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What Surprising Bodily Function Do Most Mammals Have in Common?

Most mammals share a remarkable ability to thermoregulate, maintaining a stable internal temperature regardless of the environment. This biological thermostat is a testament to the intricate design of mammalian physiology. Intrigued? Discover how this shared trait unites us with the animal kingdom and what it reveals about our own bodies. What might this mean for our understanding of life on Earth?

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.

Despite vast differences in size, most mammals larger than a rat take around 21 seconds to empty their bladders.
Despite vast differences in size, most mammals larger than a rat take around 21 seconds to empty their bladders.

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:

  • The urethra is a flow-enhancing device. As urinary systems evolved, the urethra scaled up without its function being compromised.

  • Hu’s study could prove helpful for a range of applications, such as diagnosing urinary problems in animals and inspiring the design of hydrodynamic systems based on those in nature.

  • An elephant's bladder can hold up to 5 gallons (18 liters) of fluid.

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    • Despite vast differences in size, most mammals larger than a rat take around 21 seconds to empty their bladders.
      By: Eric Isselée
      Despite vast differences in size, most mammals larger than a rat take around 21 seconds to empty their bladders.