Hidden DNA found in blue whales reveals they've been mating with other species — and their hybrid offspring

Blue whales in the Atlantic Ocean are harboring a previously unknown and potentially alarming level of hybrid DNA, a new study shows. The findings hint that whale hybrids are much more reproductively viable than previously realized.

Blue whales (Balaenoptera musculus) are the world's largest animal, capable of reaching an astonishing 110 feet (34 meters) long, around three times longer than a school bus. 

The number of these giants plummeted in the early 20th century due to extreme levels of commercial whaling. As a result, blue whales are currently listed as endangered by the International Union for Conservation of Nature's (IUCN) Red List of Threatened Species, although their numbers are starting to rebound worldwide. Of the four subspecies of blue whale, B. musculus musculus, which is found in the North Atlantic and North Pacific, is one of the most at-risk.  

In a new study, published Jan. 6 in the journal Conservation Genetics, researchers analyzed the genomes of B. m. musculus in the North Atlantic for signs of inbreeding, which could impede the recovery of this group. 

The researchers created a "de novo" genome for this population, meaning it was created from scratch, by piecing together bits of DNA from different individuals. The team then used this new genetic blueprint as a template to further analyze the full or partial genomes of 31 individuals from across this population's range.

"This is a long, laborious process akin to assembling a huge jigsaw puzzle with no picture on the box for guidance," study co-author Mark Engstrom, an ecological geneticist at the University of Toronto, told Live Science in an email. But once the puzzle has been solved, it becomes much easier to do it again and again, he added.

They found that each of the sampled whales had somefin whale (Balaenoptera physalus) DNA lurking within their genomes. Around 3.5% of the group's DNA came from fin whales on average.