In humans, the sense of balance, called equilibrioception, is based on sight, vestibular balance, and proprioception (the ability to sense the body and its parts). Vestibular balance refers to the vestibules in the cochlea, or the hearing organ, of the inner ear.
When fluid moves through the semicircular canals, the brain receives signals about the direction and speed of rotation of the head. The bulbed ends of the canals contain microscopic hair cells like the ones in the cochlea that can pick up on displacement of the vestibular fluid, which provides a sense of direction and orientation. The utricle and saccule, called the otolithic organs, help us detect changes in acceleration. Both of them contain a layer of calcium crystals that move along with the movements of the head. Displacement of the calcium stones causes hair cells to bend. This is then picked up by the brain as a change in velocity.
Tails allow man’s best friend (as well as cats, mice, squirrels, and monkeys) to keep their balance in precarious situations. Tails give animals a greater margin of error when it comes to keeping their center of gravity steady. They’re the reason a tightrope walker needs a pole whereas a cat on a fence doesn’t. In other words, a cat’s tail serves the same purpose as a tightrope walker’s balancing pole: it helps stabilize the center of gravity.
Tails are the reason why arboreal animals, like squirrels and monkeys, can maintain perfect balance while moving at high speeds through treetops. Since tails are a “free” limb, they can be used to maintain weight distribution and balance. The only way for humans to achieve the same kind of balance is by extending their arms to the sides.
Plants have a sense of balance, too! Although it’s probably not anything we would recognize as balance, per se. Nearly all plants exhibit gravitropism, or the ability to detect and react to changes in gravitational pull. Plant roots exhibit positive gravitropism, meaning they grow towards the force of gravity, whereas plant stems exhibit negative gravitropism and grow against the pull of gravity.
Is this phenomenon due to the fact that sunlight is up and groundwater is down? Not exactly — various experiments have shown that plants do respond to the direction and orientation of sunlight (leaves, for example, tend to follow the light), but that gravitropism, especially when it comes to the growth of roots, is a very real thing. A plant with a source of water adjacent to its roots will unerringly send its roots downward, and a potted plant deliberately left on its side (like the one in the picture), will do the same.