A Guyana dolphin leaps to escape the attention of a bottlenose dolphin
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.
I wouldn't be surprised that they can modify their signals to mimic, and even possibly communicate with other species
Biologist Dr Laura May-Collado
Details are published in the journal Ethology.
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."
As yet, Dr May-Collado cannot be sure if both species are changing the way they communicate, or whether it is one species attempting to call more like the other.
That is because her sound equipment could only record the total calls produced by mixed species groups of dolphins, and could not separate out sounds made by individuals.
Bottlenose dolphin (rear) and Guyana dolphin (fore)
"This limits how much I can say about how much they are communicating," says Dr May-Collado.
However, dolphins are known to have an extraordinary ability to change their calls when 'talking' to other individuals, or to ensure they are heard over the din of background noise pollution.
So "I wouldn't be surprised that they can modify their signals to mimic, and even possibly communicate with other species. Particularly when their home ranges force them to interact on a daily basis, which is the case of this study," she says.