In the week centered July 22, the relative Niño 1+2 anomaly reached +3.0°C, 1.6°C above the benchmark Niño 3.4 and the widest gap in twelve weeks. The Galápagos archipelago sits in the eastern boxes, not the benchmark one. Here is what the peer-reviewed record says happens to the world's only marine lizard when that water warms, including the part the "shrinking superpower" headlines leave out.
Every few years a genuinely astonishing fact about El Niño makes the rounds: Galápagos marine iguanas survive the lean years by getting shorter. Not thinner, shorter. They appear to resorb substantial skeletal tissue, becoming measurably shorter, and later regain the lost length when food conditions improve. It is true, it is peer-reviewed, and it is almost always told as a heartwarming story about resilience.
It is not a heartwarming story. It is what starvation looks like in a lizard that cannot leave. And the coverage that carries it usually quotes the wrong number, because the index most articles cite is measured more than 3,000 kilometres from the animals in question.
ENSO is monitored through four rectangles drawn across the equatorial Pacific. Niño 3.4 (5N-5S, 170W-120W) is the benchmark, and it is the number nearly every headline uses. It is the principal region used in NOAA's operational classification of ENSO events, applied to seasonal rather than individual weekly values. It sits in the central Pacific. Niño 3 covers the eastern basin (5N-5S, 150W-90W), and Niño 1+2, the smallest box, hugs the coasts of Ecuador and Peru (0-10S, 90W-80W).
The Galápagos archipelago straddles the 90W line where those last two boxes meet. Its eastern islands, including San Cristóbal, fall inside Niño 1+2. Its western islands, Isabela and Fernandina, fall inside Niño 3. The precision goes further than is comfortable. The two study populations in the Nature paper that made the shrinking famous are Santa Fe, at 90°02'W and 0°50'S, and Genovesa, at 89°59'W and 0°19'N. Santa Fe misses the western edge of Niño 1+2 by two minutes of longitude. Genovesa clears the eastern edge of Niño 3 by one minute of longitude and sits nineteen minutes of latitude north of Niño 1+2, which puts it in neither box.
None of that means the indices are wrong. It means they were drawn to monitor a coupled ocean-atmosphere system, not to describe the water at any particular island. But it does mean that if you want a number that bears on Galápagos conditions, the eastern boxes are far closer to the point than the benchmark is. Right now those two answers are very different.
| Week centered | Niño 1+2 | Niño 3.4 | Gap |
|---|---|---|---|
| 6 May | +1.0 | +0.4 | 0.6 |
| 13 May | +1.3 | +0.4 | 0.9 |
| 20 May | +1.6 | +0.5 | 1.1 |
| 27 May | +1.7 | +0.5 | 1.2 |
| 3 June | +2.0 | +0.7 | 1.3 |
| 10 June | +2.1 | +0.9 | 1.2 |
| 17 June | +2.4 | +1.1 | 1.3 |
| 24 June | +2.5 | +1.2 | 1.3 |
| 1 July | +2.7 | +1.2 | 1.5 |
| 8 July | +2.6 | +1.3 | 1.3 |
| 15 July | +2.8 | +1.4 | 1.4 |
| 22 July | +3.0 | +1.4 | 1.6 |
Since the week centered 27 May the gap has sat between 1.2°C and 1.6°C, and it is at 1.6°C now, the widest of the twelve weeks shown. A reader looking only at the benchmark Niño 3.4 value would see +1.4°C, while Niño 1+2 in the far eastern Pacific stood at +3.0°C, more than double. For the full four-box picture and why the shape of an event matters as much as its magnitude, see our look at the widening east-west gradient.
Marine iguanas (Amblyrhynchus cristatus) are the only lizard known to forage in the sea. They graze algae off intertidal and subtidal rock, diving as deep as 30 metres, and they feed almost exclusively on algae. That single-item menu is the whole vulnerability.
The Galápagos are cool for the equator because of upwelling. Cold, nutrient-rich water rises around the islands and feeds the algae. During El Niño the thermocline deepens across the eastern Pacific and that upwelling is disrupted. Wikelski and Thom, citing Laurie's 1989 review of the 1982-83 event, describe Galápagos sea surface temperatures rising from an average of 18°C to a maximum of 32°C in El Niño years as the cold currents and cold upwellings shut down.
The consequence is not simply "less food." It is different food. The green and red algal species the iguanas prefer disappear from the intertidal zone and are replaced by brown algae, which the iguanas find hard to digest. In the 1982-83 event the intertidal was colonised by Giffordia mitchelliae, a brown alga not previously recorded in Galápagos. An iguana can be surrounded by visible algae and still starve.
This is the same physical mechanism that drives the Peruvian anchoveta collapse a few hundred kilometres east, which we covered in the anchovy and fishmeal chain. Deepened thermocline, suppressed upwelling, primary production falls, everything above it in the food web pays.
Martin Wikelski and Corinna Thom published the finding in Nature in January 2000, drawing on two long-running mark-and-recapture studies of individually marked animals, one lasting 18 years on Santa Fe and one lasting 8 years on Genovesa. The headline number: individuals became shorter by as much as 20 percent, up to 6.8 centimetres of snout-to-vent length, within two years. Body length increased again during subsequent La Niña conditions.
Three qualifications belong with that number and are almost always dropped.
The bone claim is an inference, and the authors say so. Cartilage and connective tissue together make up only about 10 percent of total body length, so a 20 percent reduction cannot be explained by soft tissue alone. Wikelski and Thom concluded that bone resorption accounts for much of it. They drew the analogy to astronauts, whose long bones lose both density and length during long spaceflights under conditions of limited exercise and high stress hormones, and noted that marine iguanas in El Niño years get very little exercise from feeding and run very high corticosterone.
And the shrinking does appear to work. Among adults longer than 300 mm measured before and after the 1992-93 event, the individuals that shrank most survived significantly longer. The reason is unglamorous: smaller grazers feed more efficiently and spend less energy. That is also why the largest iguanas have the highest mortality.
The most recent addition to this picture is from September 2025. Shahar Dubiner and colleagues at Tel Aviv University, working with the Galápagos Science Center, measured resting metabolic rates in young marine iguanas on San Cristóbal. They sampled in February 2024, near the end of the 2023-24 El Niño. Niño 1+2 averaged 27.0°C that month, following a year in which anomalies averaged +2.1°C and peaked at +3.3°C. They measured 47 juvenile and subadult marine iguanas then, and returned in March 2025 to measure a separate group of 50. After quality-control exclusions, the primary resting-metabolism analysis included 36 animals.
During the El Niño the young iguanas carried 17.4 percent less mass for their length and were 11.8 percent slimmer. That much is expected from a starving animal. The finding is what remained after the researchers corrected for size: an additional 20.3 percent reduction in resting metabolic rate. The animals were not just smaller, they were running slower for their size. The team suggests suppressed growth and shrunken high-cost organs such as heart, gut, liver and muscle, with warmer diving water plausibly contributing.
They focused on juvenile and subadult animals because those stages are highly vulnerable, and because metabolism may reveal an alternative energy-saving response in individuals that shrink less than adults do. Note also that this compares one El Niño year against one following year at one site. It is a clean result, not a long time series.
Here is the part the viral version omits. Shrinking is a response to starvation that improves the odds of an individual animal. It does not prevent the population from collapsing, and it does not make El Niño survivable in any general sense.
Reported mortality varies substantially by event, island and population. Laurie and Brown documented approximately 60 to 70 percent mortality from starvation in their study population during 1982-83, while subsequent work found losses approaching 90 percent on some islands during severe events, including a genetic study of eleven island populations following the 1997-98 event. Neither figure should be treated as a universal archipelago-wide mortality rate. Some islands lose almost everything while others get off comparatively lightly, depending on local upwelling, exposure and which algae recolonise.
Note also what these figures are not. They are historical outcomes from the 1982-83, 1992-93 and 1997-98 events, and from the 2024 event in the metabolic study. They are not a forecast for 2026. Marine iguanas can live for decades and can switch repeatedly between growth and shrinkage across a lifetime, which means many long-lived adults on those rocks today may have experienced previous El Niño food shortages.
The iguanas are a good reminder of why this site cares which box a number came from. An animal does not experience an index. It experiences the water it is sitting in, and the algae that water will or will not grow. You can track all four Niño regions as they update on our teleconnections dashboard, and read why NOAA changed the reference index in our RONI explainer.