A new study throws light on why tortoises and turtles come in such a wide range of sizes.
Associate Professor Michael Alfaro of theUniversity of California, Los Angeles and colleagues, publish their research today in The Royal Society's Biology Letters, which suggests that body size variations in chelonians can be explained by broad habitat differences.
Part of their study looked at a phenomenon called "Island gigantism", exemplified by the giant tortoises which inhabit the Galapagos Islands, the Seychelles and Mascarene Islands.
"One of the striking patterns in chelonians, that is, turtles and tortoises, is the fantastic range in body size," Alfaro, the senior author on the paper, says.
"You have some species that are not much more than a few grams, to the sea turtles which are over 600 or 700 kilograms. So we wanted to understand that diversity of size, and one of the most striking examples of size evolution within turtles is the evolution of gigantism in the island species in the Galapagos and Seychelles."
The team collected carapace measurements for 226 chelonian species. They classified the species into four basic habitat categories - freshwater, mainland, marine or oceanic island.
They then compared the data set within several modelling parameters, and calculated different optimal body sizes according to habitat.
Habitat diversity is the key
Alfaro says mainland and freshwater chelonians in general are much smaller than the sea turtles or island tortoises, but display a much broader size variation.
"I think what's going on is we've really identified that habitat, in very broad brush strokes, has something to do with the size diversity that we see in the chelonians," Alfaro says.
"If we were to collect more ecological data we probably could explain more of the size variation within the freshwater or mainland terrestrial species."
Alfaro says the team was surprised by the strong signal.
"At the start of this study," he says, "I would not have been surprised if we found that the change in size evolution, or the association between shifts in size evolution, and habitat wasn't that strong."
The researchers say previous studies had examined the relationship between body size and island endemism in birds and mammals. But theirs is the first study to show a strong evolutionary preference for gigantism as an optimal condition in oceanic island tortoises.
They say past research suggests that large size was a preadaption, which allowed the ancestors of modern island chelonians to initially reach and populate the islands.
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 looking at size evolution across all vertebrates, in order to gain a better picture of vertebrate evolution.
"Size is a good proxy for many ecological characters," Alfaro says.
"It's a character that can be measured pretty readily and compared in a reasonable way across very diverse organisms. What we're doing is constructing a time-tree for vertebrates and conducting new kinds of statistical analyses to find where the hotspots in evolutionary change are."
The researchers hope this will help answer questions such as what lineages are significant, or what time periods show rapid or slow changes in size evolution?