Platypus ( Ornithorhynchus anatinus )
|CL Bonaparte , 1838|
The monotremes (monotremes, from gr. Μόνος monos "single", "unique" and τρῆμα trema "hole", "opening"), formerly fork Animals called, are an order of mammals (Mammalia). Monotremes are the only representatives of the subclass of the mammals (Protheria). They differ from all other mammals in that they do not give birth to living offspring, but lay eggs . This taxon is divided into two families: the Echidna (Tachyglossidae) and the platypus (Ornithorhynchidae) - a total of five recent species that only live in Australia and New Guinea .
Monotremes are counted as mammals despite their oviparous mode of reproduction, as this group is defined by other characteristics that these animals all have. This includes the fur , suckling the offspring with milk and a number of anatomical details, including the structure of the temporomandibular joint and the three ossicles . In other characteristics, including the structure of the skull and the musculoskeletal system, as well as in the excretory and sexual tract , they differ significantly from the other mammals.
In terms of physical structure, the two families show different adaptations to the respective living space and way of life. The aquatic platypus (living in water) has a water-repellent fur , a streamlined body with a flattened oar tail and webbed feet, while the ant urchins that live on land have paws with grave claws and are protected against predators by spines on their back and flanks. Monotremes reach a head body length of 30 to 77 centimeters. Their weight varies from 1 to 3 kilograms for the platypus and up to 16 kilograms for the long-billed hedgehog .
Monotremes are homoiothermic , so their body temperature is the same. However, at 30 to 32 ° C, it is well below the temperature of other mammals. The thermoregulatory abilities of monotons are more limited than that of other mammals.
Skull and teeth
Their skull is elongated, the snout is covered with a leather-like cover and is reminiscent of a bird's beak . The monotons lack whiskers (vibrissae), but their beak is equipped with extremely sensitive electroreceptors . With their help, these animals can perceive the weak electrical fields that are created by the muscle movements of their prey and thus locate and prey on them. The eyes are small, auricles are only present in the long-beaked ant urchins, they are absent in the short-billed ant urchins and also in the platypus they are regressed as an adaptation to the aquatic way of life.
In the construction of the skull , monotons show a number of specific anatomical details. This applies, among other things, to the construction of the side wall of the skull, which, in contrast to other mammals, is largely formed by the petrous bone , which is also reflected in a different arrangement of the masticatory muscles and the cranial nerves . Cover bones such as the lacrimal bone and the parietal bone are missing in the facial skull . The (secondary) temporomandibular joint corresponds to that of the other mammals, the lower jaw also consists of a single bone; However, this is very slim and the muscle attachment on the ascending branch ( coronoid process ) has receded.
Adult monotons no longer have teeth , but the hatchlings still have an egg tooth , which they use to break through the egg shell. This toothlessness is not an original feature, however, as the fossil ancestors from the Cretaceous and early Cenozoic era still had teeth. Echidnas are completely toothless, which is typical of ant-eating mammals and in convergent form, for example in the anteaters and pangolins finds. In contrast, during the development of the platypus, dental appendages appear, with two molars erupting. However, they are already worn out before the animals are fully grown and are replaced by horny chewing plates.
Remaining skeleton and locomotion tract
Like most mammals, they have seven cervical vertebrae , but in contrast to other mammals, they have cervical ribs . The shoulder girdle of monotons is massive and creates a stable connection between the front legs and the trunk. In addition to the shoulder blade and collarbone common in mammals , it also has some elements that are otherwise only found in reptiles and birds. These include the coracoid (raven bone) and the interclavicle ("intermediate collarbone"), which firmly connects the collarbone with the breastbone (sternum).
A feature that the monotremes share with the marsupial mammals are the sac bones ( ossa epubica ), two bones protruding forward from the pubic bone of the pelvis . Originally, these bones probably had nothing to do with reproduction, as they occur in both sexes, but rather served the muscle attachment for the movement of the hind limbs.
Another special feature is the position of the limbs , as the upper arm and thigh are kept almost parallel to the ground, which causes reptile-like locomotion. However, the stomach does not drag on the floor, which means that you can run quickly. The limbs of the animals are short and strong and each end in five toes, which are webbed in platypus and grave claws in anthers. A characteristic is the poison apparatus, which occurs only in male animals. This consists of a gland in the thigh, an outlet duct and a horn sting about 1.5 centimeters long that is fixed to the calcaneus . The effectiveness of the poison is not exactly known, there are reports that a domestic dog that was supposed to retrieve a shot platypus died from it. Presumably it is also used in rival fights for the mating privilege, but the poison is fatal for conspecifics in the rarest of cases. It is generally less effective on ant urchins; in female monotons the poison apparatus has regressed.
Internal organs and reproductive tract
The female reproductive organs , ureters and intestines flow into a single opening, the cloaca , a perineum region , which is missing. The monotremes owe their German and scientific name to this feature (Monotremata means "Einlochtiere"), even if other mammals - such as the tenreks - also secondary to the formation of a cloaca. The structure and function of the internal organs ( heart and blood circulation , respiration and digestion ) differ only slightly from the other mammals. Special features can be found in the structure of the larynx , in which the thyroid cartilage is made up of two separate pairs of arches, and in the stomach . This has no glands, so chemical digestion is restricted to the small intestine .
Males have a penis that is split at the tip and that only carries semen - urination occurs through the cloaca. When not erect, the penis rests in a pouch in front of the cloaca. One scrotum (scrotum) is missing, the testicles are located within the abdominal cavity near the kidneys . Females have paired ovaries , but in platypus, as in birds, only the left ovary is functional, the right one is regressed. The ovaries are connected to a uterus via a fallopian tube , these open into the urogenital canal, which leads to the cloaca.
Like all mammals , female monotons have mammary glands , but in contrast to other mammals, the teats are missing , instead the milk is secreted through numerous openings onto the mammary gland area ( areola ) of the abdomen, from which the young animals ingest it. Echidna, but not platypus, form a breeding bag ( incubatorium ) on the abdomen during gestation , in which the eggs are incubated and the hatchlings are kept after their birth. The brood pouch has no connection with the pouch bones and its structure also differs significantly from that of the marsupial .
A peculiarity of monotons is the set of sex chromosomes , which consists of five and not one pair of chromosomes: In the platypus it was discovered in 2004 that the females have ten X chromosomes and the males 5 X and 5 Y chromosomes, while most of the others Mammalian species (including humans) only have two (XX in females and XY in males). The research on the Echidna Echinacea is still ongoing, but it can be said that the chromosome system of these animals is in some respects similar to that of birds , which is believed to have evolved independently of that of mammals.
Way of life
The two families of monotons, platypus and echinids , have adapted to different habitats and thus show considerable differences in their way of life. Here only a rough overview is given, for more detailed information see the respective articles.
Platypuses lead an aquatic (water-related) way of life, their habitat are standing or flowing freshwater systems in eastern and southeastern Australia . With their webbed feet and paddle tails, they are well adapted to this habitat, they retreat to rest in earthworks, which are mostly on the embankment. Echidna, in contrast, are terrestrial (land dwelling). They do not make any special demands on their habitat and can be found in desert regions and tropical rainforests as well as in mountains at over 4000 meters above sea level. In general, monotremes tend to be crepuscular or nocturnal, but the time of activity of the ant urchins also depends on the climate and food. It is known from both families that they fall into a torpor (cold rigidity) in cool weather and correspondingly little food supply . All monotons live solitary outside of the mating season; they are loyal to their location and no pronounced territorial behavior is known.
These animals are carnivores . The platypus eats crustaceans , insect larvae and worms , which it prey on underwater. Echidna feed primarily on ants , termites and earthworms . As mentioned above, they use electroreceptors on their beak to locate the weak electrical fields that are created by the muscle movements of their prey.
The monkeys' predators include colored monitors , carpet pythons and introduced predators such as the dingo and the red fox . Echidna can become wedged in the ground in the event of an attack or, like hedgehogs, curl up into a ball of prickles; It is not known exactly whether the platypus defends itself against predators with its poisonous sting or whether it is primarily used to fight for the mating privilege.
Courtship and mating behavior
Corresponding to their solitary way of life, the monotons have a complicated courtship and mating behavior . So-called "free march columns" are known from the ant urchins, several males often follow a female for weeks and lose up to 25% of their body weight. When the female signals that she is ready to mate, the males dig a real "mating ditch" around the female and then try to push each other out of it until the strongest animal finally comes into play. "Unlike most other mammals, mating is performed belly against belly".
The courtship behavior of platypus is less complex; among other things, the male grabs the female's tail with his beak and they sometimes swim in circles for days. Mating itself takes place in the monotremes in a similar way to the usual way for mammals: the male inserts his penis into the female's cloaca.
Germ development and incubation
The follicles differ from those of other mammals in the absence of secondary and tertiary follicles ; the egg cell is surrounded by a single layer of follicular epithelium. The fertilization takes place in the fallopian tube instead, from then migrates the zygote in the uterus on. There the egg continues to grow, and the outer, parchment-like egg shell is also given off by the uterine glands. For platypus there are 12 to 14 days between mating and oviposition, and three to four weeks for anthers. The eggs of the monotremes are small, they are around 10 to 15 millimeters in diameter, have a whitish or cream-colored shell and a large yolk . While Echidna usually only lay one egg, the platypus has up to three. After laying, the female incubates the eggs for around ten days. The mother platypus uses an earth structure padded with plant material for incubation.
For the purpose of incubation, female ant urchins develop a brood pouch in the abdominal area in which the temperature is 2 ° C above the mother's body temperature and in which the young animal is incubated. "The cloaca protrudes so far that the mostly one or two eggs can be deposited directly in the incubator, which only gapes from the middle of the abdominal wall during the incubation period."
Hatching and rearing
The degree of maturity of newly hatched monotremes corresponds roughly to that of the marsupial . Hatchlings are hairless, around 15 millimeters long and weigh 300 to 400 milligrams. The front limbs are already strong when hatched, the rear only weakly developed. After hatching, the young animals are fed with milk like all mammals ; its composition largely corresponds to that of the other mammals. Since the females have no teats , the young lick or slurp the milk from the mammary gland on the mother's belly.
Young ant urchins stay in the incubator for around seven to eight weeks after hatching. As soon as their spines grow, they have to leave it. The mother then lays them in a nest and returns every five to ten days to suckle them. Young platypuses remain in their mother's den for around five months. The males of the monotremes do not take part in rearing young. The platypus is weaned at around three and a half months, and the ant urchins at around seven months. Sexual maturity occurs at one to two years.
While anthers are relatively long-lived and can reach an age of 50 years in human care, the average life expectancy of platypus in the wild is estimated at five to eight years, with a maximum age of 17 years in captivity.
Today's species of monotremes are divided into two families with three genera and five species:
- The platypus ( Ornithorhynchus anaticus ) is the only recent member of the platypus family (Ornithorhynchidae).
- The family of the Echidna (Tachyglossidae) consists of two genera, the short-billed urchin ( Tachyglossus aculeatus ) and the three species of the long- billed urchin ( Zaglossus ).
Fossil ancestors of the monotremes have been documented since the Cretaceous period for around 120 million years. The basic representative of this group is the Kollikodon , which is classified in its own family, the Kollikodontidae . Platypus ancestors have also been documented since the Cretaceous, including the genera Steropodon and Teinolophos . However, these still had teeth, as did the Obdurodon genus known from the Miocene .
The discovery of a single tooth from a platypus relative called Monotrematum sudamericanum from the Paleocene , which was made in Argentina , is the only evidence so far of a spread of these animals outside of Australia. From a paleobiogeographical point of view , South America was settled via the Antarctic continent , which was connected with Australia until the Eocene and had a much warmer climate at that time. Probably there were monotons on this continent as well, even if no fossils have been found so far .
Fossil ant urchins have been documented since the Pliocene; these can be classified in the current genera Tachyglossus and Zaglossus . Some of the animals reached larger dimensions than they do today, and Zaglossus hacketti was the largest ant urchin found to date. It was about one meter long and 30 kilograms in weight and lived in the Pleistocene in Western Australia until about 15,000 years ago.
The phylogenetic origin of the monotons is controversial. It used to be assumed that they developed from a different branch of the Therapsids ("mammal-like reptiles") than the other mammals, this is hardly represented today. Most researchers believe that they have a common ancestor with the rest of the mammals, making them a monophyletic group.
Where the monotremes separated in the mammalian family tree and with which extinct taxa they are more closely related, however, is controversial. There are several theories about this, see also evolution of mammals .
- A now outdated concept divided the mammals into two taxa, the Theria ( marsupial mammals and higher mammals ), and the primordial mammals in the broader sense (Prototheria), to which, in addition to the monotremes, a number of primitive groups such as the Morganucodonta , the Docodonta , the Triconodonta and others belong. This theory is hardly widespread any more, today the monotremes are considered more closely related to the other mammals than many primeval groups of mammals, which makes the "Prototheria" a highly paraphyletic group.
- Another hypothesis put the monotremes in a close relationship to the Multituberculata , a once species-rich group of mammals spread from the Jurassic to the Oligocene , which outwardly showed similarities with the rodents . Most scientists today, however, take the view that the Multituberculata represent a branch of the mammals that is alien to the monastery.
- In 2001 Luo and others introduced the Australosphenida taxon . According to this hypothesis, they would have developed as an independent group of mammals in the Mesozoic era in what was then the southern continent of Gondwana . This theory is based on some newly discovered fossil finds whose teeth are believed to have similarities to the teeth of platypus ancestors. Other scientists, however, vehemently deny this theory, they see in the monotone an isolated side branch.
- All of the above hypotheses contrast the monotremes with the theria , a taxon from sacral and higher mammals. But there are also hypotheses that assume a common ancestor of the monotonous and marsupial mammals. This taxon is called Marsupionta and thus forms the sister taxon of the higher mammals (Eutheria). The Marsupionta thesis is, however, a minority opinion, most researchers see in the monotremes the sister taxon of the Theria.
There is no generally accepted thesis about the history of development and the relationships between monotons and other mammals. The differences of opinion can also be explained by the fact that only teeth or jaw parts were found of many fossil species, which makes an interpretation difficult. New fossil discoveries may bring more clarity to this issue.
Monotons and humans
At first, Europeans reacted with astonishment and skepticism to reports about these animals, which seem to combine characteristics of different taxa. The platypus was initially mistaken for a fake, the work of a skilled taxidermist. And the English name of the Echidna , Echidnas, is derived from the Greek mythical figure Echidna , who was half human and half snake. The English zoologist George Shaw provided the first scientific descriptions of both animals at the end of the 18th century , which were astonishingly accurate. However, their true mode of reproduction was not recognized until the 19th century. Even the well-known German zoologist Alfred Brehm assumed in his work Brehms Tierleben that monotremes are viviparous; he dismissed anything but “fables, some of which owed their origin to the reports of the natives” and, following the example of the Australian zoologist George Bennett, came to the conclusion: “Nowhere was anything to be found which could lead to the assumption that the Boys come out of eggs, and the eggs would have been carried away by the old ones. One could no longer be in doubt that the beak-billed animal gives birth to living young. Bennett does not believe that the natives have ever seen the mother suckling, and so excuses her for her false account of laying eggs. "
It was not until the end of the 19th century that it was confirmed that monotremes actually lay eggs. To this day, laypeople consider them the epitome of biological curiosities and are often counted among the so-called " living fossils ".
Friedrich Engels confessed in a letter to Conrad Schmidt in 1895 that he "had seen the platypus' eggs in Manchester in 1843 and, with haughty narrow-mindedness, mocked the stupidity as if a mammal could lay eggs, and now it has been proven!" The theory of value by Karl Marx asked whether “the terms prevailing in natural science are fictions because they by no means always coincide with reality? From the moment we accept the theory of evolution , all of our concepts of organic life only approximately correspond to reality. Otherwise there would be no change; on the day when concept and reality absolutely coincide in the organic world, development comes to an end. The term fish includes aquatic life and breathing with gills; how do you plan to go from fish to amphibian without breaking through this term? And it has been breached, and we know a number of fish that have developed their air bladder into their lungs and can breathe air. How do you plan to move from the egg-laying reptile to the mammal bearing living cubs without bringing one or both of the terms into conflict with reality? And in reality we have a whole subclass of egg-laying mammals in the monotremes ”, which is why his reader“ should not do the same to the concept of value, which is why [he, Friedrich Engels] had to ask the platypus for forgiveness afterwards! "
The threat situation of the individual species is different. While the short-billed urchin is common and common across much of Australia, the long-billed urchins that live in New Guinea are threatened because their meat is considered a delicacy and they are often hunted with dogs. The platypus , which used to be hunted for its fur, is now fully protected; Because of its high demands on the habitat in Australia, it is considered "common but vulnerable".
- Ulrich Zeller: Monotremata (Prototheria). In W. Westheide and R. Rieger: Special Zoology. Part 2: vertebrates or skulls. Spektrum Akademischer Verlag, Munich 2004, ISBN 3-8274-0307-3 , pp. 471-484.
- Bernhard Grzimek: Monotremes. In: Grzimek's animal life. Encyclopedia of the Animal Kingdom. Volume 10, Bechtermünz, Augsburg 2001, ISBN 3-8289-1603-1 .
- Malcolm C. McKenna, Susan K. Bell: Classification of Mammals: Above the Species Level . Columbia University Press, New York 2000, ISBN 0-231-11013-8 .
- Ronald M. Nowak: Walker's Mammals of the World . 6th edition. Johns Hopkins University Press, Baltimore 1999, ISBN 0-8018-5789-9 (English).
- Drawing of the anatomical relationships of the reproductive organs in monotremata after David Peters: From the Beginning: The Story of Human Evolution. William Morrow & Co (1876) 
- For comparison, the drawing of the anatomical relationships of the reproductive organs in Marsupialia and Placentalia . 
- Urania Tierreich, 1st ed. 1969, Bd. Mammals, p. 23.
- Urania Tierreich, 1st ed. 1969, Bd. Mammals, p. 23
- Zhe-Xi Luo, Zofia Kielan-Jaworowska, Richard L. Cifelli: In quest for a Phylogeny of Mesozoic mammals. In: Acta Palaeontologica Polonica. Volume 47, No. 1, pp. 1-78.
- Alfred Brehm: Die Säugethiere 1. In: Brehm's Thierleben, 1864–1869.
- Friedrich Engels : Letter to Conrad Schmidt in Zurich . March 12, 1895.