Vocal tract length and formant frequency dispersion correlatewith body size in rhesus macaques
W. Tecumseh Fitch
Program in Speech and Hearing Sciences and Department of Anthropology, 33 Kirkland Street,
Room 1036, Harvard University, Cambridge, Massachusetts 02138~Received 11 December 1996; accepted for publication 14 April 1997!
Body weight, length, and vocal tract length were measured for 23 rhesus macaques ~Macaca
mulatta! of various sizes using radiographs and computer graphic techniques. Linear predictive
coding analysis of tape-recorded threat vocalizations was used to determine vocal tract resonancefrequencies ~‘‘formants’’! for the same animals. A new acoustic variable is proposed, ‘‘formant
dispersion,’’ which should theoretically depend upon vocal tract length. Formant dispersion is theaveraged difference between successive formant frequencies, and was found to be closely tied toboth vocal tract length and body size. Despite the common claim that voice fundamental frequency(F 0 )provides an acoustic indication of body size, repeated investigations have failed to support sucha relationship in many vertebrate species including humans. Formant dispersion, unlike voice pitch,is proposed to be a reliable predictor of body size in macaques, and probably many other species.
© 1997 Acoustical Society of America. @S0001-4966~97!01808-0#
PACS numbers: 43.80.Ka, 43.80.Jz, 43.70.Aj, 43.64.Bt @FD#

INTRODUCTION

The acoustic characteristics of animal vocalizations havebeen postulated to provide information about many important attributes of the vocalizer, including its size, age, sex,
and reproductive status and emotional state. A particularly
important set of parameters may relate to the size of the
vocalizer. Body size is an extremely important variable inanimal physiology, mortality, and ecology ~Peters, 1983;
Schmidt-Nielsen, 1984; Harvey, 1990; Alexander, 1996!,
and in many types of animal social behavior, including mostprominently aggressive interactions ~Parker, 1974; CluttonBrock and Albon, 1979! and mating behavior ~Clutton-Brocket al., 1977!. The size of various components of the soundproduction apparatus ~e.g., the lungs, vocal folds, and vocal
tract! has an important effect on the acoustic output ~Fant,
1960; Lieberman, 1984!. Because the size of these production components may in many cases be related to the overall
weight or length of the animal, there is good reason to expectthat some aspects of the acoustic signal may provide cues tothe size of the vocalizer.
The most frequently cited acoustic parameter which
could provide a cue to body size is mean and/or lowest fundamental frequency ~Darwin, 1871!. In particular, the lowest
producible fundamental frequency of phonation (F 0min) is
determined by the length and mass of the vocal folds: thelarger the folds, the lower is F 0min . If the size of the vocal
folds were related to the vocalizer’s body size, F 0minwould
provide a good cue to body size ~Morton, 1977; Hauser,
1993!. This indeed appears to be the case in some species,
including some toads and frogs ~Martin, 1972; Davies andHalliday, 1978; Ryan, 1988!. However, such a relationshipbetween body size and vocal fold size does not seem to be
typical in other vertebrates. For instance, there is no correlation between F 0 and body size in adult humans ~ Lass andBrown, 1978; Künzel, 1989; Cohen et al., 1980; van Dommellen, 1993!, red deer ~McComb, 1991!, and other amphib1213

J. Acoust. Soc. Am. 102 (2), Pt. 1, August 1997

ian species ~Asquith and Altig, 1990; Sullivan, 1984!. This
lack of correlation in adult humans is particularly surprising
given the widespread assumption that a ‘‘deep’’ or lowpitched voice indicates large body size.
The lack of correlation between F 0 and size seems lesssurprising when the anatomy of the vocal folds is considered.
The folds are housed within the flexible cartilaginous larynx,
which itself floats at the top of a trachea and is unconstrainedin size by neighboring bony structures @the hyoid bone,
though ossified, grows as a unit with the larynx, Schön~1971! and Schneider et al. ~1967!#. Thus the larynx andvocal folds can grow independently of the rest of the head orbody, as indeed occurs in human males at puberty ~Negus,
1949; Goldstein, 1980!. At puberty, androgen receptors in
the laryngeal cartilages respond to increased circulating testosterone with a profound growth spurt ~Tuohimaa et al.,
1981; Beckford et al., 1985!. The result is a typical F 0 for
adult males which is about half that of adult females, despite
an average difference in body weights of only 20% ~Hollien,
1960!. Hypertrophy of the male larynx, out of all proportion to body size, is carried to an absurd extreme in animals
such as the howler monkey @Allouatta seniculus, Schön~1971!#, in which the larynx and hyoid together are the sizeof the entire skull, or the hammerhead bat @Hypsignathus
monstrosus, Kingdon ~1974! and Schneider et al. ~1967!#,
where the larynx of the male occupies virtually the entire
thoracic cavity.
When such developmental flexibility is present thereis clearly no a priori reason to expect vocal fold size ~andthus F 0 ! to be well-correlated with body size ~Fitch, 1994;
Fitch and Hauser, 1995!. Despite the common claim thatvoice pitch provides an accurate cue to body size ~e.g., Morton, 1977!, these data suggest that the larynx is illsuited to provide dependable cues to body size.
A different potential acoustic cue to body size derives
from the fact that, in most vertebrates, the sound signal cre-

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ated in the larynx passes through the supralaryngeal vocal
tract ~hereafter, simply ‘‘vocal tract’’! before being radiatedinto the environment. The column of air in the vocal tract hascertain natural modes of vibration or formants, which affectthe resultant output signal ~Fant, 1960! ~the terminology ofspeech scientists is adopted here, using ‘‘formants’’ as opposed to the more general term ‘‘resonances,’’ despite thefact that the sounds produced by nonhumans differ in significant ways from human speech!. If the cross-sectional areafunction of the vocal tract is constant, the primary determinant of formant frequencies is the length of the vocal tract~Fant, 1960; Lieberman and Blumstein, 1988!. In particular,a lengthening of the vocal tract tube will lead to a decrease in
the average spacing between successive formants, or ‘‘formant dispersion.’’Several researchers in speech science have postulated a
relationship between body size and formant frequencies~Fant, 1960; Nearey, 1978; Lieberman, 1984; Peterson andBarney, 1952!, and Peterson and Barney provided evidence
suggestive of such a relationship in humans ~children havehigher formant dispersion than adults, and women greaterthan men!. Because the mammalian vocal tract is made up of
the pharyngeal, oral, and nasal cavities, which are firmly
bounded by the bones of the skull, and skull size is closely
tied to overall body size ~Morita and Ohtsuki, 1973;
Dechow, 1983; Janis, 1990; Valkenburgh, 1990; Alcantara
et al., 1991; Sharma, 1990; and see data below!, vocal tract
length should be much less free to vary independently ofbody size than larynx size. Thus we can expect vocal tract
length and the attendant acoustic cue of formant dispersion
to provide a correspondingly more robust cue to body size.
Systematic investigation of the role of the supralaryngeal vocal tract in nonhuman vocalization was inaugurated
with the work of Lieberman ~Lieberman, 1968; Lieberman
et al., 1969; Lieberman, 1975; Lieberman, 1984!, who wasinterested primarily in the differences between the humanand nonhuman vocal tract. Lieberman showed that production of the full range of vowels, in particular the /i/ and /u/vowels which are ubiquitous in human language, is impossible without the ‘‘two-tube’’ vocal tract of modern humanbeings. However, Lieberman ~1968, p. 1576! also described
modifications of monkey calls via changes in vocal tract
length, and did not rule out the possibility that such changes
have communicative significance. Further suggestions of a
possible communicative role for vocal tract resonances in
nonhumans came from spectrographic analyses of baboons~Papio hamadryas!, geladas ~Theropithecus gelada!, andvervets ~Cercopithecus aethiops! ~Andrew, 1976; Richman,
1976; Seyfarth and Cheney, 1984, respectively!.
More recent work has documented the role of supralaryngeal articulation in modifying acoustic characteristics of
nonhuman vocalizations. Bauer ~1987! built on Marler andTenaza’s ~1977! technique of frame-by-frame video analysis
of vocal production in chimpanzees ~Pan troglodytes! toshow that F 0 is related to mouth-opening size during chimp
vocalizations. However, whether any causal acoustic mechanism underlies this correlation remains unclear. Hauser et al.
~1993! used video analysis to analyze rhesus macaque calls,
showing that changes in the first resonance frequency were
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J. Acoust. Soc. Am., Vol. 102, No. 2, Pt. 1, August 1997

well-correlated with changes in mandible position. A similar
result was documented in cat ~Felis domesticus! vocalizations by Shipley et al. ~1991!. Finally, Hauser and SchönYbarra ~1994! experimentally eliminated vocal tract elongation via lip movements ~using injections of the nerve-blocker
xylocaine into the peri-oral region of rhesus macaques!.
They found that resonance frequencies were significantly
higher than normal in the ‘‘coo’’ vocalization, which is normally accompanied by rounded lips. No changes were observed in F 0 or call duration. These experimental data arethus consistent with the predictions of source–filter theory~Fant, 1960!, and indicate independence of source and filterin these calls.
Overall, a wealth of data suggests that the principles ofsource–filter theory and acoustic phonetics, originally developed for human speech, are applicable to nonhuman vocalizations as well. Source and filter appear to be independent inmost cases, and movements of the articulators affect the vocal tract filter in the predicted ways. A variety of vertebrates
can use the differences in formant frequency to discriminate
synthesized vowels ~baboons: Heinz and Brady, 1988; dogs:
Baru, 1975; cats: Dewson, 1964; blackbirds and pigeons:Heinz et al., 1981!, and macaques can perceive formantswith accuracy rivaling that of humans ~Sommers et al.,1992!. However, the only conclusive evidence that supralaryngeal acoustic cues are utilized in nonhuman communication comes from Owren’s work with vervet monkeys ~Cercopithecus aethiops!. Owren and Bernacki ~1988! used linear
predictive coding ~LPC! analysis of vervet ‘‘snake’’ and‘‘eagle’’ alarm calls to isolate characteristics of source waveform, presumed vocal tract filtering functions, and temporal
patterning which distinguished these calls. Owren ~1990b!
then used an operant paradigm to test classification of synthetic calls in which each of these characteristics was modified independently. The results indicated that spectral characteristics played the dominant perceptual role indistinguishing the two call types, suggesting that supralaryngeal articulation conveys distinctive information in thesecalls.
The literature reviewed above suggests that vocal tract
length and formant frequencies may be correlated with bodysize in many vertebrate species, and that nonhumans possess
the perceptual machinery to make use of this correlation.
However, despite the fact that several researchers have suggested such a relationship ~Fant, 1960; Lieberman, 1984;
Peterson and Barney, 1952!, no study has addressed this issue directly in any species. The goal of this study was tomeasure body weight and body length, the lengths of the oral
and nasal vocal tracts, and formant frequency dispersion incalls, and to quantify the relationship~s! between these variables, using rhesus macaques as the study species.
I. METHODS
A. Study animals

The subjects were 20 healthy rhesus macaques ~Macaca
mulatta!, housed at the New England Regional Primate Research Center in Southboro, MA. Animals were between 1
and 9 years of age ~mean 4.7 yr!, and between 2.6 and 15.6
W. Tecumseh Fitch: Vocal tract length in macaques1214

FIG. 1. Schematic of anatomical features ~left! and morphometric features ~right! used in this study: G: glottis, H: hyoid bone with subhyoid air sac, OC: oralcavity, LO: lip opening, NC: nasal cavity, NO: nostril opening, T: tracheal lumen, TB: tongue body, VTL: vocal tract length.

kg in weight ~mean 7.13 kg!. Both males ~N511! and females ~N59! were studied. Animals were fed primate chow,
fresh fruit, and ad libitum water, and were maintained attheir ad libitum body weight.
B. Anatomical measures

Animals were anesthetized with 5–7 mg/kg of Telazol~a standard veterinary anesthetic mixture of tiletamine andzolazepam!, or 10 mg/kg of Ketaset ~ketamine hydrochloride!, rendering them unconscious for approximately 1/2 h.
They were weighed on a Mettler Toledo SM 30-K digitalelectronic balance to an accuracy of 610 g, and their crown–
rump length was measured to centimeter accuracy. Each animal was laid upon its side on the radiographic table, andmidsagittal radiographs ~hereafter x rays! were made of thehead and neck region ~1/40-s exposure time, 100 mA, 92–
116 kV depending on animal size!. To allow accurate determination of absolute size from the x-ray films, a 3-cm leadreference strip was placed within the area of exposure, held
above the table by a cardboard support at the animals’ midsagittal height.
X-ray clarity was sufficient to delineate the outlines ofboth the oral and nasal vocal tracts, as well as the location of
the glottis ~Fig. 1!. The location of the glottis was madeclearer by the presence of a subhyoid air sac in this species~Geist, 1933!, which opens into the larynx via a thin tube
directly above the level of the ventricular folds, and wasclearly visible in the x-ray images.
Vocal tract length ~VTL! was determined from tracingsof the x-ray images using a Wacom ArtZ UD 0608 digitizing
tablet and NIH Image software ~version 1.52, available freeon the Internet from NIH!. A curvilinear line equidistantfrom the medial and external walls of the oral or nasal vocal
tract was drawn from the middle of the glottis to the openingof the lips or external nares, respectively ~Fig. 1!. This measure of VTL is acoustically motivated: It follows the path ofplane-wave propagation of sound from the glottis to the oral
or nasal radiation site, and should on theoretical grounds be
associated with formant frequency dispersion ~Fant, 1960!.
The VTL, in pixels, was recorded along with the length of1215

J. Acoust. Soc. Am., Vol. 102, No. 2, Pt. 1, August 1997

the 3-cm lead strip, in pixels, which was later used to derive
the actual VTL in cm. The length of the entire skull from theoccipital ridge to the front of the incisors was also recorded.
The accuracy of this technique was very high: ten repeated measures of one animal’s x ray yielded standard deviations of 5 pixels or less ~1 mm or less!. Standard errors
were a fraction of a millimeter for all x-ray measures.
C. Acoustic measures

Recordings were made with a Sony WM-D3 Walkman
Professional recorder and Sony PC-62 microphone using
Maxell XL II high-bias cassette tapes. Animals were recorded in their home cages before being captured or anesthetized. Lip configuration was carefully observed during recording, because macaques, like humans, can substantiallyshorten their vocal tract by retracting the lips, or lengthen itby protruding the lips ~Hauser and Schön-Ybarra, 1994!.
Animals typically emitted aggressive ‘‘pant-threats’’ and‘‘pant-barks’’ ~Hauser et al., 1993! when faced or stared atby an unfamiliar observer. These vocalizations were madewith an almost completely closed mouth and no lip protrusion ~the ‘‘tense-mouth face’’ of van Hoof, 1967!, and thus
should be closely registered with the anatomical measure oforal vocal tract length employed in this study. Threat calls
are brief, noisy, coughlike calls ~Fig. 2 and Bercovitch et al.,
1995!, with a very broadband source, making them wellsuited for formant frequency measurement ~unlike a highfrequency tonal call, where formants are only detectable if aharmonic and formant coincide, see Lieberman and Blumstein, 1988; Ryalls and Lieberman, 1982!. Therefore although some other call types were recorded ~mainly coos!,
all acoustic analyses were performed on these threat vocalizations.
Recordings were digitized at 16 bits quantization and
22-kHz sampling rate using the built-in sound input on an
Apple Power Macintosh 6100/60, using Macromedia
SoundEdit 16 software. Formant frequencies were measured
using LPC analysis ~Markel and Gray, 1976; Wakita, 1976!,
implemented via autocorrelation in Matlab 4.2 ~using the
Matlab Signal Processing Toolbox!. LPC analysis has been
W. Tecumseh Fitch: Vocal tract length in macaques

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FIG. 2. Spectrograms of threat vocalizations from three different individual M. mulatta of increasing size. Note the decreasing formant dispersion ~from left
to right: 2.6 kg female; 5.3 kg female; 9.2 kg female! ~2048-point Hamming window FFT, 50% frame overlap, 176 Hz bandwidth!.

used to analyze similar primate vocalizations ~Owren andBernacki, 1988; Rendall, 1996!, and its applicability has
been verified by Owren ~1990a, b!.
Vocal tract length variation between monkeys should
theoretically result in a variable number of formants below
the Nyquist frequency, with short vocal tracts producing few
formants and long tracts producing many. Using a simple
quarter-resonator tube model ~Lieberman and Blumstein,1988!, a 5-cm vocal tract would have three formants below
our Nyquist frequency of 11 kHz, while a 10-cm vocal tract
would have seven. A variable number of poles was used in
the LPC analysis ~3–9; LPC order58–20!. All LPC measurements were visually verified by superimposing the LPCderived frequency response over an 512-point fast Fouriertransform ~FFT! of the same time slice, allowing the user to
empirically determine the optimum number of poles for eachcall by trial and error.
The 512-sample arrays from locations chosen interactively by the user were input to the LPC function ~no preemphasis, no weighting window!. At least five LPC spectra
were derived by analyzing successive time-slices of eachcall. Due to the noisy and random nature of the glottal sourcein pant-threats, all formants were not excited in each timeslice; therefore, the successive LPC spectra were averaged
together resulting in a single long-term averaged LPC spectrum. Typically, formant locations did not change appreciably during the course of a call; occasionally there was slight~6100 Hz! variation in a given formant frequency over thecourse of the call ~probably due to slight mouth closing or
opening!.
Animals varied greatly in the number of usable vocalizations produced, with some producing only a few and others producing dozens. However, due to noise in the animalroom from cage movements and other animals, most callrecordings obtained were unanalyzable. Fortunately, formant
frequency measurements are extremely consistent from one
call token to the next ~see Results!. Thus for most analyses,
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the single best call ~highest signal-to-noise ratio! was chosen~if there were multiple possibilities, the first one on the tape
was chosen!. A single call was used for each animal because
for many animals only a single good-quality call was available, and this avoided the statistical complications caused bymixing single measurements for some animals with meanvalues for others.
Formant dispersion (D f ), which is the average distance
between each adjacent pair of formants, was calculated using
the following formula:
Df5

( N21
i51 F i11 2F i

N21

,

~1!

where D f is the formant dispersion ~in Hz!, N is the total
number of formants measured, and F i is the frequency ~in
Hz! of formant i.
II. RESULTS

Summary data for each variable measured are given in
Table I, and a summary of all the correlations examined inthis study is given in Table II. In general, intercorrelations
were very strong between vocal tract length ~VTL!, formant
dispersion, and both measures of body size. All of the variables measured were roughly normally distributed, so the useof parametric statistics ~i.e., regression analysis! was justified.
Although males tended to be slightly larger than females, there was no significant sex difference in any anatomical or acoustic variable measured ~unpaired t tests,
p.0.05!. Therefore, unless otherwise noted all of the analyses reported here combine data from males and females.
A. Anatomical correlations

Extremely tight correlations were found among measurements of body size, skull length, and vocal tract length.
Body weight and body length were highly correlated
W. Tecumseh Fitch: Vocal tract length in macaques

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TABLE I. Basic descriptive data for acoustic and anatomical variables. ~s is the standard deviation, S.E. thestandard error of the mean, N the count, and ‘‘min’’ and ‘‘max’’ the minimum and maximum values!.

Age ~yr!
Weight ~kg!
Body length ~cm!
Skull length ~cm!
Oral VTL ~cm!
Nasal VTL ~cm!
# Formants
Formant D ~kHz!
Log10wt

Mean

s

S.E.

N

Min.

Max.

# Missing

4.667
7.128
47.417
12.204
7.850
8.915
4.800
2.146
0.792

2.590
3.774
6.576
1.558
1.290
1.348
1.399
0.700
0.243

0.610
0.844
1.550
0.348
0.289
0.301
0.313
0.157
0.054

18
20
18
20
20
20
20
20
20

1.000
2.600
35.000
9.254
5.514
6.739
3.000
1.250
0.415

9.000
15.58060.00015.0229.739
10.891
7.000
3.510
1.193

2
0
2
0
0
0
0
0
0

seven ~mean 4.8!, and as predicted by theory was correlatedwith oral vocal tract length ~r50.828, p,0.0001)and inversely correlated with formant dispersion (r520.918, p,0.0001). Because formant density is an integer value, it isless sensitive than formant dispersion and is not further used~see Sec. III!.
There were always at least three formants for any given
call. Therefore, acoustic analyses were done in two ways foreach call: including all formants present ~thus making use of
all information available for any given call!, or including
only the lowest three formants per call ~assuring equality ofanalysis between animals!. The pattern of results was identical for the two analyses, and therefore only the measure offormant dispersion defined in Eq. ~1!, which includes all
available formats, is reported.
Formant dispersion was correlated significantly andnegatively with VTL, as predicted by the source–filter theoryof vocal production ~oral VTL, r520.915, p,0.0001; nasal VTL, r520.852, p50.0001!. Given the anatomicalcorrelations described above, it is thus unsurprising that formant dispersion correlated with log10 body weight(r520.868, p,0.0001! and body length (r520.816,
p,0.0001!. Stepwise multiple regression analysis indicatedthat log10 body weight accounted for the most variance informant dispersion ~partial correlation coefficients: age
20.713, body length 20.821, log10 body weight 20.876!,
and that it alone accounted for all significant variance informant dispersion.
The formant dispersion ~D pred, in Hz! predicted by asimple one-tube model of the vocal tract ~without end correction, since no data was available on size of the oral opening! is~r50.911, p,0.0001!, as were body length and log10 bodyweight ~r50.953, p,0.0001! ~Fig. 3!. Because bodyweight should theoretically be proportional to the cube of alinear dimension, log10 body weight is used hereafter ~logarithms were not taken for all measurements because the relationships between body length, skull length, VTL, and formant dispersion appear to be linear!.
Skull length was correlated with body length(r50.974, p,0.0001) and log10 weight (r50.955,
p,0.0001!. VTL was correlated with skull length ~oral VTL,
r50.957, p,0.0001; nasal VTL, r50.927, p,0.0001!,
as was expected given that the main dimensions of the vocal
tract are delineated by the skull ~Fig. 3!. As expected due tothe intercorrelations between body size, skull length, andVTL, VTL correlated with log10 body weight ~oral VTL,
r50.947, p,0.0001; nasal VTL, r50.906, p,0.0001! andbody length ~oral VTL, r50.950, p,0.0001, Fig. 3; nasal
VTL, r50.928, p,0.0001).
B. Acoustic correlations

The number of usable pant-threat calls obtained varied
greatly among animals ~range 1–10, mode51, for four animals!. For four animals, seven or more calls were obtained,
allowing an analysis of consistency in formant locations
among the calls of a particular individual. The formant values for these monkeys are shown in Fig. 4, which illustrates
the impressive consistency of formant locations for each individual, and the substantial differences between individuals.
Therefore, the rest of the analyses were performed with only
one call per animal, as explained in the Methods section.
There was variation in the number of formants found for
different animals. The number of formants below the 11 kHz
Nyquist frequency ~‘‘formant density’’! varied from three to

TABLE II. Correlation coefficients between the various acoustic and anatomical variables measured in thisstudy. All correlations are significant at the p,0.0001 level.

1. Log10 wt
2. Skull length
3. Oral VTL
4. Nasal VTL
5. F1
6. Formant D
7. Body length

1217

1

2

3

4

5

6

7

1.000
0.942
0.944
0.904
20.72320.886
0.953

0.942
1.000
0.963
0.939
20.711
20.869
0.974

0.944
0.963
1.000
0.972
20.72420.922
0.950

0.904
0.939
0.972
1.000
20.62520.8680.928

20.72320.71120.72420.6251.000
0.699
20.70220.88620.86920.92220.8680.699
1.000
20.816

0.953
0.974
0.950
0.928
20.70220.8161.000

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W. Tecumseh Fitch: Vocal tract length in macaques

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FIG. 3. Bivariate plots illustrating intercorrelations of the base 10 logarithm of weight ~kg!, oral vocal tract length ~cm!, and formant dispersion ~kHz!.

D pred5

c
,
2L

~2!

where c is the speed of sound ~335 m/s! and L is the vocal
tract length in m. This prediction is the same for open or
closed tubes ~where the resonances are related as 2,4,6,...!, or
half-open tubes ~where the relation is 1,3,5,...!. The formant
dispersion observed did not differ significantly from that predicted by Eq. ~2! ~paired t tests, t50.63, p50.53). Thus
although the x-ray data indicate that the monkey vocal tract
is not a tube of exactly uniform diameter, use of a more
complex model of vocal tract anatomy than a simple tube ~assuggested by Shipley et al., 1991! does not seem warrantedby the current data. The good fit between predicted and actual values also provides further evidence of the usefulnessof low-order LPC for isolating and measuring spectral peaksof nonhuman vocal tracts ~Owren and Bernacki, 1988;
Owren, 1990a,b!. Because the excitation signal for the threat
vocalizations analyzed here is coughlike and noisy ~presumably generated by noise at the glottis!, it is plausible to interpret the low-order LPC spectrum as characterizing the vocal tract filter function, and thus the spectral peaks in the
LPC spectrum as formant frequencies.
These results were consistent across both ages andsexes. When only adult animals were included in the analysis~i.e., animals aged 4 years or older!, the correlation between
log10 body weight and formant dispersion dropped ~due to

FIG. 4. Consistency of formant frequency measurements across different
calls for each of four individual monkeys.
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the smaller sample size! but was still very significant statistically ~N511, r520.73, p50.008). The correlation also
held for juveniles only (N57, r520.84, p50.015). Similarly, the correlation between log10 body weight and formant
dispersion was strong in the 11 males considered separately(r520.896, p50.0002), as in the nine females separately~r520.913, p50.0006!. Thus the relationship between
body size and acoustic output described here does not result
simply from differences between adults and juveniles, or between males and females. Instead, it appears to result from a
direct anatomical relationship between body size, vocal tract
length, and formant dispersion.

III. DISCUSSION

The results reported here indicate that formant dispersion, which is determined by vocal tract length ~VTL!, isclosely correlated with body weight and length in rhesus
macaques. As predicted by source–filter acoustic theory, intandem with the tight anatomical correlations reported
above, the spacing between formants provides an accuratemeans of predicting a vocalizer’s body mass and/or length.
Formant dispersion in the threat vocalizations of rhesus
macaques thus provides an excellent potential cue to bodysize in the population of rhesus macaques studied here ~r 2
greater than 75%!. Whether rhesus macaques use this information remains unknown, but a growing body of data ~reviewed in the Introduction! strongly suggest that nonhumananimals attend to formant cues and make use of them in their
species-specific communication systems ~Dewson, 1964;
Lieberman, 1968; Baru, 1975; Andrew, 1976; Richman,
1976; Heinz et al., 1981; Seyfarth and Cheney, 1984; Heinz
and Brady, 1988; Owren and Bernacki, 1988; Owren,
1990a,b; Sommers et al., 1992; Hauser and Schön-Ybarra,
1994; Hauser et al., 1993; Fitch, 1994; Fitch and Hauser,
1995; Rendall; 1996; Owren et al., 1997!. Rendall ~1996!
suggested that one use of formant information in macaquecommunication is to provide a relatively stable indicator ofindividual identity. The data in this paper support this idea
~because formants were found to be very stable over multiple
calls!, and suggest that another potential use for formant information is to evaluate a vocalizer’s body size.
W. Tecumseh Fitch: Vocal tract length in macaques1218

A. Reliability of formant dispersion

Despite the common conception that F 0 provides a cue
to body size ~e.g., Morton, 1977!, the preponderance of evidence now suggests that F 0 does not provide an accurate
indication of size in adult humans ~Lass and Brown, 1978;
Künzel, 1989; Cohen et al., 1980; van Dommellen, 1993! orin several other animals ~McComb, 1991; Asquith and Altig,
1990; Sullivan, 1984!. I suggest that this lack of correlationis due to the relative independence between body size andlarynx size ~which determines vocal fold length and hence
lowest F 0 !. The cartilages of the larynx can grow independently of the rest of the body, and experience a hormonedependent growth spurt in many mammals ~Tuohimaa et al.,
1981; Beckford et al., 1985; Hollien, 1960! that leads to a
decoupling of body size and F 0 , particularly in males. In
contrast, vocal tract length is anatomically dependent uponskull size, which is in turn closely correlated with body sizein all mammal species which have been examined ~Morita
and Ohtsuki, 1973; Dechow, 1983; Janis, 1990; Valkenburgh, 1990; Alcantara et al., 1991; Sharma, 1990!. The results of the current study indicate that ~a! body size, skull
size, and VTL are anatomically correlated in macaques, and~b! VTL determines formant dispersion, which thus provides
an acoustic indication of body weight and mass. Hence, unlike F 0 , formant dispersion should provide a robust, reliablecue to body size in most mammals and perhaps terrestrial
vertebrates in general.
Two key factors underlie this reliability: the link between formant dispersion and VTL, and the link between
VTL and body size. Each of these are discussed in turn.
Because tract length affects the overall pattern of formant
frequencies, any given formant provides some information
regarding tract length. Thus, for example, the lowest formant(F1) could potentially be used as a cue to tract length. However, using one formant as the sole cue to VTL would entail
several problems. First, if the frequency band of the chosenformant is obscured by environmental noise or degradation,
no VTL information would be available. In contrast, formant
dispersion capitalizes on the redundancy of the formant spacing pattern, and is thus robust to the degradation of information in any one ~or even several! formants. Second, individual formants are more sensitive to changes in the vocal
tract transfer function ~as seen in human vowels and, to alesser degree, in baboon grunts: Owren et al., 1997!. As astatistical measure encompassing all formant information,formant dispersion is less sensitive to deviations in a single
formant.
Another potential cue to VTL is ‘‘formant density’’ ~thenumber of formants in a particular frequency range, e.g., inthis study the total number of formants below the 11-kHz
Nyquist frequency!, which is inversely related to formant
dispersion. The main problem with density as an acoustic
cue to tract length is that, as an integer measure, it is considerably less sensitive to changes in VTL than formant dispersion, which is as accurate as discrimination of individual
formants @formant difference limens are 1%–5% in both humans and macaques, Sommers et al. ~1992!#. Of course, itwould be possible to devise more accurate formant density
indices, but the simplicity and numerical tractability of for1219

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mant dispersion recommend it. Thus formant dispersion isaccurate, robust to environmental deterioration, and resistantto errors due to variability in individual formants.
B. Source of unreliability for vocal tract cues

When it comes to providing vocal tract information, not
all calls are equal. If the source contains energy at only afew, widely spaced frequencies ~e.g., a high-pitched tonal
call!, it will provide little information about the vocal tract
transfer function. Thus low-pitched calls provide for better
resolution of formants than do sounds with high F 0 ’s ~Ryalls
and Lieberman, 1982!. Better yet, a noisy or impulsive
source is ideally suited for accurately outlining the transfer
function ~e.g., Tartter and Braun, 1994 with whispers!. The
accuracy of formant dispersion as an acoustic cue to bodysize will thus depend on the particular type of call, withharsh, noisy or impulsive calls ~like the threat vocalizations
studied here! being ideal, and high-pitched tonal calls being
worst. As well as having clear methodological implications,this observation leads to an interesting behavioral prediction.
In contexts where accurate information about size is favored~e.g., a large animal demonstrating its size to competitors or
mates!, noisy or low-pitched calls might be expected since
they provide more accurate delineation of vocal tract cues~see also Peters, 1984!. This provides alternative grounds for
Morton’s ~1977! ‘‘motivational-structural rule’’ that low,noisy sounds accompany aggression.
Another potential source of unreliability is changes invocal tract length due to articulatory movements. Opening or
nearly closing the mouth, or ~in animals with mobile lips!
protruding or retracting the lips will alter tract length to somedegree ~around 20% in humans, Fant, 1960!, as will raisingor lowering the larynx. However, the manipulation of VTL
via lip movements does not necessarily render it ~or related
acoustic cues! unreliable. There is still a maximum VTL,
fixed by skull dimensions, which is attained when the mouth
is nearly closed and the lips are pursed. This articulatory
position is characteristic of the threat vocalizations studied
here, along with threats in a wide variety of other species~see below!. The observed correlation between lip rounding
and threat raises an interesting possibility. If listeners associate long vocal tracts with larger bodies, an animal thatelongates its vocal tract while vocalizing would maximize its
perceived body size. This suggests that aggressive vocalizations made with nearly closed mouths and protruded lips
would be more threatening, and submissive vocalizations
made with lips retraced more appeasing, and that the evolutionary origin of certain facial expressions is not purely visual, but arises also from their acoustic effects ~Ohala, 1980;
Fridlund, 1994; Fitch, 1994; Fitch and Hauser, 1995!.
Vocal tract elongation is associated with threat behavior
in many species. van Hooff ~1967; p. 18!, in his generalreview of primate facial displays, cited the ‘‘tense-mouth
face’’ in which the ‘‘mouth corners are brought forward... asa result the mouth often looks like a narrow slit.’’ This display is usually performed by a dominant animal immediately
preceding an attack, and is associated with a low-pitchedbark in at least some species ~chimpanzees and baboons!.
Similar ‘‘pouts’’ or ‘‘pucker’’ behavior is associated with
W. Tecumseh Fitch: Vocal tract length in macaques

1219

threat in other primates ~Kaufman, 1967; Fitch, 1994; Epple,
1967!, seals ~Miller, 1975!, bears ~Pruitt and Burghardt,
1977!, and in dogs and other canids ~Fox and Cohen, 1977!.
Similarly, the use of mouth corner retraction to signal
submission and absence of threat appears to be extremely
widespread in mammals. A vast number of species, including dogs, seals, ungulates, and almost all primates share asubmissive ‘‘grin’’ facial display ~van Hooff, 1967, 1972;
Miller; 1975; Fox and Cohen, 1977; Oppenheimer, 1977;
Marler and Tenaza, 1977; Gautier and Gautier, 1977;
Walther, 1977, 1984; Goodall, 1986; Preuschoft, 1992,
1995!. This expression is variously ascribed a defensive or
submissive role and is occasionally seen during play. The
human smile appears to be an example of this display, although its use has been extended into nonaggressive situations to denote an unthreatening or friendly attitude ~vanHooff, 1972; Preuschoft, 1992, 1995!. These observations
prompted Ohala ~1980! to suggest that shortening the vocal
tract in order to seem smaller would provide an acoustic~rather than visual! function for the human smile. The data inthis paper are consistent with this hypothesis.
Finally, the anatomical correlation between VTL andskull size reported here and suspected in most mammals does
not hold for all vertebrates. In birds, the sound-producing
source is the syrinx, which lies at the base of the trachea near
the lungs ~Nowicki and Marler, 1988!. Thus the trachea is an
integral part of the bird vocal tract, and the length of thetrachea must be added to the length of the oral/nasal cavities
to derive the bird’s total vocal tract length, which is not
limited by the size of the skull. The trachea ~like the larynx!
is a flexible structure, floating free from any rigid skeletal
attachments. It is thus interesting that a wide variety of birds
exhibit tracheal elongation where the trachea is looped ingreat coils within the chest or sternum ~Berndt, 1938;
Neimeier, 1979!. This bizarre character is found only in large
birds, and often only in males, suggesting that it may represent an adaptation for acoustically exaggerating body size
~Fitch, in preparation, 1994!.
A second potential exception is anatomically modern
Homo sapiens. Our species has abandoned the structural pattern typical of mammalian vocal tract anatomy. In humans
the larynx has descended from its normal position at the backof the mouth to a position deeper in the throat ~Negus, 1949;
Laitman and Crelin, 1976; Leiberman, 1984; Flügel and Rohen, 1991!, which allows the tongue body to move freely
back and forth, thus creating a wider variety of vocal tract
area functions than are possible in nonhumans ~Lieberman,
1968; Lieberman et al., 1969!. Though typically viewed as
an adaptation for articulate speech ~Lieberman, 1975, 1984!,
this speech advantage would only be gained with considerable laryngeal descent ~several cm!. Presuming that this anatomical reconfiguration proceeded gradually over evolutionary time, what selective advantage was provided by thedescent of the larynx before the conditions for improved
speech were met?
The descent of the larynx from the standard mammalian
position has the effect of elongating the vocal tract, possibly
freeing human tract length from the skeletal size constraints
described above for most mammals. Since vocal tract length
1220

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is used by human listeners as a cue to body size ~Fitch,
1994!, it is possible that the original function of the descending larynx in early hominids was to exaggerate body size.
This idea gains support from the observation that there issexual dimorphism in the degree to which the human larynx
descends ~a full vertebra lower in males than females, Senecail, 1979; Harrison, 1995!, with no accompanying advantage for males’ vowel clarity over that of females. The degree to which vocal tract length, formant dispersion, andbody size are related in humans remains an interesting matterfor further research.
C. Vocal tract cues and human speech perception

One key phenomenon in human speech perception that
may be related to the data in this paper is vocal tract normalization, which is the use of overall formant pattern to ‘‘normalize’’ the vowels of a given speaker ~Ladefoged andBroadbent, 1957; Nearey, 1978; Lieberman, 1984!. Becauseof variations in vocal tract length, the F1-F2 vowel space ofa child is quite different from that of an adult ~Peterson andBarney, 1952!. A result is that children imitating adults, or
adults listening to children, need to adjust their perception ofa speaker’s vowels to their VTL-related formant space. If amechanism for estimating body size from formant dispersion
existed in our prelinguistic ancestors, this could have provided a preadaptive basis for vocal tract normalization inhumans.
The current results joins a collection of recent studies
suggesting that the role of the supralaryngeal vocal tract innonhuman animal communication may be more prominent
than traditionally realized ~e.g., Bauer, 1987; Owren,
1990a,b; Owren et al., 1997; Shipley et al., 1991; Hauser
and Schön-Ybarra, 1994; Owren and Bernacki, 1988; Rendall, 1996!. Although formant frequency perception plays afundamental role in human speech perception ~Joos, 1948;
Lieberman and Blumstein, 1988!, the evolutionary basis forthis is poorly understood, mainly because we have so little
information about the uses to which formant frequencies
may be put in animal communication systems. The results ofthis work suggest that the use of formant frequency patterns
to gauge body size could have been a factor promoting theoriginal evolution of formant perception, a capability laterput to such extensive and sophisticated use in human language.
ACKNOWLEDGMENTS

Many thanks are due to David Lee-Parritz for the use of
the facilities at the New England Primate Center. I am very
grateful to Dr. Lee-Parritz, Barbara Brewer, Virginia Rainville, and Nirah Shomer for their invaluable help in gathering
data. Miguel Schön-Ybarra provided assistance in interpreting the x-ray data. Comments by Marc Hauser, Caroline
Henton, Nelson Kiang, Daniel Lieberman, Philip Lieberman,
Michael Owren, Ken Stevens, and an anonymous reviewer
are gratefully acknowledged. This work was supported by an
NIH Postdoctoral Fellowship to the author.
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