Island Lizards are 'Tame', Researchers Confirm Overlooked Darwinian Theory

Compared to their relatives on the mainland, lizards living on islands are more "tame," according to new research that builds off an idea presented by Charles Darwin as he was forming his theories of evolution.

The Galapagos Islands served as a major source of inspiration for Darwin as he developed his ideas about natural selection. A corollary of his theory of evolution was that as animals faced the need to evolve, they would at times lose structures, functions and behaviors that were no longer needed as environmental circumstances changed.

Darwin observed anecdotal evidence that island animals seemed more tame than their mainland counterparts, presumably because they evolved so after coming into a situation with a lack of predators.

This casual observation came to take a backseat to Darwin's more prominent evolutionary theories. But now, a team of researchers from several US universities have picked up where Darwin left off on this theory by investigating the temperament of lizards in various locations.

The researchers report that, just as Darwin observed, island lizards are more tame than their mainland counterparts, as evidenced by their willingness to be approached.

"Our study confirms Darwin's observations and numerous anecdotal reports of island tameness," said study co-author Theodore Garland, a professor of biology at University of California, Riverside. "His insights have once again proven to be correct, and remain an important source of inspiration for present-day biologists."

To substantiate Darwin's theory, the researchers conducted an analysis of the relationships of the predator-prey distance when the prey starts to flee on mainland and island lizard species.

The analysis included mainland lizard species from five continents and island species from the Atlantic and Pacific Oceans and the Caribbean and Mediterranean Seas.

The researchers learned that island tameness does indeed exist, as evidenced by the fact that the distance that a lizard species flees a potential predator decreases the farther away from the mainland they live. The farther the island was from the mainland, the more accessible the lizards living there were, the researchers observed.

"The suggestion by Darwin and others that prey on oceanic islands have diminished escape behavior is supported for lizards, which are distributed widely on both continents and islands," Garland said.

The need for lizards to quickly escape is reduced for island species due to their natural lack of predators in those environments, Garland said.

The researchers also observed a correlation between lizard size and escape behavior.

"When prey are very small relative to predators, predators do not attack isolated individual prey," Garland said. "This results in the absence of fleeing or very short flight initiation distance."

"It is possible that other factors favor island tameness. For example, if food is scarce on islands, the cost of leaving food to flee would favor shortened flight initiation distance," Garland said.

The research is published in the journal Proceedings of the Royal Society B.