Facultative bipedalism

Facultative bipedalism is a specific way that animals move. Most facultative bipeds normally walk or run on four limbs. However, they can switch to using two legs during special or discretionary circumstances. This is very different from obligate bipedalism. Obligate bipeds, such as humans, use two legs as their primary method of movement. Facultative bipedalism is a flexible tool that animals use to adapt to their immediate needs.

There are several distinct techniques for moving on two legs. Walking involves an evenly spaced gait where one foot is always touching the ground. Running includes an aerial phase, which is a moment when both feet are off the ground at once. Skipping is a movement where both feet hit the ground immediately after each other, but the trailing foot changes with every step. Galloping is very similar to skipping, but the trailing foot stays the same throughout the movement.

Primates show a wide variety of bipedal behaviors. Sifakas, which are lemurs from Madagascar, use a vertical clinging and leaping strategy in trees. On the ground, they walk on two hind legs to conserve energy. They are unique because they can both walk and gallop bipedally. Capuchin monkeys are arboreal quadrupeds, meaning they walk on four limbs in trees. On the ground, they use a spring-like walk that lacks an aerial phase. Because they do not have a pendulum-like gait like humans, their energy costs for bipedalism are very high.

Other primates use bipedalism for specific survival tasks. Chimpanzees often stand on two legs to gather and transport food. This method allows them to carry more than twice as much weight as they could while walking on four limbs. They also use bipedalism to reach fruit in shorter trees. Gibbons are considered low-speed obligate bipeds when they are on the ground. Their anatomy is specialized for leaping, but they use specific back muscles to move bipedally on the ground.

Lizards also demonstrate facultative bipedalism through a complex physical mechanism. When a lizard accelerates its hind legs rapidly, it creates a ground reaction force. This force causes the lizard's trunk angle to open and shifts its center of mass. This shift in weight causes the front limbs to lift off the ground. While some thought this was a passive physical property, recent research suggests lizards may actively attempt this movement.

This behavior may help them increase acceleration to escape predators.

Prehistoric animals also practiced this form of movement. Many dinosaur species, such as ornithopods and marginocephalians, were facultative bipeds. Scientists believe bipedalism in dinosaurs may have evolved in smaller carnivores. These animals needed speed and agility to compete with much larger predators. This need likely led to the development of larger hind-limb muscles. Over long periods, these ecological pressures might have eventually led to species becoming obligate bipeds.

Even extinct species like the Australopithecines show signs of this behavior. Their physical structure, including a human-like pelvis and arched feet, suggests they could walk on two legs. However, they also had short hind limbs and a broad pelvis. These traits meant that walking bipedally required a lot of energy for them. They likely used bipedalism only when the benefits of the movement outweighed the high energy costs. This shows that facultative bipedalism is a complex balance between physical ability and energy use.