Iguanas repeatedly deplete and then regrow bone during and following times of food stress.
Introduction
Nearly two centuries after Charles Darwin’s eye-opening visit, the Galápagos islands continue to open our eyes to nature’s ability to adapt to unusual and extreme conditions.
The archipelago’s marine iguanas (Amblyrhynchus cristatus) depend nearly completely on species of red and green algae for food, which they forage for in intertidal areas, clambering over rocks or diving in the sea. The availability of their preferred algae can change dramatically and repeatedly over the iguanas’ lifetimes. When El Niño weather events cause the temperature and freshwater influx into the ocean to increase, they disrupt currents that normally bring nutrient-rich water up from the ocean depths, and foster the growth of the iguana’s preferred algae. During these events many iguanas starve. However, many other survive, due to an unexpected and remarkable .
The Strategy
When the iguana’s food source decreases, many iguanas get smaller in size, by as much as 20% of their regular length. By decreasing in size, iguanas increase the relative benefit of the available food. In larger animals, a greater proportion of food calories just go to maintaining existing biomass, leaving less for energetic activities (such as foraging). Smaller iguanas thus feed more efficiently than larger iguanas.
Because cartilage and connective tissue only account for about 10% of body length, it appears the shrinking in iguanas is also due to a loss of bone tissue. Moreover, when the food supply increases again, iguanas regain their original length, apparently rapidly rebuilding their bones. Scientists don’t yet understand the physiological processes by which these changes occur, only that they do.
The Potential
In an uncertain world, changing size to fit the available resources is a useful design strategy. At present, cities constantly face the challenge of either under- or overbuilding, for instance, resulting in unnecessary environmental impact and dramatic swings in things like housing prices. Imagine if bridges swelled in size during rush hour, or if hotels decreased in size and energy use with a decrease in occupancy.
Less abstractly, the marine iguanas’ apparent ability to regrow bone tissue could hold important lessons of value for postmenopausal women and older people suffering from osteoporosis, as well as for astronauts spending prolonged periods of time in space.
Marine iguanas at Galapago islands. August 2008.
A marine iguana stands tall on a rock, its body looks elongated, but it can shrink under various circumstances.
