Bottlenose dolphins in South America may be splitting into two different species right before our eyes. Their distinct genetic variations may have come about due to their differing habitats.
Along the coastlines of southern Brazil, Uruguay and Argentina in the South Atlantic Ocean, common bottlenose dolphins (Tursiops truncatus) that live near the shore don’t look like their cousins that live further out to sea. They are longer, lighter and have a triangular dorsal fin, in contrast to the offshore dolphins’ hooked dorsal fin and darker skin.
The two kinds of common bottlenose dolphin live differently from one another, too. Those in the coastal “ecotype” form small groups in bays and estuaries and don’t stray too far from home, while those in the offshore ecotype live in pods of hundreds and roam widely.
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When Ana Costa at the University of Glasgow in the UK and her team looked at the dolphins’ skeletons, they found still more differences: the offshore dolphins had shorter and more plentiful vertebrae than the coastal variety.
“This is a charismatic, worldwide species, and it is one of the most well-studied cetacean species. Thus it is surprising that, up until now, these differences went undetected,” says Costa.
The dolphins were so physically and behaviourally different that Costa wondered if the two groups had gone down different evolutionary paths, separating into two species – meaning that the ecotypes had stopped interbreeding.
To find out, she and her colleagues collected more than 250 tissue samples from common bottlenose dolphins of both ecotypes off Brazil’s coast and analysed the DNA. They also compared the skulls of 106 dolphins from the same region. The genetic and skeletal differences were closely grouped together by ecotype, and genetic analysis revealed little evidence of recent interbreeding between the two varieties.
Costa says the results suggest that the dolphins’ opposing habitat preferences are driving the evolution of coastal and offshore species. The genetic analysis included differences in a small portion of the dolphins’ genomes that experience fast rates of change over evolutionary time. Costa says she wants to compare more genes from the two ecotypes in the future.
“All the findings so far are demonstrating that we are observing two dolphin groups in process of speciation,” she says.
Marie Louis at the University of St. Andrews in the UK agrees that the two ecotypes may be splitting into two species. She notes that the genetic differences are stark, as the South Atlantic’s offshore dolphins are more closely related to offshore varieties in the North Atlantic than to their coastal neighbours in the south.
Costa says the findings support the designation of the two ecotypes as different subspecies: T. truncatus gephryeus for those near shore and T. truncatus truncatus for those offshore. She and Louis both say the findings reveal information that is crucial for conservation of the dolphins.
“Anthropogenic pressures on the two ecotypes are likely different, as they live in different habitats,” says Louis. “It is essential to manage [the populations] separately to preserve the existence of the two ecotypes.”
Journal of Evolutionary Biology
