What living things grow very quickly
Mammalian body size : Grow to extinction
A fall from a great height ends in very different ways, depending on how big an animal is. “An ant remains unharmed, a person breaks his bones, a horse bursts,” summarized the British biologist J. B. S. Haldane in 1928.
The experiment would probably not allow an ethics committee, but scientists are still using other methods to research the advantages and disadvantages of a large body. For example, a team of biologists, paleontologists and ecologists has investigated how the body size of mammals has changed in the course of evolution. The result, which they present this week in the journal “Science” (Volume 330, Page 1216): Mammals have undergone an enormous growth spurt.
The researchers inspected fossils of long-extinct mammal species from the past 65 million years. The sudden end of the dinosaurs heralded the rise of mammals. "We went through every epoch and said: O. k., What's the largest animal in this group of mammals?" Says John Gittleman of the University of Athens, Georgia.
When mammals shared the earth with dinosaurs, their body size was between three grams and 15 kilograms, the scientists write. But after the giant lizards became extinct, they grew a thousandfold within a few million years and gave birth to colossi like the Deinotheria, ancestors of today's elephants. This development took place equally on all continents. The growth started very quickly and then slowly flattened out 40 million years ago. 34 million years ago, the megalomania apparently ended. With Indricotherium transouralicum, a kind of giant rhinoceros without a horn, the most powerful mammal that ever set foot appeared: with a shoulder height of almost 5 meters and a weight of 17 tons.
Large herbivores have clear advantages over small ones
The reason for this explosion in body size was probably the abundance of unoccupied ecological niches that were suddenly available to the mammals and into which they literally grew. "When the dinosaurs disappeared, suddenly there was no one left to eat the vegetation, and the mammals began to take advantage of it," says Jessica Theodor of the University of Calgary. Large herbivores have clear advantages over small ones.
“This is mainly due to the digestion,” explains Theodor. Leaves and grass in particular consist of many substances such as cellulose, which mammals can only utilize with the help of bacteria. “The larger an animal is, the longer it takes for food to travel through the digestive system and the more time the bacteria have to do their job,” says the researcher. The large animals digest more slowly - and therefore have more of their food. In addition, large animals have fewer predators and use body heat more efficiently because their body surface area decreases in relation to their mass. So in the animal kingdom, size is good. As early as the 19th century, the paleontologist Edward Cope established the rule that a group of animals grows larger and larger in the course of evolution.
For carnivores, too, body size initially seems to be beneficial. After all, it's easier to hunt and kill an animal when you're in roughly the same weight class. In addition, the predators have to compete with other predators of the same and different species and the following often applies: the bigger and stronger wins. So it's no wonder that the carnivores got bigger and bigger.
Growth also means changing your diet
“But the largest carnivores never grow as big as the largest herbivores,” says Theodore. The reason: Carnivores need a lot more energy than herbivores. While they can graze and chew all day long, hunters lead a more strenuous existence. On average, a predator travels three times as much a day as a herbivore of the same size. Therefore, the growth also brings with it a change in diet. As soon as hunters reach a weight of around 21 kilograms, hunting smaller animals is no longer worthwhile. The expenditure of energy and time is simply too great.
And there is one major disadvantage: the larger a carnivore becomes, the more sensitive it is to environmental changes. The British researchers reported this week in the journal "Biology Letters". Chris Carbone of the London Zoological Society and his colleagues studied 199 populations of 11 carnivores of different sizes, foxes, weasels, polar bears, leopards, lions. They compared how changes in the number of prey affected the number of hunters. The study impressively shows how much more sensitive large animals are. "If the prey population decreased by ten percent, then a population of weasels would also decrease by about ten percent, but a population of polar bears by 50 percent," says Carbone.
It is precisely their weapons, their speed and their strength that make the great hunters so vulnerable. “Hunting is very strenuous and the animals always have to carry their hunting equipment, muscles, teeth, claws with them,” says Carbone. This can be worthwhile in good times, but in bad times the disadvantage quickly becomes a death sentence. In fact, a 2004 study in Science magazine showed that large predatory species in North America generally died out faster than other species. The big predators are so impressive, but as with falling from a great height, in the end it is the small animals that crawl away unharmed.
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