Mammals Evolve From Size Of Mouse To Elephant In 24M Generations
January 31, 2012

Mammals Evolve From Size Of Mouse To Elephant In 24M Generations

Mammals can evolve from the size of a mouse to the size of an elephant in as little as 24 million generations, although they shrink more than 10 times as fast as they grow to large sizes, according to new research reported Monday by an international team of biologists and paleontologists.

The study is the first of its kind to measure how fast large-scale evolution can occur in mammals.

The researchers explored increases and decreases in mammal size following the extinction of dinosaurs 65 million years ago, seeking to explain large-scale size changes and recovery from mass extinctions.

Their estimates are based on calculations of the most rapid increase in size observed in the fossil record after a mass extinction wiped out the dinosaurs.

The researchers found it took about 10 million generations for terrestrial mammals to hit their maximum mass: that's equivalent to a mammal the size of a cat evolving into a mammal the size of an elephant.

Changes in the size of sea mammals, such as whales, occurred at twice the rate of land mammals.

"This is probably because it's easier to be big in the water — it helps support your weight," said study co-author Dr. Erich Fitzgerald, Senior Curator of Vertebrate Paleontology at Museum Victoria and a co-author.

The researchers also discovered that 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," said Dr. Jessica Theodor, co-author of the study and an associate professor of biology at the University of Calgary.

Study co-author Dr. Alistair Evans, an evolutionary biologist and Australian Research Fellow, said the research was unique because most previous studies had focused on microevolution, the small changes that occur within a species.

"Instead we concentrated on large-scale changes in body size. We can now show that it took at least 24 million generations to make the proverbial mouse-to-elephant size change — a massive change, but also a very long time," said Dr. Evans.

"A less dramatic change, such as rabbit-sized to elephant-sized, takes 10 million generations."

The researchers looked at 28 different types of mammals, including elephants, primates and whales, from the four largest continents (Africa, Eurasia, and North and South America) and all ocean basins for the last 70 million years.

Size changes were tracked in generations rather than years to allow meaningful comparison between species with differing life spans.  For instance, a mouse only lives for about two years while an elephant lives for 80.

The researchers were surprised to find that decreases in body size occurred 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," said Theodor.

"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,” she said

Dr. Evans was also shocked by how fast mammal sizes could decrease.

"The huge difference in rates for getting smaller and getting bigger is really astounding — we certainly never expected it could happen so fast!" he said.

Many miniature animals, such as the pygmy mammoth, dwarf hippo and 'hobbit' hominids lived on islands, helping to explain the size reduction.

"When you do get smaller, you need less food and can reproduce faster, which are real advantages on small islands," Dr. Evans said.

The research sheds light on which conditions allow certain mammals to thrive and grow bigger, and which ones slow the pace of growth, something that could contribute to extinction.

The findings were published Monday in the journal Proceedings of the National Academy of Sciences.


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