Hibernation is a state of minimal (metabolic) activity. When an animal is in a state of hibernation during the summer, it is called aestivation or summer sleep. In the past, the concept of hibernation was based on an absolute drop in body temperature (often more than 32°C). Today, this is based on the decline in metabolism. During hibernation, various processes in the body slow down: body temperature, metabolism, respiratory rhythm and heart rhythm. The benefit of hibernation is that an animal can survive during periods of food scarcity (such as winter) without having to spend a lot of energy gathering food.
During hibernation, body fat is mainly used as an energy source. This fat is stored during the active period of the year. In addition, some animal species (such as the arctic ground squirrel) can recycle nutrients in the body. Research has shown that this species can break down its muscle fibers during hibernation, thereby releasing nitrogen. This nitrogen can be converted into amino acids and then into proteins, which keeps body tissue intact (source).
The length of hibernation depends on the species, ranging from a few days to many months. Some animals sleep during the entire hibernation, while others sleep for broken periods. This is due to the different types of hibernation:
These are animals that are in hibernation at fixed times every year, regardless of the ambient temperature and amount of food. These species undergo a 'traditional' hibernation: a body state in which the body temperature drops sharply along with the breathing and heart rhythm. Species that fall into this group include: reptiles, amphibians, bats, hedgehogs, (many) rodents and some insectivores.
These are animals that are only in hibernation due to environmental stressors, such as temperature, food shortage or both. Species within this group undergo both 'traditional' and 'untraditional' hibernation, depending on their body temperature or metabolic activity. Species that belong to this group are: some monkeys (e.g. dwarf lemurs), prairie dogs and bears. Bears are wrongly thought to be a 'traditional' hibernation even though this concerns a winter rest; the heart rate does slow down, but the body temperature remains fairly constant. A bear can therefore easily be woken up.
Hibernation in captivity
It is not favourable for a zoo to have no visible animals for a long period of the year. However in captivity, true hibernation is rare for animals. Often the duration of the hibernation is then greatly reduced from a few months to only a few weeks. As a result, an animal's diet can also deviate from its natural diet. To compensate for this, zoos often have a special 'bulk' program in the months before natural hibernation. This allows the animals to store energy reserves in the body to be consumed during these shortened hibernation periods.
In addition, more and more seasonal diets are being used to imitate the natural variation in the diet. In 2021, Marcus Clauss gave a presentation (link) on the work of Charles Robbins discussing the concept of seasonal diets. To summarize, both the diet composition and amount can be adjusted to mimic different seasons. Two examples of this can be seen in the charts below for different bear species. By feeding this way, it simulates the availability of food throughout the year.
A practical example of this is a study from the San Diego Zoo (link). Here, the goal was to feed bears more seasonal diets to better mimic seasonal physiological changes. After a year, the bears showed twice as much weight variation, which is more in line with natural variation.
Although this concept is emerging, it is even more important to look at the individual animal. Always adjust the diet and feeding frequency to the individual. In addition, the charts above are just examples and the ideal diet depends on the species and individual.