Nearly 150 years ago Charles Darwin, the father of evolution, made an observation: animals that lived on islands without predators tend to be tamer, for lack of a better word. They live in less fear and therefore can be easily approached. Darwin wrote about this in his classic 1859 book, "The Origin of Species," in which he notes the difference in behavior between island birds and those in his native England:

The fear of man is slowly acquired, as I have elsewhere shown, by the various animals which inhabit desert islands; and we see an instance of this, even in England, in the greater wildness of all our large birds in comparison with our small birds; for the large birds have been most persecuted by man. We may safely attribute the greater wildness of our large birds to this cause; for in uninhabited islands large birds are not more fearful than small; and the magpie, so wary in England, is tame in Norway, as is the hooded crow in Egypt.

Although this observation has been repeated for more than a century, it hasn't really been studied and quantified scientifically until now. A team of scientists decided to look into this and examined 66 lizard species to see how they might flee in the face of a predator or other potentially dangerous animal. Their goal was to calculate the "flight initiation distance" — in other words, how close the lizards would allow a predator to get to them before they turned tail and fled. Lo and behold, the scientists calculated the island-based lizard species could be approached much more closely than those on the mainland. Their research was published last week in Proceedings of the Royal Society B.

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

Other studies have looked at the flight initiation distance for individual species, but this appears to be the first major studies across numerous species around the world.

As the authors write in their paper, this "tameness" has an evolutionary advantage. If a lizard species lives on an island without the constant fear of predation, they do not need to "waste time and energy developing and performing needless escape." This allows them to put more energy into gathering food and reproducing, giving a natural selection advantage to the individuals who do not waste their energy.

Although the authors do not touch on it in their paper, their research is one more clue as to why so many island species have become threatened or gone extinct as their habitats have been invaded by non-native predators such as rats, cats, dogs and snakes.

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