Why do animals move, and how do they overcome the constant pull of gravity?

Nature · Published 3 October 2026 Swipe more like this →

In short: Movement is a fundamental biological necessity, yet the mechanics behind it vary wildly across environments. Whether crawling, walking, or jumping, every animal must solve the same physical puzzle: how to generate force while managing gravity, inertia, and drag. Discover the hidden biomechanics that allow life to traverse our planet.

At its core, animal locomotion is a feat of engineering. To move from place to place, an organism requires a propulsive mechanism—usually muscle—coordinated by the nervous system. While the specific environment dictates the challenges, the physical constraints remain constant. Gravity, defined here as the weight and inertia of a body, acts as a primary barrier to movement. To prevent their bodies from collapsing, terrestrial, aerial, and fossorial animals have evolved skeletal systems that act as rigid frameworks or flexible hydraulic supports.

The energy required to overcome these forces varies by medium. In water, buoyancy can render an animal weightless, significantly reducing the muscular work needed to overcome inertia. However, aquatic animals must still contend with drag, which is determined by their shape and the viscosity of the surrounding water. In contrast, terrestrial animals face gravity as a dominant force, while drag is generally considered negligible. On land, locomotion is primarily achieved through axial movement—where the body shape is modified to provide force—or appendicular movement, which utilizes specialized appendages.

Appendicular locomotion is the most common form of movement for vertebrates and arthropods. For vertebrates, the posture of the limbs is a critical factor in efficiency and agility. The sprawling posture, where the upper limbs are held horizontally, is considered the most primitive form and is still used by salamanders, most reptiles, and many arthropods. More advanced stances, such as semi-erect and fully erect postures, allow for different mechanical advantages. Regardless of the method, from the sidewinding of snakes on loose sand to the specialized kiting of spiders using silk, every form of movement represents a unique evolutionary adaptation to the physical demands of the environment.

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