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Small winners in the mammalian race to evolve

Posted on June 11, 2013

It takes at least 10 times as many generations for a mouse to reach elephantine proportions as for the reverse transition, reveals a vast study of mammalian evolution over the past 70 million years.

Between two and five million years ago, something akin to a giant guinea pig roamed South America. Weighing about a tonne, it would have loomed large over its modern relatives – diminutive rodents such as mice and rats.

Such extraordinary contrasts in body mass are part of the story emerging from an international study led by Monash University and published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS). The research tells the story of mammalian body size over the past 70 million years.

Body size plays a critical role in survival, explains lead author Dr Alistair Evans, senior research fellow with the Monash School of Biological Sciences. Being large, for example, can help you regulate body temperature in a cold climate. Being small can help you survive when there is fierce competition for food.

“Believe it or not, the ancestors of elephants were once as small as mice,” Dr Evans says. “So we were curious to find out how long it would take a 20-gram mouse to evolve into a two-million-gram elephant … and vice versa.”

The researchers pieced together the lineages of nine families of large mammals – including those that contain elephants, apes, deer and whales – from the often-fragmentary traces left in the fossil record (the diminutive rodent family was included because of its gargantuan guinea pig relative). By calculating the body mass of each family, they could show how the animals have grown or shrunk over the years. They found that it can take 24 million generations to build an elephant-sized creature out of a mouse, but perhaps only two million generations to create a mouse-sized animal from an elephantine beast.

Previous studies have examined the rate of body mass increase and decrease, but have focused on particular animal lineages; this research looks at entire branches of family trees – the extinct ancestors and all of their descendants. The earlier studies also often focused on shorter time spans, up to only a few million years. The period from 70 million years until present day begins at the point at which mammals started to resemble what they are today, and captures the fascinating complexities in their rise.

But size is a complex matter of adaptability, and Dr Evans explains that bigger is not always better.

“It isn’t just about sheer size – it is also about how quickly or slowly you can adjust your size to the changing conditions around you,” Dr Evans says. “This can play a crucial role in your chances of survival.”

Source: Monash University

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