Flaviu Bodea

Centre for Bat Research and Conservation

Bats live their lives in colonies. They form some of the biggest colonies from Europe right here, in Romanian caves. These colonies are often comprised of hundreds, thousands, or even tens of thousands of individuals. They can be like living carpets on the cave ceiling.

Living in a colony (like in any other gathering of the individuals of a species) has many advantages that can help a species survive throughout harsh periods. However, like any other living system, it also has disadvantages, which can lead to grave consequences for a colony, if several factors are present at the same time. In this text, we summarize some scientific articles by which we will describe why bats evolved to use this form of organization: the colony. And implicitly: why is it important that we protect these colonies.

The first thing that we would like to underline is that bats are a fission-fusion type of animals. It sounds complicated, but it is not. It only means that colonies are not permanent: they form only at certain points in time and for specific purposes. For example, in Romania, we have two major types of colonies: hibernation colonies and maternity colonies. Hibernation colonies are formed during winter (December – March), with maternity colonies forming during summer (May – July). Of course, there are differences between the behaviors of different species, but most of the species from the temperate zone adhere to these patterns.

Now that we’ve established what kind of main colonies bats form, it is time to go to the next step and see exactly what advantages of living in a colony brings. We will analyze this aspect by taking into consideration two major obstacles that a species must overcome to survive: physiological and ecological obstacles.

Living in a colony, especially during cold periods, has exceptional thermal advantages for bats. By hanging in a remote place with their bodies tightly packed together, they loose less corporal heat, which helps them maintain their body temperature constant. In this way, they spend less energy to increase their temperature. This is a crucial aspect for survival during the harsh conditions of winter, when all the resources they have are the fat layers on their body, which formed by eating insects. By “sharing” heat between individuals, they do not have to gather such large amounts of fat. Even if some individuals did not manage to feed so much during autumn, they still have a chance to survive. Because of the colony.

Another important physiological advantage the bats have by living in a colony is the greater chance of reproduction. Most of the time, the places where hibernation colonies form are also suitable places for mating. In these colonies, a large species diversity is gathering during autumn, but most important is that both males and females are gathering at the same place. In this way, the time required for a male and a female to meet and reproduce is greatly reduced. Also in this way, they are saving a lot of energy by not having to search a long time for a mate, and then to fly long distances towards suitable places for hibernation.

The ecological constraints that are imposed on bats are represented mainly by the availability of resources for the colony and the amount of space they require to survive. These two factors determinate the chances of survival a species has in a certain place. If there is not enough food, then the number of individuals in a colony is going to drop, or some of them are going to migrate. If the physical space is too small, then again, some bats are going to migrate, possibly starting a new colony.

Still, living in a colony also helps with the previously described ecological obstacles. Since their size is small and the groups formed by them are tightly packed, especially during the hibernation period, the actual space occupied is also small. For example, on a surface of just one square meter, you can find up to 2.000 individuals of the common bent-wing bat (Miniopterus schreibersii). Still, it can happen that some bats leave the colony, even if the required factors for a good life are fulfilled.

Living in a colony also brings advantages when it comes to finding food, due to communication between individuals and information sharing about good feeding sites. So even finding food becomes easier. A good example is the case when mothers pass information to their pup about good feeding spots. Even vampire bats (yes, those who feed with the blood of animals) share food with those “colleagues” in the colony, who were unable to feed enough during the night. Another less obvious effect of this behavior is the creation of a strong social bond between individuals, avoiding unnecessary competition between them. Sharing food and adequate feeding spots locations is good, yet this can also be a disadvantage: helping individuals survive (even if they should not have survived) reduces the effects of the natural selection.

Another important aspect of living in a colony is the socialization between individuals. This way of living strengthens socialization, because individuals develop together from birth and so they create strong social bonds between them. The unusual longevity of bats (the current record being at over 40 years old for Siberian bat) further strengthens this aspect, because multiple generations overlap. In this way, the information is passed on to the next generation and the colony becomes even more aggregated. The bond between bats is getting stronger. Most probably this the main advantage that keeps the colony a suitable way of organization over the ages.

The fact that bats live in a colony and the strong social bonds they create only shows us how evolved they are, how well they adapted to their environment. In this way, we can conclude that the protection of bats can not be achieved without the protection of the colonies themselves, wherever they might be: in caves, in churches or in a 150 year old hollow tree. And the simplest methods of protection can be followed by everyone.


  • Dietz M., Kalko E. K. (2006): Seasonal changes in daily torpor patterns of free-ranging female and male Daubenton’s bats (Myotis daubentonii). Journal of Comparative Physiology B, 176(3): 223-231.
  • Fenton M.B. (2001). Bats. Checkmark Books.
  • Fenton M.B., Simmons N.B. (2015). Bats: A World of Science and Mystery. University of Chicago Press.
  • Kerth G. (2008): Causes and consequences of sociality in bats. Bioscience 58(8): 737–746.