The desert iguana, Dipsosaurus dorsalis, is one of the 66 species of lizards the researchers studied in their analysis.
Credit: Theodore Garland, UC Riverside
Humans are not the only animals that can be calmed by the solitude of island life: Island-dwelling lizards are less skittish around humans and other potential predators than their mainland counterparts are, a new study shows.
Biologists have thought that island animals might generally be tamer than mainland animals ever since Charles Darwin first observed this phenomenon on the isolated, equatorial Galapagos Islands more than 150 years ago during his pursuits on the HMS Beagle.
Darwin's explanation for the phenomenon was fairly straightforward: Animals living on islands with few predators waste less time and energy preparing to flee than those living in mainland regions with more numerous and frequent threats. Still, the theory has not been experimentally tested until now. [Album: Bizarre Frogs, Lizards and Salamanders]
Researchers based at Indiana University-Purdue University Fort Wayne studied skittishness in 66 different lizard species from around the Atlantic and Pacific oceans, and Caribbean and Mediterranean seas by measuring the distance at which the animals fled from approaching humans.
The researchers found that compared with mainland lizards, island lizards were calmer and let humans approach them closer before fleeing — the same observation Darwin had made.
"Our study confirms Darwin's observations and numerous anecdotal reports of island tameness," study co-author Theodore Garland, a biologist at the University of California, Riverside, said in a statement. "His insights have once again proven to be correct, and remain an important source of inspiration for present-day biologists."
The team analyzed existing data of other predator-prey relationships in island and mainland populations, and found a similar trend in which the greater the distance an animal lived from the mainland, the closer that animal allowed predators to approach before fleeing — a measurement known as flight initiation distance.
The team also analyzed whether island size affected flight initiation distance, but did not find a significant relationship.
The researchers pointed out that in addition to the number of predators, confounding factors may also result in the apparent tameness of island animals.
"It is possible that other factors favor island tameness," Garland said. "For example, if food is scarce on islands, the cost of leaving food to flee would favor shortened flight initiation distance."
Since the researchers did not approach the lizards while the animals were eating, the team was unable to determine the influence of this factor, and will need to conduct future studies to sort out the role of this factor, the researchers said.
The study findings were detailed last week in the journal Proceedings of the Royal Society B.