Lunge-feeding baleen whales use 'oral plug' to eat without drowning

When rorqual whales feed, they lunge through the water with mouths open wide, taking in vast patches of their tiny prey suspended in a volume of water as big as their massive bodies in one giant gulp. In the process, their mouths experience enormous physical forces. Now, researchers reporting in the journal Current Biology on January 20 have discovered how the whales do this without so much as a choke.

“We discovered a structure in fin whales, which likely exists in all lunge-feeding whales, or rorquals,” says Kelsey Gil of the University of British Columbia. “We’ve termed it the ‘oral plug’ and found that it blocks the channel between mouth and pharynx. It means that when a whale lunges, the entrance to the pharynx and thus the respiratory tract is protected.”

Because almost all the water that enters the mouth during a lunge is filtered out before their prey is swallowed, she adds that the whales’ oral plug also keeps prey mixed with water from being forced into the pharynx and down the esophagus during a lunge.

Before Gil and her colleagues got started, the mechanics of lunge feeding was well understood, but not much was known about how prey such as krill are manipulated and swallowed after a lunge. The general anatomy of the larynx, a hollow organ that forms an air passage to the lungs and holds vocal cords, was also known, and scientists had some information about how the larynx in whales makes sound. But they didn’t know much at all about the rest of the pharynx, the critical junction between respiratory and digestive tracts—or what happens when a whale swallows.

“It’s impossible to study this in a living whale, so we rely on tissue from deceased whales and use functional morphology to assess the relationship between a structure and its function,” Gil says. The researchers closely examined the whale’s anatomy, both intact and by carefully dissecting the pharynx. They manipulated the various structures to see how they could move. They also looked at the direction of muscle fibers to understand how they’d move when muscles contract and shorten.

“The oral plug is a part of the soft palate, so simply considering the gross anatomy, there is only one direction it should move,” she says. “When the oral plug moves backward and upward to block the nasal cavities, it means that no other structure can be in that same spot below the nasal cavities. This forced us to then examine the larynx, since that spot is where we would expect to see the larynx during breathing, but it cannot be in that position during swallowing.”

Instead, she explains, the larynx is positioned at the floor of the pharynx, where laryngeal cartilages close to protect it. A muscular sac attached to the bottom of the larynx is forced upwards into the laryngeal cavity to completely block the lower respiratory tract.

The findings also show that the pharynx in fin whales—and likely all rorquals—can only be used by either the respiratory or the digestive tract at one time. Interestingly, according to the researchers, no structure like the oral plug has been reported in any other animal. “There are very few animals with lungs that feed by engulfing prey and water, so the oral plug is likely a protective structure specific to rorquals that is necessary to enable lunge feeding,” Gil says.

The findings are a reminder that there still are many unknowns about whales. Gil’s team plans to continue studying the pharynx of rorquals and the branches of the respiratory and digestive tracts that lead to and from it in even more detail.

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This work was supported by NSERC.

Current Biology, Gil et al.: “Anatomical mechanism for protecting the airway in the largest animals on earth” https://www.cell.com/current-biology/fulltext/S0960-9822(21)01727-9

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