Do Marine Iguanas Shrink? Surviving El Niño Famines

What did scientists discover?

Martin Wikelski and Corinna Thom studied marine iguanas on two islands of the Galápagos, measuring and marking thousands of animals, about 6,000 in all, across 18 years and four El Niño events: 1982–83, 1987–88, 1992–93 and 1997–98. They measured snout-to-vent length, the standard way to measure a lizard’s body without its tail.

Over and over, some adults came out shorter than they had been a year or two earlier. At first the researchers assumed they had simply made measuring errors. But the shrinking always coincided with El Niño years, and in more than 100 marked iguanas it was clearly reversible. Some lost as much as 6.8 cm (2.7 in) of body length within about two years, far beyond any plausible error.

When the results were published, marine iguanas were hailed as the first vertebrates shown to shrink in length as adults and then grow back.

Why does El Niño starve marine iguanas?

Marine iguanas live almost entirely on red and green algae, which grow well in the cold, nutrient-rich water that normally wells up around the Galápagos. During El Niño, warm, nutrient-poor water spreads across the eastern Pacific. The iguanas’ favored algae die back and are often replaced by brown algae that they cannot digest properly.

The result is famine. On some islands, El Niño events have killed up to 90 percent of marine iguanas. Large males, which need the most food, tend to suffer the heaviest losses. More about their diet is in How do marine iguanas feed?

How can an animal get shorter?

Losing weight in hard times is common, but losing length is unusual, because body length depends on the skeleton. Wikelski and Thom calculated that shrinking of soft tissue, such as cartilage and connective tissue between the bones, could account for only about a 10 percent reduction. To shrink by 20 percent, they argued, the iguanas’ bones themselves must get shorter, probably as bone material is broken down and absorbed by the body.

The researchers suggested that stress hormones, especially corticosterone, which rises during starvation and can break down bone, might drive the process. When food became plentiful again, survivors first put on weight and then grew longer, rebuilding what they had lost. Exactly how the bones shrink and regrow is still not fully understood, and some scientists have argued that X-ray studies of individual iguanas are needed to settle it.

How does shrinking help iguanas survive?

A smaller body needs less food, which matters when algae are scarce. The researchers also noted that shrinking iguanas became slimmer and had smaller mouths, which might make them more efficient at cropping the short, sparse algae left during a famine.

Their data supported the idea that shrinking is linked to survival:

• During the 1992–93 El Niño, large adults that shrank more lived longer than those that shrank less.
• Larger iguanas, those over about 30 cm (11.8 in) snout-to-vent length, shrank the most.
• In the 1997–98 El Niño, females shrank more than males, and bigger animals shrank more than smaller ones.
• Even about 1 cm (0.4 in) of shrinkage was associated with noticeably better survival.

Stress hormones and survival

A follow-up study by L. Michael Romero and Martin Wikelski, published in 2001, measured the stress hormone corticosterone in iguanas from six populations during the 1998 El Niño famine and the following La Niña, a time of plentiful food. Iguanas had higher hormone levels during the famine, and the average levels on each island predicted how many iguanas there would survive. On some islands only about 15–20 percent died, while on others 65–70 percent did.

These findings fit the idea that the hormonal stress response is closely tied to how iguanas cope with famine, including, perhaps, shrinking. Later research has also found that during El Niño, marine iguanas are much thinner for their length and have lower metabolic rates, saving energy in every way they can.

Is shrinking an adaptation or just starvation?

Not every scientist was convinced that shrinking is a clever survival strategy. Some researchers pointed out that adult marine iguanas spend only a small fraction of their daily energy diving for food, so being a little shorter might not save much. They suggested that shrinking could simply be a side effect of severe starvation, with bigger animals shrinking more because they starve harder.

Others, who had seen hints of “de-growing” in other lizards and salamanders, noted that such changes are usually too small to separate from measurement error. The marine iguana stands out because the shrinkage is large and repeated across many years. Whether shrinking evolved as an adaptation, or is a lucky consequence of how the body copes with hunger, is still discussed.

Other animals that shrink

Marine iguanas are not the only animals to change size seasonally. Common shrews and some other small mammals, such as weasels, shrink their skulls and even their brains in autumn and winter, then partly regrow them in spring. This is called Dehnel’s phenomenon, after the Polish zoologist who described it in 1949, and it probably helps these tiny animals save energy when food is scarce.

Will climate change make things harder?

Marine iguana populations are resilient. When El Niño cuts their numbers by 30–50 percent, they generally recover within about four years. But some climate models suggest that strong El Niño events may become more frequent as the world warms, which could give iguanas less time to recover between famines. Combined with introduced predators and other threats, this is one reason the IUCN lists the marine iguana as Vulnerable.