True hibernators

Many of us think of bears when talking about hibernating animals. True hibernators, like the eastern chipmunk, maintain reduced body temperatures to around 40 degrees Fahrenheit, with reduced heart rates of from around 350 beats per minute to about five while in hibernation.

Bears maintain a near normal body temperature during their winter dormant period, which is similar to a long sleep. While a bear's heart rate drops from a normal active average of about 50 beats per minute, to as low as eight, bears can awaken rapidly from their lethargic sleep when disturbed, since their body temperature has not dropped significantly. In midwinter, pregnant female bears wake to give birth, then go back to sleep while their newborn cubs nurse. Still, most bears sleep through the winter if left undisturbed.

In our area, striped skunks, opossums and raccoons are other animals that sleep in winter maintaining body temperature, rather than being true hibernators. Reptiles, such as turtles and snakes; amphibians, including toads, salamanders and frogs; fish and insects hibernate differently than mammals, since their body temperatures match their environment. These cold-blooded or exothermic animals survive in a cold or even frozen state to thaw out and become active in higher temperatures, since their bodies require the warmth of the environment to run their metabolic processes.

Groundhogs, like most mammalian hibernators, gradually drift into hibernation and then gradually drift out of hibernation. Similar to other hibernators, groundhog body temperatures drop in the midst of hibernation to just above the temperature of the winter burrow, as low as 40 degrees. Groundhog heart rates drop from about 80 while active to around five beats per minute in hibernation. In late winter, body temperatures gradually elevate while they drift out of hibernation, while receiving nutrients from stored body fat.

In early to mid-February in this area, groundhogs will start to venture out sporadically to seek mates. Groundhogs have one litter per year. They typically mate soon after hibernation ends, during March to early April, producing a litter of two to six hairless, blind young, after a one-month gestation period. Mating too early would result in the litter being born before a plant food supply is readily available, and late mating would not give the newborn animals enough time to put on weight to get them through winter hibernation.

Both the male and female remain in the same burrow following mating, with the male leaving the burrow after the young are born. Groundhogs are great at burrowing, and can excavate up to 50 feet of tunnels, as deep as 5 feet, with several entrances.

Other winter hibernators in our area include several species of bats. The little brown bat has an average heart rate of about 400 beats per minute, yet maintains a hibernating heart rate of about five beats per minute. Bats often find a winter cave in which to hibernate and, similar to other hibernators, will maintain a reduced body temperature as low as 40 degrees, near the winter temperature of the cave.

Other bats in our area include the Eastern red bat, the Eastern pipistrelle, the Northern myotis and the big brown bat. Since big browns can survive at lower temperatures than other bat species, including below freezing, they can be discovered hibernating in less secluded places such as barn attics. It is best not to disturb hibernating animals due to several concerns including rabies, interruption of the maternity cycle, and risk to the health of the animal from it having to use up its winter fat reserves during a wakened period.

Nature Notes for 11/25/2012