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LANGUAGE DISCRIMINATION BY LARGE-BILLED CROWS

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Language discrimination has previously been found in human infants, cotton-top tamarin monkeys, rats, and Java sparrows. This ability might also be relevant for the crow, a social passerine with extensive auditory perceptual skills living in close contact with humans. In this experiment we tested whether crows autonomously pay attention to spoken language, and whether they can discriminate a familiar, locally spoken language (Japanese) from an unfamiliar language (Dutch) without training. When presented with sentences spoken by multiple speakers, the crows showed significantly more responses to the Dutch than to the Japanese, which suggests that they discriminate two languages with distinctive linguistic features, and that they might also be more attentive to an unfamiliar language, Dutch, compared to a familiar one, Japanese. These results further extend the hypothesis that language discrimination is based on a general perceptual mechanism that predates the evolution of language and show that crows can voluntarily apply this mechanism to language outside of experimental setups .
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LANGUAGE DISCRIMINATION BY LARGE-BILLED CROWS
SABRINA SCHALZ*1, and EI-ICHI IZAWA2
*Corresponding Author: SS3903@live.mdx.ac.uk
1Department of Psychology, Middlesex University, London, UK
2Department of Psychology, Keio University, Tokyo, Japan
Language discrimination has previously been found in human infants, cotton-top tamarin
monkeys, rats, and Java sparrows. This ability might also be relevant for the crow, a social
passerine with extensive auditory perceptual skills living in close contact with humans. In
this experiment we tested whether crows autonomously pay attention to spoken language,
and whether they can discriminate a familiar, locally spoken language (Japanese) from an
unfamiliar language (Dutch) without training. When presented with sentences spoken by
multiple speakers, the crows showed significantly more responses to the Dutch than to the
Japanese, which suggests that they discriminate two languages with distinctive linguistic
features, and that they might also be more attentive to an unfamiliar language, Dutch,
compared to a familiar one, Japanese. These results further extend the hypothesis that
language discrimination is based on a general perceptual mechanism that predates the
evolution of language and show that crows can voluntarily apply this mechanism to
language outside of experimental set-ups.
1. Introduction
Although language as a whole is unique to humans, some cognitive abilities
necessary for language are shared with other species. Ramus and colleagues
(2000) showed that both human newborns and cotton-top tamarin monkeys
(Saguinus oedipus) can discriminate between two languages from different
rhythmic classes. They used Japanese (mora-timed) and Dutch (stress-timed)
sentences spoken by four different female speakers per language and presented
them to the infants and monkeys in a habituation/dishabituation design. In a
second experiment, they synthetized these sentences to only include prosodic
characteristics and removed lexical and phonetic information, as well as speaker
variability (see Ramus & Mehler, 1999 for full description). They found that
human infants failed to discriminate the natural stimuli, but successfully
discriminated the synthetized stimuli containing only prosodic information. On
the other hand, the tamarin monkeys were able to discriminate both types of
stimuli despite speaker variability, although they performed better with the natural
sentences than with the synthetized sentences. When presented with the
synthetized stimuli played backwards, both the tamarins and the infants failed to
discriminate the stimuli sets. The authors conclude from these observations that
the ability to extract and process cues relevant for language discrimination likely
preceded human speech, although humans and tamarins may use different cues
for this task.
Taking into account a conceptual replication by Toro, Trobalon and Sebastián-
Gallés (2003), this perceptual mechanism may date back even further. Their work
with Long-Evans rats (Rattus norvegicus) using lever-press training with the
original stimuli by Ramus et al. (2000) showed that just like human newborns and
tamarin monkeys, rats are able to discriminate between synthetized stimuli of
different rhythmical classes (stress-timed Dutch and mora-timed Japanese) when
they are played forwards, but not backwards. Further research by Toro, Trobalon
and Sebastián-Gallés (2005) showed that the rats were able to generalize
previously learned prosodic cues to novel stimuli, and that they could only
discriminate natural sentences produced by a single speaker, but not those
produced by multiple speakers. It is indeed curious that all three species tested
performed equally well with the synthetized stimuli, while there seems to be large
variations in their performance with natural stimuli, possibly due to irrelevant
information introduced by speaker variability.
The great number of parallels between birdsong and human language make
passerines a well-suited model organism for biolinguistics (see Doupe & Kuhl,
1999), and their sensitivity to acoustic features can be extended to human
language as well. To name just a few examples, Java sparrows can discriminate
between prosodic patterns in Japanese spoken either with admiration or suspicion
and generalize them to new sentences if the prosody remains familiar (Naoi,
Watanabe, Maekawa & Hibiya, 2012), and discriminate English and Chinese
sentences spoken by a bilingual speaker and generalize this discrimination to new
sentences and a new speaker with training (Watanabe, Yamamoto & Uozumi,
2006). Zebra finches can discriminate between familiar and novel infant-directed
songs and speech in English and Russian (Philmore, Fisk, Falk & Tsang, 2017),
discriminate between trochees and iambs (Spierings, Hubert & ten Cate, 2017),
use formant frequencies to discriminate the words wit and wet despite speaker
variability (Ohms et al. 2009), and abstract prosodic patterns of human speech
with prosodic stress on either the first or final syllable and generalize them to new
stimuli (Spierings & ten Cate, 2014). Spierings and ten Cate (2014) conclude from
this that “the sensitivity to prosodic cues is not linked to the possession of
language and might have preceded language evolution, possibly originating from
a pre-existing sensitivity to meaningful variation in pre-linguistic communicative
sounds.”
Crows live in social groups or fission-fusion societies (Clayton & Emery, 2007),
which requires them to vocally communicate with group members and identify
conspecifics based on auditory cues. They can discriminate conspecifics based on
their unique vocal signature (Kondo, Izawa & Watanabe, 2010), discriminate
reliable and unreliable conspecifics based on their individual call (Wascher,
Hillemann, Canestrari & Baglione, 2015), and recognize group members using
audio-visual cues (Kondo, Izawa & Watanabe, 2012). In addition to conspecific
calls, crows also discriminate between familiar and unfamiliar human voices,
possibly because they often live in close contact with humans (Wascher, Szipl,
Boeckle & Wilkinson, 2012).
Taking into account these extensive capabilities related to the auditory
discrimination of individual calls and the self-motivated attention to voices of
both conspecifics and heterospecifics, crows may also be attentive to linguistic
features of languages spoken in their surroundings. The purpose of this
experiment is therefore to examine whether crows autonomously pay sufficient
attention to spoken language to discriminate a familiar from an unfamiliar
language without prior training. Foregoing training and keeping the experimental
set-up as naturalistic as possible has the advantage of showing more accurately
the linguistic capabilities and the degree of attention to linguistic features wild
urban crows living in close contact with humans display on their own. We used
the same stimuli previously used in Ramus et al. (2000), and Toro et al. (2003,
2005) to allow for a more accurate comparison between the findings of this
experiment and the previous language discrimination experiments with human
infants, cotton-top tamarin monkeys, and rats. Such a comparison might highlight
the analogies and heterogeneities between these species, and thereby provide
further insights into the evolution of the mechanisms necessary for language
discrimination.
2. Method
Eight large-billed crows (Corvus macrorhynchos; four males and four females)
between the ages of two and four years were tested. One female crow was
excluded from analysis due to lack of response. All subjects were caught in the
prefectures Tokyo, Chiba, and Ibaragi with the permission from the
Environmental Bureau of the Tokyo Metropolitan Government. The crows were
housed in individual stainless steel-mesh home cages with a total of twenty-four
crows in the room of the animal experimental facility at Keio University. Both
caretakers and previous experimenters were native Japanese speakers. Before and
after the experiment, they had access to food and water ad libitum.
The experiment was carried out in an outdoor aviary (W1.5 × D2.7 × H1.6 m). In
the aviary, four perches were installed in the back, middle, front and the front-
right corner approximately 1m above ground. A water basin was placed on the
ground. Outside the aviary, a wireless loudspeaker (Sound Link Mini, Bose, USA)
was placed next to the front-right corner for stimulus presentation, and a video
camera (Handy-Cam HDR CX535, Sony, Japan) for recording the crows’
behaviour was placed at 50 cm from the front end of the aviary.
We used twenty Dutch and twenty Japanese sentences as stimuli. They were all
declarative, adult-directed, approximately 2.5 seconds long, and spoken by four
female native speakers. After the habituation to the aviary on three consecutive
days, the crows were tested for their responses to the Dutch and Japanese stimuli
in a total of eight trials which were distributed over four days (i.e., two trials per
day). Four crows received Dutch stimuli for the first four trials and Japanese
stimuli for the last four trials, while the other three crows were assigned the
opposite language order. Before the start of each trial, the crows were given 35
min for familiarization to the surroundings. Each trial consisted of four blocks of
stimulus presentation with inter-block intervals of a 12-min silent period. Within
each block, a set of ten sentences spoken by two different speakers was
continuously presented twice in a random order. A 30 min silent period was
inserted between the trials each day. The trial schedule including stimulus
presentation was controlled by the programme PsychoPy 3 (Peirce, 2007). The
sound level was set at a range between 70 and 80 dB across the perches.
According to the different behavioural responses to 1,000 Hz and 1,600 Hz tone
stimuli between individual crows in a pilot experiment, either of two behaviours
as response for each crow was measured during the stimulus presentation blocks
from the video-recorded data: the amount of time they had their head lifted at least
above the horizontal line, or the amount of time they sat on the right half of the
front perch or on the perch in the front right corner close to the loudspeaker.
Response times were coded in BORIS (Friard & Gamba, 2016). To normalize the
response time to the stimuli varying slightly in their durations for each crow, we
calculated the relative value of response time per 10 seconds to the total stimulus
duration in each (see equation 1).
1)   
 
The results were analysed using a generalized linear mixed model with an inverse
Gaussian error distribution and a log link function. The model included the
relative response time as an independent variable, the language as a fixed effect,
and the individuals and the blocks within each trial as random effects. These
analyses were performed using the free software R v.3.6.1 with the lme4
package (Bates, Maechler, Bolker & Walker, 2015). Significance of the
independent variable was tested based on the Wald tests at the 0.05 level. Animal
housing and the experimental protocols adhered to the guidelines of the Animal
Care and Use Committee of Keio University.
3. Results
The model analysis produced a significant effect of the language variable with a
negative coefficient for Japanese (p < 0.001, t = -4.90, β ± S.E. = -0.39 ± 0.08;
figure 1). This result suggests that the crows were significantly more attentive to
the Dutch sentences than the Japanese sentences.
Figure 1. Relative response time per 10 seconds during the Dutch and the Japanese stimulus blocks.
The crows showed more responses to the Dutch sentences than to Japanese ones.
At the individual level, five out of the seven crows clearly showed more responses
to the Dutch stimuli that to the Japanese one (figure 2).
Figure 2. Individual relative response time per 10 seconds of each crow to the Dutch (left) and the
Japanese (right) stimuli. The response behaviour for the crows in the first row was the amount of time
their head was raised above the horizontal line, the response behaviour for the crows in the second
row was the time they sat in the area next to the speaker.
4. Discussion
The results show that crows can discriminate between Dutch and Japanese
sentences despite speaker variability and without prior training. A priori, crows
should not be more interested in one language over the other. The initially higher
attention to Dutch suggests that the crows were already familiar with Japanese
before the experiment. Since all of them were caught in highly urbanized areas in
Japan in and around Tokyo and were then in contact with Japanese experimenters
and/or caretakers on a daily basis, it is safe to assumed that they were exposed to
Japanese for their entire lives. This would support the hypothesis that crows
actively listen to human speech of their own accord and without experimental set-
ups to a degree that would enable them to identify and later recognize key features
of Japanese independent of the individual speaker that distinguish it from other
languages. Dutch, on the other hand, would likely be completely new to them and
thus prompt them to pay more attention to it at first. This reaction would then be
expected to gradually decline as they habituate more and more to it, and
eventually their attention to Dutch should be equal to their attention to Japanese.
The individual differences between the crows may be partially due to experience.
“WW” and “Blu”, who were almost equally attentive to the two stimuli sets, are
also the youngest crows at two and three years respectively and are considered
juvenile, while the other crows are four years old. The shorter exposure to
Japanese due to their young age might be the reason for their failure to
discriminate it from Dutch, although further research is needed to verify this
hypothesis.
Crows in urban areas such as Tokyo live in close contact with humans and speech
would therefore be relevant to them, as it conveys information about the speaker’s
identity and helps them determine whether they already know the specific person
(Wascher et al. 2012) and whether that person might pose a threat. The perceptual
abilities required for their extensive repertoire of vocalizations to communicate
with conspecifics (Conner, 1985) and to discriminate group members based on
their vocal signature (Kondo et al. 2010) may also be extended to the perception
and categorization of human speech. Further experiments with crows from urban
areas in other countries as well as crows from uninhabited areas are necessary to
see whether the increased attention to the non-local language, or rather any
language for crows from uninhabited areas, is consistently present.
These results stand in clear contrast with those obtained from human infants, who
failed to discriminate the natural Dutch and Japanese sentences prior to the
removal of non-prosodic information (Ramus et al. 2000). Speaker variability is
likely the reason for this, as the rats successfully discriminated natural sentences
spoken by only one speaker but failed when they were spoken by different
speakers (Toro et al. 2005). The crows as well as the tamarins’ successful
discrimination despite speaker variability points towards a more robust extraction
of relevant linguistic features disregarding irrelevant information than that
displayed by human infants and rats. Bird song and the vocalizations of New
World monkeys show several similarities (see Snowdon, 1989), such as the
repertoire of chirps and whistles used by cotton-top tamarin monkeys to convey
different messages (Cleveland & Snowdon, 1982). Toro et al. (2005) argued that
their experience with this type of vocalization, experience that rats do not have
and infants have yet to gain, facilitates the discrimination task for the tamarin
monkeys, which might also be the case with the crows. These results further
support the previous findings in mammals and passerines that language
discrimination is not a uniquely human ability and is instead based on a general
perceptual mechanism that evolved prior to human language.
Taken together, the results obtained in this experiment show that crows living in
close contact with humans are sufficiently attentive to spoken language out of
intrinsic motivation to extract and recognize linguistic features distinguishing
different languages from each other despite variation introduced by speaker
variability. The crows’ self-motivated attention to language could point towards
an adaption to sharing their habitat with humans, as an increased attention to
human vocalizations might provide information on danger, comparable to
eavesdropping on heterospecific alarm calls observed in multiple species (e.g.
Meise, Franks & Bro-Jørgensen, 2018). This attention to linguistic features may
not be limited to language discrimination or the recognition of familiar voices.
Further experiments are necessary to see which elements of language animals
living in urban areas are also sensitive to, and whether there are any differences
compared to individuals from rural areas.
Acknowledgements
We are very grateful to Frank Ramus for kindly supplying the original stimuli
used in Ramus et al. (2000, 2005) and Toro et al. (2003, 2005). This experiment
was carried out as part of the Thesis@Keio programme at Keio University. The
study was financially supported by JSPS KAKENHI #17H02653 and Keio
University Grant-in-Aid for Innovative Collaborative Research Projects
#MKJ1905 to E.-I. I.
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... We thank Ei-Ichi Izawa for enabling the incidental recording of the calls during an unrelated project (Schalz & Izawa, 2020) that was carried out as part of the "Thesis@Keio" programme at Keio University and financially supported by JSPS KAKENHI #17H02653 and Keio University Grant-in-Aid for Innovative Collaborative Research Projects #MKJ1905 to E.-I. I. ...
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