Journal Archive > 2001 > October

Paradise Lost

Marine iguanas provide clues to causes of stress

On a traffic-choked highway, a weary driver is trying to make it home for dinner. He's hot; he's tired; he's losing time—and maybe his temper.

On a beach in the Galapagos Islands, a marine iguana is trying to find some algae for its dinner. But the iguana is coated with oil, the result of a fuel spill. The feeding ground, too, has been infused with an oily residue.

Biologist Michael Romero with a marine iguana on the Galapagos Islands.

The cranky commuter shares in the plight of the iguana: They're both under stress. And while the causes and consequences of their dilemmas may differ, they're undergoing similar physiological changes brought about by that stress.

Learning about what causes stress in iguanas could help scientists better understand what causes stress in humans, says Michael Romero, assistant professor of biology.

Partners in stress
"People and animals, [stress] is what we all share," says Romero. "We all have common hormones released during stress."

For the past half-century, scientists and lay people alike have focused on the effects—often detrimental—of stress on humans. But there's still little understanding of the physiological causes of stress, Romero says.

And it's often forgotten that stress evolved as a coping mechanism to help humans survive dangers like storms, famines or predators. Animals still function in this way, and Romero's work has concentrated on the stress response in wild animals.

One of his most recent projects involves marine iguanas in the Galapagos Islands and their response to two types of environmental disasters—one caused by nature, the other by humans.

Romero and a colleague started out studying the iguanas' stress response to a natural calamity—the loss of their food supply due to the El Nino weather system in 1997-98. But the researchers also had a chance to examine the iguanas' response to a man-made calamity—the spill of nearly 200,000 gallons of oil from a grounded tanker this past January.

"The two aspects came together in the Galapagos," said Romero. Working with Martin Wikelski of Princeton University, Romero went to the Galapagos, an ecologically unique archipelago off the coast of Ecuador made famous as the inspiration for much of Darwin's work. The project was funded by Tufts, the National Science Foundation and the Charles Darwin Station in the Galapagos.

The islands were feeling the effects of El Nino, during which the trade winds cease to blow and the normally cool water surrounding the islands heats up. This produces a lot of rain, and the desert terrain of the Galapagos becomes rich in plant life. But the now-warm water does not contain as many nutrients as it did previously.

That's deadly for the marine iguanas, which survive solely on algae that need cold, nutrient-rich waters to breed. As the islands became green and lush, the iguanas, faced with an 18-month famine, began to die off.

But not all the iguanas perished. "The question was: Why didn't they all die?" Romero said. "We were very interested about why, within a bunch of animals that were in deep trouble, some would be OK."

People and marine iguanas have something in common—their physiological response to stress.

Hormonal indicators
Romero and his team measured the levels of corticosterone, the stress hormone specific to iguanas that is the equivalent of cortisol in humans. They discovered that the higher the hormone level, the more likely the iguana was to die.

"The really new thing about the study is that we sampled blood from iguanas on six different islands," Romero said. "The corticosterone level in all the iguanas still alive determined the survival rate during the El Nino. We could predict the percentage survival rate on the islands, depending on the level of corticosterone in the animals on the island.

"On some islands, 15 to 20 percent died. On some others, 65 to 70 percent died," he said. "Those with the highest levels of corticosterone had the highest mortality."

This past January, with El Nino over, Romero and Wikelski returned to the Galapagos to collect more blood samples from the iguanas. Three days after their return to the United States, the Jessica, a Jamaican ship flying under the Ecuadorian flag, ran aground on San Cristobal Island in the Galapagos, discharging almost 200,000 gallons of diesel and bunker oil throughout the island chain. Ironically, the ship was transporting fuel for eco-tourism boats in the area.

"It was incredibly rare, an opportunity to measure the effect of a manmade disaster," Romero said.

At first glance, it was not regarded as a disaster of epic proportions. "It wasn't like the Exxon Valdez," Romero said. "You didn't have lots of animals washing up. Everyone thought: ‘We avoided disaster.' It was really downplayed in the media."

Romero and Wikelski enlisted the help of researchers and graduate students based at the Darwin station in the Galapagos, who collected blood samples from the post-spill iguanas. Two-thirds of the reptiles had visible oil on their bodies; one third looked normal, Romero said.

"Yet all the stress hormones in the animals were elevated from the previous visit," Romero said. "It didn't make any difference whether they were covered with oil or not."

Mortality predictors
Romero and his colleagues speculated that all the iguanas might have been ingesting oil through their diet of marine algae—an indirect, but substantial result of the oil spill.

"If we can rely on the stress levels recorded during El Nino, we can predict how many will die from the after-effects of the oil spill," Romero said. "We predict that 30 percent should die." The researchers do not yet have enough follow-up data to know whether their prediction was correct.

The lesson, Romero said, is that the repercussions of environmental events can carry on for years.

"Take how [the oil spill] was portrayed in the media and by the oil companies—as no big deal," he said. "The long-term outlook on the wildlife may not be obvious at all, if you simply look at the oil."

Regardless of their initial physical condition, he said, all the iguanas will suffer from the spill, and all have demonstrated that they are under stress, "even from an oil spill that ‘wasn't that bad.' "