Size matters, if you’re a tortoise

For the first time, a team of scientists has shown that islands like the Galapagos strongly favor the evolution of giants – giant tortoises, that is.

Lots of us tend to favor the underdog, the little guy. But I’m not sure Mother Nature does the same. For example, a study published today in Royal Biological Society Letters shows that evolution favors tortoises that are giants among their species.

Image Credit: Yotcmdr at Wikimedia Commons

The study was led by Michael Alfaro of UCLA. His team showed, for the first time, that there’s a strong evolutionary preference for gigantism in tortoises on oceanic islands like the Galapagos or the Seychelles.

His paper explains that there’s an incredible range in body sizes of turtles and tortoises. They can range in weight from just a few grams to over 700 kilograms. Dr. Alfaro set out wanting to understand why that diversity of size existed. As Carl Holm of ABC Science reports:

The team collected carapace measurements for 226 [turtle and tortoise] species. They classified the species into four basic habitat categories – freshwater, mainland, marine or oceanic island.

They then compared the data set within several modeling parameters, and calculated different optimal body sizes according to habitat.

Their finding – which isn’t too surprising – is that habitat is the driving force between size differentiation in these creatures. And that, in general, mainland and freshwaters species have more size diversity than sea turtles or island tortoises.

The more interesting finding is why gigantic turtles got so big, and why they stayed that way. The scientists say their large size was a “preadaptation”. In other words, their heft and heartiness allowed them to reach and populate islands like the Galapagos. But, as Carl Holm at ABC Science writes:

But the fact that they kept their large size suggests that size was at least selectively maintained in the descendants of the initial immigrants, Alfaro and co-authors say. Their size is thought to be linked to a lack of predators, lack of competition for resources and adaptation to potentially erratic environmental fluctuations on islands.

They draw attention to the fact that oceanic islands are susceptible to unpredictable periods of adverse conditions, and that their larger size would make it easier to survive times of reduced food supply.

Alfaro says he and co-author Graham Slater are now turning their attention to size evolution across all vertebrates, to learn more about vertebrate evolution. That is, evolution in creatures more similar to us.

I like to think he’ll find both creatures large and small – but especially the small ones – are brilliantly suited to life on Earth. It might be a stretch.

Rebecca Lewison: Sea creatures snared in fishing gear

Carl Safina on the state of the world’s oceans

Beth Lebwohl