The marine iguanas (Amblyrhynchus cristatus) of the Galápagos Islands are famous for their remarkable foraging behavior. Adept swimmers, they venture into the open ocean—sometimes diving as deep as 25 meters (82 feet) on a single breath—to graze on algae. Only the largest individuals can afford to descend to these extreme depths, however. Smaller iguanas lose body heat too quickly in the cold water so must stay in the shallows. In normal years, when the red and green algae these lizards prefer are abundant enough to support large populations (and competition for the easy-to-access seaweed is stiff), it’s advantageous for the iguanas to be large so they can forage in deeper—and greener—pastures. But when El Niño arrives at the islands, everything changes, including the iguanas themselves.

The rich seas around the Galápagos Islands are normally fed by an upwelling of colder, nutrient-rich water that fuels the region’s marine food chain. During El Niño years, however, this upwelling is cut off, causing surface temperatures to rise from an average of 18 degrees Celsius (64 degrees Fahrenheit) to a maximum of 32 degrees Celsius (90 degrees Fahrenheit). Red and green algae are among the many species impacted by these changes; they tend to disappear from the intertidal zone as the water warms, while brown algae moves in to take their place. Unfortunately, marine iguanas have a hard time digesting brown algae and extracting the energy and nutrients they need to survive.

Some individuals, like this intrepid forager photographed by Roy Mangersnes off the coast of Fernandina Island during the 2015–2016 El Niño, venture to increasingly deep waters in search of red and green algae. But they expend so much energy on these dives that they often can’t consume enough calories to offset the effort. The largest lizards are always the first to die in these conditions, since they require more calories and process food less efficiently than smaller animals. But all of the iguanas struggle. A strong El Niño event can reduce the population by as much as 90 percent. And while the population is currently able to recover in between El Niño events, which typically occur once every 3–7 years, recent models suggest that these events may become more frequent and intense in the coming decades because of global climate change. Indeed, the 2015–2016 El Niño was the strongest on record, and the National Oceanic and Atmospheric Administration (NOAA) predicts that it will soon be followed by another El Niño later this year—a return interval that has occurred only once before, in the 1960s.

While these circumstances may sound bleak, marine iguanas have proven to be quite resilient in the face of change—a necessity given their long average life span (28 years or more) and the frequency with which El Niño revisits the islands. One of their key survival strategies is a phenomenon that had never been documented among vertebrates until scientists at University of Illinois began studying how marine iguanas respond to El Niño conditions. Remarkably, the scientists discovered that when the lizards’ preferred meal grows scarce, they shrink by as much as 20 percent—not just becoming skinnier, but absorbing their own bone and reducing the length of their skeletons. Because smaller bodies require fewer calories, shrinking can give iguanas an advantage during challenging times, increasing their odds of surviving until more hospitable conditions return. And unlike humans, for whom bone regrowth after osteoporosis is largely impossible, the iguanas can grow new bone again once El Niño retreats.