Size matters: evolutionary changes in body size measured

For the first time, scientists have measured how quickly large-scale evolutionary changes in body size occur. Intriguingly, while it takes 24 million generations for a mouse-sized animal to evolve to the size of an elephant, shrinkage is a much more rapid process. The findings, which focus on increases and decreases in mammal size following the extinction of the dinosaurs 65 million years ago, are reported in the Proceedings of the National Academy of Sciences.

Land-based mammals exhibited the slowest growth rates, while sea mammals such as whales took about half the number of generations to hit their maximum. But rates of shrinkage were much more rapid. The researchers noting that it only took 100,000 generations for very large decreases, leading to dwarfism, to occur.

Dr Alistair Evans, of Monash University, Australia, said the study was unique because most previous work 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. A less dramatic change, such as rabbit-sized to elephant-sized, takes 10 million generations.”

Evans and his co-researchers looked at 28 different groups of mammals, including elephants, primates and whales. Size change was tracked in generations rather than years to allow meaningful comparison between species with differing life spans.

Evans said he was most surprised to find that decreases in body size occurred more than ten times faster than the increases. “The huge difference in rates for getting smaller and getting bigger is really astounding – we certainly never expected it could happen so fast.”

Many of the species which shrunk, such as the dwarf mammoth, dwarf hippo and dwarf hominids (found on the Indonesian island of Flores) eventually became extinct. The researchers speculate that their dwarfism was directly related to their environment.

“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 co-researcher Jessica Theodor, of the University of Calgary.

The study also notes that many of the miniature species lived on islands. “When you do get smaller, you need less food and can reproduce faster, which are real advantages on small islands,” Evans explained.

The researchers say their work furthers our understanding of conditions that allow certain mammals to thrive and grow bigger and circumstances that slow the pace of increase and potentially contribute to extinction.

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Source: University of Calgary, Monash University

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