Skull of (a) narwhal, (b) the hybrid analysed in the study, and (c) beluga. Mikkel Høegh Post, Natural History Museum of Denmark.

Danish and Canadian scientists have reported the first known evidence of potential hybridisation between beluga whales (Delphinapterus leucas) and narwhals (Monodon monoceros).

Modern technology allowed them to analyse the genes from a whale skull found in West Greenland in 1990 and stored since then in the Natural History Museum of Denmark.

After comparing DNA extracted from the skull’s teeth with DNA from the genomes of eight live belugas and eight live narwhals from the same area, they determined it was a first-generation male offspring of a female narwhal and a male beluga – and about 54% beluga.

“Our finding of hybridisation between belugas and narwhals is unexpected, as a recent genomic analysis showed that gene flow between the two species ceased 1.25-1.65 [million years ago],” the scientists write in a paper published in the journal Scientific Reports.

“However, hybridisation among cetacean species is relatively common; there are at least 16 described cases of hybridisation between wild or captive cetacean species.”

The research was led by the Mikkel Skovrind and Eline Lorenzen from Denmark’s University of Copenhagen.

With colleagues, they used the ratio between the number of X chromosomes and the number of non-sex chromosomes – a common method to determine the sex of an individual – to infer that the hybrid was male.

Analysis of the mitochondrial genome, inherited exclusively along maternal lines, suggested the hybrid’s mother was a narwhal.

The authors also analysed isotopes of the carbon and nitrogen contained in bone collagen extracted from the skull and compared it to a reference panel of 18 narwhal and 18 beluga skulls.

The concentration of carbon isotopes in the target samples was higher than that in the other skulls, suggesting that the hybrid’s diet was different from either parent species.

From this, the authors infer that the animal foraged closer to the bottom of the sea floor – in the benthic zone – than either narwhals or belugas.

Belugas, also known as white whales, and narwhals, which boast a single massive horn, are the sole representatives of the Monodontidae family. With bowhead whales (Balaena mysticetus) they are the only three of the 89 extant cetacean species found in Arctic waters year-round.

Belugas and narwhals are similar in size – growing to between 3.5 and 5.5 metres long – and have similar migratory and mating patterns, but there are also significant differences, including in appearance, prey selection and diving capabilities.