Compared to many other mammals, bats are rather small. Fruit bats are among the largest members of this family. They can have a wingspan of 1.7 m and weigh 1.5 kg (e.g. Pteropus vampyrus). The smallest bat and at the same time one of the smallest mammals of the world is the bumblebee bat (Craseonycteris thonglongyai). With a wingspan of 13 cm and a weight of only 1.7 g, it is just slightly larger than a human thumbnail. Incidentally, it is unlikely that any mammals smaller than the bumblebee bat will evolve. The reason for this is that small animals have a very large body surface in relation to their body weight, through which they lose body heat and thus energy.
One of the most characteristic features of bats are their wings. These have evolved from the mammalian forelegs and therefore still have all the skeletal components of these forelegs. The enormously elongated metacarpal bones, which serve to stabilise the flight membranes, are striking (Figure 1). Only the first finger, the thumb, did not have any elongation of the bones. It is the only finger that can move freely. The flight membrane itself consists of two layers of skin and connects the body with the hands, fingers and feet. This flying skin contains blood vessels, nerves, sensory cells as well as ligaments and muscles. It is divided into different parts by the bats' extremities. The part that lies between the 5th finger and the trunk and the small piece of flight skin near the shoulder bear the weight of the body during flight. The flight skin between the fingers to the wing tip serves to propel the animal forward during flight. Between the legs and tail of many bat species is the caudal flight membrane, which can be very important for catching prey, but can also perform other functions such as slowing down in flight. For its optimal function, it is important that the flight membrane does not dry out. Therefore, bats try to keep it moist and supple by secreting secretions from the mouth area.
In the course of their development, bats developed such strong shoulder muscles for wing movement that they only need seven of the 20 muscles normally found in the mammalian hand, which saves them an enormous amount of weight. When the animal is at rest, the wings are folded. Flying foxes and horseshoe bats often wrap their wings around their body like a cloak.
The legs of bats are small, thin and not particularly muscular. Unique to mammals, however, is that they are rotated 180°. So their knees point backwards! This makes it easier for bats to control their flight as well as their unusual head-down sleeping position. The leg skeleton and hips are also adapted to the bats' hanging posture. However, this position of the legs makes crawling locomotion difficult and only a few bats are good on their feet. When crawling, the legs protrude sideways under the body, like a reptile.
As bats are small animals with typically very high metabolisms, they have to take great care of their energy balance. Therefore, it is important that they use as little energy as possible when just hanging out in the roost. Similar to birds, which can hold on to a branch without exerting force, bats have developed a mechanism by which they can hang upside down and cling on with tightened toes without having to use muscle power. This so-called tendon locking mechanism works so well that even dead animals can remain suspended in their quarters for months without falling off.
The faces and ears of bats show an amazing variety of shapes. Many bats have fleshy skin appendages on their nose and mouth. These can vary greatly in shape and size and resemble structures of leaves (e.g. in leaf-nosed bats) or horseshoes (in horseshoe bats). These so-called nasal attachments fulfil important functions for echolocation and thus for the orientation of the animals (see chapter Echolocation).
Hearing is one of the most important sensory perceptions for the majority of bats. Therefore, the ears are important parts of the body for bats, especially if, like most species, they orientate themselves acoustically by echolocation and also hunt their prey in this way. This can be seen in the prominent position of the ears in many bats' faces. Some bats, e.g. the brown long-eared bat (Plecotus auritus), have unusually long ears. These are an adaptation to the way they feed, as long-eared bats specialise in finding their prey by their rustling noises in the vegetation and on the ground. A closer look at the auricle of many bat species reveals that there are folds and notches that are likely to play an important role in perceiving the echoes of the ultrasonic sounds emitted by the animals. Many bats also have distinctive ear cap-like structures at the entrance to the auricle, which are called tragus or antitragus depending on their location (Figure 3). These cartilaginous skin structures are crucial for locating objects.
The English saying, "to be blind as a bat", is anything but true. For contrary to many rumours, bats are not blind at all and can see in black and white. In addition, more and more nectar-eating bat species are known to be able to see in the UV range.
Depending on the feeding habits of the animals, there are strong variations in the faces and dentition, which represent adaptations to the respective way of life of the animals. For example, many nectar-eating bats have elongated snouts and extremely long tongues to reach the nectar in the calyxes. Incidentally, the plants pollinated by bats have also adapted to their pollinators, as can be seen, for example, from their often very long calyxes. The dentition of nectar-eating bats is also often greatly reduced. Fruit-eating bats, on the other hand, have dentitions that are adapted to soft food, which is evident from the flat cusps on their teeth. Dentitions reminiscent of predators are typical for insectivorous, but especially for carnivorous bats, such as the Central and South American great spear-nosed bat (Vampyrum spectrum), which can prey on birds up to the size of a pigeon.
The digestive system of bats allows for rapid passage of food. This typically includes a relatively large stomach followed by a relatively short intestine. The cardiovascular system of these animals is designed for high performance. The bat heart is significantly larger than the hearts of other mammals of the same size. In addition, the bat's blood has an enormously high content of red blood cells, which transport the oxygen in the body. The lungs of bats are also much more effective at extracting oxygen than the lungs of all other mammals. This is especially important in flight, when the body has a particularly high oxygen consumption.