- Written by Nadia O'Carroll
During the dormant period growth, development, physical activity are suspended as the metabolic rate decreases, body temperature drops and breathing slows. Dormancy may occur on a regular basis such as every winter, or it may occur irregularly, as a response to adverse conditions such as drought.
The triggers for dormancy include external factors such as weather, lower atmospheric pressure and temperatures, humidity, water temperature, photoperiod (length of day), reduced food supplies or drought. Internal factors also play a part in dormancy. Living organisms have body clocks - regular biochemical, physiological and behavioural processes that produce a pattern of Circadian (daily) biological rhythms and Circannual (annual) biological rhythms and these stimulate dormancy.
Dormancy is much more than a long, deep sleep. Sleep may produce slight reductions in heartbeat, breathing rate and body temperature but dormancy produces profound changes. Sleep also causes changes in brain activity and brain waves, but the brain waves produced while hibernating resemble wakeful brain patterns, and many animals appear sleep deprived when emerging from hibernation.
There are a number of dormant states:
Hibernation before hibernation begins; a large amount of food is eaten to build up fat reserves. The endocrine system releases hormones and the metabolic depression begins. Body temperature can drop significantly eg squirrels can drop their temperature from usual 37degrees C to 4 degrees C, (compare this with humans who will suffer fatal hypothermia if their temperature decreases from 37 degrees C to 30degrees C). Heart rate falls to as little as 2.5% of the usual rate, eg marmot’s heart rate drops from 140 beats per minute to 15 beats per minute. Breathing rates drop eg chipmunk’s 95 breaths per minute to 1 breath every 2 to 3 minutes and may be suspended for up to 1 hour. Body fat is utilised for energy and water is recycled from body fat in order to stay hydrated without drinking.
Estivation is a response to escape extreme hot, dry weather conditions. eg the Australian water holding frog, which burrows underground in the desert, where it can remain for up to 5 years. Lungfish also estivate if water dries up – they bury themselves in mud and secrete a water holding mucus case over their entire body, they may live in this for up to 3 years.
Brumation applies to reptiles and amphibians. In tropical and temperate climates reptiles simply slow down and become lethargic when the temperatures fall, but in cold climates brumation is like true hibernation eg tortoises in Russia dig deep burrows and hibernate for months
Torpor is short term dormancy. Many birds have daily torpor to conserve energy eg the hummingbird has a high metabolic rate and a heart rate of 1200 beats per minute but when in is in torpor it can reduce its energy needs by 95%.