<h1>Bizarre animals</h1>
Nor did Darwin witness the final outcome of the controversy
It's hard for anyone to imagine how platypus, the "hermit," has caused scientists trouble. In 1799,
The platypus first entered the field of British scientists, and even when it caused a heated debate, it lasted for about 95 years, and even Darwin was unable to witness the final outcome of the controversy.
Initially, all naturalists decided that the furry animal with its webbed limbs and flattened beak was a funny specimen that had been carefully stitched together, so much so that when the platypus specimen was exhibited at the National History Museum in London, each visitor hoped to find the "thread that had been carefully hidden." Of course, no one can get what they want.
Next, the debate turned to whether it really belonged to mammals. Although it has long hairs, it is not visible that it has nipples and mammary glands. Even more confusing, the dissection found that it had the same cloaca as birds and reptiles. Finally, scientists had to create a whole new category for it and its close relative, the echidna, under the mammalian order: monoporosis.
It wasn't until the 1830s that Li scholars discovered where the mammary glands on Duckbill had been. By 1884, William Candlewell, a young embryologist from Scotland, had visited Australia and sent back a telegram that would become famous. The content of the telegram was unusually simple: "Single-porous egg growth, egg partial egg splitting." A few words, but the whole of Britain, the academic community stunned. Hairy, able to suck milk, lizard-like limbs and laying eggs, are traits that scientists wonder: Could it be that platypuses are missing parts of evolution that are between reptiles and mammals? Darwin called this anomalous animal a "living fossil" even before he could figure out whether platypus could lay eggs.
<h1>The platypus we already know</h1>
Today, we already know that the platypus is a single-perforated mammal endemic to Australia, found between Cape York in eastern Australia and South Australia, and also inhabits Tasmania. The currently living single-for-hole animals are very rare, including only platypus and four species of echidnas.
Four species of echidna
Strictly according to taxonomic criteria, platypus is the only survivor of the family and species under the order Monotreme, and the rest of the members exist only in fossil form.
platypus
The shape is very strange, the whole body is wrapped in soft and thick brown short hairs, the limbs are very short, the five toes have hooked claws, there are film-like webs between the toes, resembling duck feet, the snout is flattened, shaped like a duck's beak, and the tail is large and flattened. Platypus are oviparous, as are reptiles and birds. Platypus is also one of the very few mammals that can defend itself with venom. Both male and female, platypus have poisonous spines at birth, but the females and the stingers disappear when they grow to 30 cm in size, only the thorns on the hind feet of the males still exist, and the poisonous juice is stored inside, and the venom is similar to that of snake venom, and the spray can hurt people. If a person is injured by a platypus stinger, it will cause immediate severe pain and take several months to recover.
The platypus's thick, short fur forms an effective barrier like a waterproof coat, maintaining a constant body temperature. Platypus' broad, flattened tail is an effective reservoir of fat. Although the platypus has short limbs, it can swim, paddling with its webbed feet on its forelimbs and controlling the direction on its hind limbs. While crawling on land, the webs between the toes of its hind limbs can be retracted. Platypus have a flat, duck-like beak, but unlike birds, birds open their mouths and close their mouths by opening and closing their beaks up and down, while platypus "duck-billed" is essentially a soft-textured snout with a surface full of nerves that can receive radio waves from other animals like radar. The eyes and ears of the platypus are hidden in the protruding posterior end of the mouth and nose, and are completely closed after entering the water. That is to say, the platypus is both deaf and blind underwater, and relies entirely on the sharp weapon of "duckbill" to find food and identify directions in the water.
Platypus weigh 0.7-2.4 kg, and males are usually much larger than females. The average length of males is about 50 cm, and the females are about 43 cm. In general, the size of the platypus shows local differences due to the difference in the region, climate and environment, whether there are natural enemies and many other factors.
Platypus is a carnivorous animal, and its main prey objects include link worms, insect larvae, freshwater crayfish and so on. Whenever they catch food in the water, they store it in their cheeks and wait until they surface before enjoying it slowly. Juvenile platypuses have teeth, but when they grow enough to climb out of their burrows, their teeth fall out. Because of its toothlessness, platypus chews only using plates that grow continuously in the gum bed. There are many raised transverse ridges on the bite surface of the cuticle plate, which can crush the shells of mollusks such as shellfish and snails, or chop up other food. Platypus eat a large amount of food, consuming at least 20% of its own body weight every day. As a result, a platypus spends an average of about 12 hours a day foraging in the water.
Platypus prefers to inhabit the periphery of rivers and lakes. Platypus built burrows, the entrance is generally about 30 cm above the water.
During the breeding season, from June to October of each year, adult female platypuses carefully build safer and more comfortable shelters. This is a special kind of cave, the depth of the cave is at least 20 meters or more, and several partitions are set along the way, which can adjust the temperature and humidity in the cave and prevent predators from invading. At the end of the passage, the female platypus will make a bed with the fallen leaves and reeds that have been pulled back, and the nest will be arranged very comfortably. In terms of marriage form, platypus belongs to polygamy. Males are usually not involved in caring for their offspring and return to their caves after mating, while females are tasked with nurturing and nurturing offspring.
The average body temperature of platypus is about 32 °C, while the average body temperature of placental mammals is usually around 37 °C. Scientists speculate that this is likely the result of platypus actively adapting to the harsh external environment during its evolution. Perhaps it was this change that made it a rare survivor of the single-for-hole family.
After tens of millions of years of evolution, the platypus has neither become extinct nor evolved much, and has always hovered in the "transitional stage", so it is full of mystery and has academic significance. This australian-only animal is often used as a mascot for various events in the country. The reverse motif of the Australian 20 cent coin is also a platypus.
<h1>Secrets hidden in the genetic code</h1>
In 2016, more than two hundred years after the platypus debuted in the scientific community, the world-famous scientific magazine Nature first published detailed analytical data on the platypus genome, providing enlightenment for revealing the secrets of the platypus hidden behind the genetic code.
From dependence on egg yolks to dependence on milk
Genetic analysis suggests that the composition of the platypus genome reveals its most authentic trait, which is traits that are both reptile and mammalian. For example, platypuses have reptile-like limbs; this trait can be read directly on the corresponding gene fragment. For example, in the genome of a reptile, there is a string of genes that specifically control the size of the fetus, and these genes are the same whether they come from sperm or eggs, while the corresponding genes of mammals (except for the single-foremen) have different activity capabilities because of their different origins: if they come from sperm, the genes are abnormally active; if they come from eggs, the gene activity is low. Studies have shown that for platypuses, the corresponding genes are the same whether they come from sperm or eggs. That is, in terms of genes that relate to fetal growth, platypuses are closer to reptiles than mammals.
Genetic studies have also shown that, like other mammals, about 2% (18,527 in total) of the genes in the platypus genome are specifically responsible for protein production, a portion of which corresponds to egg yolk and milk production. We know that reptiles use the yolk of an egg to provide all the nutrients needed for development to the developing embryo, while mammals are different: before the fetus is born, the placenta provides continuous nourishment for the developing fetus; after the birth of the pups, it provides nutrients through breast milk, which is also an important factor in the development of mammalian brain volume. So how did the early egg-laying reptiles evolve into placenta-capable mammals? The results of platypus genetic research have provided scientists with clues: the first step in this transformation must be to promote milk production, and then gradually get rid of spawning and fertility.
Female platypus lay 1-3 eggs at a time, usually two. In appearance, the eggs of the platypus resemble the eggs of reptiles, with a cortex texture, a small size, and a diameter of about a few millimeters, which is closer to round than a bird's egg. The growth cycle of a platypus egg consists of 28 days in the womb and incubation10 days in vitro
The growth cycle of a bird egg is 1 day in the intestine and 21 days in vitro incubation. After spawning, the female platypus curls up to hatch. The incubation process is roughly divided into three stages: the first stage, the embryo does not grow any functional organs, completely absorbing nutrients from the yolk; the second stage, the fingers and toes begin to develop; the third stage, the egg teeth appear.
The newly hatched pups are very weak, their eyes cannot be opened, they are hairless and completely dependent on breastfeeding.
Platypus has mammary glands but no nipples. It has a "bag" in its abdomen, and the secreted milk flows through the small holes in the inner skin for the cubs to lick and eat. Breastfeeding lasts 3-4 months. After that, the mother platypus will go out for a short time to forage. After about 5 weeks, the mother platypus is out foraging for longer and longer. After another 4 months, the juveniles began to crawl out of the hole.
<h1>Manufactures neurotoxins</h1>
The researchers also accidentally discovered a set of genes that dominate neurotoxin production in the platypus genome, further demonstrating that the ability to produce venom is a trait common to platypus and reptiles.
The researchers, whose original purpose was to explore how platypuses could effectively protect their pups in a sinister environment, accidentally found a gene fragment in the platypus genome that resembles a defense against microbial invasion. Further research found that the gene was not used to defend against microbes, but against much larger opponents, as it controlled the production of the main component of neurotropic venom.
The venom in the platypus is mainly a mixture of several proteins similar to prosthetics, three of which are unique to the platypus. These substances are produced by the platypus' immune system and are so toxic that they can kill dog-sized animals. For humans, the venom of platypus is not fatal, but it is enough to incapacitate the injured person locally. Once a person is stabbed, the area around the wound will soon become puffy, which in turn will lead to swelling and severe pain in the entire injured trunk. This severe pain can also trigger prolonged hyperalgesia, which can last for tens of days or even months.
<h1>Super keen sense of smell</h1>
Platypus seem to retain too many traits inherited from their ancestors, but their ability to adapt to their environment is equally amazing. The platypus's environment is full of water, and they spend most of their time underwater. Underwater, the platypus can neither see nor hear, relying entirely on electronic sensing receivers distributed on the platypus beak to track and detect the electronic pulses emitted by the prey.
In interpreting the platypus genome, scientists were pleased to find that platypus genes also contain fragments of the dominant odor detection system, suggesting that platypuses may be good at discovering certain water-soluble odors. According to scientists, "animals have two noses" — one that detects compounds that fill the air through olfactory sensing; the other, the so-called plough nose organ, which is like a taste bud in the nasal cavity, can detect the presence of non-volatile compounds. Dogs, for example, lick their urine from time to time and transmit the collected pheromones through their tongues to the plow nose organ located in the palate. The platypus's plow nose sensor is also exceptionally sensitive. Typical mammals have about hundreds of plow nose sensors, while platypuses have thousands of plough nose sensors. Since platypus spend 90 percent of their time underwater, scientists speculate that they are likely to rely on such sensors to detect water-soluble odors.
<h1>Elusive gender issues</h1>
Most mammals have similar sex chromosomes in appearance, larger in numbers of around 50, while reptiles and birds, unlike those, are so numerous and tiny that cytologists initially used "chromosomal dust" to describe them. When biologists first looked at platypus cells through a microscope, they were once again struck by what they saw: platypus's chromosomes were similar to those of birds and reptiles, consisting of 46 tiny chromosomes and 6 relatively bulky chromosomes.
We know that the sex of most animals depends on different combinations of sex chromosomes.
Mammals, including placentals (such as us humans) and marsupials (such as kangaroos), include X and Y, where the C chromosome is larger, the Y chromosome is smaller, XX represents female, XY represents male; the sex chromosomes of birds include z and w, where z chromosome is larger, the W chromosome is smaller, ZZ represents male, and zw represents female.
However, not all animals have different sex chromosomes. In the case of reptiles, many species of reptiles, whether female or male, have exactly the same sex chromosomes, and their sex differences depend entirely on the temperature at the time of hatching. For example, a turtle egg, if it hatches in a cool environment, is a male; if it hatches in a warm environment, it is a female. It can be seen that many animals have undergone a long evolutionary process of millions of years before gradually getting rid of the limitation of sex depending on temperature and locking the sex decision in the corresponding sex chromosomes.
<h1>Alternative mammals</h1>
The study found that platypus had 10 sex chromosomes – the females appeared in a combination of 5 pairs of xxs, and the males appeared in 5 pairs of XY. But it is worth noting that the sex chromosomes x and Y of platypus and our human sex chromosomes x and Y are completely different things: the human x sex chromosome is locally present on platypus 6 regular chromosome. This discovery confirms the academic theory that sex chromosomes evolved from conventional chromosomes.
Since the sex chromosomes of platypus are different from placental mammals represented by humans, who are they more like? One study suggested that the sex chromosome of platypus was very similar to the z in the sex chromosome of birds. Researchers have previously determined that platypus and chickens share several identical genes. Now, after careful comparison, they further determined that the platypus's sex chromosome X5 and the chicken's sex chromosome z share hundreds of identical genes, that is, the two chromosomes are identical. This conclusion really makes many people dizzy – birds and platypus are completely different branches of the biological evolutionary tree, why is this happening? Some researchers speculate that it is likely that the common ancestor of these animals used two sets of sex determination systems at the same time, namely XY and ZW, and in the subsequent evolutionary process, different offspring chose to retain different sex determination systems. Whether this seemingly inconceivable speculation is true remains to be verified.
Since the platypus debuted in the scientific community, scientists have learned more and more about this ancient and exotic animal as research has progressed, but this has often led to new controversies and confusion. Everything about platypus is still intriguing. We believe that as more animal genomes are solved, the mysteries in the evolutionary process of mammals will eventually be solved one by one.