoff points were established based on data
available for all audiograms. The divisions between low,
middle, and high areas were arbitrarily set at 1 and 8 kHz.
Areal measurements were made by importing the raw
audiogram data into IGOR PRO 4.04 wave measurement
software.
Analyses
Tests for taxonomic effects. Initial examination of
the data revealed obvious differences between haplorhines
or anthropoids and strepsirrhines in many of the variables. The significance of these suborder differences was
first tested using nonparametric Mann-Whitney U-tests,
considered significant at P ⱕ 0.05. However, traditional
statistical methods for the analysis of comparative data
fail to take into account the nonindependence of species/
genus means due to phylogenetic relatedness (Felsenstein, 1985), resulting in inflated type I error rates and
lowered statistical power (Garland et al., 1993). Methods
are now available for the incorporation of patterns of
phylogenetic relationship into statistical calculations of
ANOVA, ANCOVA, correlation, and regression. These
techniques are applied here using PDAP (Phenotypic Diversity Analysis Programs, version 6, 2002) (Garland et
al., 1993) and the results are compared with those obtained using traditional parametric and nonparametric
methods.
The phylogenetic relationships among the primates examined in this study are illustrated in Figure 6. The
topology and branch lengths in this tree differ slightly
from those in Ross et al. (2004) as explained in Figure 6.
The lack of complete data for all taxa meant that different
subsets of the tree were used in different analyses. These
trees were created by pruning from the tree those taxa for
which the relevant data were not available. For each tree,
the statistical adequacy of the branch lengths was tested
using the diagnostics in PDAP. No branch length transformations were necessary.
Tests for suborder effects on morphometric and audiometric data were performed using PDANOVA. The test
statistic (F ratio) was calculated using PDSINGLE (although a standard statistical package could have been
used) and the significance of this statistic was evaluated
using Monte Carlo simulations of the traits on the phylogenetic tree using PDSIMUL. Ten thousand simulations
were run under a speciational Brownian motion model.
This model assumes all evolution occurs at speciation
events, not along branches, so all branch lengths are set to
unity. We used this simulation model because the lengths
of the basal branches in Figure 6 are controversial at
present. For each variable, boundary conditions were set
to be slightly broader than the range seen in extant primates and the initial or starting value was the assumed
primitive condition. These simulations produce null distributions of F ratios that were imported into MS Excel
where the critical value of the F ratio at the 95th percentile was identified (hereafter referred to as Fphylo).
Audiometric analyses. Several problems confound a
phylogenetic analysis of the audiometric data: there are
only 4 strepsirrhines with audiograms compared with 14
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AUDITORY DIVERSITY IN PRIMATES
similarly sized species for which threshold data on several
individuals are available (⬎ 3).
Correlation analyses. The final step was to test the
association between morphometric data and audiometric
data to see if differences in auditory structures are correlated with differences in hearing performance. For this
stage of the analysis, 10 audiograms were analyzed (the
10 for which the species also have complete information on
outer ear shape and/or middle ear impedance transformer
ratios). Since this abbreviated data set has a maximum n
of nine, all correlations with a P value of ⱕ 0.10 were
considered significant because such small samples sizes
permit a more relaxed significance criterion (Olson and
Miller, 1958). These tests were conducted using both traditional Spearman’s rank order correlations and phylogenetically independent contrasts in PDTREE with branch
lengths transformed to unity. The degree to which the
correlations between form and function agree with predictions based on auditory theory is scrutinized below.
RESULTS
Outer Ear
Fig. 6. Phylogenetic tree of primates examined in this study. The
PDTREE files are available from the authors on request. Numbers are
ages of nodes in million years. Tree structure follows that used by Ross
et al. (2004), with the following changes in node dates: Microcebus
added as sister taxon to Mirza, diverging at 20 Myr (Yoder and Yang,
2004); age of Primates increased to 85 Myr; age of Anthropoidea increased to 50 Myr; age of Strepsirrhini increased to 68.5 Myr; age of
Malagasy strepsirrhine ancestral node increased to 62 Myr; age of basal
Cheirogaleidae node increased to 29 Myr; age of internal lemuriform
node decreased to 42 Myr; age of Lemuridae decreased to 32; and age
of basal Lorisiformes decreased to 40 Myr, all after Yoder and Yang
(2004). The divergence of the Eulemur clade from the Hapalemur-Lemur
clade was estimated at 28 Myr and of Hapalemur and Lemur at 17 Myr
based on Yoder and Yang (2004). Yoder and Yang (2004) do not support
the clade Indriidae, consisting of (Indri, Propithecus, Avahi), with Lepilemur as its sister taxon, so original clade dates from Ross et al. (2004)
are used. The age of the Haplorhini node was increased to 67.5 Myr,
midway between Anthropoidea (50) and Primates (85) nodes dated by
Yoder and Yang (2004). Various anthropoid taxa were also trimmed from
the tree and Nasalis was added to the Simias branch, diverging at 4.5
Myr, halfway from the stem node to the present.
anthropoids; there is limited overlap in the body sizes of
these groups; and several of the audiometric variables are
correlated with body mass [body mass data from Smith
and Jungers (1997)]. To control for these problems, a narrow allometric approach was used, considering only the
taxa that are in the same size range. This limited the
comparison to the three lorisoid and three platyrrhine
audiograms.
The audiometric data used in this study (and most other
studies) represent mean threshold values calculated
within species. These average threshold values may mask
underlying intraspecific variation. Comparative analyses
of mean audiograms would ideally include estimates of
intraspecific variability; however, such data are seldom
available. Here we present a comparison of the audiograms of Galago senegalensis and Callithrix jacchus, two
Table 2 presents the generic averages for the variables
used in the outer ear analysis. The pinna shape index (Psi)
is plotted by family/subfamily in Figure 7A. The values for
Cebus and Saimiri are plotted separately. This plot underscores the pattern observed in Figure 1 that the major
distinction in primate outer ear shape is between anthropoids and all other primates. There is a significant difference in outer ear shape between these two groups (U ⫽ 9;
P ⬍ 0.001) with anthropoids having lower index values
indicative of a pinna shape that is more symmetrical compared with the relatively tall and narrow pinnae characterizing nonanthropoids. This difference in pinna shape
index may be influenced more by differences in pinna
width than pinna height. Comparisons of small-bodied
primates suggest that nonanthropoids have narrower pinnae than anthropoids, although this difference is not quite
statistically significant at P ⱕ 0.05 (F ⫽ 3.88; P ⫽ 0.06).
Certainly, in this body size range, the values for pinna
height show almost complete overlap. The suborder-level
effects on Psi remain significant using phylogenetically
adjusted critical values of the F ratio (F ⫽ 72.18; Fphylo ⫽
44.05).
Middle Ear
Table 3 presents the generic averages for the variables
used in the middle ear analysis. The percentage of acoustic transmission (T1), calculated using the traditional cochlear impedance value of 5,600 dynes sec/cm3, is presented in Figure 8 grouped by family/subfamily. Once
again, subordinal level differences are apparent. The majority of anthropoids have transmission values between
81% and 92% while strepsirrhines show higher values
ranging from 92% to nearly 100%. The exception to this
dichotomy is the Callitrichinae, with all five genera having T1 values between 93% and 97%. Using the higher
estimate of cochlear impedance (11,200 dynes sec/cm3),
anthropoids exhibit T2 values ranging from 54% to 69%,
strepsirrhines from 68% to 92%, and callitrichines with
values between 70% and 76%. Despite the intermediate
values for callitrichines, the difference between anthropoids and strepsirrhines remains significant when evaluated using standard nonparametric statistics, regardless
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COLEMAN AND ROSS
TABLE 2. Height and width measurements in millimeters taken on primate outer ears
used to calculate a pinna shape index (Psi)*
Genus
Alouatta
Aotus
Arctocebus
Ateles
Callimico
Callithrix
Cebus
Cercocebus
Cercopithecus
Cheirogaleus
Daubentonia
Erythrocebus
Euoticus
Galago
Galagoides
Hylobates
Lagothrix
Leontopithecus
Loris
Macaca
Microcebus
Miopithecus
Nasalis
Nycticebus
Otolemur
Papio
Perodicticus
Presbytis
Pygathrix
Saguinus
Saimiri
Semnopithecus
Tarsius
Theropithecus
Trachypithecus
Varecia
Total
Pinna height (mm)
Pinna width (mm)
Pinna ratio (Psi)
33.9 ⫾ 2.33 (10)
27.6 ⫾ 1.58 (10)
25.0 (1)
33.0 ⫾ 3.62 (10)
25.5 ⫾ 0.71 (2)
24.4 ⫾ 1.41 (8)
35.0 ⫾ 2.29 (23)
33.5 ⫾ 2.12 (2)
33.1 ⫾ 3.80 (7)
17.3 ⫾ 0.29 (4)
75.1 (1)
43.7 ⫾ 4.89 (7)
25.3 ⫾ 0.28 (2)
38.8 ⫾ 2.80 (6)
24.1 ⫾ 1.34 (9)
31.2 ⫾ 2.95 (5)
29.8 ⫾ 1.89 (4)
27.0 ⫾ 0.63 (6)
23.8 ⫾ 1.50 (4)
38.1 ⫾ 3.59 (9)
22.5 ⫾ 0.71 (2)
28.8 ⫾ 1.72 (6)
34.5 ⫾ 3.00 (4)
23.0 ⫾ 2.18 (3)
46.3 ⫾ 11.2 (3)
50.1 ⫾ 4.94 (9)
24.0 (1)
32.5 ⫾ 2.12 (2)
37.4 ⫾ 2.61 (5)
19.0 ⫾ 0.82 (10)
22.0 ⫾ 2.16 (10)
48.0 (1)
30.0 ⫾ 2.00 (6)
50.2 ⫾ 6.14 (5)
38.5 ⫾ 3.87 (4)
41.0 ⫾ 1.73 (3)
32.4 ⫾ 9.24 (204)
24.5 ⫾ 2.22 (10)
21.5 ⫾ 1.27 (10)
18.0 (1)
23.9 ⫾ 1.85 (10)
20.5 ⫾ 0.71 (2)
18.6 ⫾ 0.92 (8)
25.3 ⫾ 1.96 (23)
27.5 ⫾ 0.71 (2)
27.7 ⫾ 2.06 (7)
12.8 ⫾ 0.29 (4)
46.0 (1)
33.9 ⫾ 2.54 (7)
15.7 ⫾ 1.27 (2)
25.3 ⫾ 2.30 (6)
13.89 ⫾ 0.65 (9)
26.8 ⫾ 2.59 (5)
21.3 ⫾ 0.96 (4)
22.7 ⫾ 1.03 (6)
15.8 ⫾ 2.22 (4)
29.8 ⫾ 2.59 (9)
15.0 ⫾ 1.41 (2)
22.8 ⫾ 1.60 (6)
27.0 ⫾ 2.16 (4)
13.5 ⫾ 0.87 (3)
29.8 ⫾ 5.48 (3)
38.9 ⫾ 4.23 (9)
15.0 (1)
26.0 ⫾ 1.41 (2)
30.8 ⫾ 1.48 (5)
14.7 ⫾ 0.82 (10)
20.4 ⫾ 2.22 (10)
35.0 (1)
18.4 ⫾ 1.50 (6)
35.2 ⫾ 4.44 (5)
28.8 ⫾ 2.63 (4)
24.3 ⫾ 3.06 (3)
24.0 ⫾ 6.91 (204)
1.39 ⫾ 0.097 (10)
1.29 ⫾ 0.057 (10)
1.39 (1)
1.38 ⫾ 0.129 (10)
1.25 ⫾ 0.077 (2)
1.31 ⫾ 0.091 (8)
1.39 ⫾ 0.084 (23)
1.22 ⫾ 0.109 (2)
1.19 ⫾ 0.082 (7)
1.35 ⫾ 0.008 (4)
1.63 (1)
1.29 ⫾ 0.154 (7)
1.62 ⫾ 0.113 (2)
1.54 ⫾ 0.118 (6)
1.73 ⫾ 0.068 (9)
1.16 ⫾ 0.066 (5)
1.40 ⫾ 0.123 (4)
1.19 ⫾ 0.070 (6)
1.52 ⫾ 0.148 (4)
1.28 ⫾ 0.062 (9)
1.50 ⫾ 0.095 (2)
1.27 ⫾ 0.088 (6)
1.28 ⫾ 0.065 (4)
1.70 ⫾ 0.126 (3)
1.54 ⫾ 0.083 (3)
1.29 ⫾ 0.131 (9)
1.60 (1)
1.25 ⫾ 0.014 (2)
1.21 ⫾ 0.076 (5)
1.30 ⫾ 0.086 (10)
1.08 ⫾ 0.103 (10)
1.37 (1)
1.64 ⫾ 0.160 (6)
1.43 ⫾ 0.064 (5)
1.34 ⫾ 0.036 (4)
1.70 ⫾ 0.209 (3)
1.36 ⫾ 0.182 (204)
*Error ranges represent one standard deviation and numbers in parentheses are number of specimens per genus.
of the estimate for cochlear impedance (T1: U ⫽ 12.5, P ⬍
0.001; T2: U ⫽ 12.0, P ⬍ 0.001).
The component of the ITR that is primarily responsible
for this pattern is the ossicular lever arm ratio. The areal
convergence ratios of anthropoids and strepsirrhines are
not significantly different (U ⫽ 77; P ⫽ 0.874), but the
difference in lever ratios is highly significant (U ⫽ 17; P ⬍
0.001). It is noteworthy that lever ratio is significantly
correlated (r ⫽ ⫺0.487; P ⫽ 0.012) with overall size across
all primates, and anthropoids and strepsirrhines differ
significantly in overall size. To determine whether the
suborder difference in lever ratio persists when this size
difference is taken into account, an ANCOVA was performed on lever ratio with size (GM) as a covariate. This
ANCOVA confirmed significant differences between anthropoids and strepsirrhines in lever ratio independent of
their size differences (F ⫽ 74.55; P ⬍ 0.001).
To reveal whether the difference between anthropoids
and strepsirrhines in lever ratios is due to differences in
malleolar or incudal lever arm lengths, ANCOVAs were
performed to identify suborder differences in lever arms
with size (GM) as a covariate. This revealed no differences
between suborders in incudal lever arm length, but significant differences in malleolar lever arm length (F ⫽
23.06; P ⬍ 0.001). Thus, suborder differences in ITR are
primarily driven by differences in length of the manubrium of the malleus (Fig. 9). Although areal convergence
ratio data are not available for tarsiers, the ossicular lever
arm ratio of 1.68 (derived from a single specimen) is
clearly within the anthropoid range (1.58 –1.86) and outside the range for strepsirrhines (1.83–2.34).
When suborder differences in T values are analyzed
using a phylogenetically adjusted critical value of the F
ratio (Fphylo ⫽ 32.96), the suborder differences are not
significant despite the high F ratio (F ⫽ 19.90). In contrast, the ANCOVA testing for suborder differences in
lever ratio while correcting for size remained significant
(F ⫽ 74.55), even relative to a phylogenetically adjusted
critical value of Fphylo ⫽ 33. ANCOVAs testing for suborder differences in malleus and incus lever arms with size
as a covariate were not significant.
Audiograms
Lorisoid audiograms were compared with platyrrhine
audiograms (Fig. 10A) since it was found that the mean
body mass for the species in each group (608 and 656 g,
respectively) is not significantly different (U ⫽ 4; P ⫽
0.827) and the species in one group have equally sized
AUDITORY DIVERSITY IN PRIMATES
1131
frequency sensitivity (U ⫽ 0; P ⫽ 0.016). However, the
values for threshold of the primary peak are nonsignificant, despite there being over 6 dB difference between the
means for the two species. A significant difference in highfrequency sensitivity was also detected (U ⫽ 0; P ⫽ 0.016).
However, this difference is obviously related to the restricted high-frequency limit in Callithrix (Fig. 10A) and
cannot be said to be characteristic of platyrrhine-lorisoid
comparisons in general.
Morphometrics vs. Audiometrics
Fig. 7. A: Pinna shape index values (Psi) for families/subfamilies
investigated in this study (Cebus and Saimiri plotted separately) showing
that the major differences in outer ear shape occur between anthropoids
and all other primates. Error bars represent two standard deviations. B:
Estimated pinna shape index values for selected nodes on phylogenetic
tree in Figure 6. Pinna shape index is plotted on the abscissa in the same
scale as A. The ordinate is time (Myr), from 85 Myr at the bottom to the
present at the top. Estimates are phylogenetically weighted means of the
tip data calculated in PDTREE. Error bars represent standard error of the
estimate. Lines connecting nodes are phylogenetic lines of descent.
counterparts in the other. The most obvious difference
between lorises and platyrrhines is in their low-frequency
sensitivity where there is no overlap between the species
from each group. Both low-frequency cutoff and low area
are significantly different (U ⫽ 0; P ⱕ 0.05) with platyrrhines showing an average increase in sensitivity of
around 15 dB in the lower range. Full area and middle
area as well as overall range were also found to be greater
in platyrrhines compared with lorisoids (all three: U ⫽ 0;
P ⫽ 0.05). These differences seem to be related to the
accentuated secondary peak evident in platyrrhine audiograms (local maxima around 2 kHz; Fig. 10A), but only
minimally evident in lorisoid audiograms. Threshold of
the primary peak is the remaining audiometric variable
that shows significant differences (U ⫽ 0; P ⫽ 0.05) and
indicates that platyrrhines are about 9 dB more sensitive
at the frequency of greatest sensitivity than are lorisoids.
When phylogenetically adjusted critical values of F ratios
are calculated, only the suborder differences in low area
remained significant (F ⫽ 30.40; Fphylo ⫽ 25.70).
To assess the impact of intraspecific variation on comparisons between audiograms, the audiograms for Callithrix jacchus and Galago senegalensis were compared
and are shown in Figure 10B. Although the galago data
are less complete and appear more variable than the marmoset data, Callithrix still shows significantly better low-
The results of the correlation analyses using both the
original (tip) data and independent contrasts are given in
Table 4. Using tip data, Psi shows a positive correlation
with frequency of the primary peak (best Hz; r ⫽ 0.604;
P ⫽ 0.042) but a negative correlation with low-frequency
sensitivity (low cutoff and area), total range, and total
audible area (r ⫽ 0.677, P ⫽ 0.023; r ⫽ ⫺0.736, P ⫽ 0.012;
r ⫽ ⫺0.631, P ⫽ 0.034; r ⫽ ⫺0.759, P ⫽ 0.009). Note that
an increasing value for low cutoff corresponds to decreased low-frequency sensitivity. Hence, a negative relationship is interpreted from positive correlation coefficient
values.
The correlation analyses between Psi and audiometric
variables using independent contrasts produced rather
different results from those obtained using tip data (Table
4). None of the correlations between pinna shape index
and the audiometric variables are significant at P ⬍ 0.05
or 0.10. The highest correlations are between Psi and total
range (r ⫽ ⫺0.476; P ⫽ 0.117) and total area (r ⫽ ⫺0.460;
P ⫽ 0.126), although both are just above the critical P
value.
Using tip data, there is a weak negative correlation
between T1 and low area (r ⫽ ⫺0.626; P ⫽ 0.092). Across
all primates, there is a positive correlation between T1
values and the thresholds of the primary peak (best dB;
r ⫽ 0.636; P ⫽ 0.087), i.e., as transmission percentage goes
up, so does the loudness level (dB) required to hear a
sound at the most sensitive frequency. This result is paradoxical because animals should be more sensitive as
their transmission percentage increases, not less. This
result is attributable to suborder differences. Strepsirrhines have lower sensitivity compared with anthropoids,
but highly elevated T1 values (Fig. 11), producing a positive correlation between T1 and sensitivity across all primates. However, correlation coefficients calculated within
suborders reveal the expected negative correlation between T1 and threshold of the primary peak (anthropoids,
contrasts r ⫽ ⫺0.765; strepsirrhines contrast r ⫽ ⫺0.262).
As with the correlations between Psi and audiometric
variables, none of the correlations between T1 and audiometric variables were significant when contrasts were
used. The lowest P value was produced by the negative
correlation between T1 and frequency of the primary peak
(r ⫽ ⫺0.614; P ⫽ 0.135).
DISCUSSION
Phylogenetic Effects on Primate Hearing
This study identified significant distinctions in outer ear
shape (Psi), theoretical middle ear transmission properties
(T), and hearing sensitivity (areal measures, low-frequency cutoff, threshold of the primary peak) at the subordinal level. These distinctions were highly significant by
the standards of traditional statistics, but many of these
TABLE 3. Measurements taken on middle ear structures used to calculate ITR and T values*
Genus
Alouatta
Aotus
Arctocebus
Ateles
Avahi
Brachyteles
Cacajao
Callicebus
Callimico
Callithrix
Cebuella
Cebus
Chiropotes
Daubentonia
Euoticus
Galago
Galagoides
Hapalemur
Indri
Lagothrix
Lemur
Leontopithecus
Lepilemur
Macaca
Microcebus
Miopithecus
Perodicticus
Phaner
Pithecia
Propithecus
Saguinus
Saimiri
Tarsius
Varecia
Total
Tympanic
memrbane area
(mm2)
51.5 ⫾ 5.03 (25)
25.8 ⫾ 3.11 (24)
23.7 ⫾ 2.33 (2)
56.9 ⫾ 9.13 (9)
26.2 ⫾ 0.52 (3)
49.2 (1)
32.0 ⫾ 3.26 (22)
31.0 ⫾ 3.51 (17)
24.2 ⫾ 2.33 (4)
20.0 ⫾ 1.69 (20)
14.3 ⫾ 1.01 (4)
38.6 ⫾ 4.00 (57)
32.5 ⫾ 3.00 (19)
43.1 ⫾ 2.65 (5)
17.8 ⫾ 0.21 (2)
22.1 ⫾ 1.89 (18)
23.0 ⫾ 2.18 (13)
36.0 ⫾ 2.89 (5)
46.2 ⫾ 3.66 (13)
26.9 ⫾ 2.70 (26)
23.7 ⫾ 2.01 (10)
30.5 ⫾ 3.30 (8)
13.0 ⫾ 4.27 (8)
25.3 ⫾ 1.76 (17)
Stapedial
footplate area
(mm2)
Areal
convergence
ratio
1.53 ⫾ 0.14 (20)
0.77 ⫾ 0.08 (15)
0.78 ⫾ 0.11 (2)
1.57 ⫾ 0.25 (10)
0.82 (1)
1.43 (1)
1.08 ⫾ 0.01 (2)
0.90 ⫾ 0.11 (9)
0.59 ⫾ 0.09 (3)
0.55 ⫾ 0.04 (5)
0.40 ⫾ 0.02 (2)
1.06 ⫾ 0.14 (29)
0.97 ⫾ 0.13 (5)
1.32 ⫾ 0.17 (2)
22.21
22.11
20.05
23.92
21.41
22.71
19.56
22.73
27.07
24.00
23.60
24.04
22.11
21.55
0.53 ⫾ 0.06 (4)
27.52
1.05 ⫾ 0.18 (2)
1.67 ⫾ 0.18 (2)
0.67 (1)
0.65 ⫾ 0.08 (6)
0.73 ⫾ 0.15 (5)
1.17 (1)
0.37 ⫾ 0.11 (2)
22.63
18.26
26.50
24.06
27.58
23.19
38.8 ⫾ 4.50 (20)
29.5 ⫾ 2.83 (17)
20.4 ⫾ 1.16 (13)
20.5 ⫾ 2.10 (57)
0.73 ⫾ 0.06 (8)
0.54 (1)
0.99 ⫾ 0.12 (8)
0.97 ⫾ 0.11 (2)
0.45 ⫾ 0.05 (4)
0.62 ⫾ 0.06 (14)
22.87
25.87
20.07
29.92
21.82
30.7 ⫾ 2.25 (10)
30.4 ⫾ 10.44 (449)
0.95 ⫾ 0.37 (168)
21.12 (26)
Malleus
lever
length
(mm)
Incus lever
length
(mm)
Ossicular lever
arm ratio
3.57 ⫾ 0.30
2.84 ⫾ 0.14
2.90 ⫾ 0.10
3.24 ⫾ 0.08
3.31 ⫾ 0.06
3.30
2.89 ⫾ 0.26
3.02 ⫾ 0.19
2.43 ⫾ 0.10
2.47 ⫾ 0.14
2.03 ⫾ 0.04
3.05 ⫾ 0.19
2.85 ⫾ 0.13
4.78
2.35
2.67 ⫾ 0.12
2.04
3.18 ⫾ 0.13
3.65 ⫾ 0.14
3.14 ⫾ 0.25
3.41 ⫾ 0.10
2.76 ⫾ 0.08
3.44 ⫾ 0.12
3.77 ⫾ 0.36
2.44 ⫾ 0.08
3.14
2.87 ⫾ 0.18
2.73
3.01 ⫾ 0.15
4.12 ⫾ 0.05
2.47 ⫾ 0.11
2.35 ⫾ 0.09
2.47
3.62 ⫾ 0.19
2.91 ⫾ 0.43
2.24 ⫾ 0.14
1.66 ⫾ 0.14
1.44 ⫾ 0.07
1.99 ⫾ 0.25
1.59 ⫾ 0.06
2.06
1.78 ⫾ 0.22
1.68 ⫾ 0.10
1.36 ⫾ 0.03
1.36 ⫾ 0.10
1.11 ⫾ 0.06
1.90 ⫾ 0.13
1.57 ⫾ 0.10
2.20
1.16
1.19 ⫾ 0.07
0.98
1.57 ⫾ 0.04
1.92 ⫾ 0.20
1.82 ⫾ 0.09
1.62 ⫾ 0.03
1.49 ⫾ 0.09
1.48 ⫾ 0.13
2.35 ⫾ 0.05
1.23 ⫾ 0.12
1.98
1.39 ⫾ 0.08
1.49
1.76 ⫾ 0.09
2.16 ⫾ 0.23
1.34 ⫾ 0.16
1.39 ⫾ 0.06
1.47
1.98 ⫾ 0.16
1.62 ⫾ 0.42
1.60 ⫾ 0.18 (6)
1.72 ⫾ 0.10 (23)
2.01 ⫾ 0.03 (2)
1.64 ⫾ 0.24 (2)
2.08 ⫾ 0.11 (2)
1.60 (1)
1.63 ⫾ 0.10 (12)
1.80 ⫾ 0.11 (10)
1.78 ⫾ 0.04 (2)
1.82 ⫾ 0.13 (25)
1.85 ⫾ 0.08 (3)
1.61 ⫾ 0.11 (37)
1.82 ⫾ 0.17 (4)
2.17 (1)
2.03 (1)
2.24 ⫾ 0.07 (18)
2.08 (1)
2.02 ⫾ 0.04 (3)
1.91 ⫾ 0.13 (2)
1.72 ⫾ 0.06 (4)
2.10 ⫾ 0.06 (5)
1.85 ⫾ 0.11 (7)
2.34 ⫾ 0.15 (5)
1.60 ⫾ 0.18 (3)
1.99 ⫾ 0.13 (5)
1.59 (1)
2.06 ⫾ 0.05 (8)
1.83 (1)
1.71 ⫾ 0.10 (7)
1.92 ⫾ 0.23 (2)
1.86 ⫾ 0.19 (4)
1.70 ⫾ 0.08 (8)
1.68 (1)
1.84 ⫾ 0.22 (7)
1.82 ⫾ 0.23 (223)
T1
T2
0.0178
0.0152
0.0122
0.0157
0.0109
0.0170
0.0191
0.0136
0.0116
0.0125
0.0125
0.0160
0.0137
0.0097
83.3
88.3
94.1
87.0
96.4
84.9
80.7
91.6
95.2
93.4
93.4
86.3
91.1
98.1
57.1
63.0
71.6
61.8
75.7
58.8
54.4
67.4
73.4
70.5
70.7
61.1
67.0
80.0
0.0072
99.9
89.8
0.0121
0.0183
0.0085
0.0121
0.0066
94.1
82.5
99.6
94.1
99.7
71.9
56.0
84.7
71.8
92.0
0.0108
96.6
76.2
0.0102
97.6
78.1
0.0131
0.0135
0.0108
0.0158
92.0
91.6
96.5
87.0
68.7
67.5
76.0
61.5
0.0129 (26)
92.1
70.3
ITR
*T1 is the percentage of transmission through the middle ear calculated using 5,600 dynes sec/cm3 as an estimate of the specific acoustic impedance of the inner ear
and T2 is calculated using 11,200 dynes sec/cm3. Areal convergence ratio, ITR, T1, and T2 calculated using genus means. Two-thirds correction factor applied to
tympanic membrane area before calculating areal convergence ratio. Error ranges represent one standard deviation and numbers in parentheses are number of
specimens/taxa.
AUDITORY DIVERSITY IN PRIMATES
1133
Fig. 8. Theoretical percentage of acoustic transmission through the
middle ear (T) based on ITR values derived from areal convergence and
ossicular lever arm ratios. There is a significant difference in T between
anthropoids and strepsirrhines despite the intermediate values for Callitrichinae.
Fig. 10. Audiograms of similarly sized lorisoids and platyrrhines. A:
Significant differences were found between the means for these two
groups in low-frequency sensitivity, audible area (except high area),
range, and threshold of the primary peak. B: When intraspecific variation
is considered, platyrrhines (Callithrix) still show better low-frequency
sensitivity than lorisoids (Galago), but the other audiometric traits become nonsignificant. Although a difference in high-frequency sensitivity
was detected, this appears related to the reduced high-frequency sensitivity of Callithrix and is not characteristic of platyrrhines in general.
Fig. 9. Malleus-incus pairs of two similarly sized primates (body
mass and geometric mean of skull dimensions), Varecia (left) and Cebus
(right), illustrating the relatively longer manubrium of strepsirrhines compared with anthropoids. The relative lengths of the incudal lever arms
were not found to differ between these two groups, suggesting that
malleolar lever arms are ultimately responsible for the increased ITR and
T values of strepsirrhines compared with anthropoids.
differences became nonsignificant when the degrees of
freedom were adjusted to account for phylogenetic nonindependence of the data. The loss of significance to these
suborder differences is due to the marked elevation in the
critical values of the F ratio generated by the Monte Carlo
simulations. Strictly speaking, these results suggest that
the differences between the extant members of the two
suborders are not larger than those obtained in 95% of
cases of simulated evolution of these traits on the tree in
Figure 6. We investigated the sensitivity of these results
to different assumptions of the simulations—initial or
starting values, boundary conditions, etc.—and found
them to be fairly robust. The critical values of the F ratio
generated by the simulations (Fphylo) were always much
higher than those obtained using traditional statistics.
Why do the traditional statistics and the phylogenetically
adjusted statistics give such different results?
One explanation is that many of the suborder differences arose in the basal branches of the haplorhine (or
anthropoid) and strepsirrhine clades, with little further
change along descendent branches. This is shown in Figure 7B, where the evolution of pinna shape (Psi) is reconstructed at selected primate nodes. This figure illustrates
how the distribution of pinna shapes in extant primates
(summarized in the traditional box plot in Fig. 7A) may
have come about. Little change in Psi is evident in the
lineages leading to extant strepsirrhines and in many
lineages leading to extant anthropoids. The notable exceptions are the stem lineages of extant anthropoids, leading
from the haplorhine node (at 67.5 Myr) to the basal anthropoid node (50 Myr) and from there to the platyrrhine
(25 Myr) and catarrhine (35 Myr) nodes. Most of the difference in Psi between anthropoids and other primates
arose in these three basal branches, and the phylogenetically adjusted critical values of the F ratio reflect this.
However, this example is noteworthy in that the anthropoid versus nonanthropoid difference remains significant
even when phylogenetic topology is taken into account.
Similar effects on critical values of the F ratio rendered
differences in T between anthropoids and strepsirrhines
nonsignificant.
1134
COLEMAN AND ROSS
TABLE 4. Correlation coefficients (r) and probability values (P) for correlations
between audiometric variables and Psi and T*
Psi
T
Tips
Best Hz
Best dB
Low cutoff
High cutoff
Low area
Middle area
High area
Total area
Total range
Pics
Tips
Pics
r
P
r
P
r
P
r
P
0.604
0.451
0.677
0.219
⫺0.736
⫺0.814
0.055
⫺0.759
⫺0.631
0.04
0.11
0.02
0.29
0.01
⬍ 0.01
0.44
⬍ 0.01
0.03
0.357
0.013
⫺0.139
⫺0.196
⫺0.194
⫺0.391
⫺0.321
⫺0.460
⫺0.476
0.19
0.49
0.37
0.32
0.32
0.17
0.22
0.13
0.12
0.242
0.636
0.595
0.500
⫺0.626
⫺0.436
0.124
⫺0.554
⫺0.509
0.32
0.09
0.11
0.16
0.09
0.19
0.41
0.13
0.15
⫺0.614
0.147
0.130
0.106
⫺0.108
0.092
⫺0.388
⫺0.349
⫺0.437
0.14
0.41
0.42
0.43
0.43
0.44
0.26
0.28
0.23
*Results using tip data and standardized independent contrasts are given. Tips, r and P calculated using tip data; Pics, r and
P calculated using standardized independent contrasts on tree with all branch lengths equal to 1.0. See Figure 5 for
audiometric variable descriptions.
Fig. 11. The relationship between the theoretical percentage of
transmission through the middle ear and threshold of the primary peak
taken from audiograms is in the opposite direction of that expected
when considering primates as a group, but appears to agree with theoretical expectations when compared within anthropoids and strepsirrhines.
When changes are concentrated in only two basal
branches, similar results are more likely to be obtained in
the simulations than if the changes accumulated in many
branches. In effect, the highly inflated phylogenetically
adjusted critical values of the F ratios reflect the possibility that the differences between extant members of haplorhines and strepsirrhines arose in the basal branches of
the two clades and remained relatively unchanged since
that time through either phylogenetic inertia or constraint. Another possibility is that natural selection has
acted to maintain these basal differences along all descendent lineages, producing the suborder differences characterizing the extant representatives of the two suborders.
For example, it is possible that activity cycle (i.e., diurnal or nocturnal activity) accounts for the observed difference in Psi, since all but one of the prosimians are noctur-
nal (in this study) and all but one of the anthropoids are
diurnal. ANOVA of pinna ratio by activity cycle (F ⫽
27.27) is significant using a traditional critical value of the
F ratio (P ⫽ 0.001; F ⫽ 13.3) and using the phylogenetically adjusted critical value of F (Fphylo ⫽ 27.30). This
suggests that there are effects of activity pattern on pinna
ratio independent of the effects of phylogenetic relatedness. Another possibility is that requirements for group
communication and predator-prey interactions have influenced and maintained particular pinna morphologies. To
test these hypotheses, the relationships between these
morphological and audiometric variables and various ecological variables must be explored.
The effects of phylogenetic relatedness on correlations
between audiometric and morphological variables are
more severe. None of the correlations between T or Psi and
the audiometric variables were significant when phylogenetic independent contrasts were used. The reason for this
is illustrated in Figure 12, a plot of total audible area of
the audiogram against Psi. There is a significant negative
correlation between these variables when tip data are
used (r ⫽ ⫺0.760; P ⫽ 0.02), but this relationship is highly
influenced by phylogeny and is not significant using phylogenetic contrasts (r ⫽ ⫺0.460; P ⫽ 0.21). All the strepsirrhines measured have high pinna shape indexes and
low values of total area, whereas all the anthropoids have
low pinna shape indexes and high values of total area. The
use of independent contrasts takes these phylogenetic effects into account and, in this case, suggests that the
relationship between these variables incorporates significant components of phylogenetic signal. Thus, the correlation might exist because these two variables are correlated with any of the many other features distinguishing
anthropoids and strepsirrhines, or because of phylogenetic
inertia. The data do not currently allow these possibilities
to be distinguished. The strong phyogenetic effects documented by this study suggest that test of hypotheses regarding evolution of primate hearing must take phyogenetic relationships into consideration.
Morphological Effects on Primate Hearing
The most surprising finding from this study was the
apparent lack of association between theoretical percentages of acoustic transmission (based on ITR values) and
AUDITORY DIVERSITY IN PRIMATES
Fig. 12. Scatterplot illustrating the relationship between total audible
area and pinna shape index (Psi). Although there appears to be a significant correlation between these two variables, this relationship becomes
nonsignificant when evaluated using phylogenetic independent contrasts.
measures of sensitivity taken from audiograms. When the
relationship between T and the threshold of the primary
peak is examined within suborders, the general pattern
agrees with theoretical expectations, i.e., increased acoustic transmission is associated with increased auditory sensitivity. However, when comparisons are made across primates as a whole, suborder differences in T values
confound this relationship. Compared with anthropoids,
strepsirrhines have enhanced T values but reduced auditory sensitivity (Fig. 11). The component of the ITR primarily responsible for this suborder difference in T values
is the ossicular lever arm ratio. It may be that the increased lever arm advantage of strepsirrhines provides an
increase in impedance matching performance but that
this advantage is offset by other, yet to be identified acoustical factors. One possibility is that strepsirrhines have
considerably higher cochlear impedance as a group compared with haplorhines and that using a constant value
for all primates obscures the actual (versus theoretical)
percentage of acoustic energy that makes it into the inner
ear. It is also possible the mechanical impedance of the
middle ear, determined by the mass, stiffness, and frictional resistance of the middle ear components, differs
between the two groups. Until these discrepancies are
more clearly understood, this finding should serve as a
cautionary note to researchers who use ITRs as direct
measures of hearing function in comparative studies (Lay,
1972; Hunt and Korth, 1980; Masali et al., 1992).
The advantage of using a constant value for the specific
acoustic impedance of the cochlea is that it allows the
efficiency of the middle ear to be evaluated in isolation
without influence from other auditory components. Furthermore, it permits these data to be compared directly
with other studies that used a similar approach. Within
primates, the T1 values for most ape taxa (adjusted using
a 2/3 correction factor for the effective area of the tympanic membrane) (Masali et al., 1992) are similar to other
1135
haplorhines: Pongo ⫽ 80%, Pan ⫽ 84%, and Gorilla ⫽
92%. Humans are the standout among the primates that
have been tested with T1 values around 46%. Compared
with other mammalian orders, primates have fairly typical middle ear transmission properties despite the seemingly high values (T1 range ⫽ 81% to 100%). The New
World heteromyid desert rodents were found to have T
values similar to primates falling into two primary
groups: the genera Dipodomys, Microdidodops, and Perognathus range from 94% to 100, while species of Liomys
have somewhat lower values, between 78% to 80 (Webster
and Webster, 1975). Similar to primates, differences in the
lever ratio (particularly the malleus) appear to be the
primary mechanism causing differences in ITR and ultimately T values (Webster and Webster, 1975). Among
archontan taxa, dermopterans (Cynocephalus volans)
have an average T value of 91% but scandentians (Tupaia
glis) appear less efficient with a value around 70% (Hunt
and Korth, 1980).
It is unlikely that the differences in pinna morphology
completely account for the total increase in low-frequency
sensitivity witnessed in platyrrhines compared with lorisoids (and by extension anthropoids and prosimians).
Shaw (1974) and others have found that the total contribution of all three components of the human outer ear
(pinna, concha, and ear canal) adds up to approximately a
20 dB acoustic gain between 1 and 8 kHz (centered at 2.7
kHz) with considerably less gain at higher and lower frequencies. Similar values have been reported for cats (Wiener et al., 1966) and rabbits (Fattu, 1969), while guinea
pigs show an increase of just over 10 dB (Sinyor and
Laszlo, 1973). However, the amount of acoustic amplification provided by the pinna alone is only a modest 5 dB in
humans and between 5 and 10 dB in cats, rabbits, and
guinea pigs (at the best frequency). All of these studies
indicate that the ear canal and, to a lesser degree, the
concha contribute the majority of pressure gain provided
by the outer ear. It should be kept in mind that these
pressure increases are maximal in the mid frequency
range (as defined in this study) and provide significantly
less amplification at lower frequencies. Furthermore, an
acoustic gain of 5 db, for example, provided by a single
component of the auditory system does not necessarily
result in a 5 dB increase in overall threshold values in
audiograms due to the interaction of the different auditory
components (middle ear, inner ear, as well as different
parts of the outer ear). For example, the outer ear may
produce a resonance (acoustic gain) in a frequency range
that is countered by an antiresonance (acoustic loss) produced by the middle ear, resulting in little or no net
amplification.
As pointed out above, the secondary peak seen in platyrrhine audiograms seems to be the proximate mechanism
responsible for the increase in low-frequency sensitivity.
This secondary peak may be the result of the different
middle ear cavity configurations displayed by anthropoids
and strepsirrhines. Acoustic theory suggests that dual- or
multichambered middle ear cavities will result in more
than one resonant frequency with an increase in impedance at the transition between the resonant frequencies
(Dallos, 1973; Moore, 1981), resulting in enhanced auditory reception of both high- and low-frequency sounds
(Moore, 1981; Lombard and Hetherington, 1993). This is
precisely the pattern illustrated in anthropoid audiograms: two resonant peaks separated by a trough accom-
1136
COLEMAN AND ROSS
panied by increased low-frequency sensitivity without a
significant loss of high-frequency sensitivity. However, the
problem with this generalization is that while lorisoids
resemble anthropoids in having additional pneumatic
spaces off of the tympanic cavity (e.g., medial accessory
cavity sensu, MacPhee, 1981) that may function as acoustically significant accessory chambers, lorisoids resemble
lemuroids and differ from anthropoids in their relatively
diminished low-frequency sensitivity. Research is currently being conducted to investigate the influence of primate middle ear cavity volume and different cavity configuration patterns on hearing performance.
Regardless of the specific auditory components responsible for augmenting particular aspects of hearing sensitivity, the finding that platyrrhines have significantly better low-frequency sensitivity than like-sized lorisoids
could lead to a new understanding of how these groups
have adapted to their respective ecologies and habitats.
An increase in low-frequency sensitivity could result in a
considerable expansion of the distance over which primates can adequately communicate. Brown and Waser
(1984) found that a heightened sensitivity of around 10 dB
in the low-frequency range of blue monkeys (Cercopithecus mitis) results in a four-fold increase in the audible
distance of their low-frequency boom calls. The average
difference in platyrrhine and lorisoid hearing between 250
and 1,000 Hz is 14.6 dB, suggesting that New World
monkeys have the potential to benefit from a considerable
increase in the audible distance of long calls, although the
exact propagation distances are related to particular aspects of different environments (Waser and Brown, 1986).
This study reveals that reshaping the outer ears may be
one way that anthropoids have increased their low-frequency hearing sensitivity. Although the middle ear has
also changed in morphology and presumably functionality, much future research is needed to understand fully
the exact influence that these changes have on auditory
processing and what impact they had on the evolution of
anthropoids and primates in general.
ACKNOWLEDGMENTS
The authors thank Ted Garland and Bill Jungers for
advice and assistance with statistics. They greatly appreciate the attention of John J. Rosowski, who provided
helpful discussion and detailed comments on the manuscript. Tim Smith also provided useful comments on the
manuscript. Outer ear illustrations were drawn by Colleen Lodge.
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