Research out of Woods Hole Oceanographic Institution and Hampshire College in Massachusetts published on 26 June has revealed that Bottlenose dolphins, a mammal with long-term mother-offspring relationships, exhibit higher frequency vocalisations when in the presence of their calves.
Individuals of all dolphin species use a 'signature whistle' for individual identification and localisation, with calves acquiring their whistles during their first year of life. Human languages, much like dolphin vocalisations, exhibit 'enormous variation' with few of its features being under genetic control. As such, our offspring must learn our languages, and all their nuances, from scratch which often entails thousands of hours of exposure. Vocal learning is extremely rare, says behavioural ecologist Rindy Anderson, with only a few groups including seals, non-human primates, and birds learning to communicate in this way.
Child-directed communication (CDC) is a widely researched phenomenon in which animals present differences in their vocalisations or gestures when communicating with their offspring. This so-called 'motherese' is prevalent throughout human cultures, and is believed to enhance bonding and facilitate language development. Observation of CDC in nonhuman species, however, is rare. One example outside of the mammalian class is the male Zebra Finch (known in biological taxonomy as Taeniopygia guttata) which changes the acoustics of its song when in the presence of juveniles, as compared with singing alone or to females. Changes in the acoustic parameters of vocalisations during CDC often include higher frequencies and frequency ranges; think of a human parent using a 'higher pitched' voice when speaking to their child. By examining the development of CDC in nonhuman species, scientists may be able to demonstrate convergent evolution of 'baby-talk' in a nonhuman mammal for the first time.
The recordings of dolphin whistling used in this research had been accumulated into a vast database by a research programme in Florida between 1984 and 2018. Dr Laela Sayigh and colleagues produced spectrograms of these recordings, visualising the spectrum of frequencies emitted during the whistles, and revealed the use of higher frequencies by mothers in the presence of their calves vs when alone. All calves were two years old at the time of their recordings, leading researchers to believe that the use of child-directed-communication by their mothers is not to facilitate language development. It has been suggested that the females are producing these altered signature whistles to reinforce their bond with their calves, who had already acquired their signature whistles at the time of their capture. Dolphin communication is very different from that in humans, with one dolphin repeating its own signature whistle and waiting for a counterpart to respond with its own.
Despite the apparent difference in the purpose of Child-directed-communication in humans and Bottlenose dolphins, its existence in different mammals could have important implications for understanding the evolution of language and vocalisation. However, further investigation must be undertaken following this discovery, as this research only establishes 'motherese' to be used in one context.
