Penuins are some of the world’s most charismatic birds. They look like they’re wearing suits. They appear to fly underwater. And they waddle like sumo wrestlers.
Now, scientists say they’ve finally pinpointed the muscle behind this endearing gait: an enigmatic and long-misunderstood tissue that forces the bird’s lower limbs to stay close to its body near the knees, the team reports this month in The Anatomical Record. The adaptation not only contributes to penguins’ signature waddle, it also helps keep their bodies streamlined when underwater, allowing them to move efficiently on both land and sea.
“This muscle has been mentioned for over a century, but usually in a confused way,” says Caro Acosta Hospitaleche, a paleontologist and penguin expert at the La Plata Museum. In the new study, she adds, the researchers “clearly separate it and give it a distinct identity.”
Scientists have long puzzled over the penguin’s peculiar walk. Explanations have largely focused on the birds’ skeletal structure: Their knees, which are tucked into their body cavity, are bent and arranged in a perpetual squat, giving them a sturdy base for balance, but making it difficult to walk. There was still a missing piece, however: How do the birds effectively walk with such seemingly compromised anatomy?
The answer came when a team co-led by Justin Georgi and Margaret Hall, anatomists and evolutionary morphologists at Midwestern University, was dissecting two macaroni penguins (Eudyptes chrysolophus). The birds were humanely euthanized as part of end-of-life veterinary care at SeaWorld in San Diego and donated to the team to provide further insights into penguin anatomy.
The scientists compared the musculature of the penguins with that of nonwaddling birds. They identified a suite of anatomical differences, including an uncharacteristically beefy m. supracoracoideus—the muscle responsible for raising the wing. They also found a unique arrangement of muscles in penguin shoulders—the m. latissimus dorsi—that give the birds a wider breadth of movement in their flippers. These enable the birds’ wings to move up, down, and backward instead up just up and down like in flying birds, allowing them to “fly” underwater.
Then the team took a close look at the m. flexor cruris medialis, a muscle that attaches to the tibiotarsus, a long leg bone in birds. Researchers knew this tissue helped birds bend their knees, but they’ve never precisely clarified the nearby muscles and bones it connects to in penguins. The new dissections revealed a mysterious part of the m. flexor cruris medialis that attaches to a different section of the tibiotarsus than previously thought, suggesting a hitherto unknown function.
The function and exact anatomy of this muscle has been debated for more than a century, Georgi says. “All previous work has relegated [it] to just a smaller piece of some other, larger muscle” like the m. flexor cruris medialis, he says, “and as a result ignored its possible importance.”
But the new research shows the muscle is entirely separate from the m. flexor cruris medialis and serves to connect a penguin’s two knees. The muscle is not reported in any other birds, making it unique to penguins. Georgi and his colleagues have christened it m. adductor tibialis for its ability to bind, or adduct, the legs inward.
The m. flexor cruris medialis provides the birds an adept ability to more easily maneuver their cramped leg posture, Georgi says. “It stabilizes the leg against the body when the penguin is leaning to one side,” he adds, and provides extra energy to allow the animal to swing its other leg forward as it balances on the stabilized leg, completing the waddle.
“Biomechanically, it makes sense that it helps keep the legs adducted and stabilizes that wide stance penguins have when walking,” Hospitaleche says. “It’s a reasonable hypothesis.”
Beyond the waddle, the research offers a strong foundation in penguin anatomy that is otherwise difficult to glean in these aquatic birds. “Without this kind of foundation, broader questions about penguin evolution cannot be addressed in a rigorous way,” Hospitaleche says. “This kind of work might not be flashy, but it’s necessary.”