Want Your Mouse to be an Elephant? Just Wait 24 Million Generations

When you really get down to the nuts and bolts of it, it’s pretty insane that mammals range from a little tiny mouse to an elephant and even a blue whale. While they look and act differently, on the inside they have a lot of the same parts (albeit in varying sizes). With that in mind, there’s been one question plaguing scientists for ages: Just how long would it take for some as small as a mouse to evolve to the size of an elephant?

Surprise: It takes a really long time. A new study, led by evolutionary biologist Alistair Evans of Australia’s Monash University, has an answer. Evans’ group says that, while staying viable on land at the maximum velocity of change, it would take 24 million generations for a mouse to become elephant-sized. Consider that assume the maximum rate of change, large mammals took an incredible amount of time to get as big as they are, which makes them pretty darn special.

To chart the rate changes, the study — published in the Proceedings of the National Academy of Sciences — looked at 28 different mammalian orders, including elephants, primates, and whales, over the past 70 million years across a range of habitats and environments. It’s a key time frame, as mammal diversity, both in terms of species and size, exploded after the dinosaurs when extinct.

To estimate the sizes of the animals, the team looked at teeth, skulls, and limb bones from 20 time period within that 70 million year range. They compared those measurements to modern species in order to work out growth patterns.

The team of 20 biologists and paleontologists found that marine mammals grew in size at a much faster rate than their terrestrial counterparts. The study states that it only took whales 5 million generations to grow from 25 kilograms to 190 tons, the enormous weight of a blue whale. That’s likely because some major weight and size restraints — gravity and heat regulation included — aren’t as big of a deal in the water as they are on land.

“This is probably because it’s easier to be big in the water – it helps support your weight,” Dr. Erich Fitzgerald, a co-author of the paper, said.

Surprisingly, shrinking happens much faster. According to the paper, very large decreases in size, like those leading to dwarfism, would only take about 100,000 generations.

“The huge difference in rates for getting smaller and getting bigger is really astounding – we certainly never expected it could happen so fast!” Evans said.

This is likely because miniature animals often lived on islands, in which food and range scarcity pushes for smaller, more efficient creatures. Smaller population sizes on islands also could mean traits evolve and are magnified through the population more quickly.

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

The mammalian explosion is a perennially fascinating topic when it comes to evolutionary study, largely because of how diverse mammals are. When you think of other classes of animal, like reptiles and birds especially, there’s simply nowhere near as much size diversity as there are in mammals, due to a number of physiological issues. (Can you imagine a bird the size of a giraffe? Couldn’t happen.) What’s really cool about a study like this is that it goes a long way towards showing just how this diversity grew over time.