Study Confirms A 260-Year-Old Theory About Pilot Whales

Illustration of the two subspecies of short-finned pilot whales. Natalie Renier/Woods Hole Oceanographic Institution

Sometimes animals that look the same are, in fact, different. Other times, animals that look different are actually the same species. And on some occasions, science has to go out of its way to prove that two animals that look different are indeed different on a genome level. This was the case for the short-finned pilot whale, a species commonly found in the Atlantic, Pacific, and Indian Oceans. As it turns out, they are not a single species. Instead, two subspecies of pilot whales exist, which the researchers proved using genetic analysis. The findings are reported in Molecular Ecology

The idea that short-finned pilot whales are not a single species actually comes from the first description of them by Japanese naturalist Yamase. He identified the pilot whales in northern Japan as having round heads (“Shiho”) and the ones living in southern Japan as having square heads (“Naisa”). These descriptions were also found in subsequent papers by British zoologist J.E. Gray.

Yet, the Shiho and Naisa whales had so much in common that, despite physical differences, they were considered the same type of animal. The Shiho are only found in Northern Japan and the Eastern Pacific, while the Naisa are found across this gap and beyond. Despite clear physical differences, researchers assumed that they must be the same species since there was no physical barrier that would justify the emergence of two subspecies.   

“You would expect to see a different subspecies of whale in each ocean basin –  the Atlantic, Indian, and Pacific. That’s pretty common. But what we found was that short-finned pilot whales in the Atlantic are the same sub-species as those living in the Indian Ocean and western Pacific,” lead author Dr Amy Van Cise, from the Woods Hole Oceanographic Institution, said in a statement.

Distribution of short-finned pilot whales around the world. Map by Natalie Renier, Woods Hole Oceanographic Institution.

The analysis of mitochondrial DNA from 735 samples of short-finned pilot whales confirmed that the morphology is indeed a clear indication of two separate species. The wide region in the Pacific seems to have formed a barrier due to food selection possibilities.

“It seems to have separated these groups of whales for long enough that they diverged into two different types,” Dr Van Cise explained. “That means continents and land forms may not have been as significant a barrier as we thought to this species’ evolution. Instead, the oceanic ‘desert’ in the Pacific might have been more important.”

This is the first study to look at the genetic diversity of short-finned pilot whales. The genetic analysis hints at the possibly that more subspecies will be uncovered in the future. The data suggest that within the Naisa subspecies, there are two divergent populations – one living in the Atlantic and one in the Indian and Pacific Ocean.  

“You can’t manage animals globally without understanding their diversity. If you think of a group of animals as a single species, and it turns out they’re not, you could wind up accidentally losing an entire subspecies without knowing it,” Dr Van Cise added.

While these animals are not endangered worldwide, they are vulnerable to mass strandings. Just last year about 150 short-finned pilot whales washed ashore in Australia. 

 

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