July 30, 2023
“We know where all the birds are. We don’t know where all the mammals are,” Cody Lane tells me. We’re walking through a young, leafy aspen stand in northwestern Wyoming’s upper Hoback River valley, baiting live traps for small mammals. The hillside is a waist-high profusion of flowers, grasses, and aspen saplings, lit gently by the evening sun. Except for the quiet, charcoal-black snags that stand watch around us, you might not guess that an intense September fire swept across this landscape five years ago.
Cody is wading through a pale purple jungle of mountain hollyhock (Iliamna rivularis), interspersed with bright yellow clumps of tall groundsel (Senecio serra). He reaches the next live trap, a rectangular metal box slightly larger than a zucchini. A grocery-store-sized zucchini, that is, not the enthusiastic monstrosities that late-summer gardens so often produce. Cody removes the bait ball from the previous night and hands it to me, an appealing mixture of oats and peanut butter. “It looks like Captain Crunch,” he says. He replaces it with a fresh ball of bait. With luck the new offering, its peanut butter scent still strong, will attract a rodent.
Dog beds and ruffed grouse
Cody also adds a comfortable handful of insulation—in this case, the fluffy stuffing of a dog bed.
“This is the Himalayan Plateau of North America—incredibly cold, incredibly dry,” Cody tells me. He’s waited until July to start small mammal trapping here, but even so, the nighttime lows are not much above 40°F. The insulation will keep a captured rodent warm until we check the traps in the morning.
Down the slope from us, field technician Lauren Tate is going from trap to trap as well, doing the same thing we are. You wouldn’t know it among the thick growth of wildflowers and aspens, but there are a hundred live traps on this slope, arranged in a neat 90 meter by 90 meter grid.
As Lauren continues from one to the next, she flushes a ruffed grouse. The grouse flies to a heavily-charred lodgepole pine snag in a startled whir. Most of the birds have become relatively quiet in these late days of summer, but we can hear the piping calls of a flycatcher across the slope.
“Olive-sided?” I ask Cody.
“Yep,” he responds, “they breed in the burn. We caught one last year [in a mist net].”
From birds to voles
Cody is a master’s student at the University of Montana, and the area where we’re walking this evening is one of his research sites. It’s near the edge of the Roosevelt Fire, a blaze from September 2018 that burned over 61,000 acres and 55 homes before firefighting efforts and a shift in winds finally ended its run. Now, five years later, Cody is studying how the wildlife has responded.
Cody’s research focuses particularly on birds, using mist-netting and breeding-season point count surveys to compare the bird community between areas that burned and areas that didn’t. But his master’s research is part of an even larger, multi-faceted research project led by the Ricketts Conservation Foundation. Broadly speaking, the project is looking at how the animals of the Hoback River valley—from songbirds to elk to voles—respond to fire, both in aspen stands and in conifer forests. Tonight, we’re focusing on the voles and the other small mammals.
Aspens and small mammals
Aspen forests are wonderful places for biodiversity. And if I didn’t already know that, tonight’s trap-setting walk is making it clear. In the wake of the fire, the mountain hollyhock is growing with abandon, forming stands as lush as a garden. Meanwhile, the five-year-old aspen regrowth is already as high as my head. White-crowned sparrows chip in agitation from the young trees, where the tiny cup nests of dusky flycatchers hide. A litany of purple flowers are blooming across the slope: aspen fleabane (Erigeron speciosus), thick-stem aster (Eurybia integrifolia), Engelmann’s aster (Eucephalus engelmannii), and western aster (Symphyotrichum ascendens). Farther uphill, the openings between the aspens are covered with the cheery yellow flowers and sandpapery leaves of little sunflower (Helianthella uniflora).
But what about small mammals? We know that they must be here, hiding among this lush community of plants. But we could spend days walking through without seeing a single vole, let alone knowing more about which species are here. And that’s where a live-trapping effort like this comes in. We’ll return to the traps early in the morning, measuring, identifying, and marking whatever mammals we catch. Then we’ll release them to go about their lives.
“You’ll probably get to see some cool voles tomorrow,” Cody says.
How to count voles
We’re back among the aspens at 6:15 am as a stunning orange sunrise fades into pastel clouds. We split into two crews to check the traps. Alan Moss and Josh Lefever form one team. I tag along with Cody and with Brooke Bowman, a bird biologist who is as new to small mammal trapping as I am.
The first few traps are empty. Then we reach one that has a furry friend in it. Cody dumps the contents, mammal and insulation combined, into a one-gallon ziploc for a better view. Then he grips the vole securely by the scruff of its neck, wearing latex gloves (small mammals can carry a variety of diseases) and lifts it out of the bag.
“This is like the biggest vole we’ve ever caught!” he says.
He and Brooke work together to give the vole a numbered metal ear tag. Next, we paint his belly with a thick green line of marker. This small mammal study involves three consecutive nights of trapping here, and we’re especially interested in how many voles we recapture from night to night. Based on this proportion, we’ll be able to estimate population sizes for each species and compare them between sites.
Long-tailed voles and other species
The vole is thrashing wildly in Cody’s hand, trying to escape from what must appear to him as a massive predator. Fortunately for him, this unusual experience will be over soon: it only takes us a minute or two to mark, measure, and release each animal.
Based on a careful examination of the genitalia, Cody can tell that this vole is a male. Like most voles, he has a somewhat stubby tail, a grayish furry coat, and a blunt snout. But identifying voles to species is challenging.
So far, Cody’s team has found three vole species in this aspen stand. Southern red-backed voles (Myodes gapperi) are fairly distinctive, but the others—montane and long-tailed voles (Microtus montanus and M. longicaudus)—present a major identification puzzle, overlapping substantially in their appearance.
Having this vole in the hand gives us a better chance of identifying him. Even up-close, though, identification is hard. In the past, mammalogists have generally worked from dead specimens, where a close examination of the teeth helps distinguish the species.
Cody checks this vole carefully and takes some body measurements. This one has a surprisingly long tail, a whopping 70 millimeters. Most voles have much stubbier tails. This unusual proportion of tail to body makes Cody reasonably certain that this one is a long-tailed vole.
Of voles and mice
As we continue checking the traps, we find many more montane or long-tailed voles. A substantial portion of them are subadults, born earlier this year. Voles are a popular prey for many birds and other animals—from kestrels, owls, and hawks to snakes, weasels, and coyotes—so they reproduce quickly and typically die young.
Vole populations often fluctuate greatly from year to year, and this site is no exception. “We didn’t catch a single vole here last year,” Cody tells me. This year, they seem to be as common as deer mice.
A light breeze is picking up now. We can hear a dusky flycatcher singing fragmented phrases from down the slope. Two woodpeckers—either three-toed or black-backed—are drumming from the burned conifers across the creek. When we get to our next trap, we find a deer mouse (Peromyscus maniculatus). It’s easy to tell that this is a mouse rather than a vole: it has much larger ears and a long tail with gray hairs above and white hairs underneath. And while voles are tricky, identifying mice to species here is straightforward. The deer mouse is the only species that Cody and his team have found on their sites. Well-known as a raider of household pantries, this mouse is adaptable and widespread across many habitats in North America.
Cody checks the genitalia and tells us that this one is an adult male. Adult mice tend to be larger and browner, while juveniles are smaller and grayer.
As we release the deer mouse and he scampers off, Alan and Josh call to us from farther up the slope. They’ve caught a southern red-backed vole!
This one is subtly but distinctly different from our other voles, with rusty fur along the back and somewhat larger ears. It’s a species that is strongly associated with forested habitats in the mountains, particularly those with old trees and downed logs. In Wild Mammals of Wyoming and Yellowstone National Park, Steven Buskirk writes that this species was more widespread in the Pleistocene, when forests were more extensive. As the glaciers retreated and the climate warmed, the red-backed voles became restricted to the mountains. In these habitats, they remain a favored prey animal for such creatures as the pine marten and the boreal owl.
“They’re beautiful. Handsome voles,” Cody says.
A nocturnal city
By 8:00 am, we’ve checked all 100 traps and caught 26 mammals, all of them voles or deer mice. But the morning before, the crew caught a northern pocket gopher (Thomomys talpoides)—and sometimes flying squirrels and weasels visit the traps.
The morning has opened my eyes to a community that I already knew of but that I don’t often consider, a bustling city of nocturnal rodents normally seen only in fleeting glimpses. And it’s raised additional questions for me. How do all of these mice and voles share this space?
The answer is complex and not fully known, though previous studies give some hints. Mice and voles differ in their diets. Deer mice eat seeds, insects, and even bird eggs. On the other hand, voles feed largely on green vegetation—though the southern red-backed vole also feeds on fungi and invertebrates.
Competition between voles?
How might the three vole species share this space? Researching voles in nearby Grand Teton National Park in the late 1960s, Tim Clark found that these three plus the meadow vole (Microtus pennsylvanicus) overlapped in some habitats, such as aspen stands, but differed substantially in their abundance between different habitats. The meadow vole, for example, was the least commonly-caught species among the aspens, but was overwhelmingly common in wet willow swamps. Given that all four voles seem to have relatively similar diets and activity patterns, Clark suggested that the species were competing, pushing each other to the habitat extremes in which they were each best-adapted to thrive. He found that meadow voles, for instance, were most common in the wettest habitats, whereas montane voles occurred in drier situations that didn’t support other voles.
Are red-backed, montane, and long-tailed voles jostling for space in this aspen grove, shoving each other to the margins? I don’t know, but it’s an intriguing question to ponder.
A snapshot of a hidden world
As we hike back out from the trap site, the aspen leaves rustle gently in the breeze. A Lincoln’s sparrow is singing along the creek, evoking memories of previous summer days I’ve spent in places like this. In the years ahead, studies like this one will help us better understand the ways in which the living community knits back together after a fire. But already, this research has given me a remarkable glimpse into the unseen life among the aspens. This leafy slope isn’t just a home for Lincoln’s sparrows, dusky flycatchers, ruffed grouse, and swaths of mountain hollyhock. It’s also a place where deer mice store seeds for the winter, where long-tailed voles graze, where red-backed voles hide from boreal owls, and where northern pocket gophers churn the soil.
It’s a glimpse into a hidden world among the aspen leaves—and that’s pretty special.
Buskirk, S.W. (2016). Wild mammals of Wyoming and Yellowstone National Park. Berkeley, CA: University of California Press.
Clark, T.W. (1973). Local distribution and interspecies interactions in microtines, Grand Teton National Park, Wyoming. Great Basin Naturalist 33(4):205-217.
Pennsylvania Game Commission. (2021). Mice and voles. Retrieved from https://www.pgc.pa.gov/Education/WildlifeNotesIndex/Documents/MiceNVoles.pdf