Reptiles once ruled the earth for 230 million years, and in their most prosperous Jurassic and Cretaceous periods, dinosaurs ruled the earth, crocodiles hovered over rivers and lakes, pterosaurs soared in the sky, and ichthyosaurs, plesiosaurs and dragons dominated the sea. The entire earth's sea, land and air are ruled by a class of animals, which is the first time in geological history and the only time so far.
Crocodiles were the overlords of freshwater in the age of the dinosaurs
At the end of the Cretaceous Period, about 66 million years ago, a huge meteorite descended on Earth, announcing the end of the reptile age. Dinosaurs, pterosaurs, and dragons all went extinct in this catastrophe, and mammals quickly rose to become a new generation of overlords. However, crocodiles did not go extinct with them, they survived and continued to hold on to fresh water, becoming the "last emperor" of reptiles, and fresh water also became the last "reserve" of reptiles. As a pioneer of evolution, the strength of mammals is beyond doubt, whether fresh water is too special, or crocodiles are too powerful, why are mammals slow to capture fresh water?
<h1 class="pgc-h-decimal" data-index="01" > water's innate hostility to warm blood</h1>
We note that freshwater is mixed well not only with crocodiles, but also with freshwater turtles such as crocodile turtles, South American side-necked turtles and three-clawed turtles, as well as large freshwater fish such as European squid, Dasper's sturgeon, white sturgeon and Mekong giant catfish. In short, the top of the freshwater food chain is a number of warming animals, mammals can not find to compete with them, hippos, freshwater manatees and capybaras, beavers are vegetarian animals, otters eat meat, but not in the same weight as crocodiles. The only big guys who eat meat are freshwater dolphins, but they only have 4 species and are quite limited in distribution.
Bay crocodile "Luolong": body length of more than 6 meters, weight more than one ton
Why do mammals, especially large carnivorous species, wither so much in fresh water? The easiest reason to think of is the different natural properties of water and air. The specific heat capacity of water is three times that of air (that is, for every degree Celsius of water raised or decreased, it absorbs or releases three times as much heat as air), which means that the heat storage capacity of water is better than that of air, and the water temperature is stable than the air temperature. For example, at the equator, where the temperature is about 25 °C, and at the South Pole, where the temperature is as low as -50 °C, the temperature of the deep sea is maintained at 0 to 3 °C. The body temperature of the temperate animal is consistent with the ambient temperature, and the habitat in the water can avoid exposure to extreme high or low temperatures, and it can be active at physiological peaks for a longer time.
On the other hand, this also means that water has a stronger thermal conductivity than air, and hot organisms lose heat in water faster. Thermostatic animals maintain a high body temperature in any environment, and when we are in water, we must pay three times as much energy to maintain body temperature.
On land, thermostatic mammals have the advantage: we can move their heavy bodies quickly, which can exhaust the big reptiles; we can stay active in any temperature, day and night, winter or summer, while the reptiles are greatly reduced in low temperatures. But when we get into the water, these advantages become disadvantages, and just maintaining body temperature will make our energy unable to make ends meet.
Freshwater manatees of the Amazon River
However, the natural hostility of water to warm blood does not stop mammals from conquering the world, as we can see in the ocean. There are about 130 species of marine mammals and 80 species of marine reptiles living in the oceans, which seem to be of similar species, but the numbers and levels of participation in ecosystems vary widely.
Among the 130 species of sea mammals are whales and dolphins, pinnipeds, sea otters and manatees, of which the adaptability of cetaceans to the ocean is not even below fish, including today's marine overlord killer whale and the largest animal, the blue whale. More than 70 of the 80 species of sea climbers are sea snakes, in addition to the sea iguanas and 7 species of sea turtles, which only occupy a narrow ecological niche.
In order to reduce heat loss and reduce the cost of maintaining body temperature, aquatic mammals have a way. Whales, manatees and hippos are insulated by thick skin and fat layers, and freshwater manatees and hippos live only in the tropics. The otter insulates with its air- and greasy fur, which combs its fur every day to maintain its waterproof properties while increasing its metabolic rate and heat generation, which is 1.5 times higher than that of land predators of its size and 3.5 times that of sea otters.
"Yangtze River Goddess" White Dolphin
Thus, the unfriendliness of the water environment to warm blood alone could not prevent mammals from diving into the water, and mammals have conquered water, just as The Leviathan whales (an ancient sperm whale that lived in the Miocene, one of the most powerful marine predators ever built) and killer whales did in the ocean.
<h1 class="pgc-h-decimal" data-index="02" > crocodile was never absent</h1>
Looking back at evolutionary history, mammals emerged in the late Triassic period, 225 million years ago, and then suppressed by dinosaurs for 160 million years, and by the time of the catastrophe at the end of the Cretaceous, they were still nocturnal, mouse-sized insectivores. The land overlord dinosaurs were cleared by the disaster, and it was natural that this group of little guys would have no problem seizing the ecological niche vacated by the dinosaurs, but it was another matter to seize the freshwater supremacy from the crocodiles.
Age of the Dinosaurs
The food chain on Earth can be grouped into two broad categories: the green food chain, which is based on the photosynthesis of green plants; and the brown food chain, which is based on dead organic matter, can not rely on photosynthesis in the short term. Both terrestrial and shallow sea ecosystems are dominated by green food chains, while brown food chains account for a significant proportion of freshwater and deep seas are almost entirely brown food chains.
The Great Extinction at the end of the Cretaceous Period was caused by extraterrestrial visitors, the dust raised obscured the sky for an unknown number of years, photosynthesis almost stopped, and the green food chain based on it collapsed, resulting in the extinction of the top predators dinosaurs and dragons on land and shallow seas. However, long after the collapse of the green food chain, a large amount of dead organic matter was still washed into the river, and the brown food chain was maintained, feeding some small fish and crabs, and the crocodiles relied on these to survive until the dawn of the Cenozoic.
Crocodile of the Dinosaur Age: Monarch muscle crocodile, which can be more than 10 meters long
After the extinction of the dinosaurs, mammals spent 20 million years defeating reptiles trying to revive and birds that were also competing for ecological niches, becoming the sole masters of the earth, but crocodiles held fresh water for 66 million years.
<h1 class="pgc-h-decimal" data-index="03" > physiological tolerance is key</h1>
Freshwater ecosystems are not suitable for long-term fertility of large organisms due to their low productivity, environmental instability and high isolation.
The first is low productivity. Plant yields are lower in water than on land, assuming that the primary productivity (plant yield) per unit area of tropical rainforests is 100 per unit of time, temperate grasslands are 50, and freshwater is only about 10. Due to the instability of the water flow, phytoplankton floating in the water is exposed to various dangers at any time, and the earth provides a solid and reliable foundation for the growth and reproduction of plants. Plants, as producers, determine the species richness of the entire ecosystem. In food-rich environments, mammals with high metabolic rates and strong mobility predominantly, while food-deprived environments are relatively suitable for reptiles with low metabolic rates.
Fish-eating crocodiles
However, low productivity is by no means the main reason for the lack of large mammals in freshwater. Freshwater plant yields are low, but they are still higher than in the oceans, and the primary productivity of tropical oceans is only half to two-thirds of that of freshwater. This is because the remains of freshwater organisms are decomposed by bacteria and fungi after death, which can be used as nutrients to feed plants, which is similar to that on land; while the remains of marine organisms fall to the seabed and phytoplankton cannot absorb them at all.
The second is environmental instability. Compared with land and sea, the freshwater environment is extremely unstable, and floods, droughts and poor water quality are common occurrences. As a result, freshwater organisms are particularly vulnerable to food cut-off compared to land and marine life. Rivers and lakes can change a lot in thousands of years, taking China alone as an example, in recent thousands of years, the Yellow River has undergone 26 diversions, and the ancient Surabaya and Yunmeng Daze have disappeared. Thousands of years are long for humans, only a moment for biological evolution, and although land and oceans will change due to plate drift, they are still relatively stable on millions of time scales.
Finally, and crucially, there is the high degree of isolation of freshwater systems. Land and sea are not always stable, but they are continuous, while fresh water is isolated, separated from each other by land and sea. The environment on land has deteriorated and the animals can move elsewhere. Marine animals migrate more easily because the world's oceans are connected and swimming is less laborious than running.
The Great Migration of the Serengeti
For example, every year from May to September is the dry season in Tanzania's Serengeti grasslands, where the grass withers and thousands of zebras and wildebeest migrate to the Masai Mara Reserve in Kenya and back again in October when the rainy season comes. Influenced by upwelling currents, antarctic waters are well nourished and plankton are abundant, attracting large numbers of whales and seals to feed every summer.
On the contrary, once a river or lake disappears, the creatures inside will be destroyed. Among the three major systems, fresh water is still the most likely to undergo major changes in a short period of time.
For these reasons, large animals must have two skills to survive in freshwater in the medium and long term: the ability to migrate to safe shelters, and the ability to resist hunger or develop new food resources.
Freshwater mammals, with the exception of manatees and dolphins, have the ability to migrate across land, freshwater manatees and dolphins can take refuge in river estuaries, and they only live in the most stable large rivers - the Amazon River, the Yangtze River and so on. Hippos and manatees have unusually low metabolic rates, such as hippos only need to eat half of pure terrestrial herbivores (rhinos) of the same size to meet the demand. Otters are highly opportunistic predators, preying on frogs, birds and rodents in addition to fish, shrimp and crabs.
Nile crocodile
However, it is the crocodile that combines these two skills and exerts them to the extreme. Crocodiles can both cross land and travel long distances in the sea, and are more capable of migrating than any aquatic mammal. If they can't find water everywhere, their last resort is to dig holes in the silt at the bottom of the river (sometimes as deep as 8-10 meters!). ), waiting in the cave for the rain to fall.
As warmer reptiles, crocodiles have a very low metabolic rate, and they only need a small amount of food to survive for a long time, which is also a capability that large mammals do not have. Adult crocodiles can eat or drink and hide in the mud for months without starving. This extraordinary physiological tolerance is the key to the crocodile's conquest of fresh water.
mugger
Now the answer is why there is a lack of large carnivores in freshwater. Hippos and manatees reduce their metabolic rate to adapt to freshwater life, but they are herbivores, food is within reach, and carnivores must maintain a high metabolic rate like otters to catch fish, shrimp and crabs, and maintain body temperature. Freshwater, on the other hand, is scarce and unstable, making it difficult to feed large carnivores.
As a result, mammals spent 66 million years without being able to extract fresh water from crocodiles. There have been at least two attempts in evolutionary history for carnivores to expand into freshwater, namely whales and carnivores. Eventually, the whales chose to go to the ocean, while the carnivorous otters chose to miniaturize.
Slippery otters fishing
<h1 class="pgc-h-decimal" data-index="04" > great crocodile</h1>
The crocodile is far more remarkable than one might think, an ancient creature that has evolved its adaptation to fresh water to the extreme. Crocodiles have also thought about expanding into a wider space, evolving some sea crocodiles and terrestrial crocodiles, but they have failed. Similarly, no animal can compete with crocodiles in freshwater. From the extinction of the plant dragon at the end of the Triassic period, crocodiles have been the overlord of freshwater, and it has been 200 million years today.
Mammals have dominated the land for only tens of millions of years, and if viewed in terms of order, there have been several waves of overlords, from even-hoofed to hyenas to carnivorous, and now primates (humans). Never in geological history has a superordinate animal been able to dominate one side for 200 million years, and crocodiles have created a miracle. Dragonflies, cockroaches and scorpions have been around for 300 million years, but they are small animals with a very low sense of existence, and crocodiles are the hegemons of the party!
If humans are taken into account, mammals eventually conquered fresh water. Since the 20th century, the 200-million-year-old freshwater overlord is now in a desperate situation due to indiscriminate fishing, habitat destruction and crocodile skin trade, and a variety of crocodiles such as the Orinoco crocodile, the Cuban crocodile, the Siamese crocodile, the fish-eating crocodile and the Yangtze crocodile are now critically endangered.
Alligator
Big cats have only been kings of beasts for 3 million years, and the freshwater emperor of crocodiles has been doing it for 200 million years, and from this point of view, crocodiles are more worthy of respect than lions and tigers. But in fact, because of its unassuming appearance, crocodiles have always been subjected to people's secular prejudices, and some people have lamented the beauty and strength of tigers when they see the footage of tiger hunting, and when they see crocodiles hunting, they curse cruelty, and even curse crocodile extinction. Crocodiles need human attention, understanding and protection.