A mouse scurries up to six chestnuts. Three look healthy. Three have exit holes where moth larvae ate the insides before they left. What does the mouse do?
For two years, Nagoya University researchers watched wood mice make these decisions on a forest floor. They measured the time they spent selecting nuts and found that about half the mice observed spent about five seconds sniffing and comparing chestnuts before they chose, potentially increasing their exposure to predators. The other half grabbed the nearest nut and ran in one to two seconds. The study, published in Scientific Reports , confirms that mice use smell to detect moth larvae damage in chestnuts and documents this puzzling 50-50 split in behavior. Notably, in the end, even badly damaged nuts were eaten.
Two-year mice surveillance suggests being a food critic is worth the risk
The team set up feeding stations with three healthy chestnuts and three damaged chestnuts. They placed video cameras above each station and recorded overnight. Over two years, the researchers documented 125 instances of mice removing nuts from feeding stations. This method allowed them to observe natural behavior without disturbing the mice. The cameras captured which mice visited, which nuts they chose, and how long they spent at each station. These experiments were repeated 12 times each year.
The researchers found that the picky eaters who took time to examine nuts were significantly more likely to select undamaged chestnuts. This suggests that the extra time investment pays off despite the increased risk.
"We cut open 100 moth-damaged nuts to understand their internal condition and found that in 95% of cases, moth larvae had consumed only 0-40% of the nut's contents, leaving the majority intact," noted first author Rui Kajita, a former graduate student at Nagoya University Graduate School of Bioagricultural Sciences .
The researchers also found that 90% of damaged nuts showed internal discoloration, and 40% contained feces from the larvae. These factors likely produce distinctive odors that mice can detect through sniffing and further reduce nutritional value. Despite this damage, most chestnuts disappeared from feeding stations in the morning, even the lowest-quality nuts.
The two mouse species observed, the large Japanese field mouse (Apodemus speciosus) and the small Japanese field mouse (Apodemus argenteus), showed both behavioral patterns. This ruled out species differences as an explanation for the split.
Possible causes include competition, individual differences, experience, or hunger levels, but these are untested hypotheses. What researchers did confirm, however, is that behavioral choices have consequences. Mice that took time to examine nuts were more likely to select undamaged chestnuts, though this required several extra seconds that could make them more vulnerable to predators.
"Owls, martens, and other predators hunt these mice, so every second a mouse spends at a feeding station could expose it to danger. Yet some of them still take time to choose the best chestnut. The choice shows that food quality matters enough to take risks," Hisashi Kajimura, professor and lead author at Nagoya University Graduate School of Bioagricultural Sciences explained.
A nose that can sniff out damaged, less nutritious food
Wood mice forage at night when darkness limits their vision. The researchers observed mice sniffing chestnuts repeatedly before making choices. This behavior suggests that smell provides critical information about nut quality that vision alone cannot detect.
This may explain why they rely so heavily on smell to evaluate food quality. Nocturnal animals often use smell more than vision to find food. Whether this represents a specific evolutionary adaptation to nocturnal foraging or simply reflects general rodent biology remains an open question.
"Despite the damage, most of the nuts disappeared from feeding stations overnight. This makes sense because chestnuts don't contain tannins, unlike oak acorns that can cause weight loss and death in rodents," Rui Kajita explained.
"Even low-quality moth-damaged chestnuts remain a valuable food source. The sniffing behavior allows mice to identify the worst damage and prioritize undamaged nuts, but ultimately they take everything because chestnuts are safe to eat."
Some moth-damaged nuts had feces near the exit holes that produce distinctive odors. The sniffing behavior may allow mice to identify these problems. While mice face a trade-off between selecting higher-quality food and minimizing the time they spend vulnerable to attack, both strategies seem to work as both types of mice continue to persist.
This research helps scientists understand how animals balance nutrition against safety and extends beyond mouse behavior. Mice disperse chestnut seeds across forests and the food choices they make affect which trees grow and where. Therefore, in response to moth infestation rates and/or habitat conditions, mouse behavior could change forest composition. Understanding how animals balance food quality against time and risk may help scientists predict ecosystem changes.
"It's a very relatable problem. Should you invest time in making better choices, or act quickly to minimize your risk?" Professor Kajimura added. "These mice have been doing this for millions of years and have two opposing strategies. Somehow both seem to work. Finding what causes these differences could help us understand decision-making in many species, including humans."
