Buzz
A rhinoceros has a significantly larger bladder than a dog and produces an impressive volume of urine. So which animal actually pees for a longer time? In 2014, researchers from Georgia Tech set out to find the answer, discovering that, in general, larger animals urinate for longer periods. They tested this theory using high-speed cameras to capture the urination habits of animals at Zoo Atlanta, along with supplementary videos from YouTube. They ended up studying the peeing patterns of 32 animals, from tiny mice to massive elephants.
To their surprise, they found that mammals weighing over 6 pounds tended to urinate for about the same duration, regardless of their size. On average, they spent 21 seconds peeing, give or take 13 seconds. “This consistency is remarkable,” the researchers noted in the journal Proceedings of the National Academy of Sciences, “especially when considering that an elephant’s bladder, with its 18-liter capacity, is nearly 3600 times bigger than a cat’s 5-milliliter bladder.”
The reason an elephant can expel the equivalent of nine large soda bottles worth of urine in the same time a cat takes to release a small spoonful is due to the difference in flow rates. The elephant’s urethra, which carries urine from the bladder to the outside, is both wider and longer than a cat’s, allowing gravity to exert a stronger force on the fluid and facilitating a faster flow.
Mice, rats, and other animals weighing less than 6 pounds don't follow the 21-second rule. This is because their urinary systems are so tiny that they must contend with capillary action, where fluid molecules cling to each other and the walls of the container, causing it to flow upward. The urine is thicker and flows so slowly that smaller animals can't produce a strong jet of pee. Instead, their urine drips out in tiny droplets.
For the larger mammals, the reason behind the 21-second rule remains unclear, despite the significant size differences. The researchers propose that it might be more of a matter of physics than evolutionary adaptation.
The researchers suggest their findings could assist in diagnosing urinary issues in animals. For example, if a zookeeper observes a gorilla urinating for a noticeably longer or shorter time than 21 seconds, it could indicate a potential health problem.
Interestingly, this unusual area of study might also impact infrastructure design. As the paper notes:
[B]y providing a water-tight pipe to direct urine downward, the urethra increases the gravitational force acting on urine and therefore, the rate at which urine is expelled from the body ... Engineers may apply this result to design a system of pipes and reservoirs for which the drainage time does not depend on system size. This concept of a scalable hydrodynamic system may be used in the design of portable reservoirs, such as water towers, water backpacks, and storage tanks.
Who knows, maybe this study will open the door to a future where ‘bladders’ play a central role.
