Do Penguins Have Knees? Inside a Penguin's Legs

Where are a penguin’s knees?

All birds share the same basic leg skeleton, and penguins are no exception. From the hip downward, a penguin has a thigh bone (femur), a knee, a long shin bone called the tibiotarsus, an ankle joint, a short fused foot bone called the tarsometatarsus, and finally the toes.

In a penguin, the femur is short and held almost horizontally, pointing forward from the hip. The knee is bent at a sharp angle and sits inside the body, roughly level with the lower belly. From there the shin bone runs down and back to the ankle, which is just above the feet. Feathers and a thick layer of fat cover the whole upper leg, so from the outside you see only a small part of the leg and the broad, webbed feet.

The backward-knee myth

Many people think birds’ knees bend backward, because the obvious joint halfway down a heron’s or flamingo’s leg bends the opposite way to a human knee. In fact, that joint is the ankle. Birds walk on their toes, with the heel raised high off the ground.

Penguin knees bend forward, just like ours. What makes penguins unusual is how low their ankle sits. The foot bone is so short that a standing penguin’s ankle is only a few centimeters above the ground, and when resting, a penguin often settles back onto that short foot bone, almost like sitting on its heels.

Why are penguin legs built this way?

A penguin’s body is shaped above all for swimming. Its legs sit far back on the body, close to the tail. In the water, this placement lets the feet and tail act as a rudder behind a streamlined, torpedo-shaped body, while the stiff flippers provide the power. Legs dangling lower down would add drag.

Keeping most of the leg inside the body also helps penguins stay warm. Exposed limbs lose heat quickly, and in cold-water species such as the emperor penguin and the king penguin, tucking the upper leg under insulating fat is a clear advantage. Special blood vessels in the lower legs further reduce heat loss through the feet.

The rear position of the legs is also what makes a penguin stand upright. To balance its weight over feet placed so far back, it has to hold its body nearly vertical, a posture that makes penguins look strikingly human.

Why do penguins waddle?

With short visible legs and an upright body, a penguin cannot take long strides. Instead, it rocks from side to side, lifting one foot while its weight shifts over the other. Biomechanics research on emperor penguins found that this sideways sway is not wasted motion: it helps the bird swing forward and recover part of the energy of each step, a little like a pendulum.

Even so, walking is expensive for penguins. Because their legs are so short, studies suggest they use roughly twice as much energy to walk a given distance as other animals of similar weight. That matters for emperor penguins, which may walk 50–120 km (31–75 mi) across the sea ice to reach their colonies.

Tobogganing, hopping and climbing

Penguins have other ways of getting around on land:

• Tobogganing: on snow or ice, penguins flop onto their bellies and slide, pushing with their feet and flippers. It is faster and uses less energy than waddling over long distances.
• Hopping: many penguins can hop with both feet together. Rockhopper penguins are known for bounding up steep, rocky slopes.
• Climbing: species such as Adélie and chinstrap penguins scramble over rocks and ice, using their claws for grip and sometimes their bills and flippers for balance.
• Leaping: penguins often shoot out of the water to land on ice floes or rocks, sometimes clearing a meter or more.

Do penguins use their legs to swim?

Penguins swim mainly with their flippers, which are modified wings with flattened, stiffened bones. The legs and tail mostly steer. This is the opposite of ducks and cormorants, which paddle with their feet. The same trade-off explains why penguins cannot fly: their wings became efficient underwater paddles instead. Learn more in why can’t penguins fly? and in our list of flightless birds.