An international team of scientists, including some from Canada, has discovered that mammals shrink at faster rates than they grow — a finding that sheds light on the conditions that potentially contribute to extinction.

Their study, published Monday in the Proceedings of the National Academy of Sciences, shows it took about 24 million generations for a mouse-sized animal to evolve to the size of an elephant, but only 100,000 generations for very large decreases leading to dwarfism to occur.

The research is unique because previous work in this area has focused on microevolution — the small changes that occur within a species.

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


Jessica Theodor is holding an elephant’s femur bone. (Riley Brandt/University of Calgary )

"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 Alistair Evans, of Monash University's School of Biological Sciences, who led a team of 20 biologists and paleontologists on the study.

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

Size change was tracked in generations rather than years to allow meaningful comparison between species with differing life spans. Changes in whale size 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 co-author Erich Fitzgerald, senior curator of vertebrate paleontology at Museum Victoria.

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

Scientists were surprised to discover how quickly body size decreased — at a rate that is more than 10 times faster than the increases.


Jessica Theodor is holding mouse's femur, still attached to the rest of the leg. (Riley Brandt/University of Calgary)

"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 co-author Jessica Theodor, a professor of biology at the University of Calgary.

Her area of expertise is the artiodactyls, hoofed mammals which include 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," said Theodor.