Reversible bone shrinkage documented in Galapagos iguanas

CHAMPAIGN, Ill. -- Measurements showing vertebrate animals getting smaller during the course of a study normally are dismissed as measurement error or not possible. Eighteen years of data from the Galapagos Islands, however, indicate such shrinkage is both occurring and reversible.

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

Contact: Jim Barlow, Life Sciences Editor
University of Illinois at Urbana-Champaign

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