Innocent Bystanders

Dormancy December 7, 2010

Dormancy=brief cessation of growth and development (plants) and physical activity (animals)

• Predictive dormancy=happens before the beginning of adverse conditions, ie. using photoperiod and temperature to predict winter

• Consequential dormancy=happens after the adverse conditions start–this is a riskier strategy as the animal may get caught with its pants down, but in a year like this one where we haven’t had much actual winter weather, this strategy would pay off

Hibernation=a type of dormancy that may be predictive or consequential–characterized by significant decreases in heart rate and metabolism leading to significant decreases in body temperature and brain activity (these animals are actually asleep)

Hibernating animals may experience “sporadic euthermic arousals” or “interbout arousals” in which they wake up, their body temperature returns to normal and they feed and pass waste products.

Winter lethargy=characterized by a decrease in heart rate and metabolism leading to a decrease in body temperature—brain activity is decreased but these animals do respond to external stimuli (see the debate below)

Diapause=predictive dormancy common in insects (although embryonic diapause is common in a variety of vertebrates) and is known to occur in fish

Aestivation=consequential dormancy due to hot/dry conditions usually in arthropods although exhibited by mammals such as the tenrec in Madagascar

Brumation=predictive dormancy practiced by herps-characterized by decreased activity, the animals still need to drink, may eat, and do not enter true sleep

1. The diamondback rattlesnake may be an exception to this and may be a hibernator (decreased brain activity      indicating sleep)

The process through which animals obtain energy during this dormancy helps determine if it is hibernating or brumating, but it’s not like you can ask an animal which process it’s using so no help there.

Daily torpor (sometimes also called temporary hibernation) is a short term decrease in physiological activity (heart rate and temperature). Depending on conditions bats will enter torpor during the day and become active again at night.

Extended torpor aka hibernation is a long term decrease in physiological activity.

A hibernacula (pl: hibernaculum) is that spot where an animal brumates or hibernates.

There is debate on whether or not bears actually hibernate as their heart rate slows to a range of anywhere from 50 bpm to 8 bpm but their internal temperature stays fairly close to normal, and they are still responsive to external stimuli. Chipmunks are considered true hibernators as their internal temperature can reach almost 0°C and their heart rate slows from 350 bpm to 4 bpm. Surface area to volume ratios account for these differences. A rodent has a nearly equal surface area to volume ratio and can achieve very low internal temperatures; a bear has a small surface area in comparison to its volume and cannot achieve the same decrease in temperature. In addition for the bear, if a decrease in internal temperature to 0°C was achieved the fat reserves are not sufficient to then bring the internal temperature back to a normal range. In a true hibernator the excretory system still works and they must wake up to pass waste products. 90% of a true hibernator’s fat reserve is used during these “sporadic euthermic arousals”. However, depending on conditions, an animal that experiences winter lethargy such as a bear may actually sleep the entire winter, never waking, as their excretory systems does not work “properly” and their body actually recycles waste products.

Certain rodents (chipmunks and groundhogs as examples) and bats are true hibernators. Bears, skunks, and opossums experience winter lethargy.

It’s all a quibble at that point, like most things in nature it’s a sliding scale.

An interesting tidbit—rodent teeth cease to grow during hibernation.

And there’s your nature shit for the month.

*Special request:

Skinbad spent a slumberous evening in Michael’s arms.