Why do farts stink?
I am answering this unsavory question mostly because the student who
asked it asked me to answer it, and he was a student who demonstrated
an admirable spirit of inquiry throughout the year.
People have a symbiotic relationship with numerous bacteria that live
in our intestines. In fact, there are more bacteria cells in our
bodies than human cells. Many of these bacteria live in our
intestines and help is to break down food. There are many gasses
produced as a byproduct of these processes, and farting (flatulence)
is our main way of expelling these gasses. It has long been debated
which of these gasses were most responsible for the smell, but recent
research has shown that it is caused by hydrogen sulfide and other
volatile sulfur compounds. This is why eating high-sulfur foods such
as cabbage or eggs can result in stinky flatulence.
There is also the question of why people find sulfur compounds so
malodorous. Flatulence does not have much sulfur in it, but we are
able to smell it quite easily. Most likely our sensitivity to
volatile sulfur compounds is adaptive because it helps us to avoid
eating rotten meat and eggs.
There are a couple of other interesting compounds that contributes a
little to the smell of flatulence, but quite a bit to the smell of
feces: skatole and indole, both of which are produced by the
bacterial breakdown of tryptophan in the intestines. In small
amounts these compounds have a flowery fragrance, and they are found
in many flowers – most notably jasmine and orange blossoms. In
larger amounts they smell like feces. They tend to be most prominent
in the feces of carnivores, and that is one of the reasons carnivore
feces generally smells worse than that of herbivores. (Another
reason is that many carnivores have glands that add distinctive
aromas to their feces to help them mark their territories.)
One of the most interesting characteristics of skatole and indole is
that they make smells linger. This is why it is so hard to get the
stink off of your shoes when you step on dog poop. It is also the
reason that people who make perfume add these chemicals to their
products. If you make your own fragrance from pure essential oils,
it will be strong at first, then will quickly fade. Commercial
perfumes can last all day, largely due to the addition of skatole and
indole.
I once speculated that animals that harbor a large amount of skatole-
and indole-producing bacteria do so because it is adaptive; it would
make the territory-marking characteristic of their feces last longer.
I cannot find any research to back up my hunch, and there seems to
be some animals (Homo sapiens, for example) that produce large
amounts of skatole but who are not known to mark their territories
this way (although we may have in our past).
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