1. Which Animals Are We Talking About, and What Do They Normally Eat?
Pill bugs and sowbugs are not insects. They are land-living crustaceans, relatives of shrimp and crabs, grouped together as terrestrial isopods. In Britain and much of Europe, these animals are commonly called woodlice. Scientists often say “isopods” for all of them, and this article will mostly say “pill bugs and sowbugs.”
Their usual meal is decaying plant matter. A 2016 paper in Open Biology describes woodlice as “generalist detritivores,” animals that eat many kinds of dead material, and notes that dead leaves are rich in tough plant fibers that are hard to digest [1]. A review of how land isopods feed adds that they chew leaf litter into pieces before swallowing, and that they prefer leaves that are already rotting over fresh ones, because rotting leaves are easier to digest [2].
So eating dead leaves makes sense. The puzzle is what happens next.
2. What Do Droppings Have That Dead Leaves Lack?
The same review reports that isopod droppings are rich in nitrogen and support dense growth of microbes, and that this microbial activity helps explain why isopods eat their own droppings [2]. Microbes matter to isopods in two ways: they release enzymes that help break down tough food, and the microbes themselves can be used as food [2]. In other words, a dropping can be a small, microbe-rich snack that is easier to digest than the leaf it came from.
A 2019 paper from Brazil lists the main ideas in its introduction. Besides microbes, it mentions that droppings might supply nutrients that a leaf-only diet does not provide enough of, and it points to copper [3]. The copper idea is the most unusual one, so it gets the next two sections.
3. Why Would a Pill Bug Need Copper at All?
Our blood is red because iron-containing hemoglobin carries oxygen. Pill bugs and sowbugs use a different oxygen carrier, hemocyanin, which contains copper. It turns blue when it holds oxygen.
The Open Biology paper reports that woodlice make hemocyanin in a four-lobed digestive organ called the hepatopancreas. That organ holds a lot of copper, which is kept in tiny compartments called cuprosomes [1]. (In that study, whole-organ copper was roughly 4,800 to 4,900 micrograms per gram of dry weight.) So copper is not a side detail. It is part of how these animals carry oxygen.
4. Is the Copper Idea Backed by an Experiment?
The Brazilian paper says the habit of eating droppings “may be related to the need to fulfill the copper demand,” because isopods are reported to be unable to extract copper from plant tissue [3]. If that is true, copper in a leaf mostly passes through the animal, and a dropping that still holds some copper is worth eating again.
The paper points to an earlier experiment by Weissenburg and Zimmer (2003). When animals were given copper-free food and prevented from eating droppings, the paper describes the copper in their bodies as dropping, with copper lost in the droppings [3]. The authors of the 2019 paper read this as evidence that eating droppings matters. I was able to read this experiment only through that paper’s introduction, not the original, so the details of the test are not confirmed here.
Notice what this does and does not show. It shows that copper can leave the body in droppings when food lacks copper. It does not show that copper is the reason the animals choose to eat droppings in everyday life.
5. Can Too Much Copper Hurt Them?
Yes, and that is part of why the story is complicated. A study in the Netherlands and Slovenia used sowbugs (Porcellio scaber) and poplar leaves, some left plain and some enriched with copper. Given a choice, the animals consistently preferred plain leaves. Even so, the more copper in the food, the more copper built up in their bodies, and animals offered both kinds took in less copper than animals offered only copper-enriched food. After long exposure to heavily enriched food, more animals died. The authors concluded that choosing food regulates copper intake only to a limited extent [4].
The copper in that test was added by people, so it tells us how the animals handle a copper excess. It is not a measurement of what they do in a normal garden. I only had access to this paper’s abstract.
6. Why Do Researchers Say the Reason Is Still Not Settled?
Look at the list: copper, other nutrients, microbial enzymes, and microbes as food. The 2019 paper says that “these explanations are not mutually exclusive and we cannot explain the coprophagous behavior yet” [3]. Several things could be true at once, and no one has measured how much each contributes.
That same paper also tested something practical. It compared three species, including the common pill bug Armadillidium vulgare, in three setups: droppings freely available, droppings removed regularly, and droppings blocked by a net. According to the abstract, the amount of food eaten did not differ significantly for any species or leaf, and neither did how efficiently the animals used their food. Growth differed in just one species on one kind of leaf, and the authors think this may be due to the short experiment and the animals’ sensitivity to environmental changes [3]. So in a short laboratory test, eating droppings did not change how much the animals ate. That is an interesting clue, but it is not an answer.
Also keep in mind where the evidence comes from. The work cited here uses European and South American species in laboratory feeding tests, mostly fed leaves such as poplar. How closely it matches a pill bug under a rock in your yard is a separate question.
7. How Can You Look for Clues in Your Own Yard?
You will probably not catch one eating a dropping, and that is fine. What you can observe is where you find them and what is nearby.
If it is light enough, gently lift a flowerpot, board, or flat stone in a damp spot, and ask an adult for help with anything heavy. Look at the animals underneath. Notice which ones curl up and which ones run. Look at what is around them: dead leaves, rotting wood, bits of bark. Then compare with a dry, sunny spot. Are there any there? Put everything back as you found it, handle the animals as little as you can, and wash your hands afterward.
Two questions are worth taking away. If copper is only part of the story, how much do microbes in the droppings matter? And do the pill bugs and sowbugs in your area behave like the ones in the studies? The sources below are a good place to start.