The crown, not the root! On teeth and dentition in Narluga, the hybrid son of a narwhal mum and a beluga dad

Published in Ecology & Evolution
The crown, not the root! On teeth and dentition in Narluga, the hybrid son of a narwhal mum and a beluga dad

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Narwhals (top) and belugas (bottom) are the two only toothed whale species endemic to the Arctic. Photos: Mads Peter Heide-Jørgensen

When you push open the large metal door and enter the mammal collections of the Natural History Museum of Denmark, University of Copenhagen, you immediately get hit by a pungent smell of dust, whale blubber and chemicals. The skull, ID MCE1356, rests in a wooden box on a table, in between mounted whale skeletons and rows of cetacean bones and skulls stacked from floor to ceiling in wooden scaffolds. 

Skull MCE1356 in the mammal collections of the Natural History Museum of Denmark. Behind it on the table, a narwhal tusk and a beluga skull
Bones, bones, bones

Skull MCE1356 differs markedly in appearance from that of any other known toothed whale. 'The Keith Richards of cetaceans!', as a colleague recently remarked. It has a row of teeth that protrude horizontally from the top and bottom jaws. When you pull the teeth out of the skull, they vary in thickness and in length, and have the most peculiar shapes. The teeth of the upper jaw have long, pointed roots several times the length of the crown. In the lower jaw, the teeth are wide and blunt, and some of them spiral to the left, just like the tusk of a narwhal. The following words are neatly inscribed in black cursive on the skull: Delphinapterus leucas x Monodon monoceros. A cross between a beluga and a narwhal. 

We call it Narluga. Because it sounds better than Belwhal.

Skull MCE1356, better known as Narluga. Photo: Mikkel Høegh Post
Weird dentition. Six Narluga teeth top left. Some long and pointy, some short and wide that spiral to the left, just like a narwhal tusk (at bottom). Three beluga teeth for comparison top right. For all teeth, roots are to the left, crowns to the right.

Three decades ago, the odd-looking skull was noticed on top of hunter Jens Larsen’s tool shed in Disco Bay, West Greenland, by a colleague of mine from the Greenland Institute of Natural Resources, Mads Peter Heide-Jørgensen. He interviewed the hunter, who said he had kept the skull on account of its odd appearance. It was from a whale he had shot during a subsistence hunt a few years prior; at around the same time, he had shot two other, similar-looking whales, although no remains were preserved from them. Jens Larsen let Mads Peter bring the skull back to Copenhagen for further examination. 

The skull looked like a mash-up of a narwhal and a beluga. They are the only toothed whales endemic to the Arctic, and each others’ nearest relatives. They diverged ~5 Mya, and recent work by my postdoc Mick Westbury on their genomes showed that gene flow ceased between the species at least 1.25 Mya. Although their ranges rarely overlap, both species migrate along the coast of West Greenland in winter, when the inuit subsistence hunts take place. By comparing the morphology of MCE1356 with a reference panel of narwhals and belugas, it was proposed that the skull either belonged to a narwhal/beluga hybrid, or to an unwitting beluga with weird dentition. 

This all happened in 1990, when the genomic methodologies that have now enabled us to determine the origin of the skull, were not yet in place. So whether it was indeed a hybrid could not be confirmed.

As luck would have it, when I started my position as mammal curator at the Museum in 2015, I was applying for funding for my research programme on Arctic marine mammals. The planned work included range-wide, population genomic analysis of both narwhals and belugas, so I was immediately intrigued by the skull. Further, I have a background in ancient DNA, the methodologies of which were necessary to successfully extract DNA from the skull; it looked pretty darned frayed after having been on a rooftop for four years, exposed to the full brunt of the Arctic elements.

In my application to the Carlsberg Foundation, I included a genomic study of the skull. I distinctly remember when my friend and colleague, who - after kindly agreeing to comment on my application - remarked ‘Looks good, but why would you want to include a study of that skull? That’s not very exciting’.   

Pah! If confirmed, this skull would be the only evidence of hybridization between these two High-Arctic cetaceans in existence.

When I went to sample the skull, I was joined by a student of mine, who was doing the DNA extraction in the clean lab. At the time, the marine part of the mammal collection was not my responsibility, and I therefore had to request access to the specimen through a colleague. The two well-preserved petrous bones were in a zip-lock bag next to the skull. The petrous bone is an exceptionally hard and dense portion of the inner ear, and has by far the best DNA preservation of any skeletal material. So for work on a degraded specimen like this skull, the petrous bone is the Holy Grail. 

Regrettably, I could do nothing to convince my colleague to let me near the petrous bones with my drill. I had to settle for access to the teeth. I wedged out the ones that were loose, and instructed my student to drill the root; this would not visually impact the morphology of the specimen, and the DNA yield is much higher in the root compared to, say, the crown of the tooth.

Unfortunately, after screening the DNA extractions, we found that there was literally no endogenous DNA in samples. After a lot of effort, and several bouts of sequencing, we finally retrieved a 0.05X genome from the skull. 

My PhD student Mikkel Skovrind analyzed the data, spending months on it. He did an amazing job and managed to get a surprising amount of information out of such little DNA. Using genotype likelihood methods and a reference panel of low-coverage genomes from soft tissue collected from narwhals and belugas, also from West Greenland, he could reliably show that the skull was the first-generation male offspring of a narwhal mother and a beluga father. 

An artist's rendition of what Narluga may have looked like, based on information provided by Jens Larsen, the inuit hunter who caught the whale. Illustration: Markus Bühler
Illustration: Markus Bühler

This was a surprise. We had expected the narwhal to be the father. Narwhal males have a characteristic long, protruding and spiraled tusk (and no other dentition), whereas belugas have a row of conical teeth. The tusk likely functions as a sensory organ, but is also used in displays, and we therefore thought it would be important for securing mates. Well, not in this case. As the paternal species is first in a portmanteau, that means the skull is actually a Belwhal. But nevermind, Narluga sounds better, so we’re sticking with that. 

Male narwhals have a characteristic, spiraled tusk, and no other teeth. Photo: Mikkel Høegh Post
Belugas have a row of equally-sized, conical teeth. Photo: Mikkel Høegh Post

In a random turn of events, while all this was going on, I was contacted by a researcher from Trent University, Paul Szpak, who was in Denmark for a few days and asked to access some polar bear skeletal material. He requested to visit the collections already the next morning, and I remember being distinctly annoyed with him when he arrived, because of the short notice, when I had so much other work to do. Well, as luck would have it, like many a memorable holiday, it’s all about the people you meet. Paul and I immediately hit it off, putting my initial irritation to shame. 

Paul’s lab utilizes isotopic analysis of faunal materials from archaeological sites across the Arctic to elucidate the palaeoecology of the region. It was a perfect match. I was super excited to include stable isotope work on the skull for our study, to address the foraging strategy of the hybrid, providing ecological context to the genomic data. By the time Paul left my office a few hours later, we had already drafted another half-dozen collaborative projects. Mikkel headed straight into the collections and promptly drilled a reference panel of West Greenlandic narwhals and belugas, and shipped them to Trent University for analysis.     

Rows of narwhal skulls in the mammal collections, which Mikkel drilled for stable isotope analysis

The stable isotope data showed that the dietary niches of narwhals and belugas differ, and that MCE1356 had a unique foraging strategy. Its diet was more benthic diet than that of either parental species; the carbon isotope signature was en par to values observed in bearded seals and walruses, which are both bottom feeders. Narwhals feed by creating a vacuum and sucking in their prey. In contrast, belugas are extremely sociable and hunt together in pods, steering shoals of fish into shallow water, and attacking them there. Due to the fusion of its cranial bones, we assume the hybrid was an adult. Our data suggested it had utilized a unique niche. Although we are unable to say anything more specific about prey items from the stable isotope data, we can only speculate whether it was the unfortunate horizontal dentition that had forced the hybrid to seek its prey at the bottom of the sea. 

When our paper was accepted, I headed to the collections to make sure all was in order with the skull. To my chagrin, I realised the teeth were not there - I had completely forgotten about them; over two years had passed since I pulled the teeth out of the skull, and I had no idea where they were. I rang Mikkel in a state of panic and told him to drop whatever he was doing and start an immediate search of the labs for the missing dentition. Of course, as he didn’t do the lab work, he had never seen the teeth before, so hadn't a clue where they were either. He rang me a few hours later to report that he had searched high and low but couldn’t find them. What an ordeal, this spelled disaster. Fortunately, though, after a few sleepless nights, Mikkel rang and said all was ok, he had located the bag of teeth, and had left it on my desk. 

Monday morning, I rocked up at my office and headed straight for the zip-locked bag. Our faculty press officer was coming in for an interview on our study, so it was fortunate enough that the teeth had shown up in the nick of time, as I wanted to show them to her. As it had been years since I last saw the teeth, I opened the bag and emptied them out on the desk. As I marvelled at their peculiar shapes, I surveyed the damage from the dremel, which had been used to drill powder out of each tooth for DNA extraction. I picked up one of the teeth and looked at it. Then another, and another. It was one of those moments, where it slowly dawns on you that something isn’t quite right. I went through all the teeth with an increasing sense of alarm. The small dremel holes were easy enough to spot. However, the holes were not in the roots of the teeth...they were all in the crowns! 

Yikes! Drill holes in the wrong end of the teeth

It had taken a year to get our paper accepted, with four very painful rounds of review, and comments from a total of six reviewers. The reviewers raised concerns regarding the reliability of our inferences, as they were based on so little DNA from the skull. Mikkel had put in a Herculean effort on the data analysis, and ended up spending so much time and effort on the paper, he had little time for anything else. Hence he never actually managed to work on the main part of what was supposed to be his PhD: a population genomic study of belugas sampled across their circumpolar range. He was unable to include that chapter when he handed in his PhD thesis last month, as he only had time to run the most rudimentary analysis. 

I sat at my desk, and let the magnitude of our DNA extraction faux pas sink in...

Well, on the bright side, the very low endogenous DNA content in the teeth now had an explanation - alas, we had sequenced the crowns, not the roots! 

...a lesson learned, what you tell someone is not necessarily the same as what they hear; next time, I'd better drill the samples myself... 

And fortunately, Mikkel is continuing his beluga work as a postdoc in my lab.

The work was funded by a Distinguished Associate Professor Fellowship from the Carlsberg Foundation

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