|The features of bats
The earliest bat fossils have been found in deposits dating back 55 million years. The direct ancestors of bats were ancient insect-eating mammals who possibly had gliding or even flying abilities. While the shape of the body and limbs of all other animals has gradually and constantly evolved, the most interesting thing with bats is that the appearance of modern species is almost the same as the appearance of their fossil forms.
The body structure and anatomy of bats is very similar to that of other mammals. The main differences are in the length and proportions of the forelimbs and the presence of a wing membrane. Though the eyes are not the main organ of orientation and foraging, all species have eyes of different sizes and can see surprisingly well in the dark.
The first finger (thumb) ends with a small nail, which helps the bat move on the ground. The wingspan of Bulgarian species reaches up to 46 cm and their weight varies between 5 and 55 g. The Giant noctule (Nyctalus lasiopterus) is the largest Bulgarian species and the smallest one is the Pigmy/soprano pipistrelle (Pipistrellus pygmaeus).
Some species have elongated wings (e.g. Miniopterus schreibersii, Nyctalus noctula), which make them faster and well manoeuvring flyers capable of covering great distances without great effort. Other species have broad and short wings, which suggest a slow, fluttering flight.
The ears of all bats (except for the horseshoe bats) have a fleshy projection called tragus, the function of which is not fully understood. The body is covered with thick and soft fur which protects the body from the cold during times of rest and when flying through cold air.
Depending on the season, bats inhabit different roosts.
During the winter all bat species inhabit roosts with a permanent temperature of between 2º to 10ºС. Such conditions can be found most often in water caves, flooded mine galleries and occasionally in the attics and basements of residential buildings.
During the spring and the autumn bats can be found in different roosts with a variable or constant temperature (such as abandoned or inhabited residential or industrial buildings, underground bunkers, galleries, discharge and ventilation shafts, pipes, chimneys, hollow posts, small and large caves, abysses, rock crevices, etc.
During the summer bats prefer dwellings with a higher temperature and this is where they breed. Species which form larger colonies can be found in caves with larger entrances so that in the evening hundreds and thousands of bats can fly in or out simultaneously.
Depending on their environment preferences, bats can be generally grouped into four main categories:
Cave-dwellers – they breed and hibernate exclusively in caves or other underground shelters. They are found mainly in karstic, volcanic or sea caves. The number of colonies can reach 100-10,000 individuals in the summer and 50 to 100,000 individuals in the winter.
Forest-dwellers – they breed mainly in hollows, crevices and under the bark of trees; some of these species can spend the winter in caves. They can be found in broad-leaved deciduous, mixed and more rarely coniferous forests which offer roosts and a supply of food. The number of individuals roosting in the hollows is usually 10 to 50, rarely more, with colonies more or less evenly distributed throughout a large woodland area.
Wetlands – due to their high biological potential and rich biodiversity the wetlands are one of the most important foraging habitats for nearly all bat species. They are particularly important during the summer months when thousands of bats hunt above the water surface and become an important part of the regional food chains. Most individuals who depend on water areas live in close proximity to or within such areas. They live in tree hollows, residential or industrial buildings, caves and other nearby roosts.
Synantropic – almost throughout their entire life cycle they live in man-made shelters such as attics, basements, shafts, chimneys, ventilation facilities, etc. They can be found everywhere – in villages, towns, resorts and other urbanized areas. The colonies of some species vary from 5 to 20 individuals and of others from 50 to 1000 individuals.
The life span of bats is uniquely high for mammals of such a small size and it can reach up to 20-30 years.
From December until the end of March bats in temperate latitudes hibernate. In early spring some species migrate to their summer roosts and prepare for reproduction. Sedentary species leave their winter hibernacula and spread evenly in the region they inhabit. During the spring all bats feed very actively to restore their fat reserves lost during the winter and gain body mass.
Depending on the geographical latitude and the climate, babies are generally born after May 20th, and more often in early June. In the first few weeks the juveniles are absolutely helpless and entirely dependent on their mothers. Within 30-40 days they have learnt how to fly on their own but continue to live in the breeding colonies, which consist of females and young males.
In late summer and early autumn bats again start to feed actively so as to gain sufficient fat reserves for the winter. During the swarming period, which starts in mid August and lasts until mid October the majority of bats actively mate. However fertilisation does not occur before the late winter (or early spring) due to delayed ovulation. In different times over the autumn, all the migratory species leave their summer roosts and fly to their winter hibernacula. On the way, they may can stop for several nights in transitional roosts.
With lower night temperatures during the late autumn, the number of insects decreases and bats become less active. They go into torpor more often and for longer periods as winter approaches.
Food and foraging
Bats can forage practically everywhere there are flying or crawling night insects. Most often they can be found around lakes, swamps and other wetlands, where the number of insects is usually the largest. In settlements they hunt around lamp posts in parks, along roads and above parks and water channels. In forests they hunt around small water bodies, near rivers, forest meadows, openings and the edges of open spaces. To reach their hunting habitats bats usually fly through other territories following the linear elements of the landscape – river, alleys, clearings, etc.
Major insect groups that form the bat menu
ats can eat up to 1/3 of their own weight per night. The larger bats in Bulgaria have an average weight of 20-30 g, which means that a colony of 300 individuals can eat some 550 kg of insects in one summer. In one night this means approximately 20,000 mosquitoes, beetles, butterflies and other insects, many of which are considered pests. This illustrates the great importance of bats in the ecosystems that they inhabit. As predators they occupy the upper levels of the food chain and this makes them particularly sensitive to the use of agro-chemicals.
A guanо hill in the Lyastovitsata cave, Glozhene village, Teteven region
Main food strategies
To avoid competing with each other bats use the space in night sky differently.
Open spaces hawking– these bats fly high above the vegetation, emitting low frequency FM signals.
Foraging in and around sparse vegetation – these bats fly in thinly vegetated woods, emitting high frequency FM signals.
Hunters in vegetation – these bats fly in or around tree crowns, have a well manoeuvred flight and intercept their victims from a close distance.
Perch-hunting – these bats feed by hanging from a tree branch or a wall, intercepting passing prey.
Gleaning and hovering – these bats usually hover over the prey, which is most often detected crawling on the ground or on tree leaves, most often by hearing the sounds generated by the prey.
All bats have eyes but in view of their nocturnal activity they use them to a lesser extent than their other sensory organs. Insectivorous bats orient themselves and do most of their foraging by using echolocation. The ultrasonic calls of bats are either frequently modulated or constant (ultra short waves) sounds with a frequency above 18,000 Hz, which makes them inaudible to the human ear. They are generated by bats, emitted into the open air, get reflected by the surrounding objects or by the prey and they are picked up by bats through their well developed ears.
The ultrasound calls are generated by the larynx, which is much more developed than in all other mammals.
The hearing sensitivity is very acute and bats can catch echoes reflected by objects smaller than 0.5 mm!
The bat brain processes information on flying victims and obstacles in the span of milliseconds.
Horseshoe bats (Rhinolophidae) emit constant frequency (CF) calls.
Vespertilionid (common) bats (Vespertilionidae) are specialized in getting directions and food with the help of frequency modulated sounds (FM) within a broad spectrum.