Bug o’the Week – And now for Something a Little Different V – To Sleep, Perchance to Dream

New Year’s Greetings, BugFans,

In the spirit of New Year’s Day entertainment, this is a rerun, an article that the BugLady wrote for the January, 2009 BogHaunter, newsletter of the Friends of the Cedarburg Bog.

The BugLady, Wisconsin born and bred, is not particularly a winter person (though her camera does prod her to suit up and go outside periodically).  Winter is monochromatic and still, and then there are the flash-frozen fingers and feet.  It’s a really tough period for wildlife to weather, too, and they emerge from it stressed, hungry, and at their lowest populations of the year.

Most bird species deal with the season by migrating.  Temperature isn’t the issue – birds, after all, invented down jackets.  They migrate because their summer food sources aren’t available in winter, and many of the birds that remain adjust their diet to include more plant material (incidentally, Googling “birds, hibernation” and “birds, torpor” is instructional).  Birds that do stay, stay silently; other than the call notes of crows, jays, and chickadees, the soft “yank-yank” of nuthatches, and the polyglot utterances of starlings, the winter landscape is a quiet one.  Bird song, after all, is designed to advance breeding, a summer pursuit.

Invertebrates are cold-blooded, and with body temperatures that very nearly match the temperature of the surrounding air or water, their winter options are few.  Outside of a handful of insects that migrate, some form of “sleep,” with cells protected by “antifreeze” from the damage caused by freezing and thawing, is the only alternative to death.  Insect eggs, larvae, pupae, and even adults spend the winter in “diapause,” a state of suspended animation during which development ceases.

When it comes to mammals, the term “hibernation” is applied too loosely.  Winter sleep – the “period of adaptive winter inactivity” – is a continuum.  At one end are the very few true hibernators, and at the other end are a group of mammals that may “hole up” briefly during really severe stretches of weather but that are otherwise active throughout winter.  In the middle are a variety of light and heavy sleepers.  Whatever the duration of the nap, its purpose is to minimize the number of calories burned.

True hibernators put on an impressive layer of fat in late summer and fall, retire to a den, and then lower their metabolism, heart rate, breathing, and body temperature.  Hibernators sleep so deeply that they are hard to rouse.  Woodchucks are true hibernators whose spring awakening is driven not by a call to forecast our weather but by the imperative to reproduce (the BugLady just read in Wikipedia that yearling woodchucks may be called “chucklings”).

Jumping mice (Zapus sp.), 13-lined ground squirrels and some species of bats are also true hibernators. The respiration of a 13-lined ground squirrel drops from about 150 breaths per minute to a single breath every five minutes, and its heartbeat goes from 350 beats per minute to five.  The debate continues about bears, whose body temperature drops relatively little and whose sleep is fitful, but who, for over half a year, may not eat, drink or eliminate.  “Torpor,” a short (sometimes just overnight) drop in temperature and metabolic rate in order to conserve body heat, energy and fat reserves, might be a more accurate term.

Deep sleepers” achieve torpor for a longer part of the winter.  They may be roused easily because their body temperature doesn’t drop very much or because they don’t put on a thick layer of fat and so must get up periodically to eat.  Chipmunks are classic heavy sleepers.  The BugLady’s chipmunks probably enter the winter with 25 pounds of birdseed stashed below-decks, and they may start their winter sleep in a nest of shredded grass piled on top of a mound of seeds.

Much of the food a chipmunk collects is cached in its underground tunnel system, which includes a room that serves as nest chamber, store room and bedroom, and, often, additional store rooms and a separate room for a latrine.  Because chipmunks are not well insulated, they must wake for a meal every few days.  By the end of winter, their grassy nest may be on the chamber’s floor, and if the cold lingers and they run out of food, they will emerge to forage while there’s still snow on the ground.

Light sleepers disappear for part of the winter but leave their tracks across the landscape during mild winter days and nights (many light sleepers enjoy the benefits of our bird feeding activities and of the warm motors of cars) (the BugLady’s mechanic loves her – she lets him keep all the mice he finds, free).  Skunks often hibernate in communal dens during the worst of winter – in some cases, up to a dozen females may cohabit with a single male, while other males may stay active and solitary all winter.  Raccoons and possums also den up during deep winter (pity the poor opossum, a southern mammal that is a relatively recent and ill-adapted arrival to God’s country, whose thin ear tips freeze and crack off during especially brutal winters).  The diet of all three of these omnivores includes a higher percent of small rodents in cold weather.

Squirrels trade their tree-top summer nests for leaf-lined winter lairs in hollow trees.  The nuts they store during the fall are fair game for any foraging animal, and the vast majority of nuts are found, but squirrels don’t bother to recover acorns that their noses tell them have rotted.  Squirrels are well adapted for short sleeps in severe weather.  Curled up with that bushy tail acting like a blanket, they can sit out a few days of harsh weather.

Deer (whose drab, winter coats are made up of hollow hairs that trap body heat), mice, voles, and cottontails stay active, as do muskrats, which live below the ice.  And what of foxes, coyotes, weasels, shrews, and other carnivores?  Most are active throughout the winter because they aren’t adapted for sleep, and there is still food available for them.  Watch for their tracks in the snow, too.

The BugLady read a neat story in the snow at Riveredge Nature Center the other day.  She found a snow-covered, sawed tree trunk lying across a thickety area.  There was a row of tracks along the length of the log where a coyote took the path of least resistance.

Kate Redmond, The BugLady

Bug of the Week archives:
http://uwm.edu/field-station/category/bug-of-the-week/