Here, you will find information on how to implement a promising, and yet little explored method for biacoustical analysis: the so called eigensound analysis developed by MacLeod, Krieger and Jones (2013) and expanded by Rocha & Romano (2021).
Since SoundShape's version 1.3.0, the function raven.to.wave allows the creation of ".wav" files based on selections made using Raven Pro, which is commonplace in bioacoustical analysis. Topic 3 below provides information on how to do it.
Eigensound is a multidisciplinary method focused on the direct comparison between homologous sounds from different species (i.e. stereotyped calls/acoustic units; Macleod et al., 2013; Rocha & Romano, 2021). It consists on applying a sampling grid over the representation of sound (i.e. spectrogram data; Figs. 1 and 2) and then translate the spectrogram into a dataset that can be analyzed similarly to coordinate sets used in Geometric Morphometrics Methods (GMM). By doing so, eigensound crosses the bridge between Bioacoustics and GMM.
Despite being well described by Macleod et al. (2013), the method
lacked a free and open platform to run the analysis. SoundShape
package was written on R platform to fill
this applicability gap. The package features functions that enable
anyone familiar with R to easily go from sound waves to principal
components analysis (PCA), using tools extracted from traditional
bioacoustics (i.e. tuneR
and seewave packages),
geometric morphometrics (i.e.
geomorph package) and
multivariate analysis (e.g.
stats
package).
Thanks for using SoundShape and enjoy your reading!
Note: Should you experience problems running any function, please feel free to report any issues here.
library(SoundShape)
# Sample data from SoundShape
data(cuvieri)
# Select acoustic unit from sample
cuvieri.cut <- seewave::cutw(cuvieri, f=44100, from = 0.05, to=0.45, output="Wave")
# 3D spectrogram
par(mfrow=c(1,2), mar=c(0,2,1,0))
threeDspectro(cuvieri.cut, flim=c(0, 2.5),
colkey=list(plot=FALSE), cex.axis=0.4, cex.lab=0.8, resfac=3)
# Semilandmarks from sampled surface
threeDspectro(cuvieri.cut, flim=c(0, 2.5), plot.type="points",
samp.grid=TRUE, x.length=70, y.length=50, main="Semilandmarks 3D",
colkey=list(plot=FALSE), cex.axis=0.4, cex.lab=0.8)
Figure 1: Graphical outputs using threeDspectro function from
SoundShape package: (left) 3D spectrogram and (right) points (i.e.
semilandmarks) sampled from 3D spectrogram data. cuvieri sample from
SoundShape package.
# Traditional oscillogram and spectrogram
par(mfrow=c(1,2), mar=c(4,4,2,1)) # view side by side
seewave::oscillo(cuvieri.cut, title="Oscillogram")
seewave::spectro(cuvieri.cut, flim=c(0, 2.5), grid=FALSE, scale=FALSE, main="Spectrogram")
Figure 2: Graphical outputs using seewave package: (left)
Oscillogram created with oscillo function and (right) 2D spectrogram
created with spectro function. cuvieri sample from SoundShape
package.
SoundShape package is currently available on R
platform through the Comprehensive R
Archive Network (CRAN)
(https://CRAN.R-project.org/package=SoundShape). Alternatively, a
development version is available from GitHub
(https://github.com/p-rocha/SoundShape).
The package can be installed using one of the following codes:
# Official version from CRAN (recommended):
install.packages("SoundShape")
# Development version from GitHub:
install.packages("devtools")
devtools::install_github("p-rocha/SoundShape")In case you wish to use and cite SoundShape package, use
citation("SoundShape").
citation("SoundShape")
#>
#> To cite package 'SoundShape' in publications use:
#>
#> Pedro Rocha (2021). SoundShape: Sound Waves Onto Morphometric Data. R
#> package version 1.1.0. https://github.com/p-rocha/SoundShape
#>
#> A BibTeX entry for LaTeX users is
#>
#> @Manual{,
#> title = {SoundShape: Sound Waves Onto Morphometric Data},
#> author = {Pedro Rocha},
#> year = {2021},
#> note = {R package version 1.1.0},
#> url = {https://github.com/p-rocha/SoundShape},
#> }In addition, we recommend citing MacLeod et al. (2013) and Rocha & Romano (2021):
-
MacLeod, N., Krieger, J., & Jones, K. E. (2013). Geometric morphometric approaches to acoustic signal analysis in mammalian biology. Hystrix, the Italian Journal of Mammalogy, 24(1), 110-125. doi: 10.4404/hystrix-24.1-6299.
-
Rocha, P. & Romano, P. (2021) The shape of sound: A new R package that crosses the bridge between Bioacoustics and Geometric Morphometrics. Methods in Ecology and Evolution, 00, 1-7. doi: 10.1111/2041-210X.13580
Since eigensound is centered around stereotyped acoustic units, the
foremost step in sound shape study is the careful definition of units
from which analysis will be conducted. However, even though several
authors argue that acoustic traits constrained by anatomical structures
(e.g. dominant frequency, pulse rate) are more conservative than those
under the direct influence of behaviour (e.g. call duration, interval
between calls), the criteria for homologous sound comparison may vary
across different animal groups (see Rocha & Romano, 2021, for examples
and more on homology between animal sounds). Therefore, we recommend a
throughout literature review before the definition of acoustic units.
Herein, we adopted a physiological definition of ‘note’, (McLister et
al., 1995; Robillard et al., 2006), which led to an unambiguous
selection of stereotyped calls from three frog species: Physalaemus
centralis, P. cuvieri and P. kroyeri (centralis, cuvieri and
kroyeri sample datas, respectively; Figs. 3 – 5).
# Samples of data from SoundShape package
data(cuvieri)
data(centralis)
data(kroyeri)
# Plot spectro from sample and highlight acoustic units
# centralis
seewave::spectro(centralis, flim = c(0, 4), wl=512, f=44100, ovlp=70, grid=FALSE)
graphics::abline(v=c(0.1, 0.8, 1.08, 1.78, 2.1, 2.8), lty=2)
Figure 3: Spectrogram image of centralis sample (SoundShape
package), containing a sequence of three stereotyped vocalizations, each
representing a comparable acoustic unit.
# cuvieri
seewave::spectro(cuvieri, flim = c(0,4), wl=512, f=44100, ovlp=70, grid=FALSE)
graphics::abline(v=c(0.05, 0.45, 0.73, 1.13, 1.47, 1.87), lty=2)
Figure 4: Spectrogram image of cuvieri sample (SoundShape
package), containing a sequence of three stereotyped vocalizations, each
representing a comparable acoustic unit.
# kroyeri
seewave::spectro(kroyeri, flim = c(0, 4), wl=512, f=44100, ovlp=70, grid=FALSE)
graphics::abline(v=c(0.16, 0.96, 1.55, 2.35, 2.9, 3.8), lty=2)
Figure 5: Spectrogram image of kroyeri sample (SoundShape
package), containing a sequence of three stereotyped vocalizations, each
representing a comparable acoustic unit.
The eigensound function (SoundShape package) focuses on the
acquisition of point coordinates (i.e. semilandmarks) from multiple
Waveform Audio File Format (WAV or
WAVE; ".wav" file extensions), with each file representing a
comparable acoustic unit (see section 1). These ".wav" files must be
stored on the same folder somewhere in your computer, which can be
created manually at your console and subsequently assigned as working
directory in R.
Alternatively, one can create the folder at e.g. the current working
directory using dir.create. Also create a subfolder to store the
upcoming outputs from eigensound function, as the following:
# Create a folder to store ".wav" files
wav.at <- file.path(getwd(), "Workflow sample")
dir.create(wav.at)
# Create subfolder to store results
store.at <- file.path(getwd(), "Workflow sample/output")
dir.create(store.at)A reasonable number of comparable acoustic units should be selected from
the sample and stored as new ".wav" files on a folder from your
console (see section 2 for folder paths). Each file should represent a
single unit selected from the original sound wave.
Since the slightest graphical change may incur in biased results (MacLeod et al., 2013; Rocha & Romano, 2021), acoustic units must be selected according to optimal signal to noise ratio (i.e. “clean” recording), and no overlapping frequencies from other individuals, species, or background noise. Editing and filtering of sound waves must be restricted to a bare minimum.
The selection can be performed on numerous softwares of acoustic
analysis outside R platform (e.g.
Audacity, Raven
Pro), or using some
functions from seewave and
tuneR packages as
exemplified below:
# Select acoustic units
cut.centralis <- seewave::cutw(centralis, f=44100, from=0, to=0.9, output = "Wave")
cut.cuvieri <- seewave::cutw(cuvieri, f=44100, from=0, to=0.9, output = "Wave")
cut.kroyeri <- seewave::cutw(kroyeri, f=44100, from=0.2, to=1.1, output = "Wave")# Export ".wav" files containing acoustic units and store on previosly created folder
writeWave(cut.cuvieri, filename = file.path(wav.at, "cut.cuvieri.wav"), extensible = FALSE)
writeWave(cut.centralis, filename = file.path(wav.at, "cut.centralis.wav"), extensible = FALSE)
writeWave(cut.kroyeri, filename = file.path(wav.at, "cut.kroyeri.wav"), extensible = FALSE)Alternatively, since SoundShape's version 1.3.0, the function raven.to.wave allows the creation of ".wav" files based on selections made using Raven Pro, which is commonplace in bioacoustical analysis. Each selection (i.e. line in table; see raven.list documentation) should represent an acoustic unit from the sample study:
###
## Using SoundShape examples:
# Export original sample ".wav" files
tuneR::writeWave(centralis, extensible = TRUE,
filename = file.path(orig.wav, "centralis.wav"))
tuneR::writeWave(cuvieri, extensible = TRUE,
filename = file.path(orig.wav, "cuvieri.wav"))
tuneR::writeWave(kroyeri, extensible = TRUE,
filename = file.path(orig.wav, "kroyeri.wav"))
# Store Raven Pro selection tables at same folder from original ".wav" files
for(i in 1:length(raven.list)){
write.table(raven.list[i], file=file.path(orig.wav, names(raven.list)[i]),
quote=FALSE, sep="\t", row.names = FALSE,
col.names = colnames(raven.list[[i]])) } # end loop
# Verify if folder has both original ".wav" files and Raven's selections
dir(orig.wav)
###
## Start here when using your own recordings
# Export a ".wav" sample for each selection made in Raven Pro
raven.to.wave(orig.wav.folder = orig.wav, wav.samples = wav.at)
# Verify samples
dir(wav.at)The semilandmarks from eigensound function (SoundShape package) are
automatically acquired, meaning that this approach does not add human
errors from LM digitalization. Despite that, the eigensound protocol
(MacLeod et al., 2013) requires some standardization to ".wav" files
that would otherwise lead to errors or biased results (e.g. sounds
overlapping the edges of a sound window; units far from the beginning of
sound window; a low signal to noise ratio; Rocha & Romano, 2021).
In light of this, Rocha & Romano (2021) proposed the following steps
that prevent errors and biased results, which are aided by the functions
align.wave and eigensound (with analysis.type = "twoDshape"), both
from SoundShape package.
First, define the sound window dimensions that encompass the whole
sample of acoustic units. These dimensions are represented by the time
(x-axis) and frequency (y-axis) limits for spectrogram images, which
are respectively defined by the tlim and flim arguments in
eigensound function.
Time limits should be based on the acoustic unit with the longest
duration within the sample, whereas frequency limits should consider the
unit with the largest frequency bandwidth. In the present sample study,
for instance, the longest units are also the ones with broader frequency
bandwidths (i.e. kroyeri sample; Fig. 6), with approximately 0.7 s
duration and highest frequencies close to 3.5 kHz. Therefore, the sound
window dimensions that encompass the whole sample can be defined with
tlim = c(0, 0.8) and flim = c(0, 4).
This can be exemplified using spectro function from seewave package:
# Spectrogram plots using standardized sound window dimensions
par(mfrow=c(2,2), mar=c(4,4,2,2))
seewave::spectro(cut.centralis, flim=c(0, 4), tlim=c(0, 0.8), main="data(centralis)",
wl=512, f=44100, ovlp=70, grid=FALSE, scale=FALSE)
seewave::spectro(cut.cuvieri, flim=c(0, 4), tlim=c(0, 0.8), main="data(cuvieri)",
wl=512, f=44100, ovlp=70, grid=FALSE, scale=FALSE)
seewave::spectro(cut.kroyeri, flim=c(0, 4), tlim=c(0, 0.8), main="data(kroyeri)",
wl=512, f=44100, ovlp=70, grid=FALSE, scale=FALSE)
Figure 6: Spectrogram images with standardized sound window dimensions.
The eigensound protocol also requires acoustic units to be placed at the beginning of a sound window before proceeding with the analysis. This ensures that variation in each semilandmark is due to energy shifts within the call, not to changes in their relative position in the sound window (MacLeod et al., 2013).
Although this arbitrary alignment could be performed on numerous
software of acoustic analysis outside R platform (e.g.
Audacity, Raven
Pro), align.wave
function (SoundShape package) provide an easy alternative to
automatically align the units at the beginning of a sound window whilst
also standardizing the durations of ".wav" files (see section 4.1),
thus preventing errors when running eigensound function.
In order to verify the alignment, run eigensound with analysis.type = "twoDshape" and plot.exp = TRUE, which will create 2D spectrogram
images and store them on the folder specified by store.at (see section
2 for folder paths), a helpful option for the verification of
appropriate alignment and sound window dimensions.
Below is the code employed for the alignment of sound units and verification of sound window dimensions:
# Place sounds at the beginning of a sound window
align.wave(wav.at=wav.at, wav.to="Aligned", time.length = 0.8)
# Verify alignment using analysis.type = "twoDshape"
eigensound(analysis.type = "twoDshape", wav.at = file.path(wav.at, "Aligned"),
store.at=store.at, plot.exp=TRUE, flim=c(0, 4), tlim=c(0, 0.8))
# Go to folder specified by store.at and check jpeg files createdIf either the alignment, or the sound window dimensions, are not ideal
(e.g. units far from the beginning of sound window; sounds overlapping
the edges of sound window), run align.wave with different values of
time.length and/or time.perc, then use eigensound to verify the
updated spectrogram outputs (see Rocha & Romano, 2021 for details).
The ideal window dimensions and the alignment of units are often
achieved after a few attempts. If this is troublesome, consider
revisiting the relative amplitude (dBlevel) as the background noise
could be interfering with align.wave (see section 4.3).
Next is the definition of a relative amplitude value (dBlevel) to be
used as background in the 3D spectrogram (MacLeod et al., 2013). This is
an iterative process that can be implemented by eigensound with
analysis.type = "twoDshape" and plot.exp = TRUE, and should lead to
spectrogram images with minimum influence from background noise (Rocha &
Romano, 2021).
In the present study sample, the curve of relative amplitude was set at
-25 dB (Fig. 5), which is expressed as an absolute value for dBlevel
arguments in SoundShape functions (i.e. dBlevel = 25).
The code below was used to create the graphs from Fig. 7:
# 2D spectrogram with curves of relative amplitude at -25 dB
par(mfrow=c(1,2), mar=c(4,4,1,1))
s.kro <- seewave::spectro(cut.kroyeri, flim=c(0, 4), tlim = c(0, 0.8),
grid=F, scale=F, f=44100, wl=512, ovlp=70, cont=TRUE,
contlevels = seq(-25, -25, 1), collevels = seq(-40, 0, 0.1))
#> This took quite a lot of time to display this graphic, you may set 'fastdisp=TRUE' for a faster, but less accurate, display
# 3D spectrogram (with a lower dBlevel for illustrative purpuses)
threeDspectro(cut.kroyeri, dBlevel=40, flim=c(0, 4), tlim=c(0, 0.8), main="",
colkey=list(plot=FALSE), cex.axis=0.4, cex.lab=0.8, resfac=2)
# Set background at -40 dB and remove -Inf values from spectrogram data
for(i in 1:length(s.kro$amp)){if(s.kro$amp[i] == -Inf |s.kro$amp[i] <= -40)
{s.kro$amp[i] <- -40}}
# Add curve of relative amplitude
plot3D::contour3D(x=s.kro$time, y=s.kro$freq, colvar=t(s.kro$amp), z=-25,
plot=T, add=T, addbox=F, col="black", lwd=1.9, nlevels=2, dDepth=0.25)
Figure 7: 2D and 3D spectrograms (left and right, respectively) with
relative amplitude contours highlighted by black lines (dBlevel = 25).
Spectrogram images from kroyeri sample.
After concluding the three steps that avoid errors and biased results
(section 4), next is the definition of sampling grid dimensions that
will be used for semilandmark acquisition (i.e. number of cells per
side; x.length and y.length arguments, eigensound function; Fig.
8).
The number of cells per side will determine the amount of sLM acquired
by eigensound. Therefore, the ideal grid should adequate the sound
sample, while also representing a dimensionality reduction from
spectrogram data (see Rocha & Romano, 2021). Herein, we opted for 70
cells on the time (x-axis, x.length = 70) and 47 cells on the
frequency (y-axis, y.length = 47), which was experimentally defined
with the aid of threeDspectro function, as exemplified below:
# Using threeDspectro to visualize sampling grid
par(mfrow=c(1,2), mar=c(1,2,1,0))
# As "surface"
threeDspectro(cut.kroyeri, samp.grid=TRUE, x.length=70, y.length=47, plot.type="surface",
dBlevel=25, flim=c(0, 4), tlim=c(0, 0.8), f=44100, wl=512, ovlp=70, main="As 'surface'",
colkey=list(plot=FALSE), cex.axis=0.4, cex.lab=0.8)
# As "points"
threeDspectro(cut.kroyeri, samp.grid=TRUE, x.length=70, y.length=47, plot.type="points",
dBlevel=25, flim=c(0, 4), tlim=c(0, 0.8), f=44100, wl=512, ovlp=70, main="As 'points'",
colkey=list(plot=FALSE), cex.axis=0.4, cex.lab=0.8)
Figure 8: Spectrogram data as (left) simplified surface, and (right)
colored semilandmarks acquired from the intersections of sampling grid
(i.e. x.length=70 and y.length=47). Spectrogram images from
kroyeri sample.
Once the three steps that avoid errors are concluded (section 4) and the
sampling grid is defined (see above), it is now possible to acquire
comparable semilandmark coordinates using eigensound function with
analysis.type = "threeDshape".
Results can be simultaneosly assigned to an R object, and/or stored as
the native file format of TPS series
(Rohlf, 2015),
a ".tps" file to be used by numerous software of geometric analysis of
shape. Here, we focus on the analysis within R platform, so the
results are assigned to the R object eig.sample, which is available
as sample data from SoundShape.
Note: eig.sample comprises all vocalizations present in the
samples of centralis, cuvieri and kroyeri, which led to three
acoustic units per species; a total of nine ".wav" files stored in the
same folder. Use help(eig.sample) and check Rocha & Romano (2021) for
details.
In the following code, eigensound is run with a logarithmic scale on
the time axis (i.e. log.scale = TRUE; see Rocha & Romano, 2021):
# Sample semilandmarks for each ".wav" file on a folder using a logarithmic sampling grid
# Export 3D graphs with semilandmarks as colored points for inspection
eig.sample <- eigensound(analysis.type="threeDshape", dBlevel=25,
f=44100, wl=512, ovlp=70, flim=c(0, 4), tlim=c(0, 0.8),
x.length=70, y.length=47, log.scale=TRUE, plot.exp=TRUE, plot.type="points",
wav.at=file.path(wav.at, "Aligned"), store.at=store.at)
# Go to folder specified by store.at and check jpeg files createdAfter employing a sampling grid to acquire semilandmarks from sound waves (section 5), the eigensound protocol proceeds to a dimensionality reduction procedure that facilitate comparison of sound shape data. Herein, we opted for a Principal Components Analysis (PCA), which allow complex sound waves to be described and plotted onto major axes (PCs) encompassing the majority of variance within the sample (MacLeod et al., 2013; Rocha & Romano, 2021).
The PCA can be performed using prcomp function (stats package), as
exemplified below:
# PCA using three-dimensional semilandmark coordinates embeeded in eig.sample
pca.eig.sample <- stats::prcomp(geomorph::two.d.array(eig.sample))
# View summary results
summary(pca.eig.sample)
#> Importance of components:
#> PC1 PC2 PC3 PC4 PC5 PC6
#> Standard deviation 125.0412 101.4575 39.39182 29.97205 17.11317 14.5307
#> Proportion of Variance 0.5407 0.3560 0.05367 0.03107 0.01013 0.0073
#> Cumulative Proportion 0.5407 0.8967 0.95041 0.98148 0.99161 0.9989
#> PC7 PC8 PC9
#> Standard deviation 4.95910 2.63948 5.762e-14
#> Proportion of Variance 0.00085 0.00024 0.000e+00
#> Cumulative Proportion 0.99976 1.00000 1.000e+00Note: At this point, consider employing a stopping rule to select which PCs should be retained as nontrivial and interpretable, and which ones should be ignored (e.g. broken stick models, vegan package) (Jackson, 1993; see Rocha & Romano, 2021 for details).
Before proceeding to the ordination of Principal Components (PCs), the
eigensound protocol also includes hypothetical sound surfaces to be
interpreted along with the ordination plots (MacLeod et al., 2013).
These surfaces are calculated using hypo.surf function and should be
interpreted along with the ordination plots, enhancing the comprehension
of how sound shape changed along with each PCs
In SoundShape package, the hypothetical sound shapes can be created
using hypo.surf function, which enables the calculation of either the
mean shape configuration from the sample (i.e. consensus shape;
Zelditch et al., 2012), or minimum and maximum deformations relative to
PCs, as exemplified below:
# Create hypothetical sound surfaces using hypo.surf
# Mean shape configuration (consensus)
hypo.surf(eig.sample, PC="mean", flim=c(0, 4), tlim=c(0, 0.8), x.length=70, y.length=47,
cex.lab=0.7, cex.axis=0.5, cex.main=1)
Figure 9: Hypothetical sound surface (acquired using hypo.surf
function) representing mean shape configuration from eig.sample sample
of data.
# Minimum and maximum deformations - Principal Component 1
hypo.surf(eig.sample, PC=1, flim=c(0, 4), tlim=c(0, 0.8), x.length=70, y.length=47,
cex.lab=0.7, cex.axis=0.5, cex.main=1)
Figure 10: Hypothetical sound surfaces (acquired using hypo.surf
function) representing minimum and maximum deformations relative to PC1
in the PCA featuring eig.sample sample of data.
# Minimum and maximum deformations - Principal Component 2
hypo.surf(eig.sample, PC=2, flim=c(0, 4), tlim=c(0, 0.8), x.length=70, y.length=47,
cex.lab=0.7, cex.axis=0.5, cex.main=1)
Figure 11: Hypothetical sound surfaces (acquired using hypo.surf
function) representing minimum and maximum deformations relative to PC2
in the PCA featuring eig.sample sample of data.
Among the benefits of employing a PCA on multivariate data is the possibility to generate ordination plots encompassing the majority of variation embedded in the sample (Fig. 12). These plots simplify description and are widely employed in exploratory data analysis, specially when one is looking for potential groups within the sample (Zelditch et al. 2012).
The ordination plot is facilitated by pca.plot function (SoundShape
package), which require the output of a PCA performed by prcomp
function (stats package) and a vector with groups to be colored.
The code chunk below exemplifies how create an ordination plot using
pca.plot:
# PCA using semilandmark coordinates
pca.eig.sample <- stats::prcomp(geomorph::two.d.array(eig.sample))
# Verify names of acoustic units from sample
dimnames(eig.sample)[[3]]
#> [1] "cut.cent1" "cut.cent2" "cut.cent3" "cut.cuv1" "cut.cuv2" "cut.cuv3"
#> [7] "cut.kro1" "cut.kro2" "cut.kro3"
# Based on those names, create factor to use as groups in subsequent ordination plot
sample.gr <- factor(c(rep("centralis", 3), rep("cuvieri", 3), rep("kroyeri", 3)))
# Ordination plot
pca.plot(pca.eig.sample, groups=sample.gr, conv.hulls=sample.gr, leg.pos="bottomright", cex=1.2)
Figure 12: Ordination plot using eig.sample data acquired from the
samples of centralis, cuvieri and kroyeri.
In order to fully comprehend how sound shape changes along the studied sample, the PCA outcome should be interpreted along with the visualization of hypothetical sound shapes (Figs. 9 – 11) and the ordination plot (Fig. 12).
The ordination plot (Fig. 12) represent 89.7% of the whole variance in our dataset, which yielded a clear structuring of units from different species. In addition, the hypothetical sound surfaces from the main axis of variation (i.e. mean shape and PCs) clearly represented the sound shapes of acoustic units employed in the study.
The higher positive values of PC1, for instance, corresponded to
acoustic units with clear harmonic structure and broad frequency
bandwidth (Fig. 10), a hypothetical sound shape remarkably similar to
three-dimensional spectrograms from kroyeri sample. Not coincidently,
the units from kroyeri scored high positive PC1 values. Lower and
negative values of PC1, on the other hand, were less obvious, with a
hypothetical shape that gather sonic information from cuvieri and
centralis samples, both species with negative PC1 scores. A similar
pattern is observed in PC2 axis (Fig. 11), with positive PC2 values
referring to broad frequency bandwidth and no harmonic structure (i.e.
centralis sample), and negative PC2 values representing short
durations and clear harmonic structure (i.e. cuvieri sample).
Jackson, D. A. (1993). Stopping rules in Principal Components Analysis: A comparison of heuristical and statistical approaches. Ecology, 74(8), 2204-2214.
MacLeod, N., Krieger, J. & Jones, K. E. (2013). Geometric morphometric approaches to acoustic signal analysis in mammalian biology. Hystrix, the Italian Journal of Mammalogy, 24(1), 110-125. doi: 10.4404/hystrix-24.1-6299
McLister, J. D., Stevens, E. D., & Bogart, J. P. (1995). Comparative contractile dynamics of calling and locomotor muscles in three hylid frogs. The Journal of Experimental Biology, 198, 1527-1538.
Robillard, T., Hobel, G., & Gerhardt, H. C. (2006). Evolution of advertisement signals in North American hylid frogs: vocalizations as end-products of calling behavior. Cladistics, 22(6), 533-545. doi: 10.1111/j.1096-0031.2006.00118.x
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