Male dolphins appear to "sing" together, matching one another's tempo and producing calls in-sync, to foster cooperation and coerce females into sex, say scientists.
In a study published in Proceedings of the Royal Society B, researchersdescribe synchronous performances as the "hallmarks" of many animal species—from fireflies' pulsing flashes to humans' military marches and ballroom dances.
However, it was thought that humans were unique in terms of the purpose of these displays. Many animals perform in competition with one another—often to attract a mate—but humans perform to encourage cooperation and in-group bonding, and reduce aggression between competitors. The latest findings suggests dolphins will also put on performances as part of a group effort to woo mates.
Co-operation among male dolphins is important because the rely on long-term alliances—sometimes lasting decades—with other males to improve their mating odds. Alliances of four to 14 individuals appear to work together to herd and coerce ovulating females, as well as steal and defend females from other groups.
Previous research has suggested these allied males use movement to foster cooperation, not competition. Groups have been observed moving in unison—swimming side-by-side, breaking the surface within milliseconds of each other and performing acrobatic flips and dives simultaneously.
"We propose they use vocal coordination in the same way," Stephanie King, Senior Lecturer from Bristol's School of Biological Sciences, who led the research, told Newsweek.
Dolphins are known to make "pops"—vocalizations male dolphins make during mating rituals with females called consortships. King and colleagues analyzed recordings of pop trains—a groups of pops made in quick succession—from male Indo-Pacific bottlenose dolphins in Shark Bay, Western Australia.
"Pops are only produced by male dolphins when they are herding females. It's a coercive vocalization that induces the female to stay close to the popping male," said King.
A male might start producing pops if a female attempts to move away, encouraging her to get closer again, she explained. Males also produce pops when they "guard-switch," where the closest male to the female changes.
The group has 30-years' worth of data on the dolphins that live in the bay. Recordings used in the study were collected between 2016 and 2018, and contained 453 pop trains from seven dolphin alliances, which between them involved 59 individuals.
Trains that included more than one male tended to be longer than solo performances and contained more overlap between singing dolphins than would be expected by chance—the overlap of trains averaged at 83 percent.
Collaborating dolphins were found to synchronize at tempos of 600 beats per minute (BPM), outperforming humans, who can synchronize comfortably up to around 200 BPM.
"Synchronous behaviours are common in many different animal taxa, but the study adds by demonstrating acoustic synchronous behavior to facilitate social cooperation among males to gain access to females—which has not been previously reported in wild dolphins," Fernando Diaz-Aguirre in the Cetacean Ecology, Behaviour and Evolution Lab at Flinders University, Australia, who was not involved in the study, told Newsweek.
"They also demonstrated that producing synchronous pop vocalizations is common among allied males in the population and not restricted to just a few individuals."
The authors say there could be an unknown, external factor prompting the dolphins to vocalize in this way, but add this is unlikely. The researchers could not find an alternative acoustic signal that could be triggering the pop trains and visibility in the harbour was too poor to suggest there may be a visual signal.
Instead, they hypothesize the synchronous singing could be the result of hormones. Specifically oxytocin, which has been found to forge trust and cooperation in animal species, including meerkats and gray seals, but not dolphins.
"Outside of humans, very few animals coordinate both vocal signals and physical movement when working together. We have shown that allied male dolphins not only synchronise their movements, but also coordinate their vocal behaviour when cooperating together in alliances," said King.
"The only other example that stands out is the coordinated displays of long-tailed manakins. These displays aren't synchronous, but they do comprise song duets and dance displays between unrelated male dyads in a cooperative context."
