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Abstract

Previous work suggested that titi monkeys Callicebus nigrifrons combine two alarm calls, the A- and B-calls, to communicate about predator type and location. To explore how listeners process these sequences, we recorded alarm call sequences of six free-ranging groups exposed to terrestrial and aerial predator models, placed on the ground or in the canopy, and used multimodel inference to assess the information encoded in the sequences. We then carried out playback experiments to identify the features used by listeners to react to the available information. Results indicated that information about predator type and location were encoded by the proportion of B-call pairs relative to all call pairs of the sequence (i.e., proportion of BB-grams). The results suggest that the meaning of the sequence is not conveyed in a categorical but probabilistic manner. We discuss the implications of these findings for current theories of animal communication and language evolution.
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ORGANISMAL BIOLOGY
Titi monkeys combine alarm calls to create
probabilistic meaning
Mélissa Berthet1,2*, Geoffrey Mesbahi1, Aude Pajot1, Cristiane Cäsar3,4,5,
Christof Neumann1,6†, Klaus Zuberbühler1,3†
Previous work suggested that titi monkeys Callicebus nigrifrons combine two alarm calls, the A- and B-calls, to
communicate about predator type and location. To explore how listeners process these sequences, we recorded
alarm call sequences of six free-ranging groups exposed to terrestrial and aerial predator models, placed on the
ground or in the canopy, and used multimodel inference to assess the information encoded in the sequences. We
then carried out playback experiments to identify the features used by listeners to react to the available information.
Results indicated that information about predator type and location were encoded by the proportion of B-call
pairs relative to all call pairs of the sequence (i.e., proportion of BB-grams). The results suggest that the meaning
of the sequence is not conveyed in a categorical but probabilistic manner. We discuss the implications of these
findings for current theories of animal communication and language evolution.
INTRODUCTION
One reason to study animal signals is to understand how linguistic
reference has evolved. One relevant question is whether animals
can use parts of their signal repertoire to refer to external events.
Pioneering evidence has been provided by fieldwork with vervet
monkeys (1), which triggered an important debate about whether
animal signals really refer to external events or whether they are
mere reflections of some unspecified internal states, elicited by the
external events. This debate partly originates from the fact that little
is known about whether or how animals represent the external
world as mental concepts and whether this differs from the way humans
do (2). More recently, an additional complexity has been added to
the debate, due to the fact that some animal signals are organized
sequentially (3), providing a further potential source of information
based on the combinatorial properties of signal sequences.
Black-fronted titi monkeys Callicebus nigrifrons have contributed
to this literature because adults produce two alarm calls, the A- and
B-calls (fig. S1), which can be combined into complex sequences. A
previous study (4) suggested that alarm call sequences varied not only
with predator type (A-calls were mainly given to aerial predators,
while B-calls were given to a large set of disturbances, including terres-
trial predators) but also with predator location: When aerial predators
were on the ground, B-calls were interspersed within the A-call sequences.
When terrestrial predators were detected in the canopy, B-call sequences
were always introduced by a single A-call. However, this study was
based on a small sample size and investigated a few encoding mechanisms,
and there was no experimental evidence that the encoded dual in-
formation (predator type and location) was perceived by receivers.
In the present study, we were interested in how titi monkeys
produced and perceived information in their alarm sequences. To this
end, we carried out systematic predator model presentations following
a 2 × 2 design (two predator types crossed with two locations) and
playback experiments (four types of response sequences) with observer-
habituated wild titi monkeys. We analyzed the alarm call sequences
given in response to experimental stimuli by extracting 15 quantitative
variables (referred to as “sequence metrics”; see table S1 and Materials
and Methods) and assessed what information was conveyed by these
metrics using multimodel inference. We compared behavioral re-
sponses to the broadcasting of different call sequences to determine
which information and sequence metrics titi monkeys attended to.
RESULTS
What do alarm sequences encode?
In the first experiment, we presented models of two predator types
(terrestrial and aerial predators) placed in two different locations
(on the ground and in the canopy) to 34 individuals from six groups of
monkeys. We obtained n=50 alarm call responses and characterized
each sequence by 15 different sequence metrics. We used multimodel
inference (5) to investigate whether each metric conveyed information
about predator type and/or location. We used model weights (w),
derived from Akaike’s information criterion (6), which represent
the probability that each hypothesis (i.e., each predator type and
location combination) is best supported by each metric, ranging
from 0 (weak support) to 1 (strong support).
We found that several metrics encoded for predator type
(Fig.1,AandD,b), predator type combined with location (i.e.,
predator location acting in the same way for aerial and terrestrial
predators; Fig.1,AandD,c) and the interaction between predator
type and location (i.e., predator location acting in different ways for
aerial and terrestrial predators; Fig.1, AandD,d). No metric
encoded for location only, and for several metrics, the null models had
the highest weights (Fig.1A). Overall, these results suggest that titi
monkeys mainly encode predator type with added or interactional
information about predator location.
What information do monkeys attend to?
In a second experiment, we played back alarm call sequences of titi
monkeys (n=28 trials on 14 individuals), originally given in
1Institute of Biology, University of Neuchâtel, Neuchâtel, Switzerland. 2Institut Jean
Nicod, Département d’études cognitives, ENS, EHESS, CNRS, PSL Research University,
Paris, France. 3School of Psychology and Neuroscience, University of St Andrews,
St Andrews, UK. 4Natural Sciences Museum PUC Minas, Belo Horizonte, Brazil. 5Bicho
do Mato Research Institute, Belo Horizonte, Brazil. 6Laboratoire de sciences cognitives
et de psycholinguistique, Département d’études cognitives, ENS, EHESS, CNRS, PSL
University, Paris, France.
*Corresponding author. Email: melissa.berthet.ac@gmail.com
†The se auth ors contributed equally to this work.
Copyright © 2019
The Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. No claim to
original U.S. Government
Works. Distributed
under a Creative
Commons Attribution
NonCommercial
License 4.0 (CC BY-NC).
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response to natural or experimental predator encounters. Again, we
used multimodel inference to investigate whether gaze direction of
listeners was influenced by the origin of the sequence (i.e., sequence
given to a terrestrial predator on the ground, a terrestrial predator
in the canopy, an aerial predator on the ground, or an aerial predator
in the canopy).
We found that monkeys attended most to information about
predator type and location (model with main effects for type and
location included, w=0.86; Fig.1B) and less to information about
predator type only (w=0.13). The remaining models representing
information about the interaction between predator type and location,
predator location only, or no information about predator type or
Fig. 1. Results of the multimodel inference analyses. Circle colors in (A) to (C) refer to the Akaike’s weight, i.e., the probability that a given model supports the hypothesis
(white: w = 0, weak support; red: w = 1, strong support; n.c.: the model did not converge). (A) Information encoded in titi monkey alarm sequences: Metrics are presented
row-wise, and information hypotheses are presented column-wise. For simplicity, the null and urgency models were combined as “control,” and their weights were added.
For the metric “probability that first call is A,” models that addressed the possibility that predator type and location were encoded are not relevant because the first call
can only be one of two possibilities and, thus, can only provide information about predatory type or location. (B) Gaze reaction of titi monkeys to the information
contained within the playback stimuli sequences, i.e., the original condition during which broadcasted sequences were recorded. For a graphic representation of the best
model (interaction between predator type and location), see Fig. 2. (C) Gaze reaction of the titi monkey to the metrics extracted from the playback stimuli sequences. For
a graphic representation of the best model (proportion of BB-grams), see Fig. 4. (D) Illustration of sequence metrics that support each hypothesis. Letters refer to the
corresponding model weights in (A). (E) Illustration of experimental design of the predator presentations.
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location (null and urgency models) had a combined weight of 0.01
(Fig.1B).
We then analyzed how the origin of the broadcasted sequence
influenced the gaze reaction of the subjects. When hearing a sequence
recorded from an encounter with an aerial predator, titi monkeys
looked more upward and less toward the speaker than when the
sequence was recorded from an encounter with a terrestrial predator
(Fig.2 and fig. S2). In addition, sequences recorded from encounters
with predators in the canopy elicited more gazing upward and less
toward the speaker than sequences recorded from predators on the
ground (Fig.2 and fig. S2). Looking upward is an appropriate
response when expecting an aerial predator (that is usually in the air
or in the canopy) or a predator located within the canopy. Looking
toward the speaker is appropriate when expecting a terrestrial predator
or a predator on the ground: Because of the density of the lower
strata of the forest, spotting a predator on the ground can be difficult,
and looking at the caller’s behavior and gaze direction can provide cues
about the exact location of the threat. Overall, these playback results
suggest that titi monkeys can extract information about both predator
type and location in an additive fashion from alarm sequences.
What sequential metrics do monkeys attend to?
In a final analysis, we assessed how the metrics characterizing the
different call sequences used as playback stimuli affected the time
listeners spent looking in predator-relevant directions, also using a
multimodel inference approach. Here, we ignored the information
content of the sequences (i.e., their origin) to focus on their sequence
features only.
Model weights indicated that listeners reacted strongly to the
proportion of BB-grams, i.e., the proportion of two contiguous B-calls
among all the contiguous pairs of calls of the sequence (w=0.79),
and somewhat to the proportion of A-calls (w=0.17) (Fig.1C). All
other models (including the null model) had a combined weight
of 0.03 (Fig.1C). Further inspection of the metric revealed that
the proportion of BB-grams was substantially lower in sequences
elicited by aerial predators than by terrestrial predators (Fig.3).
In addition, the proportion of BB-grams was slightly lower in
sequences elicited by predators in the canopy than by predators on
the ground (Fig.3).
With regards to reactions toward playbacks, as the proportion of
BB-grams in a sequence increased, listeners spent increasingly more
time looking toward the speaker and increasingly less time looking
upward, indicating that they expected more a terrestrial predator
than an aerial predator and/or more a predator on the ground than
in the canopy (Fig.4). Thus, the playback results suggested that titi
monkeys attended to the proportion of BB-grams to extract infor-
mation about the predator type and location.
DISCUSSION
Our analysis shows that titi monkeys encode information about
both predator type and location in their alarm sequences, albeit
in ways that, to our knowledge, have not yet been described. Preda-
tor type and location were redundantly encoded by several sequence
features, but none of the sequence metrics we investigated encoded
for predator location only (Fig.1A). To test whether recipients
were able to attend to the information conveyed by these sequences,
we carried out a playback experiment, with results showing that
titi monkeys appeared to attend to the proportion of BB-grams
(Figs.1C and 4), i.e., the proportion of two contiguous B-calls,
among all the contiguous pairs of calls of the sequence, that
provided them with information of both predator type and location
(Figs.1A,1B,2, and 3). The proportion of BB-grams mainly encoded
predator type and less predator location (Fig.3), but our playbacks
suggested that receivers were able to extract both information
(Figs.2 and 4).
Proportion of time looking upward
Aerial
ground
Aerial
canopy
Terrestrial
ground
Te rrestrial
canopy
0
0.2
0.4
0.6
0.8
1
Fig. 2. Proportion of time the listener spent looking upward across original
recording conditions of the playback stimuli. The figure shows raw data (one line
per individual), as well as estimates per condition (black circles) and bootstrapped
estimates (colored circles, 1000 bootstraps) of the model testing how gaze reaction
depends on both predator type and location (main effects). Subjects looked more
upward when they were presented with sequences elicited by an aerial predator
(compared to a terrestrial predator) or elicited by a predator in the canopy (as
opposed to a predator on the ground). For simplicity, we displayed the most
salient reaction, i.e., looking upward. Results for other looking directions can be
found in fig. S2.
Proportion of BB-grams
Aerial
ground
Aerial
canopy
Te rrestrial
ground
Te rrestrial
canopy
0
0.2
0.4
0.6
0.8
1
Fig. 3. Proportion of BB-grams in the alarm call response depending on the
eliciting stimulus. The figure shows estimates (black circles) and bootstrapped
estimates per condition (colored circles, 1000 bootstraps) of the model testing
how the proportion of BB-grams encodes both predator type and location (main
effects). The proportion of BB-grams is higher in vocal responses to terrestrial predator
than to aerial predators and higher when the predator is on the ground than when
it is in the canopy.
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These results corroborate earlier work that proposed that titi
monkey alarm sequences encode predator location and type (4).
Cäsar etal. (4) described three encoding mechanisms at the
sequence level: the call rate and the proportion of A- and B-calls encoded
for predator type only, and the insertion of either B-calls into an
A-sequence or one single A-call at the beginning of a B-sequence
(which is partly captured in the “transition probability from A to B”
metric we used) encoded for both predator type and location. Our
study corroborates these findings as we also found metrics that
encode for predator type and for both predator type and location,
but not for location alone (Fig.1A), albeit by investigating a more
comprehensive set of sequence features with an increased sample size.
Building on these results, our study showed experimentally that titi
monkeys extract this information but that the underlying mechanisms
appear to be more complex than those proposed earlier (4).
The most relevant conclusion from our study, which contrasts
earlier work on titi monkeys and other primates (7), is that information
appeared to be conveyed probabilistically. The proportion of BB-grams,
a continuous sequence feature, encoded categorical information
about predator type and location. Receivers are likely to have
extracted this information because they reacted in an appropriate
but continuous fashion to playback experiments: the smaller the
proportion of BB-grams, the more likely that subjects were looking
upward, i.e., responding to an aerial predator or to a predator in the
canopy, and the less likely that they were looking toward the speaker,
i.e., responding to a terrestrial predator or a predator on the ground
(Fig.4) (8). Therefore, the proportion of BB-grams conveyed gradual
information about a categorical event and elicited a graded reaction
from the subjects.
Human and nonhuman animals (hereafter referred to as animals)
live in environments where most stimuli appear in a continuous
form, but perception is often categorical (9). For example, although
rainbows consist of continuously changing wavelengths, they are
perceived by humans as color bands. Similar effects are found in
communication systems, including human speech. Acoustically,
the human vocal tract can gradually alter the second formant of the
syllable from the sound “b” (as in “beer”) to “d” (as in “deer”) and
then to “g” (as in “gear”), although they are perceived in sharply
categorical ways by listeners (10). Another example comes from the
American Sign Language, where the hand configuration gradually
differs between the words “please” (the thumb and all the fingers are
selected) and “sorry” (only the thumb is selected) but is perceived
categorically by deaf signers (11).
Similarly, animal vocal repertoires often produce graded vocaliza-
tions [e.g., (12)], with evidence that these signal systems are perceived
categorically by conspecific recipients (13). For example, female
túngara frogs Physalaemus pustulosus categorize the mating calls of
males as conspecific or not, although the calls exhibit graded variation
in seven different acoustic parameters (14). By categorizing their
environment, individuals can apply the same response to stimuli
belonging to the same category, which results in an improvement of
their fitness (e.g., by mating with potential sexual partners) and survival
(e.g., by fleeing when exposed to a predator) (15). Thus, categorical
perception is a crucial cognitive capacity with high fitness relevance
in a physical world that is largely gradual.
Although the notion of categorical meaning is intuitively compelling,
it is not necessarily the default mode of animal perception. Categorical
perception has been a major theoretical pillar in animal communication
research, particularly because of its intuitive link to linguistic theory.
For example, Macedonia and Evans [(16), p. 179] presupposed that
external events are processed in categorical terms (“…all eliciting
stimuli must belong to a common category”). Although this approach
has been fruitful and productive, it has also generated enigmas
suggesting that the underlying theory may have to be revised. For
example, in a seminal paper, Cheney and Seyfarth (17) were puzzled
by the fact that animals appeared to have very few categorical semantic
labels, mostly limited to predator classes and a few social events.
One possibility is that graded meanings are the default way of animal
communication [e.g., (18)], although this hypothesis has been much
ignored and considered as less interesting than categorical perception
(16). Our study suggests that explaining animal communication on
categorical terms alone may be too restrictive and anthropocentric
and may explain the struggle to extract meaning from some animal
communication systems.
Proportion of looking time
0.0 0.1 0.2 0.3 0.4 0.5 0.6
0
0.2
0.4
0.6
0.8
1
A
Downward
0.0 0.1 0.2 0.3 0.4 0.5 0.6
0
0.2
0.4
0.6
0.8
1
B
Speaker
Proportion of BB-grams
0.0 0.1 0.2 0.3 0.4 0.5
0.6
0
0.2
0.4
0.6
0.8
1Upward
C
Fig. 4. Listener’s gaze reaction, depending on the proportion of BB-grams of the alarm sequence. Proportion of time listeners spent looking downward (A), toward
the speaker (B), and upward (C), depending on the proportion of BB-grams of the playback stimuli. The figure shows raw data (circles), as well as estimates (black lines)
and bootstrapped estimates (colored lines, 1000 bootstraps) of the model testing how gaze reaction depends on the proportion of BB-grams. Listeners spent more time
looking toward the speaker (B) and less time looking upward (C) when there were more BB-grams in the sequ ence. The time looki ng downward (A) was not affected.
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Our data show that the titi monkey alarm system most likely
relies on call combinations at the sequence level, which potentially
allows individuals to convey rich information with a limited set of
calls (3). Since the listener needs to wait for the emission of enough
calls to choose an appropriate reaction, this strategy may be seen as
inefficient in predatory contexts where information should be
quickly conveyed. When looking carefully at the alarm sequences of
titi monkeys, it seems likely that predator type is the predominant
information that can potentially be quickly extracted by the receivers:
It is encoded by the first call in a sequence (A-calls for aerial predators
and B-calls for terrestrial predators; Fig.1,AandD,b) and is redundantly
encoded later in the sequence through the proportion of BB-grams
(Figs.1A and 3). Predator location, on the other hand, seems to be
secondary information: It is not encoded alone by any of the metrics
we investigated (Fig.1A) and only appears over the course of the
sequence through the proportion of BB-grams (Fig.3).
This imbalance of information can be explained by the fact that
predator type and location typically are correlated (aerial predators
attack from the canopy and terrestrial predators attack from the
ground), suggesting that providing information about the predator
type might be sufficient and would allow receivers to react quickly
and efficiently to the threat in most predator detections. However,
this system is not the most effective when a detected predator is not
at its typical location (e.g., a bird of prey on the forest ground): In this
case, titi monkeys add information about predator location at the
sequence level using a call combinatory sequence feature (BB-grams),
which elicits an appropriate reaction from the listeners (Figs.2 and 4).
Thus, alarm systems such as that of titi monkeys can provide some
flexibility by conveying complex information with only few calls.
We have shown that information about predator type and location
are encoded at the sequence level in a probabilistic manner. However,
we only tested two locations (ground versus canopy), and further
experiments might reveal whether titi monkeys also encode further
predator locations (e.g., airborne). Moreover, at least two other
encoding mechanisms can convey additional information about
predation events. First, variation of spectral features of calls can
convey rich information about external events (12,19) and were not
addressed in the current study. Second, we did not investigate
whether interactions among sequential and/or spectral metrics
affected the information transfer and the probabilistic form of the
alarm sequence. For example, spectral features could also convey
information about predator type and location, in a fashion that
allows the receiver to react more quickly and more efficiently to the
threat than with the proportion of BB-grams. These possibilities
remain to be tested in the future.
Our study on titi monkeys is, to our knowledge, unique in the
way it provides empirical evidence of probabilistic meaning in an
alarm call system. It is unclear whether this mechanism applies
exclusively to titi monkeys and is absent in other taxa or whether
other species have simply not been studied in the framework of
probabilistic meaning attribution, something that will have to be
resolved by future research. If common in other taxa, then a relevant
next question to address is whether probabilistic meaning is the
ancestral state and whether human categorical meaning evolved
from it. An important general point emerging from this work is that
the animal communication theory should be extended beyond the
classic linguistic framework to encompass communicative capacities
that are not commonly found in humans to better understand what
makes language unique.
MATERIALS AND METHODS
Study subject and site
Our study was conducted from May 2015 to August 2016 at the
“Reserva Particular do Patrimônio Natural Santuário do Caraça”,
an 11,000-ha private reserve in the Espinhaço Mountain range,
State of Minas Gerais, Brazil (20°05S, 43°29W), where previous
studies on titi monkeys already took place (4,8,20,21). The two
Atlantic forests of interest, Tanque Grande and Cascatinha, are
located 1 km apart from each other in the core of the reserve
(transition zone between Cerrado, Atlantic forest, and Caatinga),
with an elevation of around 1300 m.
Subjects were sampled from six groups of habituated black-fronted
titi monkeys C. nigrifrons. Five of them (A, D, M, P, and R groups) were
habituated to human presence between 2003 and 2008 (20); one addi-
tional group (S group) was habituated during the study period in 2015
(table S2). Titi monkeys typically live in family groups comprising an
adult heterosexual pair and up to four offspring. Both sexes disperse after
reaching sexual maturity, at around 3 to 4 years of age (22). Thus, the
group compositions changed since 2003, with only some paired adults
still present in our study (table S2). We considered an individual as an
adult from the age of 30 months, as a sub-adult between 18 and 30 months,
as a juvenile between 6 and 18 months, and as an infant if less than
6 months old [see (20)]. Recognition of individuals was based on morpho-
logical cues, such as size, fur pattern, and facial or corporal character-
istics. The territories of the six habituated groups overlap with habituated
groups and nonhabituated groups. This research was conducted in com-
pliance with all relevant local and international laws and has the approval
of the ethical committee CEUA/UNIFAL (Comissão de Ética no Uso
de Animais da Universidade Federal de Alfenas), number 665/2015.
Predator presentations
The experiments followed a protocol developed by Cäsar etal. (4).
Predator presentations were conducted between May 2015 and
August 2016. We used the following four taxidermy predator models as
stimuli: two models of caracaras Caracara plancus (aerial predator), one
model of tayra Eira Barbara, and one of southern tiger cat Leopardus
guttulus (terrestrial predators). The models were borrowed from
the collection of the Natural Science Museum of the Pontifícia
Universidade Católica de Minas Gerais. Each species was presented
twice to each group, once in the canopy and once on the ground,
i.e., 36 expected trials in total. The order of presentation was
randomized across groups. Presentations were separated by at least
10 days for each group, and monkeys were monitored between
trials. Before each trial (i.e., detection of the model by an individual),
we monitored subjects for at least 30min and, if possible, for another
30min after the end of a trial (i.e., after the entire group had stopped
calling or left the area). We made sure that no duet, group encounter,
loud calls from a lost individual, or predator encounter occurred in
the 30min preceding the experiment; otherwise, the trial was aborted,
and we waited for another 30min to set up the equipment again.
For canopy presentations, we placed the model at 3 to 10 m off
the ground (mean±SD = 6.3±1.6 m), depending on the structure
of the arboreal strata. For ground presentations, we placed the model
on the forest floor (i.e., at 0 m). We considered a trial as failed if
more than one individual emitted the first 10 calls (n=1) (this trial
was removed from the dataset during the analyses and, thus, was
not rerun), if the recording quality was insufficient (cicadas noise;
n=1), if model detection took place during setup (n= 5), if the
model was detected by an individual of less than 2 years old (n=2),
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if another species gave alarm calls before visual detection by subjects
(n=2), if an individual bumped into the model before detection
(n=1), and if a real predator was encountered before detection of
the model (n=1). If a trial was scored as failed, we waited for at least
2 months before we retested the group, except for one case (35 days).
Here, the monkeys responded to vegetation movement in the canopy
(caused by the installation of the tayra model), although they probably
did not see the model (M group). One experiment (Caracara in the
canopy, D group) failed three times, and we decided to not rerun
the experiment a fourth time. Therefore, the total number of successful
trials was n=34.
Vocal reactions were recorded in WAV (Wavesound Audio File)
format with a Marantz solid-state recorder PMD661 (44.1-kHz
sampling rate, 16-bit accuracy) and a directional microphone Sennheiser
K6/ME66 or K6/ME67 (frequency response, 40 to 20,000 Hz±2.5
dB). Distance of detection (i.e., distance between the first individual
to call and the model at the time of detection, in meters) and identity
of the first caller were noted for each trial.
Vocal reaction dataset
Since we focused on sequences, we discarded responses composed
of single calls (n=3). We completed our own dataset with all alarm
sequences recorded by Cäsar etal. (4) (n =20) and another n=5
sequences in response to the tayra model on the ground from Cäsar
(20). For consistency, we discarded any sequence in which individuals
were already calling at something else before detection of the model
(flying bird, n=1), if more than one individual emitted the first
10 calls (n=3), if another species gave alarm call to the observers or to
the model just before visual detection by the monkeys (n=1), and
vocal reaction consisted of only one call (n=1). As a result, we
included n=19 sequences from Cäsar to our n=31 sequences, i.e.,
the total dataset was composed of n=50 sequences (table S3).
Some monkeys were probably present during both Cäsar’s and
our experiments (table S2) (potentially six individuals that emitted
n=16 sequences in total). However, groups were not systematically
monitored between 2010 and 2015, so identification was not entirely
reliable. Yet, since at least 5 years passed between the two sets of
experiments, we found it unlikely that the responses to our stimuli
were dependent on the monkeys’ potential earlier experience with
the paradigm. Thus, we considered these six callers as different
between our study and Cäsar’s study. In addition, in n=4 sequences
from Cäsar, the identity of the caller was unknown. For those, we
considered the caller as a new individual that had not called in any
other trials.
Stimuli preparation for playbacks
Broadcasted alarm sequences consisted of 10 calls recorded during
predator presentations or during natural predator encounters. We
did not broadcast sequences recorded by Cäsar because most of the
group members were different or older from those recorded at that
time, which could lead to bias in the experiment.
For the terrestrial predator in the canopy condition, we only
managed to record two sequences corresponding to the pattern
described by Cäsar etal. (4) out of 12 trials, and both were of poor
quality. We thus created artificial sequences by adding an A-call
from one given individual at the beginning of a B-call sequence
from the same individual [as detailed in (4)]. The intercall intervals
between the single A-call and the nine B-calls were measured on
our recorded sequences and on two of Cäsar’s sequences (4), and
the length of the silent gap for each of the artificial sequences was
randomly chosen among these four measures. We sometimes had
to replace bad quality calls with other calls from the same sequence
(table S4). We filtered background noises and normalized all the
sequences at −1 dB. We cut and edited the sequences using Praat
5.3.84 (23), Raven 1.5 (24), and Audacity 2.0.6. (25).
The total stimuli set was composed of 22 sequences: n=6 aerial
canopy, n=4 aerial ground, n=6 terrestrial canopy, and n=6
terrestrial ground sequences. One terrestrial canopy sequence was
of bad quality, so we removed the corresponding trials from the
final dataset (tables S4 and S5).
Playback procedure
Seven females and seven males were tested from January to August
2016 (table S5). Each individual was exposed to one set of stimuli
corresponding to a predator type in two different locations (aerial
canopy, aerial ground, terrestrial canopy, and terrestrial ground),
corresponding to a total of 28 trials. The presentation of the stimuli
was randomized among individuals. No more than two trials were
run on the same day within a given group and never for 2 days on a
row to avoid habituation. No stimulus was broadcasted more than
twice to limit pseudoreplication.
Stimuli sequences were recorded from a member of the family of
the subject or from a member of one of the neighboring groups.
There is no evidence that reactions of titi monkeys to others’ alarm
sequences is affected by the identity of the caller (8), possibly due to
the fact that the pending danger requires a more urgent reaction
than the caller identity. As it is still possible that monkeys recognize
each other by spectral features, we made sure that if the playback
sequence was from a member of the same group, then the caller was
out of sight and the speaker was positioned so that the calls came
from the direction of the caller. For neighboring alarm sequences,
we played the stimuli in the overlap area between the subject’s territory
and the neighbor’s territory to avoid bias due to intrusion, except in
one case (sequence from the D group was played to the R group in
the overlap between the S and the R groups’ territories).
We monitored the group at least 30min before and after the
experiment. During the 30min before a trial, we made sure that no
duet, group encounter, loud calls from a lost individual, or predator
encounter occurred; otherwise, we waited for another 30min. We
waited for the tested individual to be in low strata (1 to 8 m high)
and in an open area to ensure a good visibility. The angle between
the subject, the camera, and the speaker was about 90°, with the
subject facing the camera. The speaker was covered with a camou-
flage net and held at the same height of the tested individual with a
perch or, if not possible, at a maximum of 7 m high so that the angle
between the horizontal line, the tested individual, and the speaker
was less than 45° and as close as possible to 0° (mean=8.1, SD=7.1)
(fig. S3). We made sure that no monkey was able to see the speaker.
The reaction of the monkey was videotaped during twice the length
of the broadcasted stimulus. Stimuli were played using an Anchor
AN-Mini loudspeaker (audio output, 30 W; frequency response,
100 Hz to 15 KHz) connected to an iPhone 4.2.1, and videos were
recorded using a camera Canon SX50 HS. We held the volume of
the loudspeaker at a constant level matching the natural volume of
a titi’s vocalizations to a human hear. To test the setup, the territorial
call of a white-shouldered fire-eye (Pyriglena leucoptera) was played.
This bird call is common in the study area and elicits no reaction
from the monkeys.
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We considered a trial as failed if it was not possible to code most
of the gazes of the monkey because it moved during the experiment
(n=6) or if the stimulus quality was too bad (n=2; the stimulus was
then removed from the analysis). If a trial failed, then we waited at
least 8 days before rerunning it, except in one case (tested individual
MR, aerial canopy trial: Only a few calls were played, so the subject
did not hear the full stimulus and the trial was run again 4 days
after) (table S5).
Vocal repertoire
We used the vocal repertoire established by Cäsar (21). The two
main soft calls emitted during a predator encounter are the A-call,
arch-shaped with a down-sweep modulation, and the B-call, S-shaped
with an upsweep modulation (fig. S1). To estimate the accuracy
of the call classification, we (M.B. and C.C.) tested between-rater
reliability. We used a subset of 200 randomly selected calls that each
of the two observers labeled. Between-rater agreement reached a
sufficient level (Cohen’s κ ≥0.8).
Metric extraction
We applied the same procedure to extract metrics from the sequences
recorded during predator presentations and to the sequences
broadcasted during playbacks. For the sequences recorded during
predator presentations, we only focused on the first 10 calls of each
sequence: The duration of emission of the first 10 calls ranges from
3.0 to 133.4s (mean=18.2, SD=23.8), which we considered long
enough to convey urgent information about a pending threat.
One observer (M.B.) labeled each of the calls and measured the
duration of each call interval, i.e., the silence between each call, by
using Praat 5.3.84 (23) (Spectrogram, Hanning window; time reso-
lution, 5 ms; frequency resolution, 88 Hz).
On the basis of previous studies, we identified the 15 variables
to characterize titi monkey alarm call sequences (table S1). Since
proportions are often distorted by rare events and small sample sizes,
we used a Bayesian approach to estimate the occurrence of rare and
common events (26). The procedure is based on a two-step process,
which starts with a theoretically motivated prior distribution of
events (never or always observed), which is then updated to create
an empirically motivated posterior distribution (values approaching
0 or 1). We used the Dirichlet distribution as the prior distribution
with =1 [see (26) for more details on the technique]. The resulting
Bayesian posterior mean for the occurrence of i is mean=count of
event i+/(total number of events + k), where k is the number of
possible events. In the Bayesian framework, the only probabilities
being equal to 0 or 1 are those set by the design based on our prior
assumptions and that correspond to impossible or mandatory
events, respectively. Thus, the few metrics that have a counterpart
in (4) and that were extracted using the Bayesian approach (26) are
expected to display a lower value than in (4) if they are common
events or a larger value if they are rare events.
We calculated 15 metrics for each sequence: (i) “Proportion of
A-calls” using the Bayesian method. We chose this variable because
it has been suggested to carry information about predator type (4).
(ii) “Slope of elements” (the probability of observing an A-call at
each place in the sequence, followed by a linear regression, with the
coefficient representing the slope). Negative slopes indicate that
A-calls are less likely to occur as the sequence progresses. (iii) “Mean
call interval” of each sequence and (iv) “coefficient of variation
of call interval” (SD/mean). Low coefficients indicate high regularity of
call emission. We chose this variable because temporal structures of
sequences can convey context information (19). (v to viii) “Proportion
of 2-grams”. In two-signal systems, such as titi monkey alarm calling,
the proportion of all four possible 2-grams (AA, AB, BA, and BB)
can be determined as the number of each 2-gram/total number of
2-grams, followed by a Bayesian correction for small size sample.
(ix) “Slope of 2-grams” [graphic representation of probability of
each 2-gram (27,28) by decreasing probability and extraction of the
coefficient of regression (later referred to as 2-gram slope)]. When
the 2-gram slope is different from 0, then one 2-gram is more
represented in the sequence. (x) “Slope of entropy”. Shannon entropy
uses principles of the information theory to measure complexity into
a sequence and has been successfully used in animal communication
(29,30). Entropy evaluates the unpredictability of a sequence, i.e.,
the degree of randomness in the sequence. Several values can be
considered: The zero-order entropy evaluates the diversity of the
vocal repertoire with H0=log2 N, where N is the repertoire size; the
first-order entropy assesses the proportion of different elements in
the sequence, with H1=− p(x) log (x), where p(x) is the probability
of a syllable x occurring in the sequence; the second-order entropy
measures the proportion of different combinations of two elements
in the sequence, with H2= p(xy) log (xy), where p(xy) is the
probability of a syllable y following a syllable x in the sequence. If
one plots the entropic values for the different orders (from 0 to 2),
then the slope provides a measure of organizational complexity
(30). A negative slope indicates an important sequential organization
and, thus, high communication capacities, while a slope of zero
indicates a random organization, with a low communicative capacity.
(xi to xv) Transition probabilities. Markov chains are often used for
sequence order analysis (3,27,30). The Markov paradigm assumes
that probabilities of future events are dependent on a finite number
of previous events. A transition matrix M can be derived from this
assumption, in which Mi,j represents the probability that an event
j follows an element i. Chains of events are often represented with a
state “Start” at the beginning and a state “End” in the end [e.g.,
(26)]. However, recent analysis suggests that Markov chains are not
the most powerful tool to highlight structure in animal sequences
(27). Moreover, Markov chains require exponential distribution
of the durations, which is not our case. To address this issue, we
conducted semi-Markov analysis (31). Semi-Markov analysis requires
that the distribution of durations of the states is independent of the
previous states or its place in the sequence. We verified with graphical
assessments that the place of the call did not influence its duration.
In our study, the titi sequences can be presented as a chain of events
A- and B-calls with an artificial “Start” state at the beginning of
the chain but no “End” state in the end, since we did not study
the whole sequences. Then, we extracted the Bayesian transition
probabilities from Start to A (also referred to as “probability that the
first call is A”), A to A, A to B, B to A, and B to B for each sequence;
Start to B was not considered here since it is negatively correlated
with Start to A.
Two-grams and transition probabilities provide complementary
information, the first one describing the probability of occurrence
of a two-call syllable and the other one describing the probability
that one call follows another one. For example, in a sequence
AAAAABA, the BA-gram has a probability of occurrence of one of
six, while the transition probability from B to A is of one. Metrics
were extracted from each sequence by using the R software version
3.4.1 (32) and the cfp package (33).
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Video analysis
The 28 videos recorded from the playback experiments were coded
with the software Elan 4.9.4 (34). The reaction of the caller was
analyzed during and after the playback experiment, for a total duration
of twice the duration of the stimulus (i.e., the duration of the playback
plus the same amount of time after the end of the stimulus). We
extracted the duration (in seconds) and direction of each gaze, i.e.,
from the moment the subject looked to one direction until it looked
to another direction. Directions of the gaze were categorized as (i)
upward (the subject had the head orientated at least at 45° above the
horizontal line and looked further than one body away from him),
(ii) downward (the subject had the head orientated at least at 45°
under the horizontal line and looked further than one body away
from him), (iii) toward the speaker (the subject had the head orientated
within 45° relative to the line between the subject and the speaker
and looked further than one body away from him), and (iv) elsewhere
(the subject looked in another direction or less than one body away
from him (e.g., food, body part, etc.). When the eyes of the subject were
not visible, the gaze direction was noted as “not visible” and excluded
from calculations of proportions. The proportion of time looking
in each direction was calculated as the duration the monkeys spent
looking in each direction divided by the time the subject was visible.
Videos were analyzed by a coder blind to the experimental
conditions (A.P.). To assess rater reliability, two raters (A.P. and
M.B.) coded three videos (10% of the total dataset). We calculated
Cohen’s κ to assess the reliability in direction and duration
coding of the gazes. An overlap matrix was created with the conditions
(gaze directions) in rows and columns (35). Agreements were tailed
on the table diagonal (same duration and same direction), and
disagreements were tailed on off-diagonal cells: When one coder
noted a duration as one gaze bout (e.g., “elsewhere” from 12 to 13s,
coder 1) and the other coded two (or more) gaze bouts for the same
duration (e.g., “elsewhere” from 12 to 12.5s and “down” from 12.5 to
13s, coder 2), the gaze bout of the first coder was cut into two bouts to
facilitate comparison with the other coder’s results (e.g., “elsewhere”
from 12 to 12.5s and “elsewhere” from 12.5 to 13s, coder 1; “elsewhere”
from 12 to 12.5s and “down” from 12.5 to 13s, coder 2; agreement from
12 to 12.5 s and disagreement from 12.5 to 13 s). The level of between-
rater agreement was considered as substantial (κ=0.79) (36), but it
should be stressed that this method has limits since a long agreement
of several seconds counts as much as a short disagreement of half a
second, so the statistical agreement is lower than reality. We thus
considered that the inter-rater agreement was good.
Statistical analysis
We used multimodel inference within an information-theoretic
framework (5). This approach can be used to compare relative support
for each model in a set of models by using model weights w, derived from
Akaike’s information criterion (6). This weight gives the probability that
a model is the best among the set of considered models, ranging from 0
(weak support for being the best model) to 1 (strong support).
To graphically represent statistical uncertainty around the model
estimates, we used a nonparametric bootstrap procedure: We created
1000 datasets that were drawn from the original dataset by selecting
observations with replacement so that each dataset comprised as
many observations as the original dataset. For each dataset, we
refitted the model and extracted and plotted model predictions.
All statistics were conducted using the R software version 3.4.1
(32). Linear mixed models (LMMs) were fit using the lme4 package (37)
and generalized LMMs (GLMMs) using the glmmADMB package
(38), model selection was performed with the MuMIn package (39), and
bootstraps were performed with a custom function (resamplefunction)
from the cfp package (33). Collinearity of the variables was checked
for each model using the package car (40).
What do alarm sequences encode?
To investigate whether each metric conveyed information about
predator type and/or location, we created six models for each metric.
Each of these six models corresponded to a combination of predator
type and location. The first two models included only predator type
or location as predictors, which addresses the possibility that sequences
encoded for predator type or location only. The next two models
addressed the possibility that sequences contained information
about predator type and location: One model contained both main
effects; the other model additionally contained the interaction term
for location and type. In all these models, we controlled for distance of
detection (in meters) to avoid a bias due to urgency. Last, in two control
models, we considered the intercept only (null model) and the dis-
tance of detection only (urgency model). In all models, the sequence
metric was the response variable. All models were mixed-effects
models in which the identity of the caller was fitted as random inter-
cept. Descriptions of the general set of models are given in table S6.
For five metrics and their corresponding model sets, we used
LMMs. The remaining metrics were fitted as GLMMs with a beta,
gamma, or binomial error structure (table S1). For each metric, we
ranked the set of six candidate models using Akaike’s weight w. If,
for a metric, at least one model did not converge (n=10 models,
five metrics), then we performed the ranking with the Akaike’s
weight of the converging models only.
What information do monkeys attend to?
To assess how the combination of eliciting predator type and location
of the played back sequences affected the time listeners spent looking
in predator-relevant directions, we created six models. The first two
models only included the predator type or location as predictors,
respectively, which addressed the possibility that listeners only
attended to either predator type or location. The second two models
addressed the possibility that listeners attended to predator type
and location: One model contained both main effects and the other
additionally contained an interaction term for location and type.
In all models, we controlled for the height of the listeners (i.e.,
the distance from the ground, in meters) to address perceived
differences in urgency. Last, in two control models, we only considered
direction of gaze (null model) and height of the individual and direc-
tion of gaze (urgency model). In all models, the response variable
was the proportion of time the listeners looked to one direction. All
models were mixed models (GLMMs) in which the identity of the
listener and the broadcasted sequence were fitted as random intercepts
with a binomial error structure (table S6). We ranked the set of six
candidate models using Akaike’s information criterion and interpreted
model weights w (5).
What sequential metrics do monkeys attend to?
To assess how the metrics characterizing the call sequences used as
playbacks affected the time listeners spent looking in predator-relevant
directions, we created 15 models, each containing one metric as
predictor variable. In all models, we controlled for the height of the
listeners (in meters) to address perceived differences in urgency.
We also designed two control models that only contained direction
of gaze (null model) and height of the individual and direction of
the gaze (urgency model) as predictor variables. In all models, the
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response variable was the proportion of time the listeners looked to
one direction. All models were mixed models (GLMMs) in which
the identity of the listener and the broadcasted sequence were fitted
as random intercepts with a binomial error structure (table S6).
Again, we ranked the set of candidate models using Akaike’s infor-
mation criterion and interpreted model weights w (5).
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/
content/full/5/5/eaav3991/DC1
Fig. S1. Soft alarm calls of titi monkeys.
Fig. S2. Listener’s gaze reaction depending on the eliciting stimulus of the sequence.
Fig. S3. Location of the speaker during the playback experiments.
Table S1. Design of the set of models for each metric.
Table S2. Composition of the six titi monkey groups during our study and that of Cäsar et al. (4).
Table S3. Description of the final dataset of predator presentations.
Table S4. Playback stimuli.
Table S5. Playback experiments schedule.
Table S6. Models formulas.
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Acknowledgments: We thank N. Buffenoir, A. Colliot, G. Duvot, C. Ludcher, F. Müschenich,
C. Rostan, and A. Pessato for help with data collection. We acknowledge logistic support from
the Santuário do Caraça. The Natural Sciences Museum of the Pontifícia Universidade Católica
de Minas Gerais (PUC Minas) lent us the predator models. We thank S. J. Alger for providing
statistical script. Rogério Grassetto Teixeira da Cunha (UNIFAL) for helping to obtain research
permits, and A. Kershenbaum, S. Townsend, R. Bshary, E. Chemla, and P. Schlenker for helpful
discussions and comments on the early drafts of this manuscript. Funding: Our research was
funded by the European Research Council under the European Union’s Seventh Framework
Programme (FP7/2007–2013)/ERC grant agreement no. 283871. We acknowledge further
funding from the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC
Grant Agreement No. 324115–FRONTSEM (PI: Schlenker) and the Institut d’Etudes Cognitives,
Ecole Normale Supérieure, PSL Research University (grants ANR-10-LABX-0087 IEC and
ANR-10-IDEX-0001-02 PSL), from the Swiss National Science Foundation, and from the
University of Neuchâtel. The research leading to the data from 2008 to 2010 received funding
from the CAPES-Brazil, FAPEMIG-Brazil, S.B. Leakey Trust, and the University of St Andrews.
Author contributions: Conceptualization: K.Z., C.N., and M.B. Formal analysis: C.N. and
M.B. Funding acquisition: K.Z. Investigation: M.B., G.M., A.P., and C.C. Resources: C.C., C.N., and
K.Z. Competing interests: The authors declare that they have no competing interests.
Data and materials availability: All data (raw videos, raw vocal sequences, audio stimuli used
in playbacks, and data files) and statistical codes have been deposited in the Figshare data
repository at the following address: https://figshare.com/projects/Probabilistic_meaning_in_
titi_monkeys_alarm_call_sequences/30488. All data needed to evaluate the conclusions in the
paper are present in the paper and/or the Supplementary Materials. Additional data related to
this paper may be requested from the authors.
Submitted 12 September 2018
Accepted 9 April 2019
Published 15 May 2019
10.1126/sciadv.aav3991
Citation: M. Berthet, G. Mesbahi, A. Pajot, C. Cäsar, C. Neumann, K. Zuberbühler, Titi monkeys
combine alarm calls to create probabilistic meaning. Sci. Adv. 5, eaav3991 (2019).
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Titi monkeys combine alarm calls to create probabilistic meaning
Mélissa Berthet, Geoffrey Mesbahi, Aude Pajot, Cristiane Cäsar, Christof Neumann and Klaus Zuberbühler
DOI: 10.1126/sciadv.aav3991
(5), eaav3991.5Sci Adv
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... First, computational linguistics offers mathematical tools that can help to detect underlying structures in complex vocal sequences (see Kershenbaum et al. 2014). Such methods have been successfully applied to several communication systems (Kershenbaum 2014;Alger et al. 2016;Berthet et al. 2019). Second, formal linguistics provides tools to investigate the combination rules linked to the meaning of individual calls, in order to determine the semantics of the resulting sequences. ...
... Titi monkeys (Callicebus nigrifrons) are an ideal candidate species for this exercise. Their alarm vocal system has been well investigated by biologists and linguists (Cäsar et al. 2012a(Cäsar et al. , 2012b(Cäsar et al. , 2013Schlenker et al. 2017;Berthet et al. 2018Berthet et al. , 2019Commier & Berthet 2019). ...
... obs.; Cäsar 2011). Because these long and multi-caller sequences are difficult to investigate with current methods, previous studies (Cäsar et al. 2013;Berthet et al. 2019) have focussed on the first 10 and 30 calls of the alarm sequence (or respectively, during the first 18 and 37 sec): these calls are emitted by one caller only, and are likely to convey enough information about the predatory event for kins to adopt a sound reaction. These 10-and 30-alarm sequences are mostly composed of two alarm soft calls, A-and B-calls. ...
Article
The emergent field of animal linguistics applies linguistics tools to animal data in order to investigate potential linguistic-like properties of their communication. One of these tools is the “Urgency Principle”, a pragmatic principle stating that in an alarm sequence, calls providing information about the nature or location of a threat must come before those that do not. This theoretical principle has helped understand the alarm system of putty-nosed monkeys, but whether it is relevant for animal communication systems more generally remains to be tested. Moreover, while animal communication systems can convey information via a large set of encoding mechanisms, the Urgency Principle was developed for only one encoding mechanism, call ordering. Here, we propose to extend this principle to other encoding mechanisms and empirically test this with the alarm call system of black-fronted titi monkeys (Callicebus nigrifrons). We investigated how information about the context of emission unfolded with the emission of successive calls. Specifically, we analysed how contextual parameters influenced the gradual sequential organization of the first 50 calls in the sequence, using methods borrowed from computational linguistics and random forest algorithms. We hypothesized that, if the extended Urgency Principle reflected the sequential organization of titi monkey alarm call sequences, mechanisms encoding urgent information about the predatory situation should appear before encoding mechanisms that do not. Results supported the hypothesis that mechanisms encoding for urgent information relating to a predator event consistently appeared before mechanisms encoding for less-urgent social information. Our study suggests that the extended Urgency Principle applies more generally to animal communication, demonstrating that conceptual tools from linguistics can successfully be used to study nonhuman communication systems.
... In some animal species, callers respond to dangers with complex series of alarm calls that encode information at the sequence level (Schel et al. 2010). For example, black-fronted titi monkeys (Callicebus nigrifrons) produce two types of alarm calls in long sequences that refer to both predator type and location (Berthet et al. 2019), a phenomenon also found in non-primate species (meerkats, Rauber 2020; Japanese great tits (Parus major), Suzuki 2014; pied babblers (Turdoides bicolor), Engesser et al. 2016). In some species, predator alarm calls strongly resemble aggressive calls given to conspecifics, suggesting that listeners consider contextual information to infer the cause of an event (titi monkeys, Cäsar et al. 2012;lemurs, Fichtel and Kappeler 2002;Fichtel and van Schaik 2006; saddleback tamarins (Saguinus fuscicollis), Kirchhof and Hammerschmidt 2006;vervet monkeys, Price et al. 2015). ...
... Early reports have suggested that male alarm calls contain subtle, contextspecific acoustic variability (Kambiré 2015;Kouassi 2008), similar to what has been described for Barbary macaque (Macaca Sylvanus) or chamca baboon (Papio cynocephalus ursinus) alarm calls (Fischer et al. 2001(Fischer et al. , 2010. Meaningful acoustic variations have also been reported in other species' alarm calls, such as Siberian jay (Perisoreus infaustus) alarm calls to different predator behaviours (perched, searching or attacking hawks; Griesser 2008), titi monkey alarm calls to different predator locations (Berthet et al. 2019), black-capped chickadee (Poecile atricapillus) alarm calls to different predator sizes (Templeton 2005) and meerkat, Campbell's monkey and blue monkey (Cercopithecus mitis) alarm calls to different event urgencies and predator types (Lemasson et al. 2010;Townsend et al. 2012;Murphy et al. 2013). ...
... In order to address this hypothesis, it would be necessary to carry out playback experiments, to test whether recipients react differently to calls recorded to different predator types (e.g., Zuberbühler 2000). Furthermore, acoustic variations in alarm calls might not be associated with the type of danger but rather with other contextually relevant cues, such as predator behaviours (e.g., Griesser 2008), sizes (e.g., Templeton 2005), colours (Slobodchikoff et al. 2009), attack directions (e.g., Berthet et al. 2019) or response urgencies (e.g., Lemasson et al. 2010;Townsend et al. 2012;Murphy et al. 2013). Finally, and although we consider this also unlikely, future research should verify if any of the above information is encoded at the sequence level, similar to what has been described for Guereza colobus monkeys (Colobus guereza, Schel et al. 2010). ...
Article
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Forest monkeys often form semi-permanent mixed-species associations to increase group-size related anti-predator benefits without corresponding increases in resource competition. In this study, we analysed the alarm call system of lesser spot-nosed monkeys, a primate that spends most of its time in mixed-species groups while occupying the lowest and presumably most dangerous part of the forest canopy. In contrast to other primate species, we found no evidence for predator-specific alarm calls. Instead, males gave one general alarm call type (‘kroo’) to three main dangers (i.e., crowned eagles, leopards and falling trees) and a second call type (‘tcha-kow’) as a coordinated response to calls produced in non-predatory contexts (‘boom’) by associated male Campbell’s monkeys. Production of ‘kroo’ calls was also strongly affected by the alarm calling behaviour of male Campbell’s monkeys, suggesting that male lesser spot-nosed monkeys adjust their alarm call production to another species’ vocal behaviour. We discuss different hypotheses for this unusual phenomenon and propose that high predation pressure can lead to reliance on other species vocal behaviour to minimise predation. Significance statement Predation can lead to the evolution of acoustically distinct, predator-specific alarm calls. However, there are occasional reports of species lacking such abilities, despite diverse predation pressure, suggesting that evolutionary mechanisms are more complex. We conducted field experiments to systematically describe the alarm calling behaviour of lesser spot-nosed monkeys, an arboreal primate living in the lower forest strata where pressure from different predators is high. We found evidence for two acoustically distinct calls but, contrary to other primates in the same habitat, no evidence for predator-specific alarms. Instead, callers produced one alarm call type (‘kroo’) to all predator classes and another call type (‘tcha-kow’) to non-predatory dangers, but only as a response to a specific vocalisation of Campbell’s monkeys (‘boom’). The production of both calls was affected by the calling behaviour of Campbell’s monkeys, suggesting that lesser spot-nosed monkey vocal behaviour is dependent on the antipredator behaviour of other species. Our study advances the theory of interspecies interactions and evolution of alarm calls.
... A number of mammal (Townsend & Manser, 2013) and bird (Gill & Bierema, 2013) species emit alarm calls in response to specific predators. Among mammals, these species include different monkey species, e.g., vervet monkeys (Seyfarth et al., 1980), Campbell's monkeys, (Outtara et al., 2009), putty-nosed monkeys (Arnold & Zuberbühler, 2006) and titi monkeys (Berthet et al., 2019) as well as prairie dogs (Kirazis & Slobodchikoff, 2006) and meerkats (Manser, 2001). ...
... Interestingly, probabilistic communication systems are indeed not only found in human language. For example, Berthet et al. (2019) present evidence that some primate communication systems, such as Titi monkey call sequences, are systems that generate probabilistic meaning. ...
... In addition, again the limitation of merge-like processes might be explained not in all-or-nothing terms but be captured by working memory constraints and increasing capacities for pattern finding and abstraction (cf. Hurford, 2012;Zuberbühler, 2019). Such a position is also in line with criticism from within biolinguistic approaches that explanation of language evolution often demonstrate an over-reliance on merge to the detriment of other important processes (Progovac, 2019b;cf. ...
Article
In recent years, multiple researchers working on the evolution of language have put forward the idea that the theoretical framework of usage-based approaches and Construction Grammar is highly suitable for modelling the emergence of human language from pre-linguistic or proto-linguistic communication systems. This also raises the question of whether usage-based and constructionist approaches can be integrated with the analysis of animal communication systems. In this paper, we review possible avenues where usage-based, constructionist approaches can make contact with animal communication research, which in turn also has implications for theories of language evolution. To this end, we first give an overview of key assumptions of usage-based and constructionist approaches before reviewing some key issues in animal communication research through the lens of usage-based, constructionist approaches. Specifically, we will discuss how research on alarm calls, gestural communication and symbol-trained animals can be brought into contact with usage-based, constructionist theorizing. We argue that a constructionist view of animal communication can yield new perspectives on its relation to human language, which in turn has important implications regarding the evolution of language. Importantly, this theoretical approach also generates hypotheses that have the potential of complementing and extending results from the more formalist approaches that often underlie current animal communication research.
... In particular, information about predatory threats and an individual's related level of arousal has been demonstrated to be reflected by changes in the number of repeated elements or the inter-element intervals [5,6]. A few primate species have furthermore been described to combine distinct call types into larger sequences, where the proportional distribution of calls and transitional probabilities among call types contain contextual information [7,8]. However, the precise mechanisms underlying how the information in these large sequences is conveyed are often less clear. ...
... Sequences can contain identity information, both on an individual level [12,13] and on a group level or local scale, i.e. neighbours versus strangers [14]. In contrast, call combinations have been demonstrated to contain meaningful, context-specific information based on the temporal ordering of the units contained in longer sequences [3,7]. For example, bonobos (Pan paniscus) combine five acoustically graded call types into longer, mixed sequences containing information about the type of food encountered [8]. ...
... min). To avoid any bias due to very short recordings, we removed all recordings with less than 10 calls resulting in 162 recordings from 60 adults (median 4 recordings per individual, range 1-10) and 129 recordings from 39 subadult individuals (median 5 recordings per individual, range [1][2][3][4][5][6][7][8]. Although some individuals were only recorded once or a few times, they were still included in the analysis as it has been demonstrated that these data points are still valuable to assess betweenindividual variation [40]. ...
Article
Full-text available
Background: The ability to recombine smaller units to produce infinite structures of higher-order phrases is unique to human language, yet evidence of animals to combine multiple acoustic units into meaningful combinations increases constantly. Despite increasing evidence for meaningful call combinations across contexts, little attention has been paid to the potential role of temporal variation of call type composition in longer vocal sequences in conveying information about subtle changes in the environment or individual differences. Here, we investigated the composition and information content of sentinel call sequences in meerkats (Suricata suricatta). While being on sentinel guard, a coordinated vigilance behaviour, meerkats produce long sequences composed of six distinct sentinel call types and alarm calls. We analysed recordings of sentinels to test if the order of the call types is graded and whether they contain additional group-, individual-, age- or sex-specific vocal signatures. Results: Our results confirmed that the six distinct types of sentinel calls in addition to alarm calls were produced in a highly graded way, likely referring to changes in the perceived predation risk. Transitions between call types one step up or down the a priory assumed gradation were over-represented, while transitions over two or three steps were significantly under-represented. Analysing sequence similarity within and between groups and individuals demonstrated that sequences composed of the most commonly emitted sentinel call types showed high within-individual consistency whereby adults and females had higher consistency scores than subadults and males respectively. Conclusions: We present a novel type of combinatoriality where the order of the call types contains temporary contextual information, and also relates to the identity of the caller. By combining different call types in a graded way over long periods, meerkats constantly convey meaningful information about subtle changes in the external environment, while at the same time the temporal pattern of the distinct call types contains stable information about caller identity. Our study demonstrates how complex animal call sequences can be described by simple rules, in this case gradation across acoustically distinct, but functionally related call types, combined with individual-specific call patterns.
... Finally, meaning in animal communication can also be transmitted by signal combinations. Examples are unordered sequences where meaning resides in the distribution of bigrams (e.g., duplications): in titi monkeys, listeners react to the proportion of one type of bigram relative to others, allowing them to infer the type and location of danger (Berthet et al., 2019). Similarly, bonobos produce call sequences with bark or peep bigrams to preferred foods and peepyelp and yelp bigrams to non-preferred foods (Zuberbühler, 2020). ...
Article
Full-text available
Spoken language, as we have it, requires specific capacities—at its most basic advanced vocal control and complex social cognition. In humans, vocal control is the basis for speech, achieved through coordinated interactions of larynx activity and rapid changes in vocal tract configurations. Most likely, speech evolved in response to early humans perceiving reality in increasingly complex ways, to the effect that primate‐like signaling became unsustainable as a sole communication device. However, in what ways did and do humans see the world in more complex ways compared to other species? Although animal signals can refer to external events, in contrast to humans, they usually refer to the agents only, sometimes in compositional ways, but never together with patients. It may be difficult for animals to comprehend events as part of larger social scripts, with antecedent causes and future consequences, which are more typically tie the patient into the event. Human brain enlargement over the last million years probably has provided the cognitive resources to represent social interactions as part of bigger social scripts, which enabled humans to go beyond an agent‐focus to refer to agent–patient relations, the likely foundation for the evolution of grammar. This article is categorized under: Cognitive Biology > Evolutionary Roots of Cognition Linguistics > Evolution of Language Psychology > Comparative Animals and humans may perceive events in similar ways but most likely process them differently in line with their respective representational abilities, ranging from agent‐biased actions, anchored in the present, to agent‐patient social scripts that extend into the future.
... Because nothing in the present paper hinges on this issue, we present the richer meaning. 5 SeeBerthet et al. (2018Berthet et al. ( , 2019 for more recent and complicated data on Titi calls. ...
Article
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In recent years, the methods of formal semantics and pragmatics have been fruitfully applied to the analysis of primate communication systems. Most analyses therein appeal to a division of labor between semantics and pragmatics which has the following three features: (F1) calls are given referential meanings (they provide information about the world rather than just about an action to be taken), (F2) some calls have a general meaning, and (F3) the meanings of calls in context are enriched by competition with more informative calls, along the lines of scalar implicatures. In this paper, we develop highly simplified models to independently assess the conditions under which such features would emerge. After identifying a sufficient condition for (F1), we find a range of conditions under which (F2) and (F3) are not evolutionarily stable, and discuss the consequences for both modeling and empirical work.
Chapter
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Chapter
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Science is about discovering new things, about better understanding processes and systems, and generally furthering our knowledge. Deep in science philosophy is the notion of hypotheses and mathematical models to represent these hypotheses. It is partially the quantification of hypotheses that provides the illusive concept of rigor in science. Science is partially an adversarial process; hypotheses battle for primacy aided by observations, data, and models. Science is one of the few human endeavors that is truly progressive. Progress in science is defined as approaching an increased understanding of truth – science evolves in a sense.
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Some experiments on birds, fish and insects, in which long records of steady‐state behaviour are obtained, are described and the relative merits of three simple models of behaviour considered. As a first approximation, semi‐Markov chains seem to offer a reasonable way of summarizing the data and provide suitable null hypotheses against which to test ethological theories.
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Vocal complexity is an important concept for investigating the role and evolution of animal communication and sociality. However, no one definition of ‘complexity’ appears to be appropriate for all uses. Repertoire size has been used to quantify complexity in many bird and some mammalian studies, but is impractical in cases where vocalizations are highly diverse, and repertoire size is essentially non-limited at realistic sample sizes. Some researchers have used information-theoretic measures such as Shannon entropy, to describe vocal complexity, but these techniques are descriptive only, as they do not address hypotheses of the cognitive mechanisms behind vocal signal generation. In addition, it can be shown that simple measures of entropy, in particular, do not capture syntactic structure. In this work, I demonstrate the use of an alternative information-theoretic measure, the Markov entropy rate, which quantifies the diversity of transitions in a vocal sequence, and thus is capable of distinguishing sequences with syntactic structure from those generated by random, statistically independent processes. I use artificial sequences generated from different stochastic mechanisms, as well as real data from the vocalizations of the rock hyrax Procavia capensis, to show how different complexity metrics scale differently with sample size. I show that entropy rate provides a good measure of complexity for Markov processes and converges faster than repertoire size estimates, such as the Lempel–Ziv metric. The commonly used Shannon entropy performs poorly in quantifying complexity.