Chimpanzee calls are closer to human language than we thought

(ORDO NEWS) — It has previously been suggested that animals have a very limited ability to produce flexible yet structured vocal sequences, such as human language sentences.

A team of German and French scientists analyzed about 900 hours of sound recordings of common chimpanzee calls to prove that this is not so.

The origin of human language has always been one of the main mysteries of evolution. Scientists are especially interested in the question of how the unique human ability to combine a limited set of sounds into words, and words into structured sentences arose, which allows us to use an infinite variety of phrases and, as a result, their meanings.

Other animal species, such as birds and bats , also use a finite set of sounds to communicate, but their ability to combine these sounds into flexible yet structured sequences has so far seemed very limited.

The authors of the study, whose results are published in the journal Communications Biology , decided to study the structure of the vocal signals of common chimpanzees ( Pan troglodytes ), it is known that they can use short calls both separately and as part of longer “phrases”.

The scientists analyzed 4,826 recordings of 46 wild adult chimpanzees (belonging to three family groups) from the Tai National Park in Côte d’Ivoire (900.8 hours of recording) and identified 390 unique sound sequences.

Sequence flexibility, orderliness, and recombinativity (ie, the ability to incorporate short signals into longer sequences, making AB part of the ABCD sequence) were evaluated.

Previously, these signs were observed separately in the signals of various animal species (for example, flexibility in eastern tits and orderliness in gibbons ), but it was assumed that only in human language did they appear together.

Chimpanzee calls are closer to human language than we thought
The ratio of the length and number of different signals. The horizontal shows the number of vocal units in the sequence, the vertical shows the number of such recordings (in logarithmic transformation). Sequences containing one to seven units (highlighted in dark blue) were reproduced by both sexes, while sequences with eight or more individual units (highlighted in blue) were reproduced only by females. The numbers above the columns indicate the number of chimpanzees that spoke the sequence of that length. Red and orange frames indicate bigrams and trigrams used for further analysis

Most of the signals reproduced by chimpanzees separately were also reproduced in two-part sequences (bigrams), which, in turn, were included in three-part sequences (trigrams): in total, bigrams and trigrams accounted for about 80% of all recorded signals.

First, the flexibility of chimpanzee vocal cues was assessed using bigrams by tracking how widely individual vocal units combined with other units (for example, squeals with grunts, pants, or roars).

Then the ordering was evaluated: it was necessary to find out which bigrams were encountered more often than usual, and in them to analyze which units more often occupied the position at the beginning, and which ones at the end.

Finally, in order to assess recombinativity, the authors of the work studied trigrams: they identified the most common among them and looked at whether the most “popular” bigrams are included in such trigrams. Then the researchers checked where the selected digram is located in relation to the third vocal unit.

As it turned out, chimpanzee vocal signals are markedly structured. First, 11 out of 12 sounds recorded in these animals were reproduced both as separate signals and as part of more complex sequences, combined with four to nine other sounds.

Second, more than half of the chimpanzee sequences were flexible, the order of sounds in them could be different (both AB and BA), although the appearance of some sounds (like a dull “hoot”) was more predictable.

Thirdly, certain digrams most often appeared either at the beginning or at the end of a trigram and were predictably combined with certain other vocal units.

Overall, the findings suggest that chimpanzees are able to flexibly combine individual units of their vocal repertoire, organizing them into two-part compositions, and these into three-part compositions.

Scientists have found that some sounds can be combined indefinitely, while others occupy strictly defined positions in complex sequences and are always combined with other specific sounds.

Since chimpanzees seem to have some idea of ​​the “correct” sequence of sounds in a complex phrase, this confirms earlier evidence that many species of primates, in addition to humans, have “grammatical sense” , that is, they are able to sense violations of order between adjacent sound units.

Although the amount of data obtained seems significant, in fact, even more sound recordings will be required to determine the full vocal repertoire of chimpanzees and establish the importance of structured sequences in the transmission of information by these monkeys.

However, even preliminary evidence suggests that early conclusions about the very limited ability of animals to produce flexible vocal sequences with changing meanings may be premature.

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