Arboreal locomotion

Arboreal locomotion refers to the movement of animals within tree habitats.

In environments where trees are present, many species have evolved specific ways to navigate them. Some animals are scansorial, meaning they only scale trees occasionally. Others are exclusively arboreal, living their entire lives in the canopy, such as certain tree snails. Moving through a forest presents many mechanical challenges. Animals must manage balance on narrow surfaces and navigate steep inclines. They must also find ways to cross large gaps between trees and move around physical obstructions.

Managing the center of mass is a primary mechanical challenge. On the ground, an animal's center of mass might swing side to side. However, on a narrow branch, this movement could push the center of mass beyond the edge. This creates a tendency to topple over and fall. To prevent this, some animals use pedal grasping to clamp onto small branches. This provides better balance, even while the animal uses its hands to feed. On vertical or steep branches, the challenge shifts from tipping to pitching backward or slipping downward. On these inclines, animals must fight the force of gravity to move upward. Many species respond by using all four limbs and increasing their gait frequency. During descent, they must use friction as a brake to control their fall.

Animals have developed diverse anatomical specializations to solve these problems. Many arboreal species have elongated limbs to reach resources or test the firmness of a branch. Some primates and reptiles, like the spider monkey or green tree python, possess prehensile tails. A prehensile tail is a tail capable of grasping and holding onto branches. For added stability, some species like the crested gecko have adhesive pads on their tail tips. Others use claws to navigate rough, corky substrates like tree bark. Squirrels use claws to climb large, nearly flat trunks. However, claws can sometimes interfere with grasping very small branches if they prick the animal's own paw.

Adhesion is another vital method for staying attached to surfaces. This works best on smooth substrates where claws might fail. Tree frogs and arboreal salamanders often use wet adhesion, which involves suction or capillary adhesion. Geckos utilize dry adhesion through specialized toe structures. They rely on van der Waals forces to adhere to surfaces, including glass.

In some gecko species, like Lygodactylus, a specialized patch of scales under the tail supplements these forces. Primates use a different method called frictional gripping. They use hairless fingertips to squeeze branches, generating a frictional force to hold on. This grip is most effective on certain branch diameters.

Specialized behaviors also help animals navigate the canopy. Some primates, bats, and sloths achieve passive stability by hanging beneath a branch. This position makes tipping and pitching less of a concern. A more active method is brachiation, a specialized form of locomotion used by primates. Brachiation involves swinging with the arms from one handhold to another while hanging beneath branches.

Gibbons are the most skilled brachiators. They can swing across gaps of up to 15 meters and travel at high speeds. To bridge even larger gaps, some animals use membranes. Flying squirrels use a patagium, or skin membrane, to glide between trees. Some frogs use toe membranes to parachute, which allows them to fall more slowly.

Even limbless animals like snakes have adapted to arboreal life. Snakes use different methods depending on their surroundings. On bare branches, they use concertina locomotion, moving slowly by expanding and contracting their bodies. When they encounter secondary branches, they switch to lateral undulation, which is a much faster mode of movement. Because of these methods, snakes often perform best in cluttered environments with small perches. This contrasts with limbed organisms, which often prefer larger perches in less cluttered spaces.

We can trace the history of these adaptations through the fossil record. The earliest known climbing tetrapod was a varanopid amniote called Eoscansor. It lived during the Late Carboniferous period in North America and had specializations for grasping tree trunks. Another early climber was Suminia, an anomodont synapsid from Russia. Suminia lived approximately 260 million years ago during the Late Permian. These ancient species demonstrate how specialized anatomy has evolved to meet the mechanical demands of life in the trees.