The State We’re In: Preparing To Hibernate?

Michele S. Byers

Michele S. Byers

by Michele S. Byers, executive director of the New Jersey Conservation Foundation

Do you have visions of crawling into a cave with blankets and snoozing until spring, emerging only when New Jersey’s forests are once again carpeted with trout lilies and spring beauties?

Many creatures in this state we’re in are having those visions right now! But hibernation is more complicated than a long winter’s nap. Animals who prepare for winter have a variety of remarkable physiological strategies.

Over-wintering bats like federally-endangered Indiana bats and some large rodents like woodchucks are New Jersey’s true hibernators. These mammals regulate their metabolism to create a torpid, cold, inactive state. Woodchucks spend many months with a constant body temperature of 38 degrees Fahrenheit. Almost never would a woodchuck wake up as early as Groundhog Day on Feb 2, unless they are living in Punxsutawney, Pa!

True hibernators slow their metabolism and lower their body temperatures independent of the outside temperature. If they wake up in mid-winter for brief periods, their metabolic rates and body temperatures go up. But arousal from slumber costs a loss of energy. That’s why white-nose fungus has taken such a serious toll on our smallest native bats. The fungus disrupts their sleep and causes them to fly and exhaust their precious fat reserves.

In the New Jersey Highlands, hibernating cave-dwelling bats have been devastated by the fungus. Seemingly healthy bats will enter their caves in the next few weeks – but very few will manage to survive until spring.
Black bears are notorious for hibernating. Bears like cramped conditions like caves, rock crevices and hollowed-out trees, and squeeze themselves into places that seem incredibly small for a creature weighing hundreds of pounds!

Unlike smaller mammals that hibernate, black bears do not drop their body temperature appreciably. They enter a state of torpor, or low metabolic activity, and, amazingly, recycle proteins so they don’t have to wake up to urinate! Bears are too big to allow their bodies to get really cold; they need to be able to wake up quickly in an emergency.

Perhaps this state’s most interesting hibernators are timber rattlesnakes. In the Ridge and Valley and Highlands regions, they hibernate in deep, rocky mountain crevices with southern exposure. In the Pine Barrens, with no rocky crevices, the rattlesnakes head toward water. They hibernate in pristine springs of cedar swamps, where the water never quite freezes. Imagine, six months beneath the moist roots of an old tree!

Northern pine snakes dig deep into Pine Barrens upland sands and hibernate about four feet below the surface. Snakes must fully digest their last autumn meal before hibernating, since undigested food in a reptile’s gut can easily lead to bacterial infection and death.

Dormancy, rather than hibernation, is the strategy of fish and many reptiles and amphibians, allowing them to survive extremely low oxygen conditions in the mud and deep water of ponds. Red-eared sliders, an invasive turtle species from the southern United States that has spread through New Jersey’s developed areas, can tolerate a lack of oxygen for exceptionally long periods.

Body chemistry helps some creatures survive the cold. Adult Mourning Cloak butterflies, for example, have “anti-freeze” compounds in their body fluids, which enable them to survive the winter inside hollow trees. These dark chocolate and cream colored butterflies are the first big springtime butterfly to emerge, usually in March or early April.

Monarch butterflies that migrate from New Jersey to the Sierra Chincua in Mexico don’t hibernate, but they respond to colder temperatures. As they gather by millions in the remaining pine forests, they engage in “muscular shivering” to raise their body temperatures above the ambient air temperatures.

New Jersey’s homo sapiens have vastly different strategies to deal with winter. We turn up the thermostat or fly off to warm destinations, both of which contribute to rising atmospheric CO2 and climate change. If by 2100 our climate is more like that of Raleigh, North Carolina, Groundhog Day may have to move to mid-January, or Punxsutawney Phil will have already awakened and left his den!

To learn more about preserving New Jersey’s land and natural resources, visit the New Jersey Conservation Foundation website at or contact me at

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