Why most mammals pee in same time span?

Most mammals, no matter how large or small, urinate in the same time span and researchers now believe that it has more to do with gravity than bladder pressure generated by muscular contraction.

"It is possible because larger animals have longer urethras," said David Hu, assistant professor at Georgia Institute of Technology in the US.

"The weight of the fluid in the urethra is pushing the fluid out. And because the urethra is long, flow rate is increased," Hu explained.

For example, an elephant's urethra is one metre in length and it urinates four metres per second, or the same volume per second as five shower-heads.

"If its urethra were shorter, the elephant would urinate for a longer time and be more susceptible to predators," Hu explained.

The findings conflict with studies that indicate urinary flow is controlled on bladder pressure generated by muscular contraction.

The researchers noticed that gravity allows larger animals to empty their bladders in jets or sheets of urine. Gravity's effect on small animals is minimal.

"They urinate in small drops because of high viscous and capillary forces. It is like peeing in space," Patricia Yang from Georgia Institute of Technology.

For the study, the research team went to a zoo to watch 16 animals relieve themselves, then watched 28 YouTube videos.

"Nature has designed a way to use gravity instead of wasting the animal's energy," Hu added.

The findings could lead engineers to design more effective water tanks, backpacks, fire hoses and other products.

The study appeared in the journal Proceedings of the National Academy of Sciences (PNAS).