Hallgatói dolgozatok

Student essays

Living in an area where these bats are fairly common I have harvested an interest in these creatures. I have frequently observed and heard them on evening walks. Until today I thought I might be delusional as it is commonly known that bats utilize ultrasound, but I have now learned they have sounds of lower frequency audible to the human ear.

According to the Norwegian Zoological Society (NZF) this species is widely distributed throughout Europe, North Africa and found as far into Asia as Xian, China. They are not threatened by extinction on a global basis or in Scandinavia, but they are considered worth attention and concern, according to the Bonn and Bern conventions. The reason is that they like to form maternity colonies in people’s houses. A habit found rather unpopular by the other occupants of the house in question. Because people wish to have bats removed from their house it threatens the bats´ multiplication. Hunting Dwarf bats use echo signals with a peak frequency of around 45 or 55 kHz. This differentiates two species, but they are ecologically similar. The males singing frequency and other social sounds deviate from these two phonetic types. The two types overlap in Europe but only the 55 kHz type is found in the north and south west.

Dwarf bats (both species) are not well studied in Norway so Swedish research is the basis of the referenced report. Scandinavian dwarf bats hunt in deciduous forest, gardens and parks, in cultivated areas and wetlands and lakes. They catch flying insects, mainly Dipterans. Most of which are associated with wetland environments. In Holland they are most common in old deciduous forests and in parks with a pond or lake. Linear flight paths are used between the resting place and hunting grounds. The mating is in the autumn and the females form maternity colonies in May. One or two offspring are born in June; twin births are more common with these species than other Scandinavian bats. The maternity colonies are in houses or other buildings. These colonies are essential to the nursing of the young as the offspring wait there while their mothers are hunting and are fed on their mothers return. The mother and pup localize each other by sound. The largest registered colony in Norway had 1000 individuals, this including yearlings. In Holland the colonies are only about 300 individuals. In Great Britain a 1900 large colony has been registered.

The males defend individual territories around their resting place throughout summer when they are solitaire, and in the mating season. Territorial male Dwarf bats actively persue intruders. The territory is in a 100m radius around the resting area. During the mating season the females gather in reproductive groups at the male´s resting place. The males mark their territory by singing and flying in a regular elliptic flight pattern over 100-200m in 2-6 meters height. This occurs 1-2 hours after sunset. The singing is interrupted by intervals where hunting in areas outside their territory presumably takes place. The singing is important reproductive behaviour because the males invest a lot of energy in this and loose weight during the mating period.

Dwarf bats hibernate in suitable places mainly over ground but have been found underground. This has been reported in south Eastern Europe. Hibernation is commonly encountered in houses.

The main threat to the dwarf bat is the eradication of colonies in buildings. All bats are protected species, but permission can be given to remove or drive away colonies in buildings by the local authorities. Since the dwarf bat forms large maternity colonies it is likely to be considered a nuisance for the owner of the house and the bats will readily be expelled. Loss of colonies during renovation and tearing down old houses and remedial timber treatment contributes to the population loss. An investigation was made in Skåne, Sweden, involving two separate bat species. A high amount of environmental poison was found in bats living in this industrial agricultural area. The population growth that year was negative. Wild life management encourages restrictions on the permissions to expel a colony. Another possibility is to create suitable day resting places by putting up bat boxes, an equivalent of bird nesting boxes thus increasing the density of males.

It is obvious that numbers are not everything when an encroachment such as expelling a whole maternity colony threatens the continued survival of dwarf bats. Today, more poignantly than ever a species must be protected before they are on the extinction line.

Bibliography:
1. Olsen, K. M. (red.). 1996. Kunnskapsstatus for flaggermus i Norge. Norsk Zoologisk Forening. Rapport 2. 210 s.
2. Solheim, R. 1990. Flaggermusene. S. 38–61 i: Semb-Johansson, A. & Frislid, R. (red.). Norges dyr. Pattedyrene 3. J. W. Cappelens Forlag, Oslo.
3. Isaksen, K., Syvertsen, P. O., Kooij, J. van der & Rinden, H. (red.) 1998.
Truete pattedyr i Norge: faktaark og forslag til rødliste.
Norsk Zoologisk Forening. Rapport 5. 182 s. (ISBN 82-7857-004-3).