A vocal repertoire of Asian elephants (Elephas maximus)
and comparison of classification methods
Sharon S. Glaeser1, Holger Klinck2, David K. Mellinger2, Yao Ren3,
Patrick J. Clemins4, Michael T. Johnson3, Randy Zelick1
(1) Department of Biology, Portland State University
(2) Cooperative Institute of Marine Resources, Oregon State University and National Oceanic and
Atmospheric Administration (NOAA) Pacific Marine Environmental Laboratory
(3) Department of Electrical and Computer Engineering, Marquette University
(4) AAAS Science and Technology Policy Fellow, National Science Foundation
Steps in conducting this research
• Conducted a bibliographic research, talked to elephant experts
• Developed research questions
• Conducted reconnaissance observations, discussed data
collection options with elephant management
• Constructed an ethogram of elephant call definitions
• Designed the study
• Type of study: observational (not experimental)
• Recording media: audio, video
• Sampling rule: behavior sampling
• Recording rule: continuous recording
• Collected data
• Analyzed data
• Interpreted results and ask new questions
Original research questions v. final goals
• Original research ideas included:
• Evaluation of calls as communication signals, but this requires
examination of the response by conspecifics.
• Evaluation of vocalizations produced by males in musth.
• Original research questions required well-defined call types:
• How are calls associated with social context, behavior, perceived
emotional state, reproductive state, or an individual?
• How is variation of acoustic parameters within a call type associated
with social context, perceived emotional state, reproductive state,
individuality, or size?
• Bibliographic research showed calls were not well defined by
acoustic parameters; rather most were defined by context.
• Reconnaissance observations showed that males in this group
were not producing a vocalization unique to musth.
Contribution to basic science
• Define an acoustic repertoire of Asian elephants based
on acoustic parameters
Goals
Ecology
Acoustics
Data
Collection
• Validate structural distinction among call types using
computerized methods
• Investigate how the repertoire is used by groups and
individuals
• Provide a basis for comparing acoustic communication
among elephant species and populations
Analysis
Sharon Stanton
Distribution
Summary
Ecology
Goals
Ecology
Acoustics
Data
Collection
Analysis
• We try to offer our captive elephants an
environment that challenges them to
utilize behaviors they would exhibit in
the wild.
• Acoustic signals serve social functions,
so it is important to take into account the
differences in social structure of groups
under study and how these differences may
impact their communication.
Distribution
Summary
• And...we are encouraged when we discover our group
in Oregon utilizes an acoustic repertoire as diverse as
is found in more natural social networks in range
countries (Glaeser, 2009; Nair et al., 2009; de Silva, 2010).
Social structure of wild elephants
• Complex, fluid society
Goals
Ecology
• Multigenerational matriarchal herds of females and young with
bulls on the periphery
Acoustics
• Mature bulls live in loose association with other bulls, except
in musth when they travel alone and seek out females
Data
Collection
• Evidence that bull society may be more structured than
previously thought
Analysis
Distribution
Summary
– Young bulls seek company of mature bulls
• Society is multi-tiered, family units form bond groups or larger
clans that share home range
– Asian elephant society may demonstrate less social connectivity
at the population level, and may be less influenced by seasonal
differences in ecological conditions
Captive elephants (N. America & Europe)
• Group structure
– Females
Goals
Ecology
Acoustics
Data
Collection
Analysis
Distribution
Summary
• Natal groups + transfers
– Males
• Adult bulls separated (few exceptions)
• Adult bulls sometimes in with females
• Social introductions
– Male/female introductions
for breeding and socialization
– Female/female introductions
with change in group structure
• Temporary separations
– Breeding
– Husbandry
Domesticated elephants (SE Asia)
• Group structure
– Females
Goals
• Natal groups + transfers
– Males
Ecology
Acoustics
Data
Collection
Analysis
Distribution
Summary
• Adult bulls work in close proximity but separated on night chains
• Adult bulls separated during musth (or maybe if known to kill)
– Mahout (handler) with elephant most of the time
– Structure can be very fluid (animals bought/sold/leased)
• Social introductions
– Common with fluid structure
– Management varies
– May be restrained
• Temporary separations
– Work, loans
– husbandry
Communication modalities
• Long-distance: low frequency vocalizations, chemical
signals [1][2]
Goals
Ecology
• Short-distance: visual, tactile, auditory, and chemical
signals [3]
Acoustics
• Seismic detection and discrimination of vocalizations
[4][5]
Data
Collection
Analysis
Distribution
Summary
[1]Payne et al. 1986 [2]Poole 1999 [3]Schulte & Rasmussen 1999 [4]O’Connel-Rodwell et al. 2006,
[5]O’Connel-Rodwell et al. 2007
Process of studying sound
Record sound
(microphone responds to
sound pressure)
Create visual
representation
of sound
Measure sound
characteristics
(similar to measuring
morphological characteristics)
How do we visualize sound?
Spectrogram:
Shows frequency change with time
Shows relative intensity with color
Waveform:
Shows amplitude change with time
Produced by measuring device (mic)
Power spectrum:
Shows relative power of frequencies
Power is averaged across signal
What did we know about acoustic communication
of Asian elephants at start of study?
Goals
African elephant
Asian elephant
Calls defined
25+ [1]
(by context)
many rumbles
6 calls [1][2][10]
(by context)
Frequency range
5Hz to 9kHz [3]
14 Hz [4] to ?
Musth call
musth rumble(M) [5]
??
Individual recognition
contact call (F) [6][7]
distinct rumble (F) [8]
??
Ecology
Acoustics
Data
Collection
Analysis
Emotional state of caller unstable pitch (F) [8]
??
Increased motivation,
intention
modifications of
basic calls [2]
Distribution
Summary
call combination
in calves [9]
[1]Olson 2005 (Elephant Husbandry Guide) [2]McKay, 1973 [3]Poole in prep [4]Payne et al. 1986
[5]Poole et al. 1988 [6]McComb et al. 2003 [7]Langbauer et al. 1991 [8]Soltis et al. 2005
[9]Stoeger-Harwath and Kratochvil 2007 [10]Nair et al., 2009
What did we discover with this study?
Goals
African elephant
Asian elephant
Calls defined
25+ [1]
(by context)
many rumbles
6 basic call types
5 combinations calls
Frequency range
5Hz to 9kHz [3]
14 Hz to 18 kHz
Musth call
musth rumble(M) [5]
??
Individual recognition
contact call (F) [6][7]
distinct rumble (F) [8]
??
Ecology
Acoustics
Data
Collection
Analysis
Emotional state of caller unstable pitch (F) [8]
??
Increased motivation,
intention
modifications of
basic calls [2]
Distribution
Summary
call combination
in calves [9]
[1]Olson 2005 (Elephant Husbandry Guide) [2]McKay, 1973 [3]Poole in prep [4]Payne et al. 1986
[5]Poole et al. 1988 [6]McComb et al. 2003 [7]Langbauer et al. 1991 [8]Soltis et al. 2005
[9]Stoeger-Harwath and Kratochvil 2007
Study sites
• Oregon Zoo
(Portland, OR)
– Urban setting
Goals
Ecology
Acoustics
Data
Collection
Analysis
Distribution
Summary
• Anthropogenic noise: highway traffic, air traffic, water, electricity,
hydraulics, machinery, train, people
• Royal Elephant Kraal
(Ayutthaya, central Thailand)
– Working village
– Urban setting
• Anthropogenic noise: road traffic,
water, machinery, people
• Elephant Nature Park
(Mae Taeng Valley, northern Thailand)
– Tourist park
– Rural setting
• Anthropogenic noise: construction,
sporadic road traffic, people
Study subjects – Oregon Zoo
Pet(f) matriarch, at age 51
Packy (m)
Tusko (m)
Rama (m)
Sung-Surin (f)
Rose Tu (f)
Chendra (f)
All adults in the study
Study subjects – Thailand
• Captive elephants
• Royal Elephant Kraal
~80 elephants
• males and females of all ages
• geriatric animals
• 10 -12 calves
• Elephant Nature Park
~30 elephants
•
•
•
•
3 adult bulls
2 sub-adult males
2 calves
females of varying ages
Recording methods
• Data collection at Oregon Zoo
Goals
Ecology
Acoustics
Data
Collection
Analysis
Distribution
Summary
– February 2005 to March 2009
– Recorded continuously (typically 1 hour)
 Context: social introductions, breeding, temporary separations,
arrival of bull, euthanasia of matriarch, routine husbandry
 56 hours of data (usable data)
• Data collection in Thailand
–
–


November & December 2009
Recorded continuously (typically 1 hour)
Context: morning release to pasture, painting, night feedings, routine
6 hours of data
• Recording equipment
–
–
–

32 to 44 kHz sampling rate
M-Audio MicroTrack II (20 Hz to 20 kHz, tested to 5 Hz)
Digital video (audio extraction, 20 Hz to 20 kHz)
Oregon zoo only
• Edirol R-09 mp3 recorder (20 Hz to 40 KHz, recorded lower)
• Racal V-Store recorder, Bruel & Kjaer 4145 microphone (2.6 Hz to 10 kHz)
Caller identification
• Localizing calls
Goals
Ecology
Acoustics
Data
Collection
– Live observations only
– Depends on distance between subjects
• Visual cues for sound production
– Air blast from trunk or mouth (e.g., sand, condensation)
– Mouth wide open
– Depressed cheeks w/ squeal and squeak
Analysis
Distribution
quiet:
cheeks relaxed
Summary
squeak & squeal:
cheeks depressed
Analysis overview
Categorized calls by
listening to calls &
viewing spectrograms
(N=2791, interobserver reliability = 93%)
Measured sound
characteristics with
acoustic software
then ran statistics
Used pattern recognition
(speech recognition)
to test categories
basic call types
Basic call types
Bark
Roar
Rumble
Bark
Squeak
Squeals
Trumpet
Combinations / Modifications
• Repetition of basic call
• Combination of basic types with continuous frequency contour
Squeak train
Squeal + Squeak
Squeak + Bark
Roar + Rumble
Trumpet + Roar
Analysis overview
Categorized calls by
listening to calls &
viewing spectrograms
(N=2791, interobserver reliability = 93%)
Measured sound
characteristics with
acoustic software
then ran statistics
Used pattern recognition
(speech recognition)
to test categories
basic call types
Measuring acoustic parameters
Steps for measuring
1.
Goals
selection box
2.
Ecology
Acoustics
Data
Collection
3.
feature box
4.
Analysis
User draws selection box
liberally around signal
Osprey creates feature box
encompassing strongest
90% of energy
Osprey extracts
28 acoustic features within
feature box
Osprey extracts
signal-to-noise ratio within
selection box
Distribution
Summary
• Low frequency calls with energy near 20 Hz
– Verified fundamental frequency by measuring harmonic spacing
– Drew selection box to match fundamental in order to determine
frequency limit at low end
Acoustic parameters (examples)
Goals
Ecology
Acoustics
Data
Collection
Analysis
•
–
–
Distribution
•
Summary
Temporal features
Duration (M5) = duration of 90% of the energy
Median time (M7) = time at with 50% energy reached
Frequency features
–
–
–
–
–
Bandwidth (M6) = frequency range of 90% of energy
Median time (M11) = frequency at with 50% energy reached
Frequency of peak intensity (M20) = frequency of peak power
Cepstrum peak width (M25) = average width of peaks in power spectrum
Upsweep Mean (M27) = direction of frequency change
Analysis overview
Categorized calls by
listening to calls &
viewing spectrograms
(N=2791, interobserver reliability = 93%)
Measured sound
characteristics with
acoustic software
then ran statistics
Used pattern recognition
(speech recognition)
to test categories
basic call types
Tested call categories using pattern recognition
• Hidden Markov Model
Goals
Ecology
– Raw sound files, no noise cancellation or noise filtering
– N=1686 (basic call types, no exclusions)
• Success rate
Acoustics
Success Rate
Data
Collection
Analysis
Distribution
Summary
Quality 1
Quality 2
Quality 3
Quality 4
Overall
85.0%
76.3%
78.7%
67.7%
72.6%
Quality 1 = No overlapping sound / less degredation by noise
Quality 2 = No overlapping sound / more degredation by noise
Quality 3 = Overlapping sound / less degredation by noise
Quality 4 = Overlapping sound / more degredation by noise
• Confusion with
– Rumbles/Roars (not easy to hear difference sometimes)
– Squeals/Trumpets (easy to hear difference, but frequency is similar)
– Squeals/Rumbles (easy to hear difference, but duration is similar)
Application for pattern recognition model
• Scenario: Recorded one group, then test on a geographically and
socially disparate group
Goals
Ecology
Acoustics
Data
Collection
Analysis
Distribution
Summary
• Trained with Oregon calls, then tested with Thailand calls
• Success rate dropped to 40.8%
• Lower success could be due to difference in repertoire usage or
to variability within call types
• Could investigate similarities and differences among groups
Are the call types distinct?
• Classification tree showed overall success of 87.8%
– Potential for classifying new calls into defined types
Goals
Ecology
• PCA showed some support for grouping call types,
but with some overlap between call types
Acoustics
• HMM showed overall successful classification 72.6%
Data
Collection
Analysis
Distribution
Summary
Are the call types significantly different
based on acoustic parameters?
YES
• Global and pair-wise Analysis of Similarity (ANOSIM)
test showed significant difference among and between
all pairs of call types
Distribution of calls: by site
Thailand (N=279)
Oregon Zoo (N=2066)
Combo
13%
Goals
Ecology
Combo
3%
Rumble
6%
Roar
11%
Bark
4%
Trumpet
12%
Rumble
18%
Trumpet
37%
Roar
12%
Acoustics
Data
Collection
Analysis
Squeak
25%
Squeak
3%
Squeal
29%
Squeal
23%
Distribution
Summary
Bark
4%
• Call repertoire is the same (except Squeak)
• Call composition is different
– Number of subjects different
– Contexts are different
N=2345
excludes:
blows
calls on command
Distribution of calls: by individual
Pet
Goals
Trumpet
0%
Sung-Surin
Rumble Roar
Combo
1%
1%
3%
Bark
0%
Combo
12%
Squeak
0%
Trumpet
0%
Rose Tu
Combo
5%
Rumble
1% Roar Bark
3% 3%
Rumble
12%
Roar
9%
Trumpet
36%
Ecology
Squeal
27%
Bark
17%
Acoustics
Squeak
54%
Squeal
95%
Data
Collection
Squeal
12%
Chendra
Analysis
Distribution
Combo
3%
Squeak
9%
Tusko
Rumble
8%
Rumble
0% Roar
6%
Trumpet
31%
Bark
11%
Squeak
0%
Squeal
0%
Trumpet
2%
Combo
0%
Summary
Squeal
0%
Squeak
0%
Bark
3%
Roar
55%
Blow -tonal
81%
N=1017
Rate of calling: by social context
• Oregon Zoo
Call Rate by Context
70
60
Ecology
Acoustics
Data
Collection
Mean Call Rate/Hour
Goals
• Rate of calling
adjusted to 60 min
50
40
30
20
10
ea
th
D
Tr
an
sf
er
ep
ar
at
io
n
af
te
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ra
tio
n
eu
ni
on
R
hm
en
t
En
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ic
In
tro
du
ct
io
n
af
te
rI
nt
ro
du
ct
io
n
eu
ni
on
R
Distribution
R
Analysis
ou
ti n
e
0
Context
Summary
N=2147
excludes:
blows
calls on command
males by themselves
Rate of calling: by social context
Call Rate by Context
70
60
Ecology
Acoustics
Data
Collection
Mean Call Rate/Hour
Goals
50
40
30
20
10
Context
Summary
ea
th
D
Tr
an
sf
er
ep
ar
at
io
n
af
te
rS
Se
pa
ra
tio
n
eu
ni
on
R
hm
en
t
En
ri c
hr
ic
In
tro
du
ct
io
n
af
te
rI
nt
ro
du
ct
io
n
eu
ni
on
R
Distribution
R
Analysis
ou
ti n
e
0
Rate of calling: by social context
Call Rate by Context
70
60
Ecology
Acoustics
Data
Collection
Mean Call Rate/Hour
Goals
50
40
30
20
10
Context
Summary
ea
th
D
Tr
an
sf
er
ep
ar
at
io
n
af
te
rS
Se
pa
ra
tio
n
eu
ni
on
R
hm
en
t
En
ri c
hr
ic
In
tro
du
ct
io
n
af
te
rI
nt
ro
du
ct
io
n
eu
ni
on
R
Distribution
R
Analysis
ou
ti n
e
0
Rate of calling: by social context
Call Rate by Context
70
60
Ecology
Acoustics
Data
Collection
Mean Call Rate/Hour
Goals
50
40
30
20
10
Context
Summary
ea
th
D
Tr
an
sf
er
ep
ar
at
io
n
af
te
rS
Se
pa
ra
tio
n
eu
ni
on
R
hm
en
t
En
ri c
hr
ic
In
tro
du
ct
io
n
af
te
rI
nt
ro
du
ct
io
n
eu
ni
on
R
Distribution
R
Analysis
ou
ti n
e
0
Rate of calling: by social context
Call Rate by Context
70
60
Ecology
Acoustics
Data
Collection
Mean Call Rate/Hour
Goals
50
40
30
Surprisingly low call
rates in some
contexts
20
10
Context
Summary
ea
th
D
Tr
an
sf
er
ep
ar
at
io
n
af
te
rS
Se
pa
ra
tio
n
eu
ni
on
R
hm
en
t
En
ri c
hr
ic
In
tro
du
ct
io
n
af
te
rI
nt
ro
du
ct
io
n
eu
ni
on
R
Distribution
R
Analysis
ou
ti n
e
0
Rate of calling: by social context
Call Rate by Context
70
60
Ecology
Acoustics
Data
Collection
Mean Call Rate/Hour
Goals
50
40
30
20
10
ea
th
D
Tr
an
sf
er
ep
ar
at
io
n
af
te
rS
Se
pa
ra
tio
n
eu
ni
on
R
eu
ni
on
In
tro
du
ct
io
n
af
te
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nt
ro
du
ct
io
n
hm
en
t
En
ri c
hr
ic
R
Distribution
R
Analysis
ou
ti n
e
0
Context
Summary
Rate of calling is not a reliable indicator of arousal
OR
What we perceive as situations that warrant increased arousal do
not for certain individuals or groups
Summary of results
• 6 basic call types that are significantly different
– Bark, Roar, Rumble, Squeak, Squeal, Trumpet
Goals
Ecology
Acoustics
Data
Collection
Analysis
Distribution
Summary
• 11 acoustic parameters differentiate the basic call types
– Duration, Lower frequency, Upper frequency, Bandwidth, Median
frequency, Frequency of overall intensity, Frequency modulation,
Frequency modulation variation, Amplitude modulation, Amplitude
modulation variation, Overall entropy
• Recorded frequency range was 14 Hz to 18 KHz
– Most calls are entirely audible
– Frequency range for only 90% of signal energy 14 Hz to 9.3 kHz
• Statistical analysis and pattern recognition model
were consistent with categorization of calls by aural
perception (what we hear)
• Individuals use the repertoire differently, so we need
to be cautious in assigning functional relevance
Summary of results
Example call description: Squeak
Sounds like:
Wet finger across rubber surface, baby alligator,
audible pedestrian crossing signal
Visual cues:
Cheeks depressed, mouth open slightly or closed
measurements of 90% of energy
Lower frequency
194 – 883 Hz (median 495)
Upper frequency
797 – 3768 Hz (median 1830)
Bandwidth
301 – 3273 Hz (median 1318)
Frequency of peak intensity
537– 2024 Hz (median 775)
Duration
0.1 – 1.2 sec (median 0.2)
Frequency modulation
1.1 – 70.5 Hz (median 4.9)
Amplitude modulation
0.8 – 44.9 Hz (median 4.7)
Overall Entropy
58 – 294 Hz (median 136)
Future directions
• Compare results with those from other study groups
• Determine functional relevance of variability (conservatively)
– Associate calls with individuals, social context, behavior, and
perceived level of arousal
Thank You
Oregon Zoo elephant handlers:
Jeb Barsh, Dimas Dominguez, Jeremy Kirby,
Pat Flora, Bob Lee, Joe Sebastiani, Pam Starkey,
April Yoder
Oregon Zoo Interum Director: Mike Keele
Oregon Zoo Conservation staff:
Anne Warner, David Shepherdson, Karen Lewis
Royal Elephant Kraal, Thailand
Elephant Nature Park, Thailand
Mandy L. Cook for teaching me how to measure sound
Jean Lea Hoffheimer & Bobbi Estabrook for scoring calls
Questions