Researchers at the Georgia Institute of Technology have found that all mammals about 6 pounds and up take the same time to empty their bladders, whether it's an elephant's 18 liters or a cat's 5 milliliters. Both animals, and everything in between (including humans), averaged about 21 seconds — with some variation, of course, depending on how much they've had to drink and the time since it was last drained. The secret isn't some mysterious shared gene or common ancestor, but simply gravity. Small animals don't get much of a gravity assist in urinating and must force the fluid out themselves — but past a certain point, gravity starts being enough, and animals have evolved ways of relieving themselves that are similar in engineering, if not the size of the pipe. Elephants' longer, wider urethra lets them go at a liter a second; if you had one like that, you'd be done in half a second — convenient, but possibly hazardous. The study was published recently in the journal PNAS.
— Devin Coldewey, NBC News