Bats are members of the order chiroptera, which means 'birds with hands'. Around 1100 different species are found worldwide, divided into Microchiroptera and Macrochiroptera. Microchiroptera includes around 930 species, generally small in size, distributed all over the planet, except in the polar regions and on some oceanic islands, and are mainly entomophagous (insect eaters). The Macrochiroptera, on the other hand, are larger, number around 186 species distributed mainly in tropical regions, excluding the Americas, and are mainly frugivores (fruit eaters). In Italy, bats are the mammal group with the most species (34 different species have been reported).
Bats fly with their hands
Their distinguishing features from other mammals include arms and hands adapted into a highly efficient flying organ. The bat's wing is derived from the transformation of the forelimbs and in particular the hands, which have metacarpal bones and very elongated phalanges. The bones of the arm (the humerus) and forearm (radius and ulna are almost completely fused in bats) form the bony frame on which the wing membrane, called the patagium, is stretched. There are other mammals, such as flying squirrels, that are capable of jumping from trees, spreading their four limbs so as to stretch the skin membrane between their wrists and ankles. In this instance, however, it involves a 'glide' rather than active flight, as observed in bats, the only mammals in the world endowed with this distinctive capability.
...and see with their ears
The 'super powers' of these special mammals are certainly not limited to their extraordinary ability to fly.
It is a false belief that bats are blind. They see just as we humans do, but they have 'chosen' to be active at night in order to take advantage of a resource poorly exploited by other animals, nocturnal flying insects. Flying around and hunting in the dark for such small and swift prey solely relying on sight is very difficult.
Today we know that bats emit ultrasound from their larynx and release it through their nose or mouth. They are also able to analyse the echo that bounces back to them and to understand whether it is prey or something else.
The first researcher who was able to fully understand this capacity was the Italian Lazzaro Spallanzani in the late 17th century and Ludwing Jurine, a doctor from Geneva. Spallanzani experimented with blinded bats, which were still able to navigate perfectly and avoid obstacles. Jurine conducted experiments by closing the ears of these animals, and in that case, they had many difficulties in avoiding obstacles. Both researchers were unable to give adequate explanations for these phenomena.
Much later, in 1920, the English physiologist Walter Hartridge proposed a hypothesis, drawing inspiration from the echo sounders used by ships to detect submarines. He suggested that bats utilise ultrasound to navigate by interpreting echoes. It was not until 1938 that this assumption was proved, thanks to the American Donald Griffith, who used a device that made the ultrasound of bats audible even to the human ear. Bats are capable of creating a kind of 'auditory map' of their environment and memorising it, just as we memorise a painting.
If you observe a bat before it takes flight, you will see that it turns its head with its mouth open quickly in every direction. In this way, it explores the environment with ultrasound, picks up the echo with its ears bent forward and memorises. If the bat orients itself in a space, only two or three sounds are sufficient, but if it detects prey, the number of frequencies increases significantly.
Why do bats use ultrasound?
The reason is that the high frequency of ultrasounds creates echoes that can recognise even small bodies, such as insects. For example, a 50000 Hz pulse corresponds to a wavelength of about 6 mm, which is just the size of an insect. If, on the other hand, the pulse were 260 Hz, the wavelength would be 1 m and this would allow the insect to move without being perceived between waves.
Hungry as a bat
The term “foraging” is used to refer to the actions bats perform as they search for and capture prey. The exceptional energy expenditure during flight makes bats voracious consumers of food. In one night's hunting they have to consume a quantity of prey equal to one third to one half of their body mass. A bat weighing 20 g, for example, eats 7-10 g of prey. If a man weighing 80 kg ate like a bat, he would need to consume 27 to 40 kg of food in one day! Bats play a very important role in ecosystems, because there are few nocturnal birds that feed on insects. Chiroptera are also perfect allies in the control of many insects that are harmful to humans. A bat can catch up to 2000 prey, mostly mosquitoes, in one night! Therefore, by encouraging an increase in the number of bats, the use of chemical insecticides can be limited, benefiting human health and the environment.
Affectionate mothers
Of all mammals, bats are certainly the most social. A colony of Minioptera consisting of about 44,000 females has been observed in Australia. Also in Italy, bats form a so-called 'nursery' to raise their young. Males and females gather in autumn and in their winter shelter. This gathering instinct is probably a trick of nature to ensure survival: in this way, bats can easily find each other during the mating season and ensure the breeding of their young. Females give birth once a year, delivering only one offspring at a time.
In the nursery, each mother attends to her own pup, but it is the entire colony that, through aggregation or dispersion, safeguards the newborns from temperature fluctuations. Italian bats start mating in mid-August or early September, soon after they have finished rearing their pups. However, the sexual cycle of males does not coincide with that of females. The females mate with the males also during this period, but their eggs are not yet mature and are therefore not fertile. Females can, however, store sperm cells, which receive nourishment from the uterus, until the following spring. The eggs of the females are ripe only in spring, when bats leave their winter roost. In all mammals, the egg is fertilised after mating and the embryo immediately begins to develop. Bats are an exception in the animal world. After mating, females keep sperm cells in a small pouch located in their sex organs. It is only at the end of hibernation that one of the stored sperm cells enters the female egg and the embryo begins to develop.
Thermoregulation
Birds and mammals maintain a constant body temperature through thermoregulatory mechanisms. The bats’ body temperature during nocturnal activity varies between 35° and 40°C. During daytime rest, however, they conserve energy and their temperature drops to 15°-20°C. If weather conditions, e.g. cold and rainy nights, do not allow night flights, the bats cool down even more and enter a semi-lethargic state called torpor, which saves further energy.
During the winter, bats go into hibernation and they adapt their body temperature to that of the shelter they are in, usually ranging between 2° and 10°C. Bats are able to regulate their body temperature autonomously and use this ability whenever they need to. For example, when there are newborns, bat mothers keep their body temperature high also during the day to keep them warm, as their pups are not yet able to regulate their temperature autonomously.
Shelters
To rest during the day, to mate, to give birth and raise their young, to spend the winter hibernating, bats use different types of shelters called roosts.
In general, the roosts used by a bat throughout the year are located within a few dozens of kilometres from one another. There are, however, migratory species such as Nathusius’ pipistrelle and the common noctule, which also move over distances of more than 1,500 kilometres. In winter, bats generally take refuge in caves to hibernate. In the warmer season many species rest in tree cavities, abandoned bird nests, spaces under raised bark, old dead trees, etc. Other species, adapting to the destruction or alteration of natural environments by human activity, have found shelter in artificial underground settings such as mines, tunnels and buildings. Bats often also rest in roller shutters boxes in houses.
Endangered bats
The conservation status of a species is an indicator of the likelihood that that species will continue to survive, and unfortunately the conservation status of bats is not good. The International Union for Conservation of Nature, the most important international organisation dealing with conservation, examined some 778 species and placed about half of them on the red list of threatened species. This precarious conservation status also affects bats found in Italy, and many species risk extinction in the medium or even short term. In Italy, bats have been considered a protected species since 1939 (according to Article 38 of the Law on Hunting No. 1016 of 5/6/1939), after their importance in keeping harmful insects under control was recognised.
This measure, however, only prevented the killing or confining of these animals, but did not concern itself with protecting the environments they live in. The main causes of the decline in bat populations are attributed precisely to the alteration and destruction of habitats, which are no longer adequate to provide refuge and foraging sites, and to direct human disturbance. This is why bats have been included as protected species in the main European Directives and Conventions, such as the Bern and Bonn Conventions, and the Habitat Directive for Nature Conservation (Presidential Decree No. 357 of 8 September 1997). Unfortunately, bat conservation regulations in Italy are often ignored because most people are not aware of these problems. Therefore, information and awareness-raising initiatives regarding these small and nice-looking mammals are essential for their preservation.
Destruction and alteration of foraging environments
During Roman times, Italy’s main lowland areas, such as the areas around Turin, Milan and Emilia-Romagna, were completely covered by lush deciduous forests consisting of several layers of vegetation, up to the top layer which was dominated by oaks up to 45 metres high (roughly the height of a 13-storey building!). The forests were interspersed with areas covered by grass and shrubs, which had been created by natural fires or by the fall of large, old trees. There were also rivers, which were free to naturally change their course, and swamps over which large swarms of water-bound insects flew. It is easy to see how ideal this was for bat populations.
In Italy, with the beginning of the Roman practice of centuriatio, these environments gave way to agricultural fields and reclaimed plains. Centuriatio was a system by which the Romans organised agricultural land. They subdivided it into geometrical plots that were assigned to private citizens to be cleared and cultivated. Later, during the Middle Ages, monks reclaimed much of the marshland. With the arrival of technological progress after the end of the Second World War, tractors and agricultural machinery made it possible to cultivate large areas of land very quickly and efficiently.
The last remaining remnants of spontaneous vegetation, which hindered the movement of agricultural machinery, were removed, and the populations of numerous forest insect species began to decline. At the same time, the breeding of beasts of burden and consequently the insect species related to them decreased. The introduction of insecticides in modern agriculture has affected the possibility for many species of bats to feed themselves. Bats not only have less available prey, which are exterminated by insecticides, but also find contaminated insects. Bats therefore take up insecticides through the insects they feed on and accumulate them in their fat reserves. After awakening from hibernation, when bats rapidly deplete their fat reserves to resume activity, the accumulated poisons enter the bloodstream, often resulting in death.
Destruction and alteration of roosts
Tree cavities, tunnels created by wood insect larvae in tree trunks, caves and crevices in rock walls are natural sites where bats rest during the day. Good alternatives to natural shelters are abandoned mines, interstices in buildings, attics and lofts, or spaces under bridges. Both artificial and natural shelters, however, are subject to continuous degradation due to deforestation, removal of dead trees, tourist or speleological visits inside the caves which disturb the bats’ rest. When bats are hibernating, the rise in ambient temperature due to human presence can trigger the awakening process and in this way the accumulated fat is partially consumed. If the awakening occurs several times during hibernation, the bats are in danger of no longer having sufficient reserves for the final awakening in spring and may therefore die. As for old buildings and mines, they are often demolished or repurposed, so more shelters are lost.
