A special form of vocal feedback in humans, infant-directed speech — also known as motherese or ‘baby talk’ — facilitates language learning and is socially beneficial by increasing attention and arousal in the child. It is characterized by high pitch, expanded intonation contours and slower speech tempo. Among animals, mothers often engage in infant-directed vocalizations too, but does this also imply voice changes? In a new study, a team of researchers from the Smithsonian Tropical Research Institute, Free University of Berlin and the Natural History Museum in Berlin wanted to investigate if they can detect a phenomenon reminiscent of baby talk in infant-directed female vocalizations of bats. Their results show that parent-offspring communication in bats is more complex and multifaceted than previously thought, with female pup-directed vocalizations reminiscent of human baby talk and male pup-directed vocalizations that may facilitate the transmission of a vocal signature across generations.
Mother-pup pair of the greater sac-winged bat (Saccopteryx bilineata) in their daytime roost. The pup (dark fur color) is holding on to the mother’s belly (light fur color). Image credit: Michael Stifter.
Infant-directed vocalizations are produced in many birds and mammals, for example in primates, bats, seals, cliff swallows, and king penguins.
The function of these vocalizations is to mediate social interactions between adults and young (parent-offspring reunions) and to influence the vocal ontogeny of offspring.
In non-vocal learning species, they can influence vocal repertoire maturation or turn-taking whereas in vocal learning species, they can influence vocal signatures.
In humans, the use of infant-directed speech by which adults address the child is a well-known phenomenon.
This ‘baby talk’ is characterized by unique universal features such as higher pitch, increased frequency range and slow tempo and is significantly different from adult-directed speech.
Studies on a similar phenomenon as ‘baby talk’ with regard to acoustic characteristics in non-human animals are extremely rare.
“I believe that bats are a very promising taxon to investigate key shared features of language, such as the vocal learning ability, and that our study will inspire further studies in the biolinguistics field,” said Dr. Ahana Fernandez, from the Natural History Museum in Berlin and the Free University of Berlin.
Dr. Fernandez and her colleague, Dr. Mirjam Knörnschild of the Smithsonian Tropical Research Institute, focused on the greater sac-winged bat (Saccopteryx bilineata), a highly social species of bat that lives in Central and South Americas in stable colonies and possesses a large vocal repertoire.
Female choice in mating is very pronounced in these bats, which probably led to the complexity of courtship vocalizations in males.
During their first three months of life, as pups start experimenting with their ‘speech,’ female and male adult bats respond to them differently.
Through sound recordings of their vocal interactions, the researchers found that mother bats interact with pups as they ‘babble,’ which could be interpreted as positive feedback to pups during vocal practice.
Much like human baby talk, the pup-directed vocalizations of adult females presented a different ‘color’ and pitch than the calls directed towards other adult bats.
Male bats also communicated with the pups, but in a way that seemed to transmit the ‘vocal signature’ of their social group.
“Pup isolation calls are acoustically more similar to those of males from the same social group than to those of other males,” Dr. Knörnschild said.
“These results suggest that adult male vocalizations may serve as guidance for the development of group signatures in pup calls.”
“These results show that social feedback is important during vocal development, not only in humans but also in other vocal-learning species like Saccopteryx bilineata,” Dr. Fernandez said.
The study was published in the journal Frontiers in Ecology and Evolution.
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Ahana Aurora Fernandez & Mirjam Knörnschild. Pup Directed Vocalizations of Adult Females and Males in a Vocal Learning Bat. Front. Ecol. Evol, published online August 14, 2020; doi: 10.3389/fevo.2020.00265
This article is based on press-releases provided by the Smithsonian Tropical Research Institute and the Natural History Museum in Berlin.
