Shrinking ploy of iguana has a lesson for us

Galapagos iguanas have developed a unique way to survive the El Nino events which disturb their Pacific Ocean home - they shrink. The iguanas can lose up to 20 per cent of their body length, about 2.7 inches, in as little as two years. Scientists from the University of Illinois have interpreted this as a method to boost survival during food shortages caused by the weather.

The shrinkage was noted in 1982, 1987, 1992 and in 1997, writes Dr Martin Wikelski, professor of ecology, ethology and evolution at the University of Illinois. The measurements were initially dismissed as errors, but a pattern was soon discovered, the research team reports in the current issue of Nature. Each of the shrinking years came when an El Nino event was under way.

"In 1997-98 the animals had shrunk too much to ignore," Dr Wikelski stated. "We thought that this couldn't be an artefact, so we plotted out the data."

The main iguanas, Amblyr hynchus cristatus, eat green and red algae, which is normally plentiful along the tidal basins of the Galapagos archipelago off Ecuador. Warmer waters delivered by an El Nino change the food supply to less digestible brown algae, however, putting the iguanas under pressure.

"In shrinking, they also get slimmer, and their mouths get smaller, making them more efficient at harvesting the tiny amounts of available algae," Dr Wikelski said. "They shrink to reach a body size where survival is high. If they shrank a centimetre or so, they already increased their survival rate by 10 per cent."

When the preferred food supply returns, the iguanas start growing again and regain their lost stature. The researchers believe that most of the reduction is accounted for by absorption of bone, so the ability to restore this lost material has scientists particularly interested. It may involve something that could be used in humans to reverse the effects of osteoporosis and restore lost bone density.

"We are looking for the mechanisms," Dr 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."