June 26, 2023
Findings are comparable to human mothers, caregivers who modify their speech to infants and children
Woods Hole, Mass. — Human mothers and other caregivers often modify their adult way of speaking when communicating with infants and young children. This “motherese,” also referred to as child-directed communication (CDC), has an acoustic signature that includes higher pitch and wider pitch range, and is thought to promote attention, bonding, and language acquisition in children.
However, humans may not be the only mammals to use motherese with their offspring.
Bottlenose dolphins appear to similarly modify their communication signals when with their calves.
An analysis of recordings made during brief catch-and-release events of wild bottlenose dolphins (Tursiops truncatus) in waters near Sarasota Bay, Florida, found that females produced signature whistles with significantly higher maximum frequencies and wider frequency ranges when they were recorded with their own dependent calves vs. not with them.
Bottlenose dolphin mothers modify their signature whistles in ways that are similar to human "motherese", a speech pattern that is nearly universal across cultures and languages in human caregivers. In this example, slowed down 8 times in order to make these high frequency sounds audible to human ears, a bottlenose dolphin mother’s signature whistle reaches a noticeably higher pitch in the second whistle, when she was with her offspring. (Audio recorded under NMFS MMPA Permit No. 20455 issued to the Sarasota Dolphin Research Program)
“These differences align with the higher fundamental frequencies and wider pitch ranges seen in human CDC. Our results provide evidence in a non-human mammal for changes in the same vocalizations when produced in the presence vs. absence of offspring, and thus strongly support convergent evolution of motherese, or child-directed communication, in bottlenose dolphins,” according to the paper, “Bottlenose dolphin mothers modify signature whistles in the presence of their own calves,” published in the Proceedings of the National Academy of Sciences (PNAS). “CDC may function to enhance attention, bonding, and vocal learning in dolphin calves, as it does in human children. Our data add to the growing body of evidence that dolphins provide a powerful animal model for studying the evolution of vocal learning and language.”
“It’s really exciting to find evidence for CDC in another mammalian species, even if we can’t necessarily speak to its function in dolphins,” said journal article co-lead author Laela Sayigh, a biologist at the Woods Hole Oceanographic Institution (WHOI). “The fact that dolphins use motherese is an excellent example of what we call convergent evolution. That is, a similar type of communicative strategy has evolved in three very different species – humans and dolphins, as well as zebra finches. This is certainly suggestive of the idea that motherese probably serves some function, though we are not in a position to test that,” said Sayigh.
The paper emphasizes that researchers “currently do not know either the mechanistic driver(s) or function(s) of CDC in bottlenose dolphins. For example, acoustic differences could arise as a consequence of physiological changes associated with lactation and/or parental behavior.”
Functions for CDC could include vocal learning, promotion of social bonds, targeting of recipients, or facilitation of caller identity, “or could be non-existent, if the observed acoustic changes are simply byproducts without adaptive value,” according to the paper. “We also are unable to determine whether dolphins are as flexible in their use of CDC as humans, who are able to modulate their vocal production on a moment-to-moment basis, depending on their audience. Further research is needed on this aspect, as well as to elucidate whether CDC is in fact preferred by calves, as it is by human infants (over adult-directed speech). Although the latter finding would still not increase our understanding of the mechanistic basis, it would nonetheless suggest that dolphins have evolved to capitalize on this preference and/or on the occurrence of CDC.”
The study population was a resident community of bottlenose dolphins in and near Sarasota Bay, Florida, which has been the focus of a research program for more than 50 years, led by the Chicago Zoological Society’s Sarasota Dolphin Research Program. The research for this study involved analyzing the recordings of 19 adult female dolphins who, during brief catch-and-release health assessments, were temporarily outfitted with hydrophones with suction cups attached directly to each dolphin’s head.
Although CDC occurs in a variety of human cultures, evidence among non-human species is very limited, according to the journal article. One example is adult male zebra finches (Taeniopygia guttata), which are known to have changed acoustic parameters of their song when singing in the presence of juveniles compared to when singing alone or to females. Other species, including adult squirrel monkeys (Saimiri sp.) and rhesus macaques (Macaca mulatta), used different vocalization types when communicating with young versus older members of their same species, but this is quite different from the more subtle changes associated with CDC in humans.
The journal article also notes that female greater sac-winged bats, Saccopteryx bilineata, produced pup-directed vocalizations that differed in timbre and peak frequency from adult-directed vocalizations; however, the structure of pup-directed vocalizations was also markedly different and it is unclear whether these differences represent the use of a different vocalization type rather than the more subtle changes within vocalization types characteristic of CDC in humans.
“It has been well documented that dolphins are capable of vocal production learning, which is a key aspect of human communication. This study adds new evidence regarding similarities between dolphins and humans. With that said, I’m hopeful that this interesting finding could increase awareness in the general public about the protection of this charismatic species,” said journal article co-lead author Nicole El Haddad, who conducted her master’s thesis project on bottlenose dolphin bioacoustics as a guest student at WHOI.
“This paper could be a springboard, inspiring other scientists to focus on child-directed communication in other species. It would be interesting to compare vocal aspects of different marine mammal mothers in presence of their offspring or not,” El Haddad said.
Journal article co-author Frants Havmand Jensen said the study “reveals an intriguing case of convergent evolution as female bottlenose dolphins modify individual vocalizations (their individually specific signature whistles) in the presence of their calves, mirroring the acoustic changes observed in human motherese.” Jensen is a senior scientist at the Department of Ecoscience at Aarhus University, Denmark, and a guest investigator at WHOI.
“Our findings also have the potential to enhance population monitoring efforts,” said Jensen. “We are developing tools to listen for the unique whistles of individual animals. If we could reliably detect the subtle changes in signature whistles when calves are present, then we can use that to understand reproductive success and overall population health of wild dolphins.”
Financial support for both field and lab portions of this research has come from a wide variety of sources, including Dolphin Quest, Inc., National Oceanic and Atmospheric Administration, Disney, Harbor Branch Oceanographic Institute, The Royal Society, the Charles and Margery Barancik Foundation, Vulcan Machine Learning Center for Impact, Allen Institute for Artificial Intelligence, Adelaide M. & Charles B. Link Foundation, and others. Journal article co-author Peter Tyack received support from several Office of Naval Research grants. Research was conducted under a series of Scientific Research Permits issued by the National Marine Fisheries Service, and was approved by Institutional Animal Care and Use Committees at Mote Marine Laboratory, University of North Carolina Wilmington, Hampshire College, and WHOI.
Authors: Laela S. Sayigha,b,1,*, Nicole El Haddada,c,1, Peter L. Tyacka,d, Vincent M. Janikd, Randall S. Wellse, Frants H. Jensena,f,g
aBiology Department, Woods Hole Oceanographic Institution, Woods Hole, MA, USA
bHampshire College, Amherst, MA, USA
cEarth and Environmental Sciences Department, University of Milano Bicocca, Milano, Italy
dSea Mammal Research Unit, Scottish Oceans Institute, University of St. Andrews, Fife, UK
eChicago Zoological Society’s Sarasota Dolphin Research Program, c/o Mote Marine Laboratory, Sarasota, FL, USA
fMarine Mammal Research, Department of Ecoscience, Aarhus University, Roskilde, Denmark
gBiology Department, Syracuse University, Syracuse, NY, USA
About Woods Hole Oceanographic Institution
The Woods Hole Oceanographic Institution (WHOI) is a private, non-profit organization on Cape Cod, Massachusetts, dedicated to marine research, engineering, and higher education. Established in 1930, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate an understanding of the ocean’s role in the changing global environment. WHOI’s pioneering discoveries stem from an ideal combination of science and engineering—one that has made it one of the most trusted and technically advanced leaders in basic and applied ocean research and exploration anywhere. WHOI is known for its multidisciplinary approach, superior ship operations, and unparalleled deep-sea robotics capabilities. We play a leading role in ocean observation and operate the most extensive suite of data-gathering platforms in the world. Top scientists, engineers, and students collaborate on more than 800 concurrent projects worldwide—both above and below the waves—pushing the boundaries of knowledge and possibility. For more information, please visit www.whoi.edu
- Humans are not the only mammals to use motherese with their offspring. Bottlenose dolphins also modify their communication with their calves.
- An analysis of recordings made during brief catch-and-release events of wild bottlenose dolphins (Tursiops truncatus) in waters near Sarasota Bay, Florida, found that females produced signature whistles with significantly higher maximum frequencies and wider frequency ranges when they were recorded with their own dependent calves vs. not with them.
- “These differences align with the higher fundamental frequencies and wider pitch ranges seen in human CDC. Our results provide evidence in a non-human mammal for changes in the same vocalizations when produced in the presence vs. absence of offspring, and thus strongly support convergent evolution of motherese, or child-directed communication, in bottlenose dolphins,” according to the paper, “Bottlenose dolphin mothers modify signature whistles in the presence of their own calves,” published in the Proceedings of the National Academy of Sciences (PNAS). “CDC may function to enhance attention, bonding, and vocal learning in dolphin calves, as it does in human children. Our data add to the growing body of evidence that dolphins provide a powerful animal model for studying the evolution of vocal learning and language.”