Meet the Mice Who Make the Forest

It’s easy to look at a forest and think it’s inevitable: that the trees came into being through a stately procession of seasons and seeds and soil, and will replenish themselves so long as environmental conditions allow.

A deer mouse, temporarily captured for a behavioral test before being rereleased to the grounds of a study site at the University of Maine in late October.

Hidden from sight are the creatures whose labor makes the forest possible — the multitudes of microorganisms and invertebrates involved in maintaining that soil, and the animals responsible for delivering seeds too heavy to be wind-borne to the places where they will sprout.

If one is interested in the future of a forest — which tree species will thrive and which will diminish, or whether those threatened by a fast-changing climate will successfully migrate to newly hospitable lands — one should look to these seed-dispersing animals.

“All the oaks that are trying to move up north are trying to track the habitable range,” said Ivy Yen, a biologist at the University of Maine who could be found late one recent afternoon at the Penobscot Experimental Forest in nearby Milford, arranging acorns on a tray for mice and voles to find.

“The only way they’re going to move with the shifting temperatures is with the animals,” Ms. Yen said of the trees. “Will personality affect that? Will there be individuals who are more likely to help?”

Ms. Yen is a doctoral student in the lab of Alessio Mortelliti, a wildlife ecologist who arrived in Maine nearly a decade ago with a peculiar interest: how seed dispersal intersected with the emerging study of animal personality.

Although researchers already studied the ways that animals move seeds across landscapes, the possible role of their personalities had gone largely unexamined. The Penobscot Experimental Forest, with its 1,800 acres of closely monitored woodlands managed according to various forestry techniques, offered a landscape-scale setting to explore this question.

Each summer for the past seven years, Dr. Mortelliti’s students have trapped deer mice and southern red-backed voles in their study plots — about 2,000 animals in all — and run them through tests that measure where they fall on a spectrum between bold and shy. Before being released, each is tagged with a microchip, not unlike those used to identify lost pets.

Ivy Yen, a Ph.D. student in ecology and environmental sciences at the University of Maine.

The tags trigger sensors, like the one that Ms. Yen had mounted above her tray of acorns. Each acorn was painted with colored bands to indicate its species: red oak, bur oak, black oak, white oak, swamp white oak, scarlet oak, pin oak, willow oak. Red oak are already abundant in the region, but the other species have arrived only recently or are expected to, as rising temperatures push their ranges north.

Whether these trees succeed in this slow-motion migration — and eventually grace new landscapes with their noble, carbon-sequestering, shelter-providing, wildlife-nourishing presence — will be a function of countless encounters between a mouse or a vole and an acorn.

Does the animal take the nut? If so, is the nut eaten promptly or saved for later? Where does the animal cache it? How often do they fail to return, either because they forget the location or — as so often happens to bite-size creatures in a forest full of hungry predators — because they are eaten first, thus giving the acorn a chance to germinate?

“People see that a forest is regenerating,” Dr. Mortelliti said. “But what people don’t see is that the forest is regenerating following the decisions of small mammals.”

In Dr. Mortelliti’s study sites, each such encounter is documented. As a mouse or vole approaches an acorn-laden tray; a sensor reads their microchip, identifying the animal; a motion-activated camera captures the moment, recording which nut they took. Over the course of this season, Ms. Yen said, she would put out more than 1,800 acorns.

On this autumn night, Ms. Yen set out five trays, each about 100 feet apart. Surrounding each she scattered a nontoxic fluorescent powder that temporarily adheres to the feet of visitors. When she returned before dawn, equipped with an ultraviolet flashlight under which the powder fluoresced, small constellations of footprints surrounded each tray and trailed off into the darkness.

People don’t realize just how many mice and voles there are, Ms. Yen said. She estimated that, for every 13 steps she took on the way to the site, she passed a mouse or vole — not out in the open but hidden beneath a leaf or cozy in a grass-lined burrow. By the light of twinkling stars and a toenail moon, the rodents had performed their quiet labor. Every acorn was gone.

One by one, Ms. Yen followed each trail. The tiny footsteps glowed under her flashlight, leading around mossy hummocks and beneath fallen branches and up tree trunks and then back down again. As the mouse walks is the opposite of as the crow flies: Some trails petered out, the powder exhausted. Others ended in a cache — a hollow beneath a root, a decaying stump, a hole dug into the earth and carefully covered back up. Ms. Yen marked the final spots with small orange flags.

Some acorns, stored for the coming winter, were intact. Others had been consumed, but from the painted shell fragments Ms. Yen identified the species. With the help of Elizabeth Pellecer Rivera, a graduate research assistant, she made notes about each one. The sensor data and camera recordings would later show that much of the gathering was performed by one notably industrious deer mouse known to the researchers as 982091062973077, a 13-gram male trapped in late September and revealed by tests to be fairly shy, although with a cautiously exploratory streak.

When the season winds down, Ms. Yen, Dr. Mortelliti and two graduate students, Maisie Merz and Brigit Humphreys, will analyze all this data and look for patterns.

Maybe certain personality types will prove more likely than others to select certain oaks. It may take an especially bold rodent to hoist a massive bur oak acorn, then stagger under its weight, vulnerable to predators, until finding a hiding spot. Perhaps shy mice will be more likely to secret them in places best suited to germinate a forgotten nut.

The results will join a procession of studies that have emerged from the experiment over the past several years, most of them led by Allison Brehm, Dr. Mortelliti’s first doctoral student and the person who taught Ms. Yen how to track.

In a 2019 study in Ecology Letters, which Dr. Mortelliti described as a “proof of concept,” the researchers showed that the personalities of small mammals influence their choice of seeds. Earlier this year the team described how some deer mice, depending on their personality, were more likely than others to cache red oak, white pine and American beech nuts in ways that promoted germination.

In turn, the personality-specific foraging strategies of rodents changed when predators were around, the researchers showed in an Oikos paper in 2021.

And land use alters these dynamics. For instance, the 2019 study found that, in areas that had been logged years earlier, small mammals tended to be bold. A study the following year revealed that a more natural forest, with a mix of habitats rather than the uniformity favored by most commercial logging, contained a greater diversity of personalities.

“This diversity of personality types is maintained in populations because it’s a good thing, just like genetic diversity is a good thing,” Dr. Brehm said.

Rafał Zwolak, an ecologist at the Adam Mickiewicz University in Poland who studies seed dispersal and animal personality, called the research “absolutely pioneering.”

“I hope their work will inspire researchers in other labs, working in other ecological systems, to focus on this topic,” he said.

Asked to define the practical implications of his research, Dr. Mortelliti said, “Preserve a diversity of personalities.” There’s no one ideal personality; rather, different individuals perform different roles. Depending on circumstance — drought, natural disturbances, fluctuations in predator populations — different personality types may come to the fore. These nuanced dynamics don’t preclude timber-cutting, Dr. Brehm said, but they do argue for taking care.

“If you have to manage a landscape, you don’t want to manage it all the same way,” she said. “You want to manage different parts differently so that you have a heterogeneous landscape.” Techniques can be used that maintain a variety of tree species, ages and sizes, attempting to mimic what would happen naturally.

Much remains unstudied, Dr. Mortelliti noted. Measures of shyness and boldness are not the entirety of animal personality; they’re just relatively well-characterized and easy to measure in the field. Oaks aside, hundreds of other plant species are shifting their ranges, each following their own animal-mediated trajectory.

As Ms. Yen finished her work, night gave way to pre-dawn twilight. A blue jay called; a red squirrel chittered. Both are seed dispersers with personalities that may affect their contributions to the forest. The same could be said of bears, foxes, crows, turtles, even ants — a whole menagerie as yet unexamined, affecting not only plants but even fungi.

“I’m only looking at two species at night,” Ms. Yen said. “It’s a very small snapshot of what is happening.” A full picture may not emerge for decades, but the outlines are already clear: It takes a lot of personalities to raise a forest.