When Homo sapiens was still only a draft of evolution, bats were already flying in the air of the tumultuous period between the Cretaceous and Paleocene, some 50-70 million years ago. The following section describes, in an atypically long form, the evolution of bats, their morphology, but also the most important ecological and behavioral aspects of their lives. For a quick overview of bat body parts, we invite you to study the wonderful picture below, photographed by our colleague Dénes Dobrosi.

Enjoy your reading!

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The ears of bats receive the echo of the ultrasound, reflected from various surfaces.

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The nose of bats is similar in function to the human nose, except the nose of horseshoe bats, who emit ultrasounds through their nose.

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The teeth of bats are excellent to chew the hard exoskeleton of insects.

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Bats are not blind. Their eyes are much better adapted to darkness than human eyes.

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The wing of bats is actually composed of very elongated fingers, and includes also the stretchy wing membrane.

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The forearm of bats is strong and elongated, being an essential part of the wing.

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The patagium is a stretchy skin membrane between the fingers, forearm, body and legs of the bat, with which bats become the masters of flight.

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The legs of bats are small in comparison to other body parts, and are used only to catch prey or to hang from the ceiling of the cave.

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Through their construction, the claws of bats are automatically activated by the animal’s weight, and make it possible for bats to hang without effort.

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The fur of bats gives their body an aerodynamic shape, and also keeps them warm.

Bats appear on the stage of life

Just before and during the Cretaceous and Paleocene, we find numerous examples of an increase in the diversity of different groups, for example in plants, insects and also in mammals, which includes bats. Also during this period the extinction of dinosaurs takes place. The first known fossil representative of bats, Icaronycteris index, comes from North America, with an age of about 51 million years ago. This species already possesses characteristics typical for bats: the ability for active flying and orientation using ultrasounds. Other fossil bats have been identified in Europe, Africa and Australia, for example Paleochiropterix and Archeonycteris. Given that these fossils already present typical bat characteristics (body adapted to flight and to emit ultrasounds), currently we do not know for sure in what order these key traits appeared and what kind of mechanisms were involved. The evolution of other unique characters (ex. gregarious and nocturnal life, hibernation) probably occurred after the appearance of active flight and ultrasound based orientation.

Many theories about the evolution of bats debate in principle the order of appearance of key traits: what did evolve first, active flight or using ultrasound for orientation? Some theories claim that the active flight appeared first, with ever greater jumps on tree branches, resulting in passive gliding and later in active flight, while the use of ultrasounds for orientation and hunting emerged only due to the opportunities offered by exploring a new environment (i.e., the air). Other theories claim that the proto bat started to locate food using ultrasounds, while hanging passively and after locating its prey tried to catch it with ever greater jumps, this activity evolving towards gliding and subsequent active flight. There are also theories that support the parallel development of both traits.

A more recent theory is the adept of the scenario that the ancestral bat actually lived at ground level, and that the evolution of flight did not include an intermediate stage of gliding, but has gone directly to active flight, propelled by high jumps from the ground. This theory is based on several physiological and adaptive aspects, namely the fact that during embryonic development all life forms go through ancestral phases of evolution. In the case of the embryonic development of bats we do not observe the intermediate stage that would favor gliding. About this theory you can read in detail in the work of Adams and Shaw (2013). The complete scenario will be known only after discovering bat fossils in intermediate stages of evolution, which show mixed traits.

Icaronycteris_index, fosilă de liliac cu o vârstă de 51 milioane de ani și având deja caractere bine evoluate (ex. degete elongate). - A mintegy 51 millió éves Icaronycteris index denevérfosszília, mely már tipikus denevér jellegeket mutat (pld. meghosszabodott ujjak). - The 51 mya old Icaronycteris index bat fossil, having already well developed bat traits (ex. elongated fingers). Photo Andrew Savedra, Wikipedia.
Desen realizat de Enrst Haeckel (1834–1919), prezentând o mică parte din diversitatea uriașă a liliecilor. - Ernst Haeckel rajza, mely a denevérek változatosságának egy apró részét szemlélteti. - Drawing made by Ernst Haeckel, presenting a small part of the bat diversity.

Bats are the only mammals capable of active flight, and during their evolution they acquired unique and diverse aspects in morphology, ecology and behaviour. A prominent example of this unique system is the difference in size between the smallest bat, the bumblebee bat Craseonycteris thonglongyai, weighing 3 grams, and the largest bat species, Acerodon jubatus the Philippines, weighing 1.5 kilograms and with a wingspan of 1.7 meters. Currently around 1.300 bat species are described, forming 19 families, the bat order representing one of the most diverse groups of mammals, preceded only by the order of rodents (Rodentia). Apart from the two poles and desert habitats, bats can be found everywhere, but, as with many groups of animals and plants, the greatest diversity is found in tropical areas.

In the classical sense, bats can be divided into two suborders. Large bats (Megachiroptera) have a tropical distribution, specifically to Africa, Asia and Australia. Their main features are that they feed exclusively on plants, have a great sized body (20-1.500 g) and only a few species are capable of ultrasound-based orientation. The suborder includes 173 species. Small bats (Microchiroptera) have a small body, are mainly insectivores and are using ultrasounds for orientation. Small bats are classified in at least 17 families and unlike their bigger brothers, they have a global distribution. In the modern sense, based on molecular studies (ex. Teeling et al. 2005), we can differentiate between Yinpterochiroptera and Yangochireoptera. The first suborder includes all large bats (Megachiroptera), and five families of small bats (Microchiroptera). The remaining small bat families are included in Yangochiroptera. The existence of these modern suborders assumes that one of the following theories must be true. The first theory says that echolocation (a trait probably hard to reproduce) evolved twice independently in the order of bats, once in Yinpterochiroptera, and once in Yangochiroptera. The other theory assumes that echolocation evolved only once in the entire order of bats, but was subsequently lost in the case of large bats.

We invite you to explore the visualization of the Tree of Life, created by Damien de Vienne (University of Lyon), and to find the place of bats in this exceptional representation.

The body of bats – adaptation to a variety of scenarios

The morphological details of the body of bats evolved in accordance with their lifestyle (active flight) and the laws of aerodynamics. The latin name of the order is of Greek origin (“Chiroptera”, cheiron – hand, pteron – wing), and suggests the underlying morphology. Their body is conical in shape and the center of gravity is located in the chest area. Ribs are ossified and are firmly connected to the sternum and spine. In the neck, the backbone is strongly curved outward, making it possible to accommodate the large and high performance heart in the chest cavity. The shoulder area includes most muscles involved in flight.

According to their lifestyle, the forearms and fingers of bats have undergone many changes, which are mainly related to their extreme elongation. Between these long bones, body and hind legs, there is the stretchy skin membrane called patagium, which is essential for flight. Depending on its location, we can define different patagia. The propatagium is located between the arm and forearm edge. The plagiopatagium is located between the arms and legs, respectively edge of the body. The membrane between the first and fifth finger is the dactilopatagium. Finally, between the legs and tail, and also involved in catching prey is the uropatagium. The wing morphology of bats has evolved according to feeding preferences and hunting grounds of different species. Species hunting in dense vegetation have short and broad wings, suitable for more precise navigation (ex. the greater horseshoe bat, gray long-eared bat). Species that hunt in open areas without vegetation have long, narrow wings (ex. the noctula, the giant noctule), this morphology being useful in fast flight. In the chest and upper arms area there are large muscles, in charge of quick movement and strong wing beats.

Se observă antebrațul lung, degetele elongate și membranele de piele între părțile corpului în cazul acestui liliac cu potcoavă, capturat în timpul monitorizării naționale de lilieci. - Ezen a patkósdenevér egyeden, mely az országos denevérmonitoring folyamán lett fogva, megfigyelhetőek a megnyúlt alkar és ujjak, valamint a különböző testrészek között feszülő szárny bőrhártyái. - We can observe the elongated forearm and fingers, as well as the wing membranes stretching between body parts, on this horseshoe bat, captured during the national bat monitoring. Photo: Szilárd Bücs.
Liliecii cu potcoavă (stânga) emit ultrasunete prin foițele nazale speciale, iar liliecii cu nas neted (dreapta) emit ultrasunete prin gura deschisă. - A patkósorrú denevérek (bal oldali fotó) a különleges orrfüggelékeinken keresztül bocsájtják ki az ultrahangokat, míg a simaorrú denevérek (jobboldali fotó) a nyitott szájukon keresztül. - Horseshoe bats (left) emit ultrasounds through their special nasal formations, while vesper bats (right) through their open mouth. Photos: Szilárd Bücs, Csaba Jére.

Relatively to other body parts, the hind legs of bats are disproportionately small. The construction of the claw and the fact that claws grip automatically based on the animal’s weight, permits a passive, energy free, effortless hanging of bat. This mechanism is apparent also in the case of dead bats, that remain hanging for a long time even after their death. The body of bats is covered with fur, which reduces the uneven areas of muscles, and gives an aerodynamic shape of the animal, serving also in thermal insulation. In most cases the coloration of the fur has no particular pattern, its color being gray, brown or the combination of both. Of course there are also exceptions, like those mentioned here.

While bats around the world have a high diversity regarding nasal morphology, bats of Romania can be divided according to the presence or lack of these nose leafs. In the case of horseshoe bats, the functions of these nose leafs is to emit and direct ultrasounds. Bats without nose leafs emit ultrasounds through the mouth. Bat ears, designed to receive the echo of emitted ultrasounds show also a great diversity. The shape and size of ears is in many cases used to clearly differentiate between species. In the case of the long-eared bats, as their name suggest, the ear length can reach the length of the body, but for example in the case of bent-winged bats, the ears are smaller than the head. Using muscles located on the skull, bats can perform very precise and rapid ear movements, being able to receive the echo of the emitted ultrasound, reflected from the prey. Many bat species have a tragus, which is actually an outgrowth positioned in front of the ear opening, and also has a role in receiving the echo of the ultrasound.

Using ultrasound for orientation and hunting

Bats orientation was, for a long time, unknown to science. But Lazarro Spallanzani’s experiments in 1793 and Donald Griffin’s in 1938, showed that bats are navigating by using so called ultrasounds, specifically using the echo reflected from the surface of objects. In fact, we are talking about a biological radar.

The sounds that facilitate orientation occur in the larynx and are emitted through the nostrils or mouth. By emitting ultrasounds through the nose (ex. for horseshoe bats), the nasal formations are designed to regulate and direct sounds. Ultrasounds emitted by bats are specific in their frequency, duration and structure. Signals with extremely high energy can reach 120 decibels for some species. The vast majority of sounds are in the category of ultrasounds, with a frequency of 20-120 kHz. These sounds are not audible to the human ear, except the sounds emitted by the noctule, which are below 20 KHz frequency. Ultrasounds emitted by other species can be heard only using ultrasound detectors.

Catching prey or obtaining food is solved in three distinct phases. The first step is to detect: bats verify whether the echo of their ultrasound is present or not, yielding information about the presence or absence of the target object. The second stage is classification: when the reflected echo provides information about the size, shape and texture of the target. This information manifests itself by changes in frequency, amplitude, or rhythm of the echo perceived. The third step is localization: information is obtained on target coordinates (altitude, speed, distance, direction), and this phase ends with capturing the prey.

Depending on the type of habitat and phases of prey capture, bats emit ultrasounds at different frequencies and intensities. Based on these aspects, we can distinguish between species that hunt in open areas (ex. the noctule), species that hunt on the edge of vegetation (ex. genus Myotis) and species that hunt inside vegetation (ex. long-eared bats). In various phases of prey capture, the frequency and intensity of sounds, as well as the interval of their emissions changes, with the actual capture ending in a short series of beeps with increasing frequency of 150-200 kHz. The total duration of the three stages of searching for prey (identification, tracking and capturing) can take in as little as 200-500 milliseconds.

Bat species of the world have an extremely diverse trophic spectrum. Some bat species are consumers of fruits,nectar, fish, rodents, insects and yes, even blood. In Romania we only have insectivorous bats, with an incurable appetite for anything resembling an insect. The following taxa are their major source of food: Orthoptera, Diptera, Ichneumonoidea, Noctuidae, Sphingidae, Tortricidae, etc.

In addition to navigation with ultrasounds, bats are able to orient also based on visual and / or olfactory stimuli, especially in the case of tropical bat species. The species in Romania (but also other species worldwide) have eyes well-adapted to darkness, allowing effective navigation at sunset. Some species (ex. the greated mouse-eared bat) selects its favorite insect prey by smell, but noise from these insects still plays an important role in food procurement.

The shape of the bat skull shows correlation with the preferred method of feeding, and can be identified based on the structure of teeth. The dentition of bats is heterodont and diphyodont. Typical is the molar divided in three and its cutting structure. The molars of the upper and lower jaw are closing, so that the slippage of opposite cutting surfaces results in the efficient breakup of insect chitin exoskeleton.

The structure of the bat brain reflects the changes related to active flight, orientation and food acquisition. In the case of several microchiropteran species there has been a regression in the brain area responsible for olfaction, while the posterior part of the brain has evolved. Auditory centers are located here, and also the coordination of complex flight manoeuvres. Inferior (caudal) colliculi, which have a role in orienting towards sounds, are more advanced than superior (cranial) colliculi involved in the sense of sight. The digestive tract is very short and only slightly differentiated. This fact results in a rapid and efficient metabolism, while also being the result of adaptation to an aerial life.

Migration

As is the case of birds, bats also show a seasonal migration pattern in the autumn and spring periods. The autumn migration is directed towards overwintering places, hibernation, which are usually areas with caves and mine galleries with constant temperatures. During spring migration bats move towards sites of nursery colonies, where females give birth to their offspring. The distance travelled between locations vary from about 10 km to several hundred, or even 1,000-2,000 km. Based on migration or distance travelled, we can define three groups of bats:

  • long-distance migratory species, with possible distances of 2,000 km (eg. Nathusius’s pipistrells, parti-colored bat, noctule)
  • medium-distance migratory species, with a distance of max. 500 km (ex. Savi’s pipistrelle, Brandt’s bat, great mouse-eared bat, greater horseshoe bat)
  • sedentary species, with distances below 100 km (ex. brown long-eared bat, gray long-eared bat, Natterer’s bat)

Reproduction

The mating period of bats, unlike for other animals, is in autumn. During this period, from mid-August to October, bats are seeking out locations of reproduction, which are basically caves, where they come into contact with other individuals from separate colonies. For single days, males form harems of 5-10 females, with which they mate. During the following night the harem’s composition is changed.

The mating period may be extended till the start of winter, but fertilization occurs only in spring, when the abundance of food sources allows the proper development of the embryo. The only exception is the bent-winged bat, in the case of which fertilization already occurs in autumn, but the development of the embryo is stopped in winter, and continued only in spring. The period of gestation for the bent-winged bat is 7 months, while for other species it is only 1.5-3 months. The gestation period can be extended by two weeks in adverse weather conditions.

Towards the end of hibernation (see below), with the arrival of spring the awakenings from deep sleep are becoming more frequent, and exploratory flights are done inside around the hibernation site. In late March hibernation colonies fall apart, with pregnant females migrating to locations of nursery colonies. After awakening fertilization occurs and the development of the embryo is commenced. When nearing the time of giving birth, females form nursery colonies. During this period, males live in isolation, usually far from the nursery. Depending on the species and weather conditions, bats give birth to pups from late May or early June. Initially hanging from the fur and mammary glands of females, pups are taken to hunting, but later, after a few weeks, they are able to hang by themselves, and are being left at the shelter during hunting events.

Pups are able to fly on their own after 6-8 weeks. Females reach the reproductive age after 3-4 months, as for males, this happens at the age of one year and a half. Bats belong to the group of K strategy animals, giving birth usually to a single, in rare cases to two offspring. Unlike other small mammals, bats live an unusually long life, which can be measured in decades.

Hibernation

Most bat species start looking for food at night, whereas during the day they get into a lethargic state. This condition is due to the decrease in body temperature, which is triggered by the increased temperature of the surroundings, and its role is to conserve energy. The lowering of body temperature is done in a gradual and controlled manner. For example, in the case of the noctule, the body temperature drops from 38.8°C to 18°C in 80 minutes. With the arrival of the winter period, the frequency and intensity of this lethargic state increases, due also to the decrease of ambient temperature and scarcity of food sources. The process itself includes periods with lengths varying from one day to several weeks, or even months, during which period the bat’s body temperature falls drastically, even to a temperature of 0°C.  The heart of bats beats only around 12 times per minute during hibernation.

The role of hibernation is to facilitate the survival of bats in periods without food sources. However, this requires that during warm periods animals should be able to accumulate enough fat reserves to survive the winter. This fat can represent 20-30% of the weight of the bat. The fat accumulation process begins in late summer and aided by favorable weather conditions may be extended until mid-November. In the fat storage phase the daily lethargic states allow for minimum energy consumption, with fat being accumulated for the winter period.

Through various studies using marked bats it was discovered that animals wake up and periodically change their places inside the cave during hibernation. Awakening from hibernation can have various durations, and depends on the length of uninterrupted hibernation and the temperature outside the roost. A pipistrelle, with body weight of 6-8 grams wakes up on average in 30 minutes, while a bat with higher body weight (ex. greater mouse-eared bat) requires around 70 minutes to get body temperature up from 5°C to 38°C. The frequency of awakenings is affected mostly by changes in temperature, but can also occur when individuals are disturbed. Small bat species hibernate at lower temperatures, and wake up less often than larger species. With the start of winter, periodic awakenings decrease in frequency, but with the arrival of spring, they are becoming increasingly common. In favorable weather conditions (the arrival of spring) bats leave their winter roosts.

Bats choose their hibernation according to the following criteria: the cave / site must be free of disturbance, must have a lot of adequate surfaces suitable for hanging, must have an average temperature between 2-10°C and high humidity (70-100%), and air currents must be present. Environments with minimal or no disruption will make possible that awakenings occur less frequently. In general terms, the more quiet shelter, the better conditions it offers for hibernation. Different species react differently to disturbance. Some species can be woken up also with tactile stimuli, while other species react to light, sound or temperature fluctuations.

Regularly used hibernation roosts have an average temperature of 2-10 ° C. Different species have also different thermal preferences (ex. 0-4°C barbastelle, 7-10°C greater horseshoe bat), but this does not exclude the possibility that species hibernate also at other temperatures. There are species that are less dependent on the ambient temperature and support broader changes. For example, the lesser horseshoe bat can be found in the same cave also in areas with lower and fluctuating temperatures, but also in areas with higher and more constant temperatures.

In principle, in the case of cave-dwelling species, which can experience dehydration, it is important that the roost selected for hibernation has a high humidity of between 70-100%. Otherwise, the animal’s metabolism cannot cope with conditions, resulting in dehydration and possible death. Air currents, in case the cave has two openings (entrance, exit), can have adverse effects on bats. In this case, temperature changes in the external environment are felt even more, with large temperature fluctuations inside the roost. Bats avoid this by choosing those roosts for hibernation which have a single opening, and are bag shaped.

In the bat fauna of Romania we have species that hibernate in isolation, solitarily (ex. the barbastelle, brown long-eared bat) or hibernating in groups (ex. bent-winged bat). The size of hibernating groups can vary from a few dozen individuals up to several thousands (even tens of thousands). Lastly, there are some species which can hibernate both in isolation or in groups. In the case of the greater horseshoe bat, some bats form colonies, but individuals do not have direct contact between them. In each case, the purpose of the hibernation group is to have a good heat regulation, by reducing three times the area of ​​contact between the body and ambient air / temperature.

Based on the preferences in choosing roosts for resting and hibernation, the bat species of Romania can be classified into three groups. We can differentiate between fitophile, litophile and antropophile species. Fitophile species (ex. brown long-eared bat, Nathusius’s pipistrelle, pipistrelle, noctule, barbastelle) choose hollow trees to hibernate. Litophile species (ex. mediterranean horseshoe bat, bent-winged bat, greater horseshoe bat, greater mouse-eared bat) use caves, crevices, abandoned mines, cellars. Antropophile species (ex. gray long-eared bat, serotine, Geoffroy’s bat) choose roosts located in settlements: bridges, houses, cracks in walls, cracks between building elements. This classification, however, cannot be exclusive, given that independently from shelters used during the summer, some species resort to the use of underground shelters during hibernation.

For further studying and understanding the bat domain, we recommend the excellent book of Dietz and Kiefer (2016).

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