|Linnaeus , 1758|
The mammals (Mammalia) are a class of vertebrates . Their distinguishing features include suckling the offspring with milk that is produced in the mammary glands of the females, as well as the fur made from hair, which, in combination with the constant body temperature, makes them relatively independent of the ambient temperature. With a few exceptions ( monotons ), mammals are viviparous . Mammals are most species-rich on land, but they also inhabit the air and water. The spectrum of behavior in mammals is broad and flexible, with some groups showing complex social structures. At the beginning of 2018, 6399 recent species were distinguished worldwide . The mammals are divided into three subclasses: the egg-laying mammals (Protheria), the marsupial mammals (Metatheria) and the higher mammals or placentas (Eutheria), which also include humans . That branch of special zoology that is devoted to the study of mammals is called mammalogy .
A fur coat made from hair is one of the most important characteristics of mammals. Even if some species (for example the whales ) are practically hairless, they have developed from hairy ancestors and show hair growth at least in their embryonic development . Most mammal species are hairy on most of the body throughout their life. Hair consists mainly of the protein keratin . The hair of the animals can have several functions:
- The fur is used for heat regulation, it insulates in cold weather and sometimes also protects in hot weather. This insulation is an important prerequisite for homoiothermia (the body temperature of the same temperature).
- A special coloration and arrangement of the hair is used to protect from sight and camouflage both prey and hunters. For this purpose, different mammal species change their coat color every season (for example mountain hares and arctic foxes ). A conspicuous coat of paint can also serve as a warning against predators (for example in the case of the Skunks ).
- The coat can mark differences between the sexes ( lion's mane , facial and chest hair in humans).
- Hair can be used for communication, for example the upright hair on the back of the neck of the wolf or the upright white tail of the white-tailed deer as an escape signal.
- Hair plays a role in the sense of touch . This function is particularly pronounced in the whiskers (vibrissae), which can be moved by special muscles and are equipped with nerve fibers and mechanoreceptors .
- In a number of mammals, such as hedgehogs , porcupines and echinids , some of the hair has developed into spines that provide additional protection against predators.
- Hair can act as a filter or trap to protect sensory organs or the air we breathe from foreign bodies such as nose hair , ear hair, eyelashes and eyebrows in primates.
Mammals are usually characterized by heterodontic teeth with four different types of teeth, the incisors , canines , and two types of molars ( premolars and molars ). The number of individual tooth types is given by the tooth formula . A heterodontic set of teeth is an important distinguishing feature from the homodontic (uniform) set of teeth of reptiles and is especially important for the classification of fossils. Most mammals have a one-time change of teeth ( diphyodontia ). First, milk teeth are created ( lacteal dentition ), which are later replaced by the "second" or permanent teeth ( permanent dentition ). Only the molars are not replaced, but only come with the permanent teeth.
A number of groups of mammals have rootless teeth that continue to grow throughout their life and are worn away by abrasion. These include, for example, the incisor teeth of rodents or the tusks of elephants , narwhals , walruses and other species.
- The ursammals (Protheria) have no teeth in adulthood, only the hatchlings have an egg tooth , comparable to that of birds , with which they pierce the egg shell.
- The dentition of the marsupial mammals (Metatheria) differs in some aspects from that of the higher mammals: all taxa with the exception of the wombats have a different number of incisors in the upper and lower jaw. The early marsupials had a tooth formula of 5 / 4-1 / 1-3 / 3-4 / 4, for a total of 50 teeth. Even today these animals have 40 to 50 teeth in many cases, significantly more than comparable placenta animals.
- The early higher mammals (Eutheria) had a tooth formula of 3 / 3-1 / 1-4 / 4-3 / 3, for a total of 44 teeth. This original tooth formula can still be found in some species, for example the wild boar . In most cases, a specialized diet has reduced the number of teeth. A few taxa, for example the anteater or the pangolin , have become completely toothless. The reverse case, an evolution-related increase in the number of teeth, has only occurred in a few cases: The giant armadillo ( Priodontes maximus ) has up to 100 pin-like teeth in its tubular snout, the highest number of all land mammals. The toothed whales represent a special case, the teeth of which have become uniform again (homodont). The number of some dolphin species can be as high as 260 teeth.
Auditory ossicles and temporomandibular joint
An exclusive feature of mammals are the three auditory ossicles, hammer ( malleus ), anvil ( incus ) and stapes ( stapes ). These are located in the middle ear ; they pick up the vibrations of the eardrum and pass them on to the oval window of the inner ear .
Phylogenetically the ossicles of ingredients original gill or arches can be derived: The stirrup from hyomandibular which, when the fish component of the suspensory and other terrestrial vertebrates as columella is formed, the anvil and hammer from the quadrate and from one part of the replaced by bone Meckel's Cartilage , the articular . The eardrum is surrounded by an almost ring-shaped bone, the tympanicum .
In the other vertebrates, the quadratum and articular form the primary temporomandibular joint , which in the mammals is replaced by a secondary temporomandibular joint arising elsewhere during fetal development . This is formed by the cover bones Dentale and Squamosum . The transition from the primary to the secondary temporomandibular joint became functionally possible when the joint axes of both coincided as a result of the increase in size of the brain or cranium in the cynodontia .
- Another exclusive feature of mammals is the suckling of young animals with milk, see the section on reproduction for details .
- Mammals are the only animal group to have a diaphragm , a flat muscle that separates the chest and abdominal cavity.
- Mammals have developed a secondary palate with an inner nostril ( choane ) lying far back . It allows breathing while chewing the food extensively and in the case of young animals while suckling, and because the nasal and oral cavities are sometimes completely separated, suckling is physically possible in the first place. A epiglottis ( epiglottis ) closes at the penetration of the food in the larynx during swallowing trachea to prevent. Except in the monotremes , the larynx is largely formed by the thyroid cartilage ( cartilago thyreoidea ).
- The brain is comparatively well developed, the neocortex is an exclusive feature of this taxon.
- The skull is a modified synapsid skull. This means that in the ancestors of the mammals there was a single cranial window in the temple area, which in the mammals is closed and can only be recognized by the presence of the zygomatic arch .
- The red blood cells of mammals have no nucleus and no other organelles .
- Mammals, like birds , have a double blood circulation : a pulmonary and a body circulation. The heart is divided into four chambers - two atria and two main chambers. The two halves of the heart, one on the left with oxygen-rich and one on the right with oxygen-poor blood, are separated by a complete septum - except in the fetus ( foramen ovale ).
- Besides birds, mammals are the only animal group in whose kidneys there are Henle's loops ( Ansae nephricae ), which means that they are able to reabsorb water from the primary urine .
Diversity in physique
In the course of their evolutionary history, the mammals have colonized almost all habitats and divided into a variety of forms. A number of species have adapted to an aquatic (water-living) way of life; the most specialized are the whales , whose physique is similar to that of fish . The forelimbs are fins ( Flipper transformed), the hind limbs are degenerated and the tail is a Fluke transformed. In other taxa, such as seals and manatees , adaptation to the water is less advanced. The bats are in addition to the birds and the extinct flying reptiles , the only vertebrates that are capable of active flying. They have very elongated fingers that stretch the skin. In addition, a number of mammalian taxa have independently developed gliding membranes that enable them to glide passively: these include the giant gliders , the glider and thorn-tailed squirrels from the group of rodents, and three families of gliding marsupials (the gliding , ring and dwarf gliding sacs ). A wide variety of mammals are adapted to a way of life that digs underground. They developed a cylindrical body with short limbs, often extended to digging tools. Numerous species lead an arboreal (tree-dwelling) way of life - these are often characterized by graspable paws with opposable thumbs and prehensile tail. Inhabitants of grasslands and other open habitats often show a reduction in the number of toes and the development of horny toes or hooves , while others have developed greatly enlarged hind legs and jumping locomotion. In contrast, many species, mainly smaller ones that live in hiding, have a stocky build with short limbs - including numerous rodents and insect eaters .
There are also considerable differences in size: the smallest mammals are the pig-nosed bat and the Etruscan shrew , each weighing only 2 grams. The blue whale, on the other hand, is considered to be the largest animal that ever lived on earth and, in exceptional cases, can weigh up to 150 tons, which is 75 million times that of the smallest mammals.
Distribution and habitats
Mammals are distributed all over the world, they can be found on all continents , in all oceans and on most islands. Ursula mammals are restricted to Australia and New Guinea , on the one hand marsupial mammals live on the Australian continent and Southeast Asia east of the Wallace Line and on the other hand in North , Central and South America . Higher mammals have a worldwide distribution, but were only represented by relatively few species until the arrival of humans in Australia, namely bats and real mice . The mammal fauna of remote islands was limited until the arrival of humans; so on many islands, including New Zealand , bats were the only mammals.
Mammals have colonized almost all regions of the world and are found in most habitats. They can be found in deserts and forests , in high mountains and also in the polar regions . The interior of the Antarctic continent is one of the few regions in which (at least with the exception of temporary human stays) no mammals can be found . Several groups of mammals, the marine mammals , have adapted to marine life; however, only a few specialized whale species are found in the deep sea .
Way of life
Ways of life
As different as mammals are in terms of their physique and their habitats, so are their ways of life. There are diurnal, crepuscular and nocturnal as well as cathemeral (active both day and night) species. There are also considerable differences in social behavior: in addition to strictly solitary species, there are others that live together in groups of up to thousands of animals. Some species have developed complex behavior patterns, they establish a strict hierarchy within the group and communicate with one another using sounds, gestures or postures. Although it is the exception, there are also mammals that use poisons for defense or hunting (see: Poisonous Mammals ).
Some mammals avoid climatically extreme times and the associated lack of food by falling into hibernation or torpor (state of rigidity), for example in cold or dry seasons. The body temperature drops almost to the ambient temperature, breathing and heartbeat slow down and the metabolism is reduced.
The sense of smell plays an important role in the way of life of the mammals, among other things in the search for food and in the reproduction, where pheromones signal the readiness to mate. The smell is also important for territorial behavior; several species mark their territory using urine, feces or special glandular secretions.
In general, hearing in mammals is well developed. A special form is echolocation , in which the own position can be determined or prey can be localized using the returning echo of emitted sound waves. The echolocation is particularly pronounced in two taxa, the toothed whales and the bats , but it is also found in other groups.
The sense of touch also serves to perceive the environment. Many species have developed special whiskers (vibrissae) for this purpose, which are extremely sensitive and can be controlled by muscle movements. The skin itself is also a sensory organ; certain parts of the body are particularly rich in mechanoreceptors , for example the fingertips of primates or the nasal or trunk region of many species. The most developed sense of touch of all mammals is generally attributed to the star mole rat . In this context, the fine electroreceptors in the beak of the monastery should also be mentioned , which react to the muscle movement of the prey. The sense of touch is also often important in social interaction, for example in the mutual grooming practiced by many animals .
The importance of the sense of sight varies greatly. However, it often only plays a subordinate role, especially in animals living underground, whose eyes are often regressed. On the other hand, cats and primates , for example, have large eyes and relatively good eyesight . The position of the eyes is also decisive: while predators usually have forward-facing eyes, which enable spatial vision and thus a more precise estimate of distance, the eyes of prey are often attached to the side, which enables an almost complete all-round view and the earliest possible detection of dangers.
A common feature of all mammals is the high energy and consequently food requirement compared to other animals of the same size, which is a result of the constant body temperature. Some species consume food close to their own body weight every day. There is a huge range of types of food: herbivores (herbivores), carnivores (carnivores) and pronounced omnivores (omnivores). The number and structure of teeth as well as the design of the digestive tract reflect the diet. Carnivores have a short intestine in order to avoid the rapidly developing putrefactive toxins in their food. Herbivores, whose food is generally more difficult to digest, have developed a number of strategies in order to be able to utilize the ingredients in the best possible way. These include a longer intestine, a multi-chambered stomach (for example in ruminants or kangaroos ) or cecotrophy , the repeated consumption of the feces in rodents and rabbits . Purely leaf-eating (folivorous) species ( e.g. koalas or sloths ) make the best possible use of their nutrient-poor food by taking extremely long periods of rest.
One form of learning behavior is imprinting ; in mammals, olfactory imprinting, i.e. sensitization to different smells, is more common than in other vertebrate groups. The embossing is often used to identify relatives, such as the mother or siblings. With experiences similar to imprinting, food preference can also be determined. Actions that have been learned can also be passed on, i.e. passed on. The prerequisite for this is life in groups with social structures. Most mammals show play behavior in their youth, some even into old age. Often there are social games with play partners in which, for example, carnivorous animals are trained to sneak up on prey or to escape in the case of ungulates. Attackers and defenders often change roles afterwards. Object games also occur in which objects are touched or set in motion.
Most mammal species are either polygynous (a male mates with multiple females) or promiscuous (males and females mate with multiple partners). Since carrying and suckling are time-consuming and energy-intensive for the females, the males could produce more young than the females can give birth. In many cases, this results in polygynous behavior in which relatively few males reproduce with many females and many males have no opportunity to mate. A consequence of this is often fierce rivalry between the males for the mating privilege and in some cases a choice on the part of the female. In many mammals, this results in complex behaviors or anatomical characteristics with regard to reproduction. Many species are characterized by a sexual dimorphism (males are often significantly larger and heavier than females), also as a result of the selection pressure of the males to improve the chance of mating.
It is estimated that three percent of all mammal species live in monogamous relationships in which a male reproduces with only a single female during the mating season. In these cases, the male usually participates at least partially in raising the young. Sometimes the mating behavior also depends on the environmental conditions: when resources are scarce, the male only mates with one female and helps with rearing, when there is plenty of food, the female can raise the young alone and the males mate with several partners.
Some species of sand graves , a group of rodents living in Africa , such as the naked or gray bull, should be mentioned . These maintain a eusocial way of life: Similar to some insects, a single female, the “queen”, is fertile in a colony and mates with several males, while the other animals, as sterile workers, perform the activities necessary to care for the group.
Ways of giving birth
The way of giving birth differs most clearly in the three subclasses of mammals.
The characteristic of the mammals is a common body opening for the excretory and reproductive organs, the cloaca . The penis of the male is exclusively sperm-carrying and split at the tip. The uterine mammals differ from all other mammals in that they are not viviparous but lay eggs . These are small (around 10 to 15 millimeters in diameter) and, with their leathery shell and large yolk, resemble more reptile eggs than bird eggs. The one to three eggs are incubated by the female for around ten days. Newly hatched mammals are naked and small and in their embryo-like state they are comparable to newborn marsupials. An example of a mammal is the platypus (Ornithorhynchus anatinus), which is native to the east coast of Australia.
The marsupials differ significantly from higher mammals in the structure of their reproductive organs. Their reproductive tract is doubled, females have two uteri and two vaginas , and the males also have a split or double penis with a scrotum in front . The gestation period is short (12 to 43 days), the record holder is the narrow-foot pouch Sminthopsis macroura with only 10.5 to 11 days. Most species do not develop a placenta , but some marsupials ( e.g. koalas or nasal sacs ) have a primitive placenta . The newborns are born through a birth canal located between the vaginae, which in many species is created specifically for birth. Newborn marsupials are small and underdeveloped compared to the higher mammals. The weight of the litter is always less than 1% of the mother's weight, the proboscis babies weigh only five milligrams and are therefore the smallest newborn mammals of all. Newborn bag mammals only have rudimentary organs, only the forelegs are well developed, as the offspring have to crawl to the mother's teats on their own.
Many, but by no means all, bag mammals have a bag in which the teats are located. The females of some species have a permanent pouch, in others it is only formed during the gestation period, in others the young hang freely on the mother's teat, only hidden by their fur or skin folds. Newborns hang on to the teat with their mouths and remain firmly attached to it for the first few weeks of life. The suckling period lasts longer compared to the higher mammals.
In the past, the marsupial mode of giving birth was regarded as a primitive method that was underdeveloped compared to that of the higher mammals. The displacement of some marsupials by introduced placentas has also contributed to this prejudice. Apart from the fact that this “progressive prejudice” towards the development of humans has been largely replaced in the modern system and that several marsupial species have very successfully expanded their range, the method of reproduction of the marsupial also offers advantages: on the one hand, the strenuous gestation period for the mother is shortened In others, a young animal can be brought into the world much faster than with placentation if the one born earlier should die.
The higher mammals, or placental animals, comprise by far most of the species. Both German names for this taxon are somewhat unfortunate: The word "higher" reflects a progress that is not sustainable in the modern system, and some marsupials also have a simple placenta.
The key feature of higher mammals is the trophoblast (the outer layer of cells in a fertilized egg). This layer represents an immunological barrier and enables long growth in the womb. Marsupials do not have a trophoblast, so the gestation period must be over before the mother's immune system becomes fully effective. The placenta of the higher mammals is characterized by the allantochorion (a villi skin ). The villi ensure more efficient nutrition for the germ.
The length of pregnancy and the number of newborns also depend on lifestyle. Nests (e.g. predators or rodents) tend to have a short gestation period and a large litter size, while those who flee the nest (e.g. artifacts and whales) have a long gestation period and a small litter size. For example, the gestation period for some hamster species is only 16 days, while for African elephants it can last up to 25 months.
The eponymous characteristic of mammals is that the female feeds the newborn children with milk , a nutrient fluid that is produced in the mammary glands . These are made up of externally definable gland complexes (“mammary complex”), each of which usually ends in a wart, the teat , also known as the nipple in humans . An exception are the mammals, where the newborns lick the milk directly from the mammary gland fields from the mother's fur. The number of gland complexes differs depending on the species and is related to the average litter size, so humans or horses only have two, whereas large tenreks have 24 or up to 32. The diet with milk is referred to as suckling or in humans as breastfeeding and is carried out for so long until the young animal is able to consume solid food.
Suckling has major consequences for young animals and females. Newborns are given a diet rich in fat and nutrients that ensures rapid growth without much effort, but in return they rely on the mother's presence. A Ammenverhalten , meaning that females suckle and foreign children, only a few species (such as lions ) are known. In most cases, suckling is accompanied by intensive brood care and a caring relationship with the young. For females, on the other hand, suckling means investing a lot of time and energy.
As different as the shape and way of life of mammals is, so is their life expectancy. In general, smaller species live less long than larger species, but bats are an exception to this pattern. While male broad-footed pouch mice consistently die around eleven months of age after they have reproduced for the first time, larger mammalian species can live for several decades. None of the species living on land comes close to the age of humans, which has now been proven to be 122 years old ( Jeanne Calment ) thanks to the improvement in medicine . In addition to humans, elephants are likely to be the land mammals with the highest life expectancy at up to 80 years of age. However, some whale species are getting significantly older, the oldest known mammal was a bowhead whale at 211 years.
Humans and mammals
Note: Although humans also belong to the mammals zoologically, they themselves will not be dealt with in the following. Instead, the relationship between humans and other mammals is discussed.
Mammals have had a decisive influence on human history. Humans have always eaten their meat and processed their fur and bones. They were used as riding and work animals; to this day they are used as milk suppliers, as guard and laboratory animals. Conversely, humans also have a formative influence on most mammal species. Some genera have increased their range dramatically in the wake of humans or have been naturalized as neozoa in foreign regions. In many cases, however, their populations have been restricted and their range drastically reduced through hunting and the destruction of their habitat. A number of mammals eventually disappeared irretrievably from the earth through direct or indirect human influence.
A number of mammal species are kept by humans for their mostly economic benefits. Animals domesticated for this purpose are called farm animals . In addition, wild animals are hunted or semi-domesticated animals are kept outdoors and later caught (examples are hat forests or cattle and horse breeding in America).
Reasons for using mammals
- One of the most important reasons for hunting or keeping mammals is to enjoy their meat , which is consumed for its protein and fat content . In the western world, beef and pork and, to a lesser extent, the meat of domestic sheep , domestic goats , domestic horses and game are common. In different cultures and regions around the world, numerous species in very different lines of development of mammals have been and are hunted for their meat, from armadillos , which are considered a delicacy in South America , to the Echidna , which are eaten in New Guinea.
- The fur and skin of various mammals are also among the resources used by humans. Sheep are sheared, the skin of cattle and other animals is made into leather , and in earlier times the hides of hunted animals were used to make clothing, blankets and much more. The fur industry is still important today ; chinchillas , mink , sable , nutrias , raccoons and many more are kept in our own fur farms . As the production of a purely luxury item , fur farming has been particularly heavily criticized by animal rights activists.
- In addition to meat and fur, other body parts were and are used by mammals. These include, among other things, antlers and bones that were used as tools and building materials, tears and whale rats of the whales , ivory and parts that are used for religious or superstitious reasons, for ceremonial reasons or as status symbols and for (at least supposed) medical reasons, such as the horn of various rhino species .
- Mammals are also kept to produce milk , with milk from domestic cattle playing the largest role worldwide at around 85%. To a lesser extent, the milk is obtained from sheep, goats, horses, house ole , water buffalo , reindeer and other species.
- Due to their size and strength, mammals are used as draft, riding or pack animals. These include horses, donkeys, cattle, water buffaloes, Asian elephants , camels and domestic dogs (" draft dogs "). Due to the motorization of agriculture and the spread of automobile traffic, this purpose has declined sharply in the western industrialized countries and is mostly only carried out as a hobby or sport. But horses are still used by the police for official purposes . In the economically less developed regions of the world, this use of animals is still widespread.
- For the same reasons, humans have used mammals for military purposes since ancient times . Until the late 19th century, the horse in cavalry formations was a prerequisite for rapid battlefield operations, which were often of vital importance. Also from ancient times to the early modern period, war elephants were used to break the enemy lines; Its use in the second Punic War by the Carthaginian general Hannibal became famous . In modern armies, mammals are used as part of special military operations, for example, during World War II, the Soviet armed forces used anti-tank dogs against German armored vehicles . More recently, for example, dolphins have been trained to deal with mines in the US military .
- Because of these properties, mammals were also used for executions from ancient times to beyond the Middle Ages, as in the Roman Empire, where convicts were killed by elephants or predators using Damnatio ad bestias . In the Middle Ages it was divided into four by horses.
- Mammals are also used in many places as hunting and guard animals, especially domestic dogs and domestic cats .
- The practice of using mammals in animal experiments is also widespread . Primates (including rhesus and squirrel monkeys ) and rodents are used for this purpose . The cognitive research and the use of animals in space flight include in a broader sense to animal testing. The controversy over the real benefits of these practices is extremely intense.
- Mammals were and are often used to entertain people, ranging from animal baiting in the Roman Empire to animal demonstrations in circuses , dolphin and seal shows to rodeos , bullfights and dancing bear performances. Since the animals are often not kept in a species-appropriate manner and the training is often associated with cruelty to animals, such practices are controversial. Even the hunt is nowadays partly entertaining, for example the fox hunt that was practiced in the British Isles into the 21st century .
- Another way of using mammals is to use their good sense of smell (for example in the form of sniffer dogs or truffle pigs ) when looking for things that cannot be technically detected.
- Mammals are also used to support the disabled. An example are guide dogs . In various types of mental disorders, dolphin therapy was used to improve the patient's condition, the effectiveness of which is controversial.
- As pets or animals to pet animals are called, which are not held in a direct economic benefit, but of joy and personal affection. Some mammal species are also or primarily kept for this purpose, including rodents such as golden hamsters , domestic guinea pigs , degus , chinchillas , mice and rats , as well as domestic rabbits . Even dogs and cats are often used today as pure pets and not because of their guard and hunting function. In the case of more exotic pets, the spectrum now ranges from chimpanzees to short-headed gliding pouches to miniature pigs . The difficult to impossible species-appropriate keeping and the transmission of diseases (in both directions) are problematic for many pet species .
- The importance of certain mammals for tourism should be mentioned at this point , for example in the African game reserves. A side effect of this practice is that the protection of animals has also gained an economic function; It is criticized that the animals are often disturbed in their natural habitat. Hunting is another variant of the tourist use of mammals. These killings, which are carried out purely as a trophy hunt , have come under heavy criticism.
For many of the reasons mentioned above, humans not only limited themselves to hunting, but also tried to keep certain animal species close to them and to breed them. The domestication of animals started at least before some 10,000 to 15,000 years ago, the house dog , however, suggest genetic studies that this process might have begun already more than 100,000 years. In the eighth millennium BC In BC wild goats , wild sheep and wild cattle , and a little later also the wild boar, were domesticated as domestic goats , domestic sheep , domestic cattle and domestic pigs . At first, livestock served primarily as a supplier of food, later animals were also used for work, for example since around 3000 BC. The house horse and the llama . The process of domestication was complex, genetic studies indicate that in many domestic animals in different regions this step took place several times independently of one another. More domesticated mammals are reindeer , camel , domestic cats , ferrets , donkey , color mouse , color rat , hamsters , rabbits and guinea pigs .
Agricultural and food pests
A number of mammals are considered to be agricultural or food pests, which means that they either feed directly in the areas used for food production or in storage places of the food produced by humans. Due to the large-scale introduction of agricultural land, there is an oversupply of food for some animal species, which results in their strong reproduction and thus further damage. This trend can be observed especially in developing countries. The most well-known food pests in this country include mice , especially the house mouse and rats such as the house or brown rat , which have joined humans as cultural followers and have achieved worldwide distribution. Some animals (including fruit bats and numerous species of rodents) feed directly on the crops, while others cause damage to the roots through their underground way of life. The livestock industry sees carnivorous animals, especially predators, as a food competition, at least two species, the Falkland fox and the pouch-wolf have become extinct due to hunting. Similarly, the fishery sees seals and other fish-eating mammals as an economic threat and pursues them.
The extent of the real threat posed by animals known as "pests" is uncertain and can often be exaggerated. Often humans are the main cause of this, as they massively intervene in the natural habitat of animals. The conversion of habitats into agricultural areas and the reduction in the food supply force many species to find new sources of food. These are then in competition with the economic interests and initiate the persecution. Nevertheless, these "pests" are hunted down with excessive huntings, poisoning and other methods, which often has fatal effects on the population.
Direct threat to humans
Humans are also sometimes exposed to direct threats from mammals. Anchored in the consciousness are predominantly the cases of large man-eating predators , with the tiger in particular enjoying a reputation as “man-eater”. However, killings by predator bites are limited to a few individual cases per year. Mammals, however, are far more dangerous as disease carriers. 40,000 to 70,000 people die of rabies every year , most of them in underdeveloped countries. The main cause of transmission is the bite by infected animals such as dogs , cats , badgers , raccoons and bats . Another notorious disease is the plague , which is transmitted, in rare cases, directly by fleas that parasitize house rats and other rodents. Plague epidemics and pandemics killed millions of people, with the pandemic known as the Black Death in the mid-14th century killing an estimated one third of people in Europe.
Cultural and historical significance
Many mammals play an important role in cultural history. Noticeably large, strong or dangerous animals serve as heraldic animals , totem or clan symbols . As “ sacred animals ”, some species are regarded as manifestations of gods and enjoyed special protection, such as sacred cows and Hanuman langurs in India or cats and jackals in ancient Egypt . On the other hand, some mammals were seen as representatives of demonic powers, such as bats and cats. Stereotype notions of properties of certain species such as the stubborn donkey or the cunning fox can be found in countless stories and fairy tales and shape partly still the dirty word treasure .
Human Threat and Extinction
Through diverse interventions in nature, humans are responsible for the population decline or the extinction of many mammal species. To what extent hunting is to blame for the extinction of numerous large mammals at the end of the Pleistocene (50,000 to 10,000 years ago) is controversial, this extinction correlates at least in part with the global spread of humans (see also the point under development history ). From reports and representations, at least a clear decline in the range of distribution for numerous species can be deduced since ancient times . The situation today is also worrying for many mammal species. A commission of around 1,700 scientists from 130 countries under the leadership of the International Union for Conservation of Nature (IUCN) came to the conclusion that today at least 20-25% - but possibly up to 36% - of all land and marine mammal species are threatened with extinction. The IUCN lists 514 species, i.e. around 10%, as critically endangered or endangered . In total, at least 1,141 of the currently 5,487 mammal species are acutely threatened. Three species, the Przewalski horse , the saber antelope and the black-footed antelope , are considered to be extinct in the wild , which means that there are only stocks in human breeding programs. The reasons for the endangerment of numerous species lie mainly in the increasing loss of habitat due to conversion into agricultural areas and settlements, in environmental pollution and in hunting, since many species are considered useful or harmful. Another factor is the damage to the natural balance caused by the intentional or unconscious introduction of neozoa . Persecution by feral domestic cats and dogs, as well as food competition from mice , rats , rabbits and others, are a major problem, especially in regions where these species were not naturally native (such as Australia or many islands).
The reasons mentioned above have led to the fact that, according to the IUCN, 73 species of mammals have become extinct in the last few centuries, including the pig's foot-nosed bucket , four types of kangaroo , the thylacine , the Falkland fox , three types of gazelle , the bluebock , the Steller manatee , twelve species of bats and numerous Rodents like many tree rats and giant hatias . It is to be expected that this list will become even longer in the next few years.
Systematics and history of development
The mammals are probably - contrary to different theories that were widespread in the middle of the 20th century - a monophyletic group: They all descend from a common ancestor and also include all descendants of this ancestor. The three subgroups, mammals, marsupials, and higher mammals , are also each monophyletic taxa . Most systematics summarize the sac and higher mammals to the taxon Theria and contrast this with the primaries. However, some researchers are of the opinion that the mammals evolved from the marsupial.
The picture becomes much more confusing when fossil taxa are included in the family tree. In addition to the usual differences of opinion among scientists, only teeth and jaw parts were found from numerous species. The detailed examination of the teeth is therefore one of the key criteria for determining the evolution of mammals.
It is undisputed that the mammals developed from the synapsids , a group of reptiles that was characterized by a single skull window and that had its heyday in the Permian age. Within the synapsids, the therapsids developed , the so-called "mammalian-like reptiles", which already showed some of the mammalian features such as differentiated teeth and possibly body hair. A group of therapsids were the cynodontia , which were characterized among other things by an enlarged brain and a special jaw shape. The mammals and their close relatives are summarized in the taxon of the Eucynodontia , the most famous representative of which was Cynognathus . The sister taxons of mammals are either the Tritheledontidae , a group of very small, carnivorous animals, or the Tritylodontidae , a group of herbivores up to 1 meter long. Certain anatomical features speak for each of the two groups, but the majority of researchers prefer the Tritheledontidae.
Mammals in the broader sense
It is controversial which animal is to be rated as the oldest mammal. Some animals show a transitional status between reptiles and mammals in the structure of the ear, the lower jaw, the temporomandibular joint and the teeth , which is why some researchers refer to them as Mammaliaformes, i.e. "mammalian" or Proto-Mammalia, and do not yet classify them as mammals in the actual sense ( sensu stricto ), others continue to grasp the mammals ( sensu lato ) and already add them to it.
|Systematics of the mammalian tribe|
|Phylogenetic system of the mammalian tribe group (Mammaliaformes)
according to Krause et al . 2014 and Shundong Bi et al. 2014:
- According to some sources, Adelobasileus cromptoni is the oldest known mammal. Portions of the late Triassic skull were found in Texas. The structure of the ear suggests that this animal represents at least a transitional status from the cynodontia to the mammals. Without further fossil finds, however, the taxonomic status of Adelobasileus cromptoni can hardly be determined more precisely.
- Also sinoconodon is sometimes referred to as the oldest mammal. Relatively well-preserved fossils of this species have been found in China; the animal lived in the early Jurassic period and already showed the characteristics of today's mammals in its jaw structure. Other factors, such as multiple teeth change , combined with lifelong growth of the skull, are still reptilian characteristics .
- The Morganucodonta were a group of shrew-like , around 10 centimeters long, presumably insectivorous animals, which are documented from the later Triassic to the Middle Jurassic and were found in numerous regions of the world. The most famous representatives were Morganucodon and Megazostrodon . In the structure of the lower jaw and the teeth (the molars are characterized by three conspicuous tips) they correspond to modern mammals, the most important difference being the double jaw joint.
- The Docodonta , the best-known representatives of which are the genera Haldanodon and Docodon , are considered to be more "mammal-like" than the Morganucodonta. They are characterized by strongly widened molars, which enable effective chewing, but still show similarities with their reptilian ancestors in the temporomandibular joint. Docodonta were widespread from the Middle Jurassic to the Early Cretaceous Period , the assignment of a find from the Upper Cretaceous ( Reigitherium ) is doubtful.
- Hadrocodium wui , the remains of which were found in China from the lower Jurassic period, is regarded as the sister taxon of the “real” mammals, sometimes it is also referred to as the “first” mammal. It was a tiny animal, probably weighing only 2 grams, but already had a secondary TMJ and an enlarged brain. The differences to the mammals lie in the details in the structure of the teeth and the lower jaw.
Mammals in the strict sense
|Systematics of the basal mammals|
|Phylogenetic systematics of basal mammals
according to Krause et al . 2014 and Shundong Bi et al. 2014:
The mammals in the narrower sense (Mammalia sensu stricto), in contrast to the mammals in the broader sense or Mammaliaformes (see above), are defined as the group that includes the last common ancestor of all modern mammals and their descendants. This taxon has been documented at least since the middle Jurassic , but the history of development within this group is highly controversial.
- It is uncertain when the ancestors of today's ursal mammals ( platypus and ant urchins ) split off from the evolutionary line of other mammals. Today the view that the mammals developed independently of the other mammals from their own branch of the cynodonta is largely rejected. A new, but controversial theory places these animals in a taxon called Australosphenida , whose representatives have spread in what was then the southern continent of Gondwana since the middle Jurassic . Other theories see them as an isolated side branch that separated from the rest of the mammals early on. Yet other researchers put the ursal mammals in a close relationship to the marsupial mammals. In any case, the earliest finds that can be assigned to an ancestor of the ursal mammals originate from the Cretaceous period.
- The Multituberculata formed a species-rich group of animals that owes their name to the numerous tips of their molars. Often outwardly similar to rodents , the earliest representatives are documented since the Middle Jurassic. The Multituberculata survived the extinction of the dinosaurs and did not become extinct until the Oligocene . The evolutionary position of these animals is controversial, some authors even suspect that they merely developed a convergence to the mammals, which arose from another branch of the cynodontia . Today, however, the majority are viewed as a sideline within the Mammalia, whose position in the family tree is, however, uncertain.
- As allotheria a group is referred to, in addition to the multituberculata haramiyida - a group probably herbivorous animals from the Upper Triassic and Jurassic - and gondwanatheria - in the Cretaceous and Paleocene in Gondwana includes - lived. This assignment is based mainly on the similarities in the construction of the molars, but is controversial as the Haramiyida have some primitive features and possibly represent a sideline that arose much earlier.
- The Eutriconodonta summarize several groups of mammals, which are characterized by three-humped molars. These include the Amphilestidae from the Middle Jurassic to Lower Cretaceous, the Gobiconodontidae from the Lower Cretaceous (which also includes the newly discovered Repenomamus giganticus , a dog-sized predator), as well as the Triconodontidae , which lived from the Upper Jurassic to the Middle Cretaceous. However, it is controversial whether these groups were really closely related.
- The taxon within the real mammals without the groups listed above is referred to as Holotheria , although some systematics include some groups of the Eutriconodonta. The Holotheria include Kuehneotherium and related species, the Kuehneotheria , which are characterized by advanced tooth structures and primitive jaw features. Many authors see a much more original genus in Kuehneotherium , so that the term Holotheria is controversial.
- Trechnotheria refers to a taxon within the Holotheria, which divides into some groups that are only covered by sparse tooth and jaw finds, such as the Spalacotheroidea and the Cladotheria.
- Within the Cladotheria there was a division into the Dryolestida , which lived in the Upper Jurassic and the Cretaceous Period, some other side branches, and a taxon called Boreosphenida or Tribosphenida.
- The terms Tribosphenida (McKenna 1975) and Boreosphenida (Luo et al., 2001) denote a very similar taxon, which is identical with the exception of a few species. In addition to a few side branches, this group includes the Theria in the proper sense.
- As Theria the taxon is referred to, the last common ancestor of the marsupial (Metatheria) and higher mammal comprising (Eutheria) and all its descendants. The oldest known representatives of both taxa come from the Lower Cretaceous (around 125 million years ago), in the case of the marsupial mammals this is Sinodelphys szalayi , in the case of the higher mammals Eomaia scansoria .
Common characteristics of Mesozoic mammals
In general, the mammals of the Mesozoic Era were small, most of them only the size of mice or rats . In most species, the teeth indicate a diet consisting of insects and other invertebrates, and the shape of the brain and sense organs indicate a mainly nocturnal way of life. The question remains why the majority of Mesozoic mammals remained relatively uniform in size, body structure and way of life, especially since there was enormous radiation in size and diet in a very short evolutionary period (around 5 million years) after the beginning of the Cenozoic . In general, this question is answered with the competition from the dinosaurs, which, as long as they existed, prevented larger mammals due to the selection pressure exerted. This view is sometimes questioned: Due to the enormous size difference and the different way of life with the dinosaurs, which were presumably diurnal, there could have been at least a number of medium-sized mammals. Various physiological limitations have therefore been postulated, for example an inadequate ability to cool the body temperature or the not yet fully developed chewing and digestive systems.
More recently, however, there have been some new finds that point to a higher specialization of Mesozoic mammals. Such was castorocauda at least partially water-inhabiting volaticotherium equipped with Gleitmembranen and fruitafossor shows a hint of anteaters adaptation to a insectivorous life. Finally, repenomamus , who lived in the Lower Cretaceous in China, reached a length of over 1 meter and its weight is estimated at 12 to 14 kilograms. It is the largest mammal known from the Mesozoic Era and has also fed on small dinosaurs.
Further development in the Cretaceous Period
The marsupials were, apart from isolated finds in East Asia, restricted to North America. One of the oldest groups still in existence today is the opossum , whose ancestors are known from this time.
The higher mammals split up into the superordinate orders determined today by molecular genetic investigations ( secondary articulated animals , Afrotheria , Laurasiatheria , Euarchontoglires ), which was promoted by tectonic shifts, including the breakup of Gondwana . However, these splits are mainly substantiated by molecular genetic calculations, fossil finds of higher mammals from the Upper Cretaceous are very rare and so far only proven from North America and East Asia. The best-known genera of this era include Asioryctes , the Leptictida , which are possibly ancestors of the insectivores , the Zalambdalestidae (possible ancestors of the rodents ), the Zhelestidae (possible ancestors of the " ungulates ") and Cimolestes (possibly an ancestor of the predators ). In general, however, the assignment to today's taxa is controversial; mammals that are undoubtedly related to today's species did not appear until the Paleocene .
Development in the Cenozoic
With the extinction of the dinosaurs , many ecological niches were vacated, which were occupied by a multitude of newly emerging groups of mammals. During the Cenozoic era , mammals became the dominant group of vertebrates on land. The current order emerged, whereby the history of development was by no means straightforward, but was shaped by evolutionary dead ends, displacement processes and once again completely extinct groups of mammals. The lines of development in some groups (for example horses or proboscis ) are relatively well documented and researched by fossil finds. South America , which was separated from other continents during most of the Cenozoic era, played a special role . Due to the island location, many species penetrated into ecological niches and a unique fauna developed, among others with Sparassodonta ("pouch hyenas"), a group of carnivorous marsupials, with the Paucituberculata , a group of marsupials with many shapes that still live on and with in the Mausopossums the South American ungulates (Meridiungulata). After the creation of the Central American land bridge, mammals invaded from the north and largely displaced the native species.
Most of the mammal orders have been recorded since the Eocene , including the ancestors of the most specialized groups, the bats and whales . In the same period the first gigantic forms such as Uintatherium were formed ; this development culminated in Paraceratherium (also known under the name Baluchitherium or Indricotherium ), the largest land mammal known at 5.5 meters shoulder height and 10 to 15 tons in weight.
The mammals reached their greatest biodiversity in the Miocene ; since then the climatic conditions have deteriorated continuously, right up to the ice ages of the Pleistocene . The climatic shifts, combined with human influences, have since led to a decline in biodiversity.
Extinction of large mammals at the end of the Pleistocene
At the end of the Pleistocene (50,000 to 10,000 years ago) there was mass extinction of large mammals worldwide. With the exception of Africa and southern Asia , all species weighing over 1,000 kilograms and 80% of all species weighing 100 to 1,000 kilograms died out. In Australia this process took place around 51,000 to 38,000 years ago, where diprotodons (rhinoceros-sized marsupials), marsupial lions ( Thylacoleo carnifex ), and giant kangaroos up to 3 meters high (genus Procoptodon ) disappeared . In Eurasia this process extended over a longer period of time, from 50,000 to 10,000 years ago, and reached its climax with the end of the last glacial period . The animals that became extinct in Europe around 10,000 BC include the woolly mammoth ( Mammuthus primigenius ), the woolly rhinoceros ( Coelodonta antiquitatis ), the giant deer ( Megaloceros giganteus ), the steppe wisent ( Bos priscus ), the cave lion ( Panthera spelaea ) and the cave bear ( Ursus spelaeus ). In America, the extinction was in a tight timeframe (around 11,000 to 8,000 years ago), here disappeared among other things, the mammoths , the American mastodon and other trunk animals , saber-toothed cats , giant sloths and giant armadillos ( Glyptodontidae ).
The extent to which climatic changes or human hunting ( overkill hypothesis ) are the main culprit is still a matter of debate. The fact that the time of extinction coincides at least in part with the global spread of humans and that in none of the earlier phases of extinction could such a reduction in size be observed speak in favor of hunting. The climatic processes at the end of the last glacial period should also have contributed to an increase in the number of species, as can usually be observed in warmer periods. Proponents of the hunting hypothesis also cite an analogous process on islands that were only colonized later. In Madagascar , where people have only lived for around 1500 years, the hippos and numerous large primate species , including the giant lemurs Megaladapis, disappeared in the centuries that followed . Opponents of the hunting hypothesis claim that the primitive hunting methods used by early humans could not have had such a great impact on population size, and point to Africa, where humans have been around for much longer and where there has been no significant mass extinction. The climatic changes were also so complex that a large number of factors had to be taken into account.
Recently, the theses that a mixture of both factors is to blame for the mass extinction have increased. For the populations already affected by climatic changes, the hunt was the decisive point for the extermination. Ecological factors may also have played a role: for example, the decimation of large grazers led to the expansion of forests, which had a fatal impact on the remaining populations. Other researchers also partially blame the extensive slash and burn operations.
In this discussion, however, not only the purely scientific aspect plays a role, but also the anthropological component, depending on whether this mass extinction is the last of a long series of natural extinction processes in nature or the first of many destructive human interventions in its own Environment.
Then a somewhat simplified cladogram of the land vertebrates, followed by more detailed descriptions of any uncertainties and points of contention.
|Land vertebrates (Tetrapoda)||
The mammals are divided into three subclasses with around 25 to 30 orders, which in turn can be assigned to two or four higher-level groups in the case of the marsupial and higher mammals. A more detailed system with all families can be found under systematics of mammals .
Subclass of mammals (Protheria)
- Order monotremes (monotremes)
Subclass marsupials (Metatheria)
- Superorder Ameridelphia
- Superorder Australidelphia
Subclass Upper Mammals (Eutheria)
- Superordinate Afrotheria
- Superordinate joint animals (Xenarthra)
- Superordinate Euarchontoglires
- Parent Laurasiatheria
Some remarks on this system:
- The Ameridelphia are considered as a paraphyletic group.
- Artifacts and whales are often grouped together to form a common order ( Cetartiodactyla ), as the whales have evolved from the ungulates, which without this group would be a paraphyletic group
- The " ungulates " (ungulata) are no longer a systematic group in this system, but rather combine different, not closely related taxa. This classification is controversial.
- The animals referred to here as Tenrek-like (Afrosoricida) were previously assigned to the insectivores, but according to widespread belief they have only developed convergent to them.
- In some classifications the bats are placed in a close relationship to the primates, sometimes they are also regarded as two merely convergent taxa, fruit bats and bats . According to more recent studies, however, both are considered to be incorrect.
- The classification of the elephants, the aardvark and the pangolin has long been controversial, but genetic studies have shown that they belong to the above-mentioned groups.
Extinct mammalian orders
The family tree shown under systematics is partly based on molecular genetic analyzes. Since this is not possible with extinct animal groups, it is difficult to classify them in the system. Existing systems, such as that of Malcolm C. McKenna and Susan K. Bell, which contain both living and extinct mammalian orders, partially contradict each other with the systematic chosen here. Therefore, the extinct mammal orders of the marsupial mammals ( Metatheria ) and the higher mammals ( Eutheria ) are listed separately.
Extinct orders of the marsupial mammals:
Extinct Orders of the Higher Mammals:
- South American ungulates (Meridiungulata) with Litopterna , Notoungulata , Astrapotheria , Pyrotheria and Xenungulata
- Dinocerata (with Uintatherium )
Older orders of mammals that belong neither to marsupials nor to higher mammals are listed above under mammals in the narrower sense .
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