El Niño events warm the surface water around the Galápagos and displace the nutrient-rich upwelling that sustains the red and green algae marine iguanas graze. What grows instead is often a brown alga they digest poorly. Starvation follows, and mortality in bad years has run very high in some populations.
Long-term mark-and-recapture work by Martin Wikelski and colleagues, published in Nature in 2000, tracked individual iguanas across successive El Niño cycles and found that adults did not merely lose condition: measured snout-to-vent length decreased, by up to around a fifth in the most extreme cases, and increased again when conditions improved. Because an adult lizard's length is set by its skeleton, this implies reversible shortening of bone, most plausibly through resorption of connective tissue and bone. Smaller animals need less food, so shrinking is likely adaptive rather than simply damage.
The mechanism at the tissue level has not been observed directly in living animals, and how the process is regulated remains unresolved.