ENHS Nature Trails March 2000 Article 2

Shrinking Iguanas (and we don't mean population)

Two biologists, Martin Wikelski and Corinna Thom, doing long-term studies of marine iguana (Amblyrhynchus cristatus) populations on the islands of the Galapagos, made an astonishing discovery which they reported in the January 6, 2000 issue of the journal, Nature. During times of famine, which coincide with the periodic El Nino events, marine iguanas can shrink by as much as 20% of their body length and then grow again when food supplies rebound after sea temperatures drop. Wikelski and Thom are not just talking about loss of body fat nor of loss of cartilage and connective tissue, but about actual reabsorption of bone.

There seem to be good reasons to favor the survival of iguanas which do shrink. Smaller animals feed more efficiently, that is, make better use of calories consumed and spend less energy foraging than larger individuals. In a group of iguanas measured during the 1992-93 El Nino event, the animals which shrank more, survived longer. In the measurements which Wikelski and Thom made during the most recent El Nino of 1997-98, females shrank more than males, and larger individuals shrank more than smaller ones.

Marine iguanas feed on both green and red algae which cannot live in the warm water temperatures caused by El Nino events. When sea temperatures soar close to 32 degrees Celsius (well into the 80's Fahrenheit), the iguanas have to feed on brown, intertidal algae, which are neither very digestible nor nutritious. With little available food, the iguanas do not exercise much, and their levels of corticosterone, which indicates stress, are high.

Similar conditions of immobility and stress cause astronauts to reabsorb bone, but, and here is a very interesting and important point, so far amongst us vertebrates, only iguanas have ever been found to regrow bone. If the physiological basis for this bone regrowth could be found, the medical implications, especially for older people, could be profound. The shrinkage and growth which marine iguanas experience must occur several times during the approximately 28 year lifespans of these humble creatures.

Their evolutionary adaptation to the extremely harsh conditions of the archipelago where they have evolved is just another of the wonders of this amazing, natural laboratory that Charles Darwin first brought to the attention of the thinking public nearly a century and a half ago.

Reida Kimmel

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