1. Do Mice Really Live on Cheese?
A little background first. The mouse in your kitchen or in cartoons is the house mouse, Mus musculus. In Japan, the mice that live in forests away from people are different species. This article calls them field mice. One of them, Apodemus speciosus, is the large Japanese field mouse. Both are rodents, but they eat differently.
For the house mouse, a New Zealand study is a good place to look. In the 1970s, a zoologist at the University of Auckland trapped house mice in two forests, one planted with pine trees and one of native kauri trees. The stomachs of 594 mice from the pine forest and 117 from the kauri forest were examined. They held largely adult insects and other arthropods, caterpillars (the larvae of moths and butterflies), and seeds [1]. The mix changed with the seasons. In the pine forest, insects and caterpillars were the main food all year, and seeds became important between spring and autumn. In the kauri forest, insects and seeds both mattered all year, and kauri seeds filled many stomachs in April and May [1].
So mice are omnivores: they eat both plants and animals, and they use whatever the season offers. Those forests are not your kitchen, and the study does not tell us what mice in a house eat. But it does show that "mouse equals cheese eater" is not a good description of how the animal lives in the wild.
2. How Do a Mouse's Front Teeth Cope With Hard Seeds?
Seeds and nuts are tough. A mouse's answer is its front teeth, the incisors. Rodent incisors have no closed root, so they keep growing throughout the animal's life [2]. Gnawing wears the incisors down, but they keep growing, so they do not run out.
They also stay sharp without help. Hard enamel covers only the front of each incisor. The softer material behind it wears away faster, leaving a slanted, chisel-like edge, which the Max Planck researchers describe as a self-sharpening device [2].
The enamel itself has a surprise. In seven rodent species, including beavers and squirrels, researchers found tiny pockets of an iron-containing material between the enamel crystals. These pockets make up less than two percent of that enamel layer by volume, yet they are decisive for its strength and its resistance to acid [2]. The orange-brown color of rodent teeth was long blamed on this iron-rich enamel, but the Max Planck team found that the color comes from the layers above it [2]. Teeth can crack a hard seed, but a seed can protect itself in another way: with chemistry.
3. Why Are Acorns Hard to Eat, and How Might Field Mice Handle Them?
Acorns are rich in calories, and rodents of many kinds collect them. But acorns of some oaks hold a lot of tannin. A Japanese oak, Quercus crispula, is one that contains tannins, and the paper's introduction, citing earlier work, gives 5.7 to 8.8 percent of the seed's dry weight for the red oak group. Tannins can seriously harm plant-eating animals [3].
The large Japanese field mouse, however, did well in a 15-year study in northern Hokkaido, Japan (1992 to 2006). Its numbers generally rose after years when oaks dropped huge acorn crops, while the clear effect was not found for two other rodents living in the same forest [3]. The authors suggest why. This mouse may reduce tannin damage with tannin-binding proteins in its saliva and with gut bacteria that make an enzyme called tannase, which breaks tannin down. Such tolerance has not been known in the other two species [3].
Notice the wording: "may". The study counted mice and acorns in a forest. It did not test the mouse's body step by step, and the paper presents the saliva-and-bacteria idea as a likely explanation drawn from earlier work.
4. What Happens to the Seeds That Field Mice Bury and Forget?
Field mice do not only eat what they find. They also carry seeds away and bury them to eat later, which is called caching. Seeds a mouse never comes back for may grow, so mice can help forests spread.
A Nagoya University team tested this with seeds of a dwarf bamboo, Sasa borealis. Bamboos of this kind flower and produce seeds across wide areas at the same time. The university says these events can be up to 120 years apart [4]. The researchers placed seeds in shallow mesh baskets on the forest floor and set up an automatic camera to record what happened. Two field mice, the large and the small Japanese field mouse (Apodemus argenteus), came for the seeds. Some ate them on the spot, and others removed them and sometimes buried them. This showed that mice cache even very small seeds [5].
What they did depended on the animal and the place. The large Japanese field mouse tended to eat seeds in places where plants gave it cover from predators, and to carry them away from open ground without vegetation. The small Japanese field mouse was more likely to carry seeds to another place even when there was cover [4]. In the Nagoya summary, the researchers suspect body size is involved. They also saw the large mouse use the bamboo seeds less in fall than in summer, which the paper links to acorns becoming available and the mice switching their diet toward them [5].
The researchers conclude that field mice are important seed dispersers for the renewal of forests [4]. The answer to "what do mice eat?" turns out to include a second job: what they do not finish eating helps decide which plants come back.
5. How Can You Look for Mouse Meals Near You?
You do not need to catch a mouse. In autumn, you might spot nutshells or acorns with round holes or gnaw marks under oaks, beeches, or pines, and a few chewed seeds left at the base of a log or stone. Many animals, including insects and birds, leave signs like these, so they are only clues, not proof of a mouse. Take photographs and leave everything where it is. Wild mice can carry germs, so watch from a distance and avoid touching droppings, nests, or animal remains. If you touch anything from the ground, wash your hands. If you want more, the sources below are worth reading in their own words, especially the Nagoya University news item, which is short and easy to follow.