Parasitic infections are an unfortunate part of life for most animals, humans included. Without the (unnaturally sterile) human standards of hygiene, our primate cousins contract parasites much more commonly.

Humans treat parasitic infections with vaccinations or medications. Treating roundworm, hookworm, pinworm, and tapeworm involves differential toxicity. The drugs we take are toxic to our bodies, but more toxic to the bodies of the parasites. This makes them an effective treatment for parasitic infections in human bodies.

So how do primates cope with parasitic infections?

Scientists have observed chimpanzees from several independent populations on the African continent eating whole leaves. Infected individuals may eat from 5 to 55 leaves, often repeating the process several times. The leaves used are from a variety of species. All of the leaves have one thing in common – trichomes, or bristly growths. The chimpanzees fold the leaves carefully and then swallow them whole. The leaves stay largely undigested through the gastrointestinal tract, and when they come out, they carry many of the parasites with them. The trichomes and the pockets caused by folding the leaves trap the adult worms. This helps to clear up parasitic infection.

Heavily infected chimpanzees often eat the leaves early in the morning, just after waking. The bristly leaves cause physical irritation on an empty stomach, which stimulates the digestive tract and causes diarrhea. This method of treating parasitic infection has also been seen in bonobos and lowland gorillas, as well as other vertebrates such as brown bears and snow geese.

The chimpanzees and other apes that use this method must have evolved some serious antigag reflexes in order to swallow the leaves whole. I suppose when no other alternative is available, the short-term discomfort is well worth it to deal with the parasitic infection.

This post is an excerpt from my book “The Nature of Human Nature.”