Grape-Doling Tourists Gave Endangered Iguanas High Blood Sugar

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Northern Bahamian rock iguanas (Cyclura cychlura), already listed as vulnerable on the International Union for Conservation of Nature’s Red List, face a grave new threat: adoring tourists who regularly offer them grapes as if they’re paying tribute to Dionysus, the Greek god of wine.

Research published today (April 21) in the Journal of Experimental Biology shows that iguanas that live on islands frequented by grape-slinging tourists are unable to regulate their blood glucose levels as well as those that live on more remote islands to which humans rarely venture. The study focused on two subspecies of the rock iguana—the Allen Cays rock iguana (ssp. inornata) and the Exuma rock iguana (ssp. figginsi)—both of which are critically endangered.

“I think this is an important model that allows us to test for these effects because this is going on for wildlife across the planet. . . in other tourist settings,” lead study author and Utah State University physiological ecologist Susannah French tells The Scientist.

When tourist boats approach an island in the Bahamas, the endangered iguanas “rush the beach,” Emily Taylor, director of the Physiological Ecology of Reptiles Lab at California Polytechnic State University, tells The Scientist. The tourists are given lettuce and grapes to hand-feed the iguanas, who scurry up to grab the food. “They’re just little garbage trucks,” says Taylor, who didn’t work on the new study.

A woman in a bathing suit, sunglasses, and a sun visor sits on the sand and leans forward to feed an iguana a piece of lettuce that she's holding in her mouth. The iguana is craning forward with its mouth open, ready to take a bite.

On a 2019 expedition, California Polytechnic State University physiological ecologist Emily Taylor witnessed a tourist feeding an iguana lettuce that she was holding from her mouth, risking the lizard’s ferocious bite.


Study coauthor Charles Knapp, vice president of conservation research at the John C. Shedd Aquarium, says the mere sound of a boat engine draws the iguanas to shore, adding that the reptiles used to be afraid of humans to the point that they were difficult to spot before tourists started visiting.

Ecologists are well acquainted with the impacts that human ecotourism has on iguanas. French previously published a study showing that Galápagos marine iguanas (Amblyrhynchus cristatus) get stressed out from human activity: as human disturbances increased, they mated less and had more elevated stress hormone levels and weakened immune systems. Furthermore, a 2011 paper from another group found that Northern Bahamian rock iguanas exposed to tourists ate diets rich in sugary liquids and, sometimes, garbage, which caused gastrointestinal problems. The new research helps paint a more complete picture of how ecotourism impacts iguanas by focusing on their metabolism.

In the lab, researchers fed green iguanas (Iguana iguana) a glucose-rich diet for 17 days, then measured their blood sugar levels during a glucose tolerance test. Much like human diabetes tests, the test involved giving the lizards a high dose of glucose and taking blood samples, which revealed that the experimental group had higher peak blood sugar and took longer to return to baseline.

The researchers also measured the blood sugar levels and performance on a glucose tolerance test of rock iguanas on tourist-visited islands and two other islands that tourists don’t visit. They found that the tourist-fed iguanas had the largest and longest-lasting spike in blood sugar, indicating that they were the least able to regulate their blood glucose levels, which the team attributes to a lifetime of sugary snacks. French says that comparisons to diabetes are premature, however. “We can’t give it a syndrome or a name yet, and we don’t know the long-term health ramifications.” Further research will hopefully elucidate the extent of the threat feeding poses, she adds.

French and Knapp both tell The Scientist they hope that this and ongoing studies will help develop a management plan that’s sustainable for both the iguanas and the tourism industry that loves them. “We definitely don’t want tourism to stop; we think that’s an important part of conserving these animals,” French says.

After all, as Taylor notes, the endangered lizards are flourishing. “The irony here is that they’re thriving here because of this [tourism],” she says, “but maybe they’re thriving in an unhealthy way.”

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