According to SmithsonianMag.com –
The team filmed rats, dogs, goats, cows and elephants urinating and gathered footage from YouTube of others relieving themselves. Combining this with data on mass, bladder pressure and urethra size, they were able to create a mathematical model of urinary systems to show why mammals take the same time to empty their bladder, despite the difference in bladder size.
DUMMR.com 