哺乳动物体型处于趋减状态

Just how big can mammals get and how fast can they get there? These are questions examined by an international team of researchers exploring increases in mammal size after the dinosaurs became extinct 65 million years ago. Research published in the journal Proceedings of the National Academy of Sciences shows it took about 10 million generations for terrestrial（陆地的） mammals to hit their maximum mass: that's about the size of a cat evolving into the size of an elephant. Sea mammals, such as whales took about half the number of generations to hit their maximum.

The team, including Dr. Jessica Theodor of the University of Calgary, also discovered it took only about one hundred thousand generations for very large decreases, such as extreme dwarfism, to occur.

"Our research demonstrates, for the first time, a large-scale history of mammal life in terms of the pace of growth. This is significant because most research focuses on microevolution, which are changes that occur within a specific species," says Theodor, co-author of the study and an associate professor of biology at the University of Calgary.

The research team looked at 28 different types of mammals from the four largest continents (Africa, Eurasia, and North and South America) and all ocean basins for during the last 70 million years. For example, one group would include the mammals related to an elephant, another group would include carnivorous（食肉的） mammals.

Researchers were surprised to learn how quickly body-size decreased: the rate is more than 10 times faster than the increases.

"Many of the species which shrunk, such as the dwarf mammoth, dwarf hippo and dwarf hominids, found in the Indonesian island of Flores, became extinct," says Theodor, whose area of expertise are the artiodactyls, hoofed mammals which include in the present day, cows, pigs, sheep, camels, hippos and whales.

"What caused their dwarfism? They may have needed to be small to survive in their environment or perhaps food was scarce and a small stature would require less nutrients," adds Theodor.

This research will help scientists to better understand mammal evolution: what conditions allow a certain mammals to thrive（繁荣，兴旺） and grow bigger and what conditions would slow the pace of growth and potentially contribute to extinction.

Researchers used generations instead of time in their study because species have different life spans. A mouse only lives for about two years and elephant for 80. They created a metric（度量标准） using the relationship between body size and generation time in living mammals and used it to reconstruct the life history of the extinct forms.