Courtesy Aquaimages, Wikimedia

Despite Charles Darwin’s reputation for being a respectful observer and recorder of animal life, he apparently had a “least favorite” list topped by marine iguanas. In great numbers, these lizards populate the Galapagos Islands, which straddle the equator 800 miles off the coast of Ecuador. Darwin visited in 1835 and wrote, “The black lava rocks on the beach are frequented by large (2–3 ft), most disgusting, clumsy lizards.” It is “a hideous-looking creature, of a dirty black colour, stupid and sluggish in its movements.”  Soon after he continues, “These hideous reptiles may oftentimes be seen on the black rocks, a few feet above the surf, basking in the sun with outstretched legs.” 

Not only was the marine iguana’s appearance offensive to Darwin, so was its behavior. “One day I carried one to a deep pool left by the retiring tide, and threw it in several times as far as I was able. It invariably returned in a direct line to the spot where I stood.”

Darwin, however, limited his disdain. “The meat of these animals when cooked is white, and by those whose stomachs rise above all prejudices, it is relished as very good food.”

In 1959 the Galapagos Islands became an Ecuadorian National Park, so no more iguana tossing or munching, but the lizards have remained intriguing. A group of modern-day ecologists have studied them and discovered an unexpected survival feature—the marine iguanas’ bodies shorten and lengthen repeatedly over years according to el Niño/la Niña climate oscillations.

When times are good and the algae on which they feed is plentiful, the marine iguanas grow longer. When their principal food source is scarce, their bodies shorten by up to 20% of their length. (That is comparable to average height humans losing a foot in stature and then springing back to their baseline height repeatedly according to dietary intake.)  In 2000, the investigators published their results in Nature, a leading scientific journal. They noted that the marine iguanas’ cartilage and fibrous tissue could account for no more than half the shift, which led them to the only plausible conclusion: the bones shrink.

In 2019, another group of ecologists published an article about shrews (mammalian mice-like critters). The scientists reported that from the shrews’ first summer to the end of their first winter, their brain cases (skulls) size shrank by 13% and then enlarged by 10% the following summer. The researchers speculated that this phenomenon saved energy, which allowed the shrews to successfully mate in their second summer.

Across zoology, regulation of skeletal size varies by animal class. Birds and mammals grow to a certain size and then stop. Fish, frogs, and reptiles, however, never completely stop growing. Humans do get shorter in old age because the cushion-like cartilage discs between our vertebrae flatten, but the bones themselves do not shrink.

Shrews and marine iguanas are therefore apparently unique in their ability to change the length of their bones, and for the latter, repeatedly. I say apparently for two reasons. Other animals could possibly do the same but have not yet fallen between the tips of an ecologist’s measuring callipers. More importantly, however, the reported observations fly in the face of what bone biologists know about how bones grow, mature, and age.  

Whereas ecologists include in their queries the interplay of animals and their surroundings, bone biologists scrutinize the cellular and molecular nature of bone formation and maintenance. For a bone to shrink, specialized bone-dissolving cells on its surface would have to remove material. Simultaneously bone-forming cells in the hollow interior would have to add substance, otherwise the bone would disappear. Then when it came time for the bone to enlarge, the opposite inside-outside activity would have to occur. Nobody has observed anything of this sort.

After both groups of ecologists reported their findings, they moved on to other environmental inquiries. Other scientists have not challenged or extended the original findings. This is understandable because the task would be problematic. For laboratory-housed shrews, a careful multidisciplinary analysis would be possible; but because the shrews naturally die during their second summer, they would not afford investigators the opportunity to follow the changes through more than one cycle.   For marine iguanas, the investigation would be herculean. It would require stumbling across rough volcanic boulders far out in the Pacific Ocean, catching, tagging, and measuring hundreds of specimens, drawing their blood for detailed high-tech analyses, studying radiographic images and biopsies of their vertebrae, and then repeating the process at several year intervals over the animals’ nearly 30-year life spans. Complicating the study further, 90% of marine iguanas may die of starvation due to climate oscillations expected over a decades-long study.

10 second video. Watch one walk.

This chasm between what the ecologists know about how environmental changes affect skeletal size and what bone biologists understand about bone formation and growth is deep. The gap highlights an aspirational principle in the design of scientific research—cross-fertilization among specialties brings multiple viewpoints to bear and likely yields deeper insights. For the present conundrum, the ecologists have one set of facts, the bone biologists have another. They should talk. Discovery and control of the involved molecular and cellular mechanisms could hold great promise for preventing and treating both weak and short bones.

Does such research sound intriguing? I think so. I’d be willing to sign on as a cook for the Galapagos expedition; but sorry, Charles, no more iguana stew.

Coming June 2023: a companion book all about muscle

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