Apodemus at heights: Repeated records of Apodemus mice in tree canopies

For decades, scientists studying small forest animals have looked almost exclusively at the ground. When researchers want to understand where mice live, what they eat, or how they move, they typically set traps or place cameras on the forest floor. This approach has built a solid picture of life in the undergrowth, but it leaves a massive, silent world above largely unexplored: the forest canopy. While we know that some forest creatures are excellent climbers, the upper branches of trees have remained a blind spot in our understanding of rodent behavior. The question of whether common forest mice regularly venture into the high branches, or if they only climb there by accident, has been difficult to answer because the tools used to study them were not built for the treetops.

A team of researchers in Germany has now begun to fill in this missing piece of the puzzle by looking up. They focused on the genus Apodemus, a group of mice that includes the yellow-necked mouse and the wood mouse. These animals are known to be agile climbers, capable of using trees and shrubs, but most scientific records of them come from the ground or very low vegetation. The researchers wanted to know if these mice were simply occasional visitors to the high branches or if they actively use the forest canopy as a regular part of their daily lives. To find out, they moved their observation tools from the dirt to the sky, setting up cameras high in the trees to watch what happened when no one was looking.

In the managed forests of Lower Saxony, Germany, the team deployed sixty non-baited cameras across twenty different forest plots. Unlike traditional studies that might use food to lure animals into a trap, these cameras were placed without any attractants, ensuring that whatever they recorded was natural behavior. The cameras were mounted on tree trunks and pointed toward branches, capturing short video clips whenever motion was detected. The study covered a period from April to November 2023, watching over a mix of European beech, Norway spruce, and Douglas-fir trees. While most of the cameras were placed at a height of three meters to catch general activity, a specific subset was installed much higher, reaching into the true canopy.

The results from these high-altitude cameras were striking. Between August 8 and September 9, 2023, the team recorded Apodemus mice on branches at three different stations, all positioned well above the ground. The highest observation occurred on a Douglas-fir tree in the Solling uplands, where a camera placed 21.3 meters up captured a mouse moving along a natural branch at 11:45 PM on August 8. This was not a one-time fluke. The researchers also found repeated evidence of mice on two European beech trees, with cameras at heights of 17.7 and 18.4 meters recording activity on three separate nights at each location. In total, they captured thirteen videos representing seven independent sightings, all occurring between the hours of 10:39 PM and 1:57 AM. Because the cameras were not baited with food, these recordings prove that the mice were not climbing up to eat a trap; they were simply moving through the trees as part of their normal routine.

While the video footage was clear enough to show the mice moving confidently along the branches, it was not sharp enough to distinguish between the two similar species of Apodemus present in the area: the yellow-necked mouse and the wood mouse. However, the researchers could make a very strong educated guess based on other data collected in the same forests. During ground-level trapping in the same areas, they found almost exclusively yellow-necked mice, with wood mice being virtually absent. Furthermore, previous studies have shown that the yellow-necked mouse is a more frequent and capable climber than its relative. When combining the ground data with the known climbing habits of the species, the evidence points strongly to the animals in the high branches being yellow-necked mice, even if the video alone could not confirm the species with absolute certainty.

This discovery challenges the long-held assumption that these mice are primarily ground-dwellers. The fact that the animals were seen repeatedly at heights exceeding 17 meters suggests that using the canopy is not an exceptional event reserved for rare emergencies, but a regular part of their behavior. The researchers noted that these observations happened in late summer, a time when seeds from beech trees might be available in the canopy before they fall to the ground, offering a potential food source. Additionally, moving through the high branches could provide a safer path away from ground predators or reduce competition with other animals that stay on the forest floor. The study also highlighted that these mice were active in a forest of Douglas-fir, a tree species not native to the region, showing that they adapt readily to different types of forest structures.

The significance of this work lies not just in the height of the climb, but in the method used to find it. For over sixty years, a single record of a mouse caught in a trap 23.3 meters high was the only proof that these animals could reach such altitudes. That record, however, relied on a baited trap placed on a metal beam, which left open the question of whether the mouse climbed there for food or out of curiosity. The new study provides direct video evidence of mice moving naturally along live branches, confirming that the canopy is a genuine part of their habitat. The researchers argue that the rarity of such sightings in the past is likely due to a sampling bias; because most studies look only at the ground, the vertical world of the forest remains invisible. By lifting their cameras into the trees, the team has shown that the forest canopy is not an empty space above the mice, but a living, active layer of their world that has been waiting to be seen.