What Surprising Bodily Function Do Most Mammals Have in Common?

Bodily functions may not be the most comfortable topic to read or talk about, but nevertheless, they are an important aspect of life. Although there are many differences in bodily functions among living creatures, when it comes to mammals, there is a surprising feature that most of them share – the amount of time it takes to empty their bladder. In fact, despite vast differences in size and anatomy, most mammals larger than a rat take around 21 seconds to empty their bladders.

The relative length of the urethra (the tube through which urine leaves the body) is the key to this common bodily function. According to David Hu, an assistant professor of mechanical engineering at the Georgia Institute of Technology, "all animals have urethras of the same aspect ratio: a length-to-width ratio of 18.” This means that animals with larger bodies have longer urethras, which increases the force of gravity, creating more pressure in the bladder and causing urine to push out faster. This is why an elephant can empty its bladder just as quickly as a cat.

Hu and a team of researchers studied the urination patterns of 34 different species via YouTube and at Zoo Atlanta, where they also collected 16 bottles of urine. They found that in small mammals like rats, the urination process occurs in a series of quick drops. By contrast, larger mammals all released jets and sheets of urine in a process that took approximately 21 seconds, regardless of the creature's size.

The science of urination: