Reversible Bone Shrinkage Documented In Galapagos Iguanas

In the Jan. 6 issue of the journal Nature, scientists studying marine iguanas of two island populations report that the herbivorous reptiles shrank as much as 6.8 centimeters (2.7 inches) -- up to 20 percent of body length -- in two-year time spans. The iguanas (Amblyrhynchus cristatus) were shrinking, the scientists report, to boost their survival during a change in the weather.

Shrinkage was noted in 1982-83, 1987-88, 1992-93 and 1997-98. The measurements were noted and dismissed, but a pattern was soon discovered: Each of the shrinking periods came in El Niño years.

“In 1997-98, the animals had shrunk too much to ignore,” said Martin Wikelski, a professor of ecology, ethology and evolution at the University of Illinois. “We thought that this couldn’t be an artifact, so we plotted out the data. It turned out to be very interesting.”

The iguanas eat algae along the tidal basins of the rocky shores of the Galapagos archipelago off Ecuador. The islands normally experience cold, nutrient-rich currents from both the west and south. During El Niño years, however, warm currents and heavy rains raise water temperatures. Less digestible brown algae replaces the iguanas’ preferred green and red algae.

In years immediately after El Niño events, surviving iguanas ate well and got fat, then started growing longer again, Wikelski said. For instance, 600 iguanas were measured and marked in 1992. Following the subsequent El Niño, they were monitored. The larger iguanas -- those more than 300 millimeters (11.7 inches) from snout to anus -- shrank the most and survived the longest.

“In shrinking, they also get slimmer, and their mouths get smaller, making them more efficient at harvesting the tiny amounts of available algae,” Wikelski said. “They shrink to reach a body size where survival is high. If they shrank a centimeter or so, they already increased their survival rate by 10 percent. If they shrink more, they can increase survivability by 35 percent.”

Wikelski and co-author Corinna Thom, a biologist at the University of Wurzburg in Germany, theorize that bone absorption accounts for much of the shrinkage, because a reduction of connective tissues between the bones cannot account for it, and that high levels of corticosterone may be involved.

Perhaps as interesting to the bone shrinkage is the renewed growth of bone. Humans suffering from osteoporosis as a result of aging or space flight are unable to recover from the loss of bone length and density, especially losses in long bones.

“We are looking for the mechanism,” Wikelski said. “Is it a certain hormone or combination of hormones, or is it some other physiological mechanism that tells bone to regrow and recalcify? I think these findings are potentially important for all kinds of vertebrates.”