Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
SAF Archives  search ask the scientists in the classroom cool science
scientists from previous shows
cool careers in science
ask the scientists

Photo of Martin Wikelski Martin Wikelski as seen on Voyage to the Galapagos: Lizards of the Sea

Click on Martin's photo to read a brief bio.

q What kind of water do the iguana drink? If salty, how does their body expel the salt? If not salty, how much rainfall is needed to maintain them? (Question sent by Tom of Simi Valley, CA)

A Hi Tom, marine iguanas ingest lots of salt water with the food. Sometimes, especially when they dive, they have almost their entire stomach full of water (ca. 80 ml in a 3000 g animal). They excrete the salt through special salt glands, the most powerful ones in reptiles. Then, they blow highly concentrated salt solution off through their nostrils. Marine iguanas do not drink fresh water. Land iguanas also excrete salts, but they ingest less concentrated salt solutions from plants. Land iguanas do not need any additional water, they suffice with the water they can get from fruits, leaves and greenery.

q After watching the show, I am curious to learn more about the marine iguana and it's amazing adaptations. What is your theory about the original ancestor of the marine iguana what is was and where it came from Thanks. (Question sent by Chrissy)

A Hi Chrissy, we know that the marine iguanas split off from the ancestor about 10 million years ago. This could have happened on the continent, or in the Galapagos, because there exist old 'islands' (now submersed) in the east of today's Galapagos islands that are as old as 15 million years. The most likely ancestor was somewhat like the Ctenosaurus lizards (Black iguanas) of Central America. An interesting note: When the marine iguanas split from their ancestor, the Panama land bridge did not exist yet, thus they could have come/floated out from the Caribbean to the Galapagos. This is likely, because the major radiation/speciation of iguanids happened in the Caribbean.

q If there is another El Nino event that threatens the iguana population in future, will you or other scientists try measures to save them (like bringing in seaweed for them to eat)? Or do you think its better to just let "nature take its course"? (Question sent by Paul)

A Hi Paul, we will let nature take its course, because we can not decide what is best for the marine iguanas. Our knowledge about history is so poor, but nature has influenced these creatures for millennia. This is why they are what they are. If we mess with the selective forces of nature, we could entirely mess up this fascinating species. Nature knows better than we do, and if we help the weak animals survive during an El Nino, there will be no chance for the strong ones to propagate their genes. Just imagine, marine iguanas would learn to eat on land if we feed them there. There would not be marine iguanas any longer, maybe. Besides all that, the National Park prohibits the feeding of animals, for exactly the reasons given.

q How many eggs can one female iguana lay? How many times a year do they lay their egg(s)? I like reptiles so I enjoyed the PBS show very much. (Questions sent by Darci and other viewers)

A Hi Darci, female iguanas lay between one and six eggs, and they do this once a year at the most, and often skip a year or two. The normal clutch size of a medium sized female (1 kg) is two. All females on Genovesa lay only one egg. Only the huge females on Fernandina (4 kg) can lay up to six eggs. Females will lay more eggs after an El Nino, when population density is lower. Overall, the entire clutch makes up about 25% of a female's body mass. Thus, in Genovesa, one egg is a quarter of the body mass of a female.

q Dear Professor Wikelski, I am studying the Galapagos Islands and I am wondering what the adult marine iguanas' predators are. (Question from Westridge School)

A Good question! There are no native predators in the Galapagos for adult males. Adult females are sometimes caught by Galapagos hawks in the nesting area, but only the weak individuals are taken. However, adult animals ARE caught by feral dogs, which is a major conservation concern!

q Dear Professor Wikelski, I was searching your web site, and discovered that iguanas are tame, so they are easy to study. I was wondering if they were tame because they are used to all of the scientists coming onto the island, or if that behavior is just natural. (Question from Ashley)

A That is a fantastic question - and one we have not answered yet in detail. The funny thing is: both answers are right: Galapagos animals are very shy on first encounter, i.e., if they have NEVER seen a researcher or a tourist. But, they get used to humans incredibly fast. So, if a Galapagos animal sees you for the second or the third time, it will already be tame, or almost tame. We do not know yet why this habituation ('getting used to') is so much faster in Galapagos animals than in other creatures.

q Do the colors on the marine iguana have an effect on which mate the females choose? If this is so, why does it? Thank you. (Question from Tyler)

A Great question! In other lizards, color makes a big difference in mate choice. Color is often an honest signal of the quality of a male, i.e. if a male can eat a lot of high-quality food, it will have a brighter color than another male. Thus, females should choose him over other males. However, in marine iguanas, color does not indicate high-quality food. Accordingly, females do not choose males in relation to their color. We tested this by painting males all different colors and subsequently looked at the reaction of females - they did not care the least! We hypothesize that the red color in marine iguanas is derived from their algae food, and the color pigments accumulate in the skin during the mating season, because males do not change their skin any more - they put all their energy into reproduction instead. Thus, color in male marine iguanas only tells the females that a particular male is in reproductive mood, but not much more. So, they use activity instead as a gauge for the quality of a male.

q Why do marine iguanas have spikes on their heads - what are they used for and how do they help them? (Question from Chandra, Crystal, and Shane, Williston, N.D.)

A The question about the spikes is a really good one - and one that I cannot quite answer, unfortunately. I can only speculate, as we have not yet conducted any detailed study on the function of the spikes. The most likely function is that they are used during head-push fights. During the mating season, iguana males fight a lot with each other and often 'bump heads'. Thus, one could view them as some form of 'antler', a weapon that helps during fights.

q How fast can marine iguanas swim? How deep can they dive? (Question from Brittany)

A Marine iguanas swim fairly slowly. The speed depends on their body length and the body undulation. For example, an animal of a total body length of 1 meter travels 0.5 meters per undulation. Since it can make about 1 undulation per second on average, its speed would be 50 cm (0.5 meters) per second, which is 30 meters per minute, or 1.8 km/hour. However, it would not be able to keep up this speed for very long. The speed of diving is faster than this, I would estimate that they can get up to 5 km/hour for short distances during diving. Iguanas can dive up to 25 meters, and probably even deeper. However, they routinely only dive to about 5 meters on St. Fernandino and 12-15 meters on Espanola.

q When you mark the iguanas does it affect anything in their life style? (Question from Jeremy, Heather and Devin)

A This is a really good question. Most other lizards are affected by changes in their skin coloration. Not so the iguanas. Or say, their social role is not affected. We tested this by painting marine iguanas with different colors, e.g., painting a red territorial male black, or a non-territorial black male red. No other iguana cared about it, i.e., the painted iguanas did not have higher or lower rates of social interaction after the painting treatment. However, the thermoregulation properties are slightly affected by color. The effect is minimal for the paint we use, but when iguanas change naturally from being black to being red (during the mating season), this really affects how fast they can warm up and how long the can stay in the sun. Normally, iguanas want to be black - they can warm up faster and thus forage for longer in the cold sea. However, during the mating season, iguanas do not (or hardly ever) forage, but have to sit in the sun all day long. So, it is better to be red then, and not warm up too fast, so that you can sit on your territory even when your black neighbor iguanas has to go into the shade.

q On your web site about marine iguanas ( you say that Marine iguanas starve very rapidly during El Nino conditions, and that you hypothesize that this is due to the costs of salt excretion. Can you explain why you have this hypothesis? (Question from Shannon, Westridge School)

A Good question! We have not tested this yet, but iguanas ingest huge amounts of salt every day during foraging. The excretion costs for salt are probably high, but we have not quantified them yet. Our hypothesis for the influence of salt excretion costs on the high mortality rates comes from a comparison of land and marine iguanas. Land iguanas can hang out for months on end during 'La Nina' conditions, i.e. when it is extremely dry on land and when they have practically no food. Marine iguanas, on the other hand, starve very rapidly during El Nino conditions. We think that this difference is largely due to the fact that marine iguanas have the additional costs of salt excretion, which land iguanas do not have (at least in that extent).

q From your point of view, what do you think is going to happen to the islands in 20 years? On the show you explained how the change in climate affect the living style of the animals. Does this mean that one day some animals on the islands will become extinct? (Question from Michael)

A This is one of the most debated questions in science right now. My personal opinion is that climate change will not affect marine iguanas much, but may potentially affect some Galapagos species like penguins. Marine iguanas can adapt to changes in temperatures and sea level easily. If temperatures increase and algae are less abundant, many individuals will die. But a lot will survive. The survivors will have smaller body sizes, but continue to feed on algae in the intertidal zone. However, the major problems for marine iguanas are introduced feral cats and dogs. Fortunately, they only occur on some islands, so other iguana populations will not be affected. Nevertheless, these feral dogs and cats should be controlled. So, overall, I think, the islands and their inhabitants have seen a lot of climate change over the past millennia. There will be some changes, but I do not expect those changes to be dramatic. The major changes will occur at those sites where foreign species have been introduced that heavily compete, and sometimes directly extinguish, the native species. The Galapagos National Park and the Charles Darwin station fight these introduced species, and the fate of Galapagos will be determined by how much money those two organizations have to successfully re-instate the pre-human situation in Galapagos.


Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.