The concept of an animal dropping from a great height often conjures images of a fatal impact. Physics suggests that any object in freefall accelerates until air resistance matches the pull of gravity. Yet, the animal kingdom offers a surprising twist to this universal law. Certain creatures are naturally immune to the consequences of high-altitude drops. This unexpected survival is not due to magical adaptation, but rather a simple mechanism rooted in the relationship between size, mass, and the air around them.
Understanding Terminal Velocity and Air Resistance
Any object falling through the atmosphere eventually reaches a constant maximum speed known as terminal velocity. This speed is attained when the downward force of gravity is perfectly balanced by the upward force of air resistance, or drag. At this equilibrium point, the net force acting on the object becomes zero, and it stops accelerating.
Terminal velocity is not a fixed number, but depends on several factors specific to the falling object. The two most important variables are the object’s mass, which determines the gravitational pull, and its cross-sectional area and shape, which determine the air resistance it generates. A heavier object requires a greater drag force to balance its weight, meaning it must fall faster to reach its terminal velocity. Consequently, a dense, streamlined object falls much faster than a light, spread-out one.
The Critical Factor of Size and Mass Ratio
The ability of small animals to survive a long fall is governed by the physics of scaling, often summarized by the square-cube law. This principle states that as an object grows in size, its volume and mass increase much faster than its surface area. For a falling animal, mass drives the gravitational force, while surface area is the primary source of air resistance.
When an animal is scaled down, its mass decreases by the cube of the change in size, but its surface area only decreases by the square of that change. This results in a proportional increase in the animal’s surface area relative to its mass. Therefore, tiny organisms generate a tremendous amount of drag force relative to their negligible weight, causing their terminal velocity to be extremely low.
The terminal velocity for a small creature is reduced to a speed that is entirely non-lethal, turning a freefall into a very slow, controlled descent. The impact force upon landing is determined by the speed and mass of the object. Since both are minimized for small organisms, the resulting kinetic energy is easily absorbed. This scaling advantage is the reason why a fall survivable for a mouse would be instantly fatal for a rat or a human.
Examples of High-Fall Survivors
The most dramatic examples of terminal velocity survival are found among insects. They are so small that the air acts almost like a thick fluid around them. An ant, for instance, has a terminal velocity so low, possibly around four miles per hour, that a fall from any height is more like a gentle flutter to the ground. Their light weight and durable exoskeleton allow them to land without injury.
Moving up the size scale, small mammals like mice and shrews also possess survivable terminal velocities. A mouse can be dropped down a mine shaft and walk away with only a slight shock. The tiny Etruscan shrew, one of the world’s smallest terrestrial mammals, has a body mass countered by a relatively large surface area. This results in a fall speed that its light, flexible skeletal structure can easily withstand.
Even slightly larger animals, such as squirrels, can survive falls from significant heights due to their ability to maximize air resistance. By splaying their limbs, they increase their surface area, effectively lowering their terminal velocity. This behavior, combined with their low body density and ability to absorb shock, makes falls that would be catastrophic for a human relatively harmless.
Why Large Animals Cannot Survive Terminal Velocity
In contrast to small creatures, the physics of scaling works against larger animals, making a fall from a great height almost certainly lethal. A human in a belly-to-earth position can reach a terminal velocity of approximately 120 miles per hour. This high speed is a direct consequence of their large mass increasing much faster than the surface area that generates drag.
At this rapid speed, the kinetic energy upon impact is immense. The force of deceleration when hitting the ground is so great that it exceeds the structural limits of the animal’s body, resulting in catastrophic structural failure. For large creatures like deer, cows, or humans, the high terminal velocity generates an impact force that instantaneously ruptures internal organs and blood vessels. The sheer momentum of a large body falling at high speed ensures a non-survivable event. The only way for a large animal to survive a freefall is if the fall is too short to reach terminal velocity, or if the impact is somehow cushioned.