(CN) — Rorqual whales feed by taking in vast patches of water and tiny prey as voluminous as their own bodies thanks to a skin flap inside their mouths that prevents choking, researchers say.
If it weren’t for the flap, or the "oral plug," these massive whales would likely choke and drown, according to a study published Thursday in Current Biology.
“We discovered a structure in fin whales, which likely exists in all lunge-feeding whales, or rorquals,” lead researcher Kelsey Gil of the University of British Columbia said in a statement. “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."
Almost all water that enters a whale’s mouth is filtered out before its prey is swallowed, Gil said. The oral plug helps keep prey mixed with water from entering the pharynx and traveling down the esophagus during a lunge.
Before Gil and her colleagues began their study, the mechanics of lunge feeding were well understood, but much remained unknown about how small prey such as krill are handled following a lunge.
Scientists also knew very little about the anatomy of a whale’s pharynx, aside from the larynx, the hollow organ that forms an air passage to the lungs and holds vocal cords.
The pharynx, Gil and her team found, is the critical junction between the respiratory and digestive tracks after 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 said in the statement.
In dissecting the whale’s pharynx, researchers manipulated the various structures and observed the muscle fibers to learn how the structures move as well as which direction they move when muscles contract or 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,” Gil said.
She added, “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 then to 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.”
The larynx, Gil explained, 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 show that the pharynx in fin whales, and probably all rorquals, can be used by the respiratory and digestive tracks at the same time.
No similar structure to an oral plug has been reported in other animals, Gil said.
“There are very few animals with lungs that feed by engulfing prey and water, so the oral plus is likely a protective structure specific to rorquals that is necessary to enable lunge feeding,” she said.
Gil did not immediately respond to an email request for more information.
Baleen whales are named after the baleen plates inside their mouths that sieve plankton and other creatures from the water.
Most baleens feed by taking in water and then forcing the water through the baleen plates with their tongue to filter the water out and keep whatever other prey they may have captured, including crustaceans, such as krill, and fish, such as herring and sardines.
Rorquals are the largest group of baleen whales. The group includes the blue whale, believed to be the largest animal that has ever lived. The blue whale can weigh up to 200 tons, while the fin whale, which was primarily used for this study and is by no measure small, can grow up to 130 tons.
All members of the rorqual family have a series of longitudinal folds on their skin that allow their mouth to expand to huge widths while feeding and enabling them to take in immense mouthfuls of water and prey in a single gulp.
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