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The underground world of karst: bats


Bats have been recorded as present in 15 speleological structures within Krka National Park

Bats make up about one-fifth of the total number of mammals in the world. They differ from all other mammals due to their ability to fly and their use of sonic waves (echolocation) for orientation. They can live up to 40 years. The body is covered with fur consisting of short dense to downy-dense hairs, and the colour is usually darker on the back than on the stomach. The neck is short, head relatively large with a large mouth and strong jaws containing large canine teeth and sharp molars. The annual cycle of bat activity is both seasonal and weather dependent.

In winter, the cold weather limits the amount of food available for insectivorous bat species. Therefore, bats usually hibernate through the winter to save energy. In spring, their body temperature rises with the increasing external air temperature, and they again become capable of flying and hunting. In spring, the embryos begin to mature in females that mated the previous fall. In summer, pregnant females form nursing colonies, while the males are most often separated from the females and young. Females usually give birth to only one offspring each year, those some species may occasionally have two. The offspring are born in June to early July and feed on milk for several weeks. The young are capable for flight after about a month. At the end of summer, the males become more active and begin courting the females. Some shelters, particularly speleological structures, are often used as gathering spaces, with bats congregating here to mate, and later they seek out new shelters and show their young the new locations. By the end of autumn, they move to their winter habitats to begin hibernation, where they remain during the cold months, individually or in colonies that can include up to several thousand individuals.

Many species combine different types of shelters (speleological structures, attics, forests, wall cracks, cliffs and trees), depending on the season, opportunities available, and the environmental conditions. Therefore, shelters are classified based on their function, into hibernation, nursing colony, aggregation, and transitional (occasional) shelters.

Speleological structures can serve as constant shelters for many generations of bats. Cave species of bat, such as the species of the genus Rhinolophus, are often faithful to the same locations throughout their lives. Bats can used underground habitats in all phases of their life cycle, depending on the air temperature within them. They prefer speleological structures with a dynamic atmosphere, with good air circulation, which also means that the temperature within them fluctuates.

From dusk until the early morning hours, bats leave the shelter in search of food. They pass over open habitats (meadows, pastures), shrubby vegetation and forest habitats, habitats near aquatic surfaces and urban areas, based on the ecological characteristics of the species.

Most European bat species primarily feed on insects. For that reasons, bats play an important ecological role as a natural pesticide, since one bat can catch from 500 to 1000 insects per hour. Bats are also considered very good bioindicators, and their abundance is a direct indicator of the health and stability of the ecosystem, while declining bat populations may be the result of the impacts of human activities.

Of the 45 bat species recorded in the European Union, 34 species are found in the Republic of Croatia, making Croatia the country with the highest bat biodiversity in all of Europe. The bat fauna in the broader area of Krka National Park has been researched sporadically since the mid 19th century. According to research to date, at least 22 bat species inhabit the park area, including the common bent-wing bat Miniopterus schreibersii, lesser mouse-eared bat Myotis blythii, long-fingered bat M. capaccinii, Geoffrey’s bat M. emarginatus, greater mouse-eared bat M. myotis, Blasius’ horseshoe bat Rhinolophus blasii, Mediterranean horseshoe bat R. euryale, greater horseshoe bat R. ferrumequinum and lesser horseshoe bat R. hipposideros. All these species use caves and pits as shelters.

In the broader area of Krka National Park, nine speleological structures have been included on the list of internationally important subterranean shelters for bats (Miljacka II, Topla pećina, Špilja izvor Krke / HE Krčić, Škarin samograd, Tradanj, Stražbenica, Mandalina, Dobra voda and Ćulumova pećina). Given the great mobility of bats and their range of movement during feeding, and their daily and seasonal migration patterns, the park area is exceptionally important for their conservation.

Bats are susceptible to changes in habitat conditions and disturbances. Loss and degradation of bat shelters is caused by a range of human activities. The decreasing numbers of insects, due to the use of various pesticides and herbicides, and due to habitat change such as the loss of wetland and forest habitats, also negatively impacts bat populations. Furthermore, there is still much misunderstanding and prejudice about bats, due to a lack of knowledge about their lives and their ecology.

Bats have been recorded as present in 15 speleological structures within the park boundaries in surveys conducted by many speleologists.

Photograph: Daniela Hamidović