The Science Of Hibernation
As seasonal temperatures drop in late autumn and the days shorten in the lead-up to the solstice, animals such as squirrels, mice and bats enter a state of hibernation. Hibernation itself is thought to be triggered by reduced temperatures and declining light levels. Hedgehogs, for example, must reach a hibernation weight of between 500 and 700 grams, and the average temperature must drop to around 5°C before they begin hibernating.
During the preceding months, these animals intentionally overeat to build up fat reserves. Once winter sets in, they retreat into their dens and enter a state of near inactivity, enduring the coldest, harshest weather. Some animals also store non-perishable food in their dens, waking intermittently to eat. This hibernation typically lasts 4 to 6 months, depending on the species and the severity of the winter.
While we often take this hibernation process for granted, it is a biologically complex phenomenon, and something humans are not even capable of. Hibernation is a prolonged state of torpor, where metabolic activity is suppressed to less than 5% of normal levels. This drastic reduction in metabolic rate allows animals to conserve energy and stretch their fat reserves and food stores throughout the winter months.
Temperature Regulation And Extreme Cooling
Hibernating animals lower their body temperature by an average of 5 to 10°C. The arctic ground squirrel, however, exhibits the most extreme example of this adaptation, cooling its body to sub-freezing temperatures. This remarkable ability is believed to be managed by levels of adenosine in the brain, a neurotransmitter that increases in ground squirrels during winter. Their brains have been shown to become more sensitive to adenosine, helping to trigger this extreme cooling.
Slowed Heart Rate During Hibernation
Another key feature of hibernation is a dramatically reduced heart rate. For instance, a hedgehog’s normal active heart rate of around 190 beats per minute drops to just 20 beats per minute during hibernation. One of the lowest recorded hibernating heart rates belongs to the dwarf lemur of Madagascar, one of the few regularly hibernating primates. Its heart rate drops from a rapid 300 beats per minute to just 6 beats per minute.
Minimal Brain Activity
Breathing also slows significantly during hibernation. Instead of taking a breath every second or two, hibernating animals may go as long as 10 minutes between breaths. In the dwarf lemur, brain activity becomes nearly undetectable during this state. However, they are far from brain-dead. By definition, brain-dead animals wouldn’t be able to wake up. Interestingly, hibernating animals don’t even display typical sleep patterns. Their brain waves more closely resemble a waking state, albeit greatly suppressed. In fact, when they finally awaken from hibernation, they often need a prolonged period of actual sleep to recover.
Hibernation is a fascinating and complex survival strategy that allows animals to endure the harsh conditions of winter by drastically reducing their metabolic functions.
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