Sleep Comes in Many Forms  

Black Bear cub being studied as part of MDIFW’s Black Bear monitoring program.
Photo: MDIFW
Editor’s note: Did you see something unusual last time you were out in the woods? Were you puzzled or surprised by something you saw? Ask our “In the Woods” columnist Kevin McKeon. He’ll be happy to investigate and try to answer your questions. Email him directly at: kpm@metrocast.net.

By Kevin McKeon, Maine Master Naturalist 

Weather too cold? Too hot? No food? No water? No problem! Find a cubbyhole and enter a state of dormancy!

Many animals survive those times of stress by lowering metabolic activities to conserve energy. Hibernation is a familiar dormancy, but there’s also torpor, aestivation, diapause, and brumation. These terms can be confusing; there are similarities among them all – and differences. (There’s also geological, or volcano, and botanical, or seed, dormancy, sometimes lasting for many months, days, hours, minutes, or even centuries. But let’s stick with critters for now.)

Around the Sanford area, decreasing hours of both daylight (photoperiod) and heat trigger winter hibernation for three critters, whose body temperatures and heart, breathing, and metabolic rates slow dramatically during these times. 

Little brown bats go to their hibernacula — a cave, mine or empty building — in September and October, and don’t emerge until early to mid-spring. Their heart rates drop from one thousand beats per minute to five! 

Groundhogs dig a den deep enough to escape the frost. Their body temperature drops to 38 degrees, heart rate to four beats per minute, and breaths to ten an hour. 

The meadow jumping mouse’s den is a small chamber lined with dry plant material dug less than two feet below the surface. The mouse will close the entry with soil until it emerges in the spring. They do not awaken to eat, drink, or eliminate wastes, so no stash of seeds or nuts is needed. 

This will come as a surprise to many people: Bears don’t hibernate! In winter, bears enter a lighter state of dormancy called torpor. They can sleep more than 100 days without eating, drinking, or passing waste — and can actually turn their urine into protein! Skunks, chipmunks, squirrels, and raccoons also go into torpor. Hummingbirds and chickadees also use this deep sleep state on a daily basis to conserve energy. During torpor, the animal can wake up quickly to avoid danger or to exit the den to feed. Mother bear awakens to give birth, and dozes on and off as she nurses her cubs.  

Estivation is a strategy that critters use to escape the heat and dryness of summer’s extremes — a sort of summertime hibernation. These animals will find a spot to stay cool, shaded, and protected. The mourning cloak butterfly uses estivation, as do salamanders, frogs, and snails. The expert is the African lungfish: it’ll burrow into the mud of a dried-up lake, cover its body with mucus which dries into a water-retaining sack, and breathe through a small tube for up to three years!

Brumation is for reptiles and amphibians. Like hibernation, it’s triggered by decreasing photoperiod and heat, which cause these critters to stay warm and safe in underground rock crevices and burrows. But unlike hibernators, they’ll awaken on warmer winter days to drink water to keep from drying out. 

Diapause is a period of suspended development used by millions of life forms, at varying and usually predictable times, in response to environmental conditions. Monarch butterflies diapause during their flight from Sanford to Mexico by suspending their egg-laying along the trip. The trip back requires three generations of egg-laying butterflies to get here! Millions of insects use diapause at different life stages. Some animals delay embryo development to ensure a more favorable time of birth. During very dry conditions, some solitary bees diapause for 10 years!