A man releases a mouse back into the wild
Alessio Mortelliti, associate professor of Wildlife Habitat Conservation at the University of Maine, releases a deer mouse back where it was captured after data was collected and it was run through a series of tests to study its personality. Mortelliti is leading a project studying the personalities of small mammals to determine how it affects their survival and the impact on the forest they live in. “The mind of a mouse can impact the whole ecosystem.” Credit: Linda Coan O'Kresik / BDN

Of all the creatures that University of Maine wildlife biology professor Alessio Mortelliti and his students have encountered during their years studying small mammals in Maine, few are as ornery, stubborn and liable to bite as the red squirrel.

Mice and shrews can be lively, voles are more relaxed, and rats are even rather nice. Gray squirrels are far less irritable than their tawny cousins, though Mortelliti and crew don’t encounter them very often when they’re out in the field in UMaine’s Penobscot Experimental Forest in Bradley.

But red squirrels do not take kindly to being trapped, tested, tagged and set free by Mortelliti and his students, who for the past seven years have conducted a long-term study to understand how small mammals and their individual personalities — from a cranky squirrel to a shy vole — affect the forest ecosystem.

Just like humans, animals have personalities that affect how they negotiate the world around them. A shy human may not venture from their home that often, or engage with strangers. Similarly, a shy mouse may quickly gobble up the seeds they find and dive back into a hole, rather than risk becoming dinner for an owl.

“Just like some humans are more outgoing than others, and some will just stand in the corner at a party, animals have distinctive personalities too,” Mortelliti said. “And how that animal behaves plays a big role in how they change their ecosystem.”

Clockwise from left: Ivy Yen, a University of Maine grad student, examines a deer mouse while collecting data Tuesday in Bradley; Maisie Merz (left) and Yen collect data from a deer mouse; Merz works in the UMaine research forest; Merz writes down information about a red-backed vole before running it through an emergence test. Credit: Linda Coan O’Kresik / BDN

It’s easy to ignore the little creatures that scamper among the leaf litter out in the woods. But these tiny mammals play an outsize role in many aspects of the ecosystem — especially seed dispersal, which Mortelliti focuses on. Mice, shrews, squirrels and other small animals disperse around 95 percent of tree seeds found in a forest. How much or how little those mammals disperse those seeds affects things like how many seedlings take root and what species of tree may come to dominate the forest.

What one mouse does with a seed will be different from what another mouse does. A bold mouse might cache seeds, willing to take the risk of being out in the open for longer periods. More outgoing animals run a greater risk of being caught by a predator, however, while more risk-averse ones are less likely to be eaten. That’s where Mortelliti’s small animal personality study comes in.

“What we want to know is how their personalities — boldness, shyness, aggressiveness, curiosity — affect forest regeneration,” Mortelliti said. “Is it all bolder animals that are caching seeds? What about the shy ones, that might escape predation? All of this affects the composition of forests.”

Each summer and fall, Mortelliti and his team place traps in sections of the Penobscot Experimental Forest, where they catch a variety of mice, voles, squirrels, chipmunks and other small rodents. When the team comes back out to see what has been caught, they put each creature through a series of tests Mortelliti devised, to determine the basic levels of shyness versus boldness, response to novelty and stress response.

In one test, the researchers place a creature in a bag to see how it responds to stress, and in the second test, they see how long it takes for it to emerge from a trap. In another, it’s placed in a large box to see whether it will hide in the corner or move toward the middle, to determine its level of curiosity or risk aversion. Once the tests are complete, each animal is tagged and released. The researchers regularly re-catch previously caught animals, ​​to assess their behavior patterns over time. Their personalities generally remain consistent.

“If a mouse moved to the center of the box in the spring, it will probably do the same thing months later,” Mortelliti said. “It’s not just their mood on that day. It’s actually their personality.”

A mouse sits in a box with equipment.
A southern red-backed vole in an emergence test on Tuesday. The test is one of three that helps researchers determine the personality of the animal and how it affects their survival and the forest they live in. Credit: Linda Coan O’Kresik / BDN

Mortelliti has a number of other projects he’s working on, including a new statewide project to track larger mammals like lynx, bobcats, fishers and martens using camera traps. He’s also involved in a science education project that draws on his research, in which high school students across New England learn about small mammal personality traits, and then go out with one of Mortelliti’s students and catch a squirrel for themselves.

“Everyone in the country has a story about a squirrel in their backyard. But they’re just kind of there, and they can fade into the background. When you see one up close, and hold it in your hands, it can all kind of come alive,” he said. “As scientists, we need to do a better job communicating science to people. Putting it right into their hands is a big part of that.”

Tracks made by a squirrel whose feet were covered in a flourescent pigment, as part of UMaine grad student Ivy Yen’s study on seed dispersal in small mammals. Credit: Courtesy of Ivy Yen

Mortelliti’s graduate students have their own projects to come out of the research, too.

Ivy Yen hides seeds painted with a fluorescent pigment, which animals then scatter across the forest. When they come back at night, the team uses a black light to find the pigmented paw prints, showing where the animals took the seeds and if they cached them or not. Yen has also begun a project to see how small animals’ personalities, such as their levels of activity, shyness or boldness, may affect how likely it is that they become vectors for Lyme disease.

After spending seven years in the woods, catching and releasing thousands of tiny creatures, Mortelliti and his team have amassed reams of data on what may or may not go through the minds of the smallest residents of Maine’s forests. The researchers have had their share of run-ins with bigger animals, including thieving raccoons and bears, who raid their traps. And they’ve become experts in handling fast little critters, who want to slip out of their hands and back into the wild.

“In some ways, you have to learn to think like a mouse,” Mortelliti said. “After a while, you know how they are going to react, and you’re ready for it.”

There are lots of ways in which the data they’ve collected could have a practical application in the years to come, from helping to determine how climate change affects the mix of species within ecosystems, to influencing how humans choose to manage forests.

“As new species move north due to climate change, the seed dispersers — the small mammals — will have to adapt to encountering new seeds,” Mortelliti said. “The smallest creatures can have a huge impact on how our ecosystems will look in future.”

Emily Burnham is a Maine native and proud Bangorian, covering business, the arts, restaurants and the culture and history of the Bangor region.