Animal urination

a study in bad science journalism


You might just want to give up reading this and just hit yourself in the face with a brick. It might actually be a more rewarding experience than this car-crash.

Some researchers from the Georgia Tech have been studying mammal urine, in particular the rate of urination across species. Co-incidentally Georgia Tech is where Kirk Englehardt is the Director of Research Communication and a talented and well-known tweeter and blogger on all things #scicomms.

So anyway, the university has issued a press release about the research which is quite well written and as might be expected it has been picked up in the media.

The paper has been published by the Proceedings of the National Academy of Sciences of the United States of America (PNAS).

The first problem is with the title of the paper — “Duration of urination does not change with body size”. This is clearly misleading given that the abstract goes on to say

we discover that all mammals above 3 kg in weight empty their bladders over nearly constant duration of 21 ± 13 s.

This implies that the actual spread of measured speed of urination ranged from 8 to 34 seconds, which is not really to say that the ‘duration of urination does not change with body size’.

The press release says

Even though an elephant’s bladder is 3,600 times larger than a cat’s (18 liters vs. 5 milliliters), both animals relieve themselves in about 20 seconds.

Which I concede might be entirely accurate — it is possible that cats and elephants both urinate for 20 seconds. But this seems to downplay the actual range of urination times reported in the paper.

As might be expected with momentous scientific news about animal urination, the news organisations have picked up this juicy titbit.

The Mail thunders “all animals from elephants to cats take exactly 20 seconds to relieve themselves” forgetting with the ‘about’ from the Georgia Tech press release and going straight for a claim that isn’t made.

Anima Khan in the Los Angeles Times does a better job, reporting

Scientists who watched dozens of different mammals from rats to elephants relieve themselves found that most of them seem to urinate in the same time frame — around 21 seconds.

Notice the difference: most animals, around 21 seconds, ‘seem to’, in the same time frame. Not ‘all animals take exactly 20 seconds’.

I suspect the Mail article has been largely influenced by a New Scientist scoop from last year which reported the findings based on a conference proceeding.

New Scientist described this as a ‘Universal Law of Urination’ and says

Elephants, cows, goats and dogs all take roughly 21 seconds to empty their bladders.
According to the team’s model, an animal’s size does make a difference to urination time — but only very slightly. Their law of urination says that the time a mammal takes to empty a full bladder is proportional to the animal’s mass raised to the power of a sixth, meaning even very large changes in mass have little effect on the time.

I suppose this comes down to how you understand the concept of a ‘very slight’ difference in urination time. I suppose we can forgive NS for some lack of precision given that the paper had not been published at the time of this report.

The press release has some interesting details:

the research team went to a zoo to watch 16 animals relieve themselves, then watched 28 YouTube videos. They saw cows, horses, dogs and more.

Lots of questions remain: how often did they watch the animals? How do they know how full the bladders of those animals were?

How reliable, actually, are these results? Watching 16 animals and 28 video clips seems to me to be a very small sample size on which to be basing a ‘universal law’.

But then it turns out that this study has not even been done to understand and quantify the speed of mammal urination but to get data to make an engineering argument — which, to be fair, sounds reasonable and may indeed mean that future flow devices are better designed.

Ultimately, though, this is not science. This is a small study extrapolated and misinterpreted by the media. Sigh.