When two dolphin species come together, they attempt to find a common language, preliminary research suggests.
Bottlenose and Guyana dolphins, two distantly related species, often come together to socialise in waters off the coast of Costa Rica.
Both species make unique sounds, but when they gather, they change the way they communicate, and begin using an intermediate language.
That raises the possibility the two species are communicating in some way.
It is not yet clear exactly what is taking place between the two dolphin species, but it is the first evidence that the animals modify their communications in the presence of other species, not just other dolphins of their own kind.
Biologist Dr Laura May-Collado of the University of Puerto Rico in San Juan made the discovery studying dolphins swimming in the Gandoca-Manzanillo Wildlife Refuge of the southern Caribbean coast of Costa Rica.
Bottlenose dolphins (Tursiops truncatus) are larger, measuring up to 3.8m long, with a long dorsal fin.
Guyana dolphins (Sotalia guianensis) are much smaller, measuring 2.1m long, and have a smaller dorsal fin and longer snout, known as a rostrum.
Bottlenose dolphins filmed harrassing Guyana dolphins (video courtesy of Dr L. May-Collado)
Both species swim in groups made up of their own kind.
When bottlenose dolphins swim together, they emit longer, lower frequency calls, that are modulated.
In contrast, Guyana dolphins usually communicate using higher frequency whistles that have their own particular structure.
But often, the two species swim together in one group. These interactions are usually antagonistic, as the larger bottlenose dolphins harass the smaller Guyana dolphins.
When the two dolphins gather, they produce quite different calls, Dr May-Collado has discovered.
Crucially, calls emitted during these multi-species encounters are of an intermediate frequency and duration.
In other words, the dolphins start communicating in a style that is somewhere between those of the two separate species.
“I was surprised by these findings, as I was expecting both species to emphasise, perhaps exaggerate, their species-specific signals,” Dr May-Collado told the BBC.
“Instead the signals recorded during these encounters became more homogenous.
“This was a very exciting discovery.”