Galapagos’ Shrinking Iguanas

THE ANIMALS on Ecuador’s Galapagos Islands have developed amazing ways to adapt to their unique environment. In the last couple of years scientists have realized that marine iguanas are doing something once thought impossible in the natural world: when their food source is particularly low, the animals shrink, sometimes as much as 20% of their body length.

“When we put all the data in the computer and found in some years these animals were decreasing body size, we thought, this is totally wrong,” says University of Illinois Professor of Ecology, Ethology, and Evolution Martin Wikelski. “We mis-measured or we entered data in the wrong way.  Then after this long-term El Niño that happened between 1992 and 1997, we found that they shrank about 20% in body length, and we decided that this is way too big to be ignored.”

In the 12 years Wikelski has been studying the Galapagos iguanas shrinkage occurred in 1987-88, 1992-93, and 1997-98 – the same years that El Niños hit. The correlation was consistent, and the connection had to be food. Marine iguanas feed exclusively on algae, and when an El Niño turns cold, nutrient-rich waters warm, massive algae die-offs occur. Wikelski speculates that shrinking is a way to cope with decreasing food sources. Iguanas are cold-blooded and need to warm up in the sun before they can start eating. With a smaller body size they can warm up faster and feed for longer time periods. In addition, their smaller body makes them more efficient foragers.

Because El Niño is a natural weather phenomenon, animals like the marine iguanas have learned to adapt to its powerful effects in a variety of ways. Although an El Niño may kill 70% of the iguana population, the animals make a fast recovery.  They reproduce more often, reproduce at a higher rate, lay more eggs per clutch, and then after 3 or 4 years the population returns to normal. Potentially, body shrinkage can now be added to the list of adaptations marine iguanas have developed to deal with El Niños.

“We don’t know yet entirely what determines the shrinkage or what causes the shrinkage,” Wikelski explains, “So what we are looking at now are certain hormones [such as] growth hormones.” Wikelski and his team have begun to x-ray their study group of some 400 iguanas. (They started with 12,000 animals in 1981, but several El Niños have hit since then, and Wikelski decided not to add any new animals to the original group.) They are also taking blood samples of hormones that could be related to shrinkage. Future research will include simulating El Niño conditions to document the animals physiological responses to changing conditions.

“The thing that is more interesting is not the shrinkage itself, but the re-growth after shrinkage,” Wikelski says. “They shrink and re-grow again. If we understand how they do that, we can actually do something about osteoporosis or the problems that astronauts face [of bone density loss] in space.”

Research on marine iguanas by Wikelski and others will continue only if there are enough iguanas to study. Unfortunately on some of the islands, introduced predators – such as feral cats and dogs – are destroying marine iguana populations. Wikelski warns that if that problem isn’t solved, in 10 or 15 years some of the islands will lose their entire iguana populations. Wikelski has limited his research to undisturbed islands, where animals face only natural challenges. “It’s an ideal situation because in most of the animal populations you have predation and competition and lots of interaction with other species, but in marine iguanas you only have this one species and its interaction with the environment. You can really test a lot of ecological and evolutionary theory there.”

—Chris Hardman

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