scuola energia e ambiente
published on 16 April 2012 in life
Bats, The Lords of Darkness
Year of the bat
The UNEP Convention on Migratory Species and The Agreement on the Conservation of Populations of European Bats (EUROBATS) celebrate 2011 as the European Year of the Bat and 2012 as the International Year of the Bat, and for this occasion have chosen the slogan, “The Year of the Bat gives wings to the world’s only flying mammals ”. In fact this is the only mammal that is able to fly, and even though it has a really horrible reputation, it is very useful to humans, and carries out various services, such as keeping the number of mosquitoes under control, it favours seed dispersing and pollination and therefore contributes to the protection of biodiversity.
Bats are called chiroptera, which means “hand wing” . There are about 1100 different species worldwide, subdivided into Microchiroptera and Macrochiroptera. There are approximately 930 species of Microchiroptera, which are generally small-sized, and are distributed all over the planet except in the Polar regions and in some ocean islands. Microchiroptera are prevalently entomophagous (insect-eaters). Macrochiroptera, instead, are bigger, there are about 186 species distributed prevalently in the tropical regions, excluding the Americas, and they are mainly frugivores (fruit-eaters). In Italy, bats are the group of mammals with the largest number of species: 34 different species have been reported.
Flying with their hands
Flying with their hands The characteristics that differentiate bats from other mammals are certainly their arms and hands, which are transformed into a perfect flying organ. In fact, the wing of the chiroptera derives from the transformation of the front limb, and in particular the hand, whose metacarpal bones and phalanges are greatly elongated. The bones of the arm (humerus) and of the forearm (radius and ulna are almost completely incorporated in bats), and form a framework of bones across which the wing membrane, called patagium, is stretched. There are other mammals, like the flying squirrel, that can jump from the trees, widening all four limbs so as to stretch the skin membrane between their wrists and ankles. In this case, however, the flying squirrel “glides” and does not actively fly, as in the case of bats, that are the only mammals in the world with this characteristic.
Seeing with their ears
The “super powers” of these particular mammals are certainly not only their extraordinary capacity to fly! It is a false belief that bats are blind. They can see just like us human beings, but they have chosen (in their evolution) to be active at night, and make use of a resource that is not greatly exploited by the other animals: the insects that fly at night. Therefore it is quite difficult to move about and hunt their prey, that are so small and fast, in the dark of the night, using only their eyesight! Today we know that chiroptera emit ultrasounds from their larynx, which are freed through their nose or mouth. And then they are able to analyze the echo that returns and understand if it is a prey or if it is something else. The first researcher who was able to fully understand this capacity was the Italian, Lazzaro Spallanzani at the end of the 17th century and Charles Jurine, a doctor from Geneva. Spallanzani carried out some experiments with blinded bats that were able to find their way perfectly, avoiding all the obstacles. Jurine, instead, tried closing the ears of these animals which, in this case, ran into a great number of difficulties in avoiding the obstacles. Both researchers were unable to give adequate explanations for these phenomena. Many years later, in 1920, the English physiologist Hartridge, thinking of the echo sounding method used by ships to identify submarines, hypothesized that the bats used ultrasounds to find their way, with the help of an echo. Only in 1938 the proof of this supposition was obtained, thanks to the American Donald Griffith who used a device with which the ultrasounds of the bats could also be heard by human ears. The chiroptera, in fact, are able to create a sort of “auditory map” of the environment in which they are, and to memorize it, in the same way as we memorize a painting. If you look at a bat before it starts to fly, you will see it turn its head, with its mouth open, rapidly in all the directions. In this way, in fact, it explores the environment with ultrasounds, and collects their echo with its ears folded forward, memorizing them. If the bat must find its way in a space, just two or three sounds are sufficient, if instead it finds a prey, the number of frequencies increases greatly.
Why do bats use ultrasounds?
The reason is that high frequency allows the production of echoes that can also identify small bodies like the insects that are hunted. For example a 50000 Hz impulse is equivalent to a wavelength of approximately 6 mm, i.e. exactly the size of an insect. If instead the impulse is 260 Hz, the wavelength would be 1 m and this would allow the insect to move about without being seen between one wave and another.
Hungry as a bat
During their feeding time, known as foraging, chiroptera hunt and capture their prey. When the bats fly, they burn an enormous amount of energy, and so bats are great food eaters. In fact in one night hunting, they feed on an amount of prey that is equal to one third or half their body weight. A bat weighing 20 g, for example, will eat 7-10 g of prey. If a human being weighing 80 kg were to eat like a bat, he would eat about 27-40 kg of food a day! For this reason bats have a very important role in the ecosystems, in fact not many nocturnal birds feed on insects. Furthermore chiroptera are perfect allies for controlling a number of insects that are harmful for human beings. A bat can capture up to 2000 prey in one night, especially mosquitoes! Therefore by favouring an increase in the number of bats, it is possible to limit the use of chemical insecticides, and this is an advantage for human health and for the environment.
Among all the mammals, bats are surely the most social. In Australia a colony of Miniopterus (long-winged bats) was observed. It consisted of approximately 44 thousand females. Also in Italy the bats form a so-called nursery to raise their young. Males and females unite in autumn in the winter roost. This aggregating instinct is probably an astuteness of nature in order to guarantee their survival. In fact, in this way the bats meet easily during the mating period, and guarantee breeding of their young . Females give birth only once a year, and only one baby is born at a time. In the nursery each mother takes care of its baby but it is the entire colony that, by aggregating or spreading out, protects the newborn babies from variations in the temperature.
Italian bats begin mating around mid August or beginning of September, immediately after having bred their young. The males’ reproductive cycle however does not coincide with that of the females. The females copulate with the males also during this period, however their eggs are still not mature and therefore are not fertile. However, the females can store the spermatozoa that receive nutrition from the uterus up to the following spring. The females’ eggs become mature only in spring, when the bats leave their winter roost. In all mammals the egg is fertilized after copulation, and the embryo immediately begins to develop. Bats are an exception in the animal world, in fact after copulation, the females store the spermatozoa in a small pocket that is in the female sex organs. Only at the end of hibernation one of the spermatozoa that were stored will enter the female egg and the embryo will begin to develop.
Birds and mammals keep their body temperature constant through thermoregulation mechanisms. In the case of bats, the body temperature during the nocturnal activities varies from 35° to 40° C. At rest, during the day, instead, they save energy and their temperature drops to 15°-20° C. If the atmospheric conditions, like cold rainy nights, do not allow nocturnal flights, the bats cool down even more and enter a state of semi-hibernation that is known as torpor, and this enables the bats to save energy even further. In winter the chiroptera really go into hibernation and their body temperature drops to the temperature of the roost they are in, generally ranging from 2° to 10°C. Bats are able to heat themselves autonomously and they use their capacity to vary their body temperature every time they need to. For example, when there are newly born bats, the mother bats keep their body temperature high even during the day in order to keep them warm, also because the babies are still unable to thermoregulate their body.
To rest at day time, to mate and deliver and rear their young, to hibernate during the winter months, the chiroptera use various types of shelters, known as roosts.
Generally, the roosts that the bat uses during the year are all within a range of ten kilometres. However there are migrating species like the Nathusius bat and the common Noctule that can travel distances over 1500 kilometres. In winter during hibernation, bats generally roost in caves. Many species in the warmer season, use the cavities of trees: abandoned bird nests, spaces under the stripped bark, or old dead trees etc. Other species, as an alternative to the natural environments that have been destroyed, or which have been altered by human activities, have adapted to living in artificial underground locations such as mines, galleries and constructions. It often happens that the bats use the boxes of external venetian roller blinds in houses.
The state of conservation of a species is an indicator of the probability that the particular species will continue to survive, and unfortunately the state of the chiroptera is not a good state of conservation. The International Union for Conservation of Nature and Natural Resources, the most important international organism that deals with conservation, after having examined approximately 778 species, added about half of them to the Red List of threatened species. This precarious state of conservation also regards the Italian chiroptera; in fact, many species may become extinct in the medium or short period of time. In Italy, bats are considered a protected species since 1939 (Article 38 of the Law on Hunting n. 1016 dated 5/6/1938) after their importance was recognized for keeping the number of harmful insects under control.
This regulation , however, only prevented the killing of these animals and detention of the same, but did not take any action to protect the environments in which they lived. The principal causes of the decline in the populations of bats in fact are attributed to the alteration and destruction of their habitat, which is no longer suited as a shelter, nor for their foraging activity, and is a source of direct anthropic disturbance. Therefore the bats have been included as protected species in the principal European Directives and Conventions, such as the Bern Convention, the Bonn Convention, the Habitat Directive on Nature Conservation (Decree n° 357 of the President of the Italian Republic dated 8 September 1997). Unfortunately the regulations regarding the protection of bats in Italy are often ignored because conservation problems are not known to most people. Therefore in order to protect them it is indispensible to promote initiatives to inform the public and make people more aware of these cute little mammals.
Destruction and alteration of the foraging areas
In Roman times, the principal plains in Italy, as for example the areas around Turin and Milan, and the Emilia Romagna region, were all covered by luxuriant broadleaf forests, consisting of various layers of vegetation right up to the vault that was dominated by oaks that could reach a height of 45 metres (more or less the height of a 13 floor building!). However the forests were fragmented by areas with grass and shrubs, created by natural wildfires or by large old trees that had fallen . There also were rivers that could freely change their natural course, and marshes where large swarms of insects that lived near the water flew about. It is easy to understand how all this was ideal for the population of chiroptera. In Italy, with the start of Roman centuriation, these lands were replaced by agricultural fields and reclaimed plains. Centuriation was the method used by the Romans to organize the agricultural territories, i.e. into lots that were identified with geometric precision, and were assigned to private parties to be deforested and cultivated. Later, in the Middle Ages, the monks drained many of the marsh areas.
With the arrival of technological progress after the end of World War II, tractors and agricultural machinery enabled the cultivation of vast areas of land in a very rapid and efficient manner. The last remaining relicts of spontaneous vegetation were eliminated as they hindered the movements of the agricultural machines, and the populations of a large number of species of forest insects started to decrease. At the same time the number of beasts of burden decreased, and as a consequence also the species of insects that were tied to them.
The introduction of insecticides in modern agriculture has conditioned the foraging activities of many species of chiroptera. In fact the bats not only have less prey available, as these are exterminated by the insecticides, but they also find contaminated insects. Therefore the bats swallow insecticides through the insects they feed on, and these accumulate in their reserves of fat. On awaking from hibernation, when the bat consumes its reserves of fat, which it had accumulated, in a very short time, in order to start its activities again, the accumulated poisons enter the bat’s blood stream, often provoking its death.
Destruction and alteration of the roosts
The bats’ natural sites for resting during the daytime are tree cavities, galleries dug into the tree trunks by wood insects’ larvae , caves and cracks in rocky walls. Valid alternatives to natural shelters are abandoned mines, narrow cracks in buildings, lofts and attics and also spaces under bridges. Both the artificial roosts, and the natural roosts are subject to a constant degradation due to: deforestation, removal of dead trees, tourist visits or speleologists entering the caves, that disturb the chiroptera’s rest. When bats hibernate, a rise in the environmental temperature due to the presence of human beings, can provoke a reawakening process and in this way the fat they have accumulated is partially consumed. If the bats are awakened various times during hibernation, in spring there is a risk that they will not have sufficient reserves for their final reawakening, and they could therefore die. With regard to the old buildings and mines, often they are demolished or destined to new uses, and so other roosts are lost.
Written by Tiziana Bosco