All mammals larger than rats take roughly the same amount of time to urinate – 21 seconds – regardless of their size, scientists say. An elephant takes the same amount of time to empty its huge bladder as a cat – despite holding 18 litres of urine. This is because their urethras are appropriately scaled to be a “flow-enhancing device,” the researchers said. Researchers hope this efficient design for a system to quickly empty the bladder will inspire smarter engineering for water tanks and reservoirs, as well as fire hoses and water-filled backpacks, Live Science reported. To compare peeing rates across the animal kingdom, David Hu, an assistant professor of mechanical engineering at the Georgia Institute of Technology in Atlanta, and his colleagues turned to the zoo and the Internet. The team looked at 28 videos of animals urinating on YouTube, and visited Zoo Atlanta to gather their own footage of 16 animals peeing and to collect urine in pieces of empty soda bottles. The researchers found that animals lighter than 3 kilogrammes, such as small rodents and bats, could not pee in streams; rather they urinated in a series of quick drops. The key factor behind this phenomenon is the length of the urethra, the researchers said. As an animal’s body gets bigger, its urethra gets longer at a predictable ratio.