Researchers studying a group of dolphins in the Bahamas have made an intriguing discovery: Most of the group had a right-side bias, much in the way that most humans are right-handed.
Dolphins are cetaceans: water-dwelling mammals. Currently, scientists recognize at least 40 species of dolphin, some of which live in seas or oceans and some of which make freshwater bodies their homes.
These cetaceans have caught the interest of the public and zoologists alike, as their playfulness, complex social networks and behaviors, and display of what may be different emotions all hint at a high level of intelligence.
These and other features have led some researchers to compare them to humans.
New observations — reported by a team of investigators affiliated with the Dolphin Communication Project, in Port St. Lucie, FL, St. Mary's College of Maryland, and Hunter College, in New York — now suggest that dolphins may resemble humans in yet another way.
In a study paper that appears in the journal Royal Society Open Science, the researchers note that, according to their observations over 6 years, most dolphins may have a right-side bias, much in the same way that most humans have a right-hand bias.
A clear preference for the right side
The research team studied a group of 27 bottlenose dolphins, one of the most widespread species of dolphin, based off the coast of Bimini, in the Bahamas. The investigators made their observations from 2012 to 2018.
They watched the dolphins as they engaged in crater feeding — a process of foraging for food that involves using echolocation to locate prey under the sand.
Echolocation involves emitting sounds that bounce off different surfaces. By listening for these echoes, the mammals can determine where their food sources might be. When a dolphin "catches" an echo while crater feeding, they dive and shove their heads into the sand to dig for prey.
As the team began to follow these dolphins, they were intrigued to note that, just before digging into the sand, the dolphins made a sharp turn with their heads.
Over the study period, the dolphins made 709 such turns. Every dolphin except for one turned their heads to the left, each time they dived in.
Only one dolphin turned its head to the right, and this was also consistent.
The bottlenose dolphins' actions as they dive to forage suggest that most of them have a right-side bias — turning to the left indicates that they prefer to face the ocean floor with the right side of their heads.
As with human right-handedness, the reason for this preference remains unclear.
In the study paper, first author J. Daisy Kaplan, Ph.D., and colleagues put forth some hypotheses as to why bottlenose dolphins may be predominantly right-sided.
"A right-sided feeding bias in dolphins may have a physiological drive," the authors write. They explain that, in dolphins, the larynx is situated to the left, "to provide room for a larger right pharyngeal food channel." This may explain why most of the observed dolphins turned their heads to the left as they dove for prey.
Another theory has to do with asymmetries in nose tissue that allows dolphins to emit sounds for echolocation — the tissue structure on the right side, the researchers explain, is larger than the one on the left.
"The bottlenose dolphin possesses the second-largest brain-to-body mass ratio of any mammalian species. Thus, we would expect a high degree of hemispheric specialization in the dolphin brain," the researchers conclude in their study paper.
However, they acknowledge that "How this hemispheric specialization correlates with lateralized behavior remains unclear."
Going forward, the investigators hope that advances in brain imaging techniques will allow zoologists to gain further insights into the equivalent of handedness in dolphins.