A new study challenges a long-held theory about bottlenose dolphins in the Mediterranean, revealing that they are divided by offshore and coastal ecotypes rather than being split into east and west populations.
“We thought that there was a general east-west split in their populations based on quite old genetic techniques that wouldn't be used nowadays,” says lead author Daniel Moore, a marine biologist at the University of Exeter.
“We started the study wanting to understand that east-west split a bit more, but using modern techniques, and what we found was just not expected at all.”
This isn't just about points on a map. “The dolphin populations can be split into two different ecotypes: offshore and coastal, just like we find in orca,” says Moore. The two types have different genetics, 'languages' and social structures. The findings are published in the journal Molecular Ecology.
Surprisingly, explains Moore, the dolphins found in Sicily are in the same offshore group as those far away in the Azores. Yet, they don’t seem to interact with coastal dolphins, despite swimming through their waters. "These dolphins have really high gene flow between Sicily and Azores, but virtually nothing between those dolphins and all the coastal populations they must pass along the way,” he says.
Local researchers had suspected that this might be the case. “When you see them in the field, you can see they're different,” says marine biologist and co-author Emily Cunningham MBE, who is married to Moore. “They're bigger, they behave differently.”
Having data to conclusively back up this was “one of those light bulb moments,” she says.
Moore believes cultural differences explain why there is so little interbreeding between the two groups. “If I wasn't married to Emily, the chances of me getting a Japanese girlfriend is very unlikely because I don't speak Japanese,” he says.
Another twist in the tale is that the researchers were only sampling dolphins in the Azores as an outlier group. “This was going to confirm that our Mediterranean dolphins were separate,” says Moore. “There was no expectation at all that, actually, some of them could be the same population.”
Yet, the choice of this location is “game changing,” he says, because without samples from the Azores, “I don't think we would have discovered this.”
Bottlenose dolphins are a protected species in the Mediterranean. Knowing that there are two different ecotypes gives us an insight into the different threats they might face in their offshore or coastal habitats.
“Understanding the population structure of protected species and how that relates to their abundance and threats, such as bycatch, may help assess the risk of extinction of certain populations,” says Dr Tilen Genov, assistant professor at the University of Primorska and founder of Morigenos – Slovenian Marine Mammal Society. This enables experts to work up suitable conservation measures.
“As ocean custodians," adds Cunningham, "we need to understand that so we can better protect them in their environment.”
Top image: Bottlenose dolphin. Credit: Tilen Genov
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