Once winter comes, the trace of bat is gone. Where have they gone?
It turns out that bats also hibernate like hedgehog and squirrel . Of course not all bat species hibernate in winter. Generally, most insectivorous bats that live in temperate areas hibernation for several months are insectivorous bats.
Temperature climate is characterized by cold winter and hot summer, and the temperature difference between winter and summer is large. Accompanying this seasonal change is the difference in availability of bat food resources. Generally, in autumn, food resources such as insects will gradually decrease, reaching the minimum in winter, and insects will not reappear until the spring of the following year. This means that bats that feed on insects face a long period of food resource shortage. Insect-eating bats are smaller in size, generally weighing between 2 and 90 grams, and most of them do not exceed 40 grams. The smaller the body size means the faster the body's heat is lost. Bats need to generate heat at a faster metabolic rate to maintain the body's temperature constant. Therefore, small-sized bats are hungry faster than larger-sized bats.
It is precisely because of its small size that the body of insectivorous bats can store limited fat. If some measures are not taken, it will be difficult to survive this difficult period of food resources in winter. Therefore, during the long-term evolution, bats have evolved the strategy of hibernation. During hibernation, the body temperature and metabolic levels of bats are greatly reduced. The body only needs to generate very little calories to maintain physiological activities in hibernation, so that bats can survive the winter smoothly.
fat is the main energy source for bats during hibernation. Bats must store enough fat before hibernation. From August to September every year, bats will prey on a large amount of food in a short period of time and "weight gain", and their weight can increase by 20% to 30%. Some individual bats can even increase their weight twice as much. The stored fat includes two forms: white fat and brown fat. White fat is mainly distributed under the skin of the bat and around the internal organs. Its main function is to store excess energy in the body in the form of fat, and it can also maintain heat. Brown fat is mainly distributed on the shoulders and back of bats, and its main function is to convert fat into calories. Since the brown fat is filled with a lot of mitochondrial , it looks brown. Brown fat is like a "thermal power company" that generates heat for the body when needed. The brown fat in bats mainly helps bats in hibernating states produce heat and are used to wake up from hibernating states. With enough fat reserves, bats can hibernate with peace of mind.
So compared with normal state, what changes have occurred in the physiological activities of bats during hibernation? In hibernation, the body temperature of the bat will drop to close to the ambient temperature (Figure 1–35) and fluctuate within a certain range. During deep hibernation, the bat's body temperature is slightly higher than the ambient temperature by 1~2℃. The bat's heartbeat is 250 to 450 times per minute in rest state, about 800 times per minute in flight state, and it will be reduced to 20 to 40 times per minute in hibernation state. In hibernation, bats' oxygen intake decreased by 99%, and they may only breathe once every hour. The hibernating mouse ear bat with a body temperature of 2℃ consumes 140 times slower than that under normal constant temperature. At such a metabolic rate, bats only need to consume about 4 mg of fat per day.
In addition, bats will stop the blood supply to their limbs in hibernation state, and excessive red blood cells are stored in the spleen, causing abnormal swelling of the spleen and causing blood pressure to drop significantly. Only some extremely important organs such as the brain and heart still maintain a normal blood supply. The brain is the commander of various physiological activities of the body. During hibernation, multiple neuron synapses in the brain will degenerate. These degradations will reduce the activity and energy consumption of nervous system , resulting in a minimization of the frequency of various physiological activities. Since all physiological activities are suppressed or even stopped, bats consume very little energy in hibernation.
When hibernation is over and bats wake up from hibernation, there will be a special protein in the brain that helps restore previously degenerated neuronal synapses, thereby restarting the brain and restoring the brain's command role.During the awakening process, the bat's breathing frequency will gradually increase, and the heartbeat will become faster and faster. The pumped blood flows to the brown fat layer, converting fat into energy, and releasing it in the form of heat, slowly warming the entire body as the blood flows. After 10 to 30 minutes, the bats can fully wake up and go out to hunt. During the hibernation period that lasts for more than 4 months or even longer, bats do not always hibernate, but will occasionally wake up, urinate, defecate or mate, and then enter hibernation.
This "hibernation-awakening-hibernation" cycle is described as a "hibernation array". The entire hibernation period of bats consists of several to dozens of "hibernation arrays". Bats wake up during hibernation at a price. Each awakening requires consumption of about 0.1 grams of fat, which is equivalent to the energy consumed by bats in deep hibernation state for about 65 days. Therefore, if awakening is too frequent, it is likely that the stored fat is not enough to support the entire hibernation period. Since hibernation itself consumes less energy, the energy consumption of bats during hibernation is mainly used for awakening during hibernation, so the frequency of awakening during hibernation determines whether it can survive the entire hibernation period safely. Both the type of bat and the ambient temperature affect the frequency of awakening during hibernation. Some bat species wake up more frequently during hibernation than others, such as treble volcanoes. The warmer the area, the more frequent the bats awaken, the more energy they consume. In the colder areas, bats need to warm the body through heat generation, and also consume energy. Therefore, choosing a hibernating habitat with the right temperature is extremely important for bats.
Usually the temperature of the hibernation habitat selected by bats is 2~8℃. Natural caves are the main hibernation site for bats. In the northern region of , the insulation walls of cellars, buildings, water wells, stone bridges, and mine caves are also good places to hibernate. In areas with relatively high temperatures in winter, it is difficult for bats to find suitable temperature caves and buildings as hibernation habitats, and at this time, hollow tree may become the choice for bats. However, the insulation effect of trees is limited, and the temperature fluctuates greatly in the tree hole, so bats will create an insulated environment through clusters. In addition to temperature, safety is also an important indicator for bats to choose hibernating habitats. Humans and bat predators not only increase the risk of bats being preyed, but also increase the number of awakenings during bats during hibernation, causing their unnecessary energy consumption. For safety reasons, bats usually choose dark, hidden, and difficult to access by predators to hibernate. The study found that some bat species living in tropical areas and the Middle East desert areas also have hibernation or dormant behaviors. This behavior usually occurs during periods of long-term drought or poor weather with scarce insect resources.
In Hawaii, bats migrate from the coastal tropical environment to the Monaloa volcano at an altitude of nearly 4,000 meters, and spend the winter in the deep lava caves with appropriate temperatures. Some hoof bats in Taiwan will also fly to mountainous areas with an altitude of more than 2,000 meters or some sand pits and underground tunnels before winter comes. By hibernating and storing fat, bats can inhabit the same area year-round and avoid long-distance migration. Some bat species, such as chrysanthemum head bat and long-eared bat, are highly loyal to their habitats. The distance between their "childcare sites", predation sites and hibernation habitats can be no more than 20 kilometers, and they may even live in the same place all year round. Some bats may migrate northward in order to choose a better hibernation habitat. A few bats, such as the European brown mountain bat and Nesser, still choose long-distance migration, sometimes a single trip even lasts up to 2,000 kilometers.
Why do these bats seek far and travel farther and travel farther to a farther place to spend the winter? Long-distance migration to warmer places means that spring comes earlier, bats ovulate earlier, and young bats are born earlier, prolonging the reproduction period of bats. After birth, young bats will be carefully cared for by female bats for longer, and can better face their first winter of "bat birth". In addition, bats born early also mature earlier, and can "marry and have children" earlier and get better reproductive opportunities. Long-distance migration is not only a survival strategy for bats but also a reproductive strategy.Like most mammals, the burden of bat breeding basically falls on the shoulders of female bats. They need to make full use of resources to feed young bats and need to migrate to "childcare sites", while male bats rarely migrate long distances like female bats.
