In nature, every morning, a bear wakes up and knows that it must expend less energy than it takes. In nature, every morning, a turtle wakes up and knows it must expend less energy than it takes in. It doesn’t matter whether you are a bear or a turtle… the important thing is that you save energy!For some animals, optimizing energy efficiency is the key to survival when out there in nature there are cold temperatures that make it difficult to find sufficient food resources to compensate for the energy loss due to the cold. In a nutshell, they do what we do when the phone is dead and there are no power outlets nearby: we activate the energy saving, cut off the internet connection, close the apps and lower the ringtone. In short, let’s put the cell phone into hibernation. Active for emergencies, but with the minimum possible consumption of energy resources, so as to last longer.
In fact, during hibernation, an animal’s metabolism slows down considerably: its heartbeat slows down, digestion slows down, it breathes more slowly (some animals even stop breathing for periods of over an hour) and its body temperature drops, in some extreme cases below zero. Reducing their metabolism therefore allows them to conserve energy.
Which animals hibernate?Small mammals, such as squirrels, dormice, hamsters, hedgehogs and bats. Also, many insects, amphibians and reptiles. Some insects, such as butterflies, ladybugs and some bees, overwinter in the adult stage. In the case of butterflies, wintering oscillates between simple torpor and diapause; although the insect is externally an adult, it may not yet be reproductively mature.That’s why rising temperatures due to climate change can be a problem for insects. However, there can be some shade for sunny forays on winter days. A brief excitation from torpor won’t necessarily harm a butterfly directly, but the energy costs expended in flying and searching for a new hibernation site could cause it stress later on. It may deplete its fat reserves and die before spring.
The problem is the temperature and, in particular, the heating in the spring. This causes hibernators to emerge too early, come out of hibernation while their fat stores are seriously depleted, and before there is enough food to sustain them in the environment.
When a hedgehog dozes off in the summer, for example, its body temperature of around 35°C drops a few degrees and its breathing will be slower but will remain constant and regular. During hibernation, however, its temperature plummets to about the level of the external environment. His metabolic rate will be 2% of his normal summer activity and his heart rate will drop from 110-150 beats per minute to between 5 and 70 beats per minute. In bats, the heart rate can drop from 400 to 11 beats per minute. At times, toads, newts, lizards, and even snakes will all gravitate to the same tether, former enemies entering a sleepy respite. All these exothermic vertebrates can be awakened by warm winter days: frogs can hunt for food and snakes bask in the weak sun.
During cold or wet weather, parent swifts have a hard time catching enough insects in the air, so their nestling chicks become cold, reducing their metabolism to go without food for 48 hours, enough to survive until the front passes. But this is semi-torpor, not hibernation. Many normally hyperactive hummingbirds do something similar, entering a state of suspended animation. Their metabolism slows down, their breathing is barely noticeable. Again, however, this is an example of numbness.
Although the physiological changes are profound, usually no hibernating animal remains completely torpid for more than about 30 days at most, as is the case with the hazel dormouse and the fat dormouse. Attacks of torpor are regularly interrupted by periods of so-called ‘euthermia’, when the animal warms up, wakes up and can move around for several hours, or even more, breaking the hibernation. This is a good occasion to throw out waste products and, under certain conditions, have a snack.
The animals prepare for winter by accumulating a lot of fat during the warmer months, but if the reserves are not sufficient, the animal could die during hibernation. Just as it can also die due to predations or premature awakening. This is why it is extremely important not to disturb hibernating animals. We don’t go looking for them, we don’t surround any dens and we avoid interrupting sleep. Hibernation is a very delicate moment in an animal’s life and therefore, as always, we must respect it.
The hibernation of the heat… the aestivation.Most animals bury themselves in the ground, which protects them from the heat. Here they wait for the rainy season or cooler temperatures. Some land snails climb trees to escape the heat of the ground, sealing themselves in their shells using the dried mucus.
A large number of aestivating animals die in periods of prolonged drought.
* Chiara Grasso is an ethologist and president of Eticoscienza