The Outside Story: Woodland Jumping Mice Are Truffle Specialists | Weekend magazine

“Shhh,” I say to my 5-year-old son, “there are animals sleeping, right under our feet.” He presses his ear against the frozen ground and hopes to hear the slow, sleepy breathing of a dozing mammal. I tell him if he could pull back the earth like a blanket and look inside, he could see a small, mouse-like ball of white, black, and yellow-orange fur curled up tightly in a nest of dried grass and dead leaves. A long, sparsely haired tail would wrap itself around this little parcel like a thin ribbon: a forest hop mouse, whose food consists mainly of truffles before hibernating.

Forest hop mice (as opposed to the closely related field-dwelling meadow hop mice) live in the coarse woody debris of hemlock hardwood and spruce-fir forests throughout New England. Their white tipped tails, which can measure 6 inches or more, make up two-thirds of the total length of their bodies. When frightened or chased by predators, these mice, with their powerful, kangaroo-like hind legs and large rear feet, leap up to 2 feet high and up to eight feet forward. You will jump several times, then stop abruptly and become completely still. These irregular movements and a fur that is good for camouflage help forest hoppers evade the claws and teeth of predators such as great horned owls, red tailed hawks, bobcats, weasels, and foxes.

The forest hop mouse is one of the smallest true hibernators in North America and relies on their supply of specialized fat to survive the winter. The shortened daylight and cooler autumn temperatures trigger a hormonal response that causes jumping mice to spend their waking moments eating as much as possible. In contrast to many other rodents, the jerboa does not hoard or store food; everything it finds to eat goes straight into its body. When they go to their underground nests for a long hibernation, these normally slender mice have gained more than 25% in weight.

In essence, a jerboa becomes their winter pantry as it converts most of the food it consumes into brown fat, which is directly converted into body heat throughout the winter. During hibernation, the brown fat acts like a slow-burning log in a wood-burning stove, preventing the woodhopper mouse’s body temperature from dropping below freezing.

In order to build up this fat reserve, wood jumping mice eat grass seeds, berries and insects such as caterpillar larvae. But more than 30% of the mouse’s diet consists of underground mushrooms, commonly known as truffles. The truffles that forest bumpers consume in the forests of New England are not the same varieties that are sniffed by highly trained pigs or dogs in Italy or France. However, the central role of these specialized underground fungi in the forest ecosystem is worth more than their weight.

Underground mushrooms are essential for forest health, according to Ryan Stephens, a small mammal and fungus researcher at the University of New Hampshire. “These fungi colonize the roots of trees and help them absorb nutrients and water,” he said. “In return, the trees provide the mushrooms with sugar.”

These mycorrhizal fungi, as they are called by scientists like Stephens, are associated with most vascular plants and trees, and their role goes beyond the nutrient cycle. These fungi have been shown to help trees and plants respond to pathogens and poor environmental conditions and regenerate after forests have been altered by things like fire and logging.

Recent research by Stephens and a team of UNH researchers in the White Mountains, New Hampshire, has shown that forest mice and other small rodents such as voles, chipmunks, mice, and shrews play an essential role in the spread of spores of underground fungi. In a way, these animals are like underground pollinators. By eating underground mushrooms and then defecating them in new locations, they contribute to the spread of the spores and enable the formation of new fungal networks with tree roots.

“Small mammals help sustain a diverse community of mycorrhizal fungi in forested systems,” said Stephens. “This is important because a diverse fungal community can help protect forests from disruptions like drought.”

I think of these hidden connections as I watch my son, holding my breath as he listens to the floor and imagines what is beneath him. There is so much of our world that we never see, yet these underground interactions make our world above the earth possible.

Susie Spikol is the community program director for the Harris Center for Conservation Education in Hancock, New Hampshire. The Outside Story is commissioned and published by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation.

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