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Study con­firms Darwin’s obser­va­tions and numer­ous anec­do­tal reports of island tame­ness, says Uni­ver­sity of Cal­i­for­nia Riverside’s Theodore Garland

Most of us have seen pic­tures and prob­a­bly YouTube videos of “tame” ani­mals on the Gala­pa­gos Islands, the bio­log­i­cal par­adise that was Charles Darwin’s major source of inspi­ra­tion as he observed nature and grad­u­ally devel­oped his ideas about the impor­tance of nat­ural selec­tion as a mech­a­nism by which pop­u­la­tions of organ­isms would change — evolve genet­i­cally — across gen­er­a­tions, even­tu­ally becom­ing bet­ter and bet­ter suited to life in their cur­rent conditions.

A corol­lary of Darwin’s rev­o­lu­tion­ary idea was that organ­isms would also evolve to lose struc­tures, func­tions, and behav­iours they no longer needed when envi­ron­men­tal cir­cum­stances changed. He noted that island ani­mals often acted tame, and pre­sumed that they had evolved to be so after com­ing to inhabit islands that lacked most predators.

But more than 150 years later that almost casual obser­va­tion remained to come under sci­en­tific scrutiny. Last week, a team of researchers from the Uni­ver­sity of Cal­i­for­nia (UC) River­side, Indi­ana Uni­ver­sity Pur­due Uni­ver­sity Fort Wayne and George Wash­ing­ton Uni­ver­sity pub­lished a study show­ing that island lizards are indeed “tame” as com­pared with their main­land rel­a­tives. The researchers were able to approach island lizards more closely than they could approach main­land lizards. The study results appeared online on 8 Jan­u­ary in the Pro­ceed­ings of the Royal Soci­ety B.

“Our study con­firms Darwin’s obser­va­tions and numer­ous anec­do­tal reports of island tame­ness,” said Theodore Gar­land, a pro­fes­sor of biol­ogy at UC River­side and one of the paper’s coau­thors. “His insights have once again proven to be cor­rect, and remain an impor­tant source of inspi­ra­tion for present-​day biologists.”

The Gala­pa­gos Islands of Charles Darwin:

(thanks to BBC, YouTube and DocumentariesForYou)

The researchers con­ducted analy­ses of rela­tion­ships of flight ini­ti­a­tion dis­tance (the predator-​prey dis­tance when the prey starts to flee) to dis­tance to main­land, island area, and occu­pa­tion of an island for 66 lizard species, tak­ing into account dif­fer­ences in prey size and preda­tor approach speed. They analysed island and main­land lizard species from five con­ti­nents and islands in the Atlantic and Pacific Oceans and the Caribbean and Mediter­ranean Seas.

Their find­ings showed that island tame­ness exists and that flight ini­ti­a­tion dis­tance decreases as dis­tance from main­land increases. In other words, island lizards were more acces­si­ble the far­ther the islands were from the mainland.

Gar­land explained that escape responses are reduced on remote islands, because preda­tors are scarce or absent there, and nat­ural selec­tion under reduced pre­da­tion favours prey that do not waste time and energy devel­op­ing and per­form­ing need­less escape.

The research team also found that prey size is an impor­tant fac­tor that affects escape behav­iour. “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.”

The researchers found no con­clu­sive evi­dence show­ing that flight ini­ti­a­tion dis­tance is related to island area. They found, how­ever, that preda­tor approach speed is an impor­tant fac­tor in lizards.

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

(Source: UC River­side press release, 09.01.2014)