Shrinking Iguanas.

• Author: Hardman, Chris
• Position: Adapting body weight to food supply - Brief Article

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 percent 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 Martin Wikelski, University of Illinois professor of ecology, ethology, and evolution. "We mis-measured, or we entered data in the wrong way. Then after this long-term El Nino that happened between 1992 and 1997, we found that they shrank about 20 percent in body length, and we decided that this is way too big to be ignored."

In the twelve years Wikelski has been studying the Galapagos iguanas shrinkage occurred in 1987-88, 1992-93, and 1997-98--the same years that El Ninos hit. The correlation was consistent, and the connection had to be food. Marine iguanas feed exclusively on algae, and when an El Nino 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 Nino 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 Nino may kill 70 percent 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 three or four 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 Ninos.

"We don't know...