6..Hibernation and migration

During the day and during the year, the activity of the organisms present maximum and minimum levels of intensity. Thus, the alternating periods of activity and inactivity of the organisms were called biorhythms. The day-night alternation was called Circadian biorhythm (for example people's night sleep, day sleep of bats in summer) and seasonal biorhythms (for example the hibernation in bats, the lethargic sleep of bears in Romania, bird migration).
We present further the changes of activity of bats, with special reference to the bats in our country and in Europe.


Their Resting Hours
In the temperate zone bats do not fly during the day because they stay hidden in dark or dimly lit places, away from noise. This period is essential for them to rest till the coming of night when they go out hunting. They rest during the day because of the high intensity of light and because of the high temperature. Diurnal rest is not yet complete. In maternity colonies there is bustling at any time.


Nocturnal Activity         
Nocturnal activities actually begin during the twilight from evening and it is extended to morning. After awakening from sleep bats execute short flights inside the roosts. Only a few get out of the shelter at first, they make a short exploration flight, then return to announce the other members of the colony. If nights are too rainy and cold bats can temporarily give up the feeding activity. Also, when leaving the shelter, they hunt for about 1-2 hours, after which they rest and digest just outside the roost, and they hunt again.

Migration
The phenomenon itself, of travelling to a long distance due to the lack of food and to the change of the thermal regime, characterizes few species. For example, the flying foxes (Pteropuspolicephalus) migrate some 1000 km from Australia due to the reduction of food resources. Also interesting, are the migrations of Lasiurus species - Lasiuruscinereus borealis-moving on the routes of traveling birds, sometimes with them, from the U.S. to Hawaii, to 4000km from the mainland.
Most species of insectivore bats do not migrate, but produce an oscillation between their winter shelter and the summer. This shift has different causes: being close to food, to separate males from females during pregnancy and motherhood. The distance on the road varies from one species to another:
- Up to 35-40 km for weak flying species: the notched eared bat (Myotis emarginatus), the water bat (Myotisdaugbentonii), the whiskered bat (Myotis daugbentonii);
- Oftenover 100 km – the common large bat(Myotis myotis), the long-winged bat (Miniopterus schreibersii) the red evening bat (Nyctalus noctula).


Hibernation
The bats from temperate areas are unable to maintain their constant body temperature regardless of external ambient temperature changes.

For bats, hibernation periods of about a month alternate with brief periods of lethargy or awakening. These short periods are necessary for individuals to change their place in the colony or inside the house, to hydrate. During hibernation, at a body temperature of about 5 degrees C, the oxygen consumption is only 0.02-0.07 ml/ g / h, or 1% of consumption made ​​during the activity. Survival is ensured exclusively by fat accumulated during summer and autumn. A particular fat deposit - brown fat- deposited on the shoulders, around the neck and on the sides of the body. This fat is a source of food but it also ensures the internal heat production (endothermic). If insectivore bats are forced to defend from the very low temperatures, other bats take steps to protect themselves from too high temperatures.During hibernation, at a body temperature of about 5 degrees C, the oxygen consumption is only 0.02-0.07 ml/ g / h, or 1% of consumption made ​​during the activity. Survival is ensured exclusively by fat accumulated during summer and autumn. A particular fat deposit - brown fat- deposited on the shoulders, around the neck and on the sides of the body. This fat is a source of food but it also ensures the internal heat production (endothermic). If insectivore bats are forced to defend from the very low temperatures, other bats take steps to protect themselves from too high temperatures.