The Physics of Pee: Animals Urinate at a Constant Duration Regardless of Size

a dog is peeing

We’ve all been in those situations when your bladder is full to capacity and you need to relieve yourself. One can only hold it in so long before nature takes it’s course. It is something we hold in common with all other mammals. Yet despite the universality of this biological function, the physics of urination among animal models is still poorly understood.

Researchers at the Georgia Institute of Technology set out to investigate the physics of pee, and recently published a study in the Proceedings of the National Academy of Sciences. They visited Zoo Atlanta and used high-speed video capture to record and analyze the flow-rate of 16 animals. Additionally, they studied animal footage from 28 YouTube videos. They discovered that despite animal size, there was a constant duration of urination lasting 21 seconds.

An elephant has a bladder nearly 3,600 times larger in volume than a cat. Larger animals also tend to pee in jet streams, while smaller mammals relieve themselves by droplets. So how can they both physically eliminate waste at the same rate? David Hu and his team suggest that despite varying bladder sizes, the differences are offset by flow rate established by the length of the urethra—the larger the animal, the longer the urethra. As this length increases, the effects of gravity also increase. This in turn puts greater pressure on the bladder, causing urine flow to speed up.

While the study of animal urination isn’t very common, it reveals much about how we relate to the animal kingdom. There are also possible applications to creating energy efficient water-based engineering systems based on the understanding of these animal models. Next time you are at the zoo, bring a timer along and observe some potty behavior—for science!

//Image source: Mike Finkelstein on Flickr.

Mariel Mohns is a research technician who is passionate about science communication and sharing ideas. She is a stereotypical INTJ, spends way too much time on the internet, and is also a classically trained pianist learning how to be a songwriting rock star. She fell in love with Darth Vader as a young girl and has been a geeky nerd ever since. Mariel blogs about her random life and musings at and tweets from @marielmohns.