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When fin whales capture their food, they end up scooping a lot of water into their mouths. Scientists in Canada have discovered a small, fatty structure in these marine mammals that may explain how they are able to engulf such vast amounts of prey-filled water without choking. 

When the researchers examined deceased whales, they identified a section of the soft palate that could shift to seal the upper airway while the whale feeds. The researchers, who dubbed the structure the ‘oral plug’ in the journal Current Biology on January 20, suspect that the plug also exists in other large baleen whales.

Fin whales are found in oceans worldwide and can grow to 85 feet long. They belong to a group of large baleen whales called the rorqual family, along with several other sea giants including the blue whale and humpback whale. Rorquals use a highly unusual strategy known as lunge feeding to capture krill, fish, and squid. During lunge feeding, the whale opens its mouth while shooting towards its prey at speeds of up to about 10 feet per second, allowing it to gulp a volume of water as large as its own body. Finally, the whale closes its mouth, pushing water out through its baleen plates, and swallows the remaining prey.

How the whales protect their airways as water floods the mouth has been a mystery, however. 

“We have a lot of knowledge about that whole process of the mechanics of lunging and engulfing all that food, and that’s pretty much where the knowledge stops,” says Kelsey Gil, a marine biologist at the University of British Columbia in Vancouver and coauthor of the findings. “We don’t know what’s going on in the throat.”

To find out, she and her colleagues examined the bodies of 19 fin whales.

“When we had the mouth open in this fin whale, we saw there was this massive chunk of tissue at the back of the mouth completely plugging the pathway that the food has to take to get to the esophagus and the stomach,” Gil says.

The almost 8 inch-wide bulbous structure was composed of fat and muscle. The researchers determined that it was part of the soft palate—the little sheet of muscle along the roof of the mouth from which the uvula hangs in humans. 

The oral plug was tightly wedged in place and could not be easily pushed free. When the researchers examined the muscle fibers of the soft palate, they concluded that the only way the oral plug could move for food to pass through during swallowing would be to shift backwards and upwards, and in doing so block the entry to the nasal cavities. 

“For these whales it’s a way to save energy,” Gil says. “It’s in its relaxed position and it’s going to be in that position most of the time and it only needs to be moved for a brief amount of time to push food through.”

The process is similar to what happens when humans swallow: The uvula is pushed back and throat muscles contract so food doesn’t go up the nose. 

“Once the nasal cavities and the upper airways are protected, you have this question of how the lower airways would be protected, [such as] the lungs,” Gil says. She and her collaborators manipulated the cartilage of the larynx, or voicebox, to see how it might move during swallowing. They found that the cartilage at the top of the larynx can come together to create a seal that prevents food or water from accidentally getting into the respiratory tract.

Additionally, Gil says, a muscular sac at the bottom of the larynx known as the laryngeal sac can create another protective barrier to block off the entry to the lungs. When the whales dive down to feed at greater depths, the pressure would push the sac upwards to plug the larynx. 

Being able to engulf massive amounts of prey is one reason that rorquals have managed to grow to such epic sizes. “The oral plug is really important for lunge feeding, and thus for allowing them to get as large as they have,” Gil says.

However, not everyone is convinced by the structure. Joy Reidenberg, a comparative anatomist at the Icahn School of Medicine at Mount Sinai in New York who was not involved with the research, says she has “serious reservations” about some of the evidence presented in the study. Based on what she has observed in dissections of rorquals, Reidenberg says, the rigid and floppy cartilage flaps at the top of the larynx wouldn’t fit together to make a particularly good seal in whales. 

Additionally, the motions of the larynx and mouth that make the protective seal and swallow food cannot both happen at the same time, she says. This is because both actions depend on moving the U-shaped hyoid bone, but in opposite directions.

Reidenberg also isn’t sold on the oral plug, which she doubts could move out of the way enough to allow food to pass by during swallowing. As is commonly observed in other animals, Reidenberg explains that it makes more sense for air to flow over the larynx and soft palate while food flows around the sides, like water parting around the bow of a boat. This would allow the whales to breathe and eat at the same time, she explains. It’s possible that the fatty bulge the team observed in the fin whale carcasses was actually caused by the weight of the larynx pushing down because the tongue was no longer there to hold it in place, Reidenberg says, although researchers would have to peek into a live whale’s mouth to find out for sure.

“I’m not convinced entirely that there is an oral plug, but if there is, I find that to be very interesting,” she says. “I’d love to see more evidence of that.”

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