Why Do Marine Iguanas Sneeze? How They Get Rid of Salt

Why do marine iguanas sneeze so much?

The marine iguana of the Galápagos is the only lizard that feeds in the ocean. It eats red and green algae, either grazed from rocks washed by the waves or cropped from the seabed during dives. Every mouthful comes coated in seawater, and the algae themselves are salty, so the iguana takes in a large load of salt every time it feeds.

Too much salt in the blood is dangerous for any animal, because it draws water out of the body’s cells. A marine iguana must therefore get rid of salt constantly, and it does so mostly through its nose. Snorting out salt is so routine that a colony of resting iguanas is often punctuated by small sneezes and puffs of spray.

Why can’t marine iguanas get rid of salt through their kidneys?

Mammals and birds have kidneys that can make urine more concentrated than their blood, which is how they flush out extra salt. Reptile kidneys cannot do this: they cannot produce urine saltier than the blood. If a marine iguana relied on its kidneys alone, it would have to lose huge amounts of water to get rid of its salt.

Salt glands solve the problem. These small organs pump salt out of the blood and release it in a fluid that is far more concentrated than the blood itself. The iguana gets rid of the salt while losing much less water.

How do the salt glands work?

A marine iguana has a pair of salt glands inside its snout, connected to the nasal passages. They are so large that the marine iguana’s skull has an unusually roomy nasal cavity compared with other iguanas.

The glands are packed with cells that actively move salt ions, such as sodium, chloride and potassium, out of the blood and into tiny ducts. The salty fluid collects in the nostrils, and the iguana expels it with a sharp, sneeze-like snort. When the spray dries, it leaves white crystals of salt on the head and around the nose, which is why many marine iguanas look as if they have gray or white hair.

The process was first studied in the 1950s, when the physiologist Knut Schmidt-Nielsen and his colleagues showed that marine reptiles, like seabirds, use special glands to excrete salt.

Is salt sneezing the same as a cold?

No. The sneezing has nothing to do with illness or allergies. It is a normal, constant part of a marine iguana’s life, most frequent after feeding, when the salt load is highest.

Visitors who sit quietly near a resting colony often hear the iguanas snort and may see the fine spray. As with all wildlife watching in the Galápagos, park rules ask people to stay at least 2 m (6.5 ft) away and never to touch the animals.

Other animals with salt glands

Many animals that live in or beside the sea, or eat salty food, have evolved salt glands, but they are found in different places on the body.

Seabirds have some of the most powerful salt glands of all, which is why gulls can drink seawater, as explained in Can seagulls drink salt water? A green sea turtle eats salty seagrass and algae like the marine iguana and clears the salt through glands by its eyes, which makes nesting females look as if they are crying.

Crocodiles show how important salt glands are. Saltwater crocodiles and other true crocodiles have salt-secreting glands on their tongues, which help them live in estuaries and the sea, while alligators have weak or no salt glands and stay mostly in fresh water. More on this in Can crocodiles live in salt water?

Why the salt glands matter for survival

Salt glands are one of the adaptations that let the marine iguana live as no other lizard does. Together with a flattened swimming tail, strong claws for gripping rocks, blunt snout and sharp teeth for scraping algae, and dark skin for warming up quickly after a cold dive, they allow a lizard to make its living from the sea.

Without its nasal glands, a marine iguana could not process its salty diet. The sneezing that amuses visitors to the Galápagos is in fact a key part of what makes this unique reptile possible.