Gigantic piddle

Although an elephant has a gigantic bladder and can produce a giant puddle of urine, it empties its bladder in no more time than, for example, a cat. In fact, all animals weighing more than three kilogrammes urinate for a maximum of about 20 seconds.
This conclusion was drawn by researcher David Hu after studying 32 different animal species. The study is remarkable. Previously it had been assumed that the speed at which urine leaves the body is determined by the pressure in the bladder. This pressure is generated by contracting muscles. According to Hu, however, the urination speed is actually determined by the length of the urethra. The weight of the fluid in the urethra pushes the urine out. And when the urethra is (very) long — as it is in larger animals — the flow rate increases.

An elephant’s urethra is about a metre long. The pressure of the fluid in the urethra is comparable to the pressure at the bottom of a swimming pool that is 90 cm deep. This pressure allows the elephant to relieve itself quickly, as the urine leaves the body under high pressure. This is important in the wild: if it were to take longer, the animal would be more vulnerable to attack by predators.

A cat takes roughly the same length of time to urinate, though its bladder is 3600 times smaller than that of an elephant. According to the investigators, all animals weighing more than three kilogrammes satisfy this ‘Law of Urination’. All of these animals urinate in a stream and all make use of gravity. The urine of animals weighing less than three kilogrammes doesn’t flow. It drips. Mice and rats take 2 seconds, and bats only a fraction of a second.