Scientists have discovered marine iguanas that can shrink themselves when food is scarce and then grow again in times of plenty.

Some of the individual animals, which live on the Galapagos Islands, Ecuador, were seen to reduce their body length by a fifth (6.8 cm). It is the first time a vertebrate has been shown to shrink and then grow back to normal size.

Martin Wikelski and Carinna Thom of the University of Illinois at Urbana-Champaign, US, report the finding in the journal Nature. They looked at two populations of Amblyrhynchus cristatus, which are known as the Diving Dragons of the Galapagos for the way they jump off rocks into the water.

They say the changes in the size of the lizards cannot be explained merely by the shrinking of cartilage and connective tissue. The researchers believe the creatures must be capable of shrinking their bones as well.

"We don't understand the mechanism yet," Dr Wikelski told the BBC. "So we don't entirely know how they shrink. To prove they shrink their bones, we need to get a portable X-ray vehicle there and take X-rays before and after shrinkage."

Warm waters

The iguanas in the study became shorter during the climate phenomenon known as El Niņo, which occurs every two to seven years. During El Niņo warm waters in the Pacific move further east than is usual and nutrient-rich upwellings are disrupted.

This results in heavier rainfall in the Galapagos archipelago and the destruction of the red and green algal blooms that are the favoured food of Amblyrhynchus cristatus. Up to 90% of marine iguanas can die of starvation during El Niņo events.

But those animals that shrink their size can live through the famine. Smaller iguanas fare better because their foraging and use of energy is far more efficient. Females also do better because of their apparent ability to shrink further than males.

The researchers found that adult marine iguanas can switch repeatedly between growth and shrinkage, depending on the environmental conditions. In adult humans, in contrast, re-growth after skeletal shrinkage, as happens in osteoporosis, is impossible.

Dr Wikelski says other animals could share the iguanas' shrinking ability as previous observations in other creatures have probably been dismissed as data errors.