Miniature mites munch around our hair follicles while filling up with feces
Among the multitudes of odd and inventive things that animals do with their poop, the strangest practice of all may be to avoid pooping in the first place. That’s not to say that these creatures have somehow achieved perfect efficiency and produce no metabolic waste— they do eat food and make feces. The big difference is that they just hold onto that waste their whole lives, until they die, eventually revealing a lifetime of poop as they decompose. Incidentally, these animals also just happen to do all this in the pores of your face, which is where they live.
The poop-hoarding creatures in question are two species of microscopic mites called Demodex folliculorum and Demodex brevis. Both arachnids are small enough to live entirely in a pore on your skin, where they stick there head into your hair follicle to eat the sebum oils produced there. They can move from pore to pore, usually under cover of darkness, but for the most part you’d never know they were there, partially thanks to the lack droppings that most animals would leave in their wake. With simple bodies consisting primarily of a mouth, claws a long, cylindrical abdomen and notably, no anus, the mites are basically forced to carry their load for the entirety of their 11-day lifespan.
Any harm for the hosts?
As unpleasant as anatomically-induced constipation sounds to us, it must meet that evolutionary standard of being “good enough” for Demodex. Aside from the inevitable creepy-crawly feeling you probably have at this point from imagining little mites toting their poop around your face, is there reason for humans to be concerned? The answer to that is less clear. The number of people who are home to either or both species of Demodex mites is high enough that if they were a big problem, we’d probably be much more aware of them. DNA samples of the mites indicate that they’ve been living with humans long enough to evolve specializations for our bodies. Most of the time, being a host isn’t a cause for concern.
On the other hand, Demodex populations have also been correlated with cases of rosacea, a skin disease generally associated with red blotches on the cheeks and forehead. While tiny arachnids eating oil off your skin seems like an easy thing to prove, there’s evidence that the mites aren’t as much of a problem as the bacteria they carry around with them. Bacillus aleronius rides around on Demodex mites, and has been found to trigger immune responses in humans that the mites, and their collection of poop, somehow never do.
Source: Everything you never wanted to know about the mites that eat, crawl, and have sex on your face by Ed Yong, Not Exactly Rocket Science