Illustration by Elwood Smith

In the Galápagos Islands, iguanas face one very big threat to their survival. Every few years, El Niño conditions wipe out the marine algae they eat. It's a tough situation: not every iguana survives.

Like humans, iguanas have hormones that regulate their stress response, and Tufts University biologist L. Michael Romero thought they might play a role in differentiating those that make it from those that don't. So back in 2002, before El Niño conditions settled in, Romero and his team captured and tagged 98 iguanas and measured the level of a key stress hormone—corticosterone—at rest and under duress, as well as the rate at which the secretion slowed to a resting level. After El Niño they returned to find that the iguanas that fared best were the ones that turned the response off fastest once they knew the danger had passed.

The key, Romero explains, is that corticosterone dials down digestive functions in order to redirect energy to stress response. As a result, the animals that were calmer stored more energy from what little food they could find. Similar stress responses may affect survival for animals in the Gulf of Mexico, Romero says. He suggests not over-handling rescued animals and making sure their pens are large enough.