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The Ecological Niche of the Marine Iguana

Amblyrhynchus cristatus

Corinne P. Sathoff

PVHS Science Research

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Abstract

Marine iguanas, Amblyrhynchus cristatus, are remarkable organisms because they are so well-

adapted to their environment. I studied the reptile’s ecological niche, “all the physical, chemical

and biological factors the organism needs to survive, remain healthy, and reproduce” (Raven and

Berg, 2006). Also, I conducted observed the evolutionary adaptations of the marine iguana in

the Galapagos Islands and was able to learn about the iguana’s physical characteristics, habitat,

and unique behaviors, such as swimming and sneezing saltwater. The trip to the archipelago

took place August 17-23, 2009, the beginning of the colder garua season on the islands with

typical overcast skies. The trip included visits to South Plaza, Santa Fe, Floreana, Española,

Santa Cruz, Rabida, Sombrero Chino, Bartolome and North Seymour where we were able to see

the marine iguanas interacting in their natural habitat.

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Introduction

Isolated just over six-hundred miles off the coast of Ecuador lies the group of volcanic

islands known as the Galapagos. The cluster of islands, made famous by Charles Darwin and his

study of the origin of species through natural selection, is renowned for a diversity of organisms

found nowhere else of the planet. Often referred to as Las Encantadas or Enchanted Isles, the

Galapagos are the definition of an inhospitable environment with few sources of freshwater,

strong currents, arid soil and isolation. This prompts the question: how did the islands come to

support such an array of life? The answer lies in the unique adaptations specific to each

organism. “These reptiles and insects, small mammals and birds, evolved unmolested on the

various islands on which they were cast into unique species adapted to the boulder-wrecked

shores, the cactus deserts of the lowlands, or the elevated jungles of the large islands’ interiors”

(Dillard, 1988).

Over time, the species of the Galapagos have changed with their environment to exist

within specialized ecological niches. The word niche comes from a French word meaning nest.

The term was first described by a naturalist, Joseph Grinnell, in 1917. One can call a niche the

sum total of all the ecological requirements that allow a species to mate and produce offspring,

thus surviving and flourishing with any undue pressures (Chupp, personal communication,

August 9, 2009). Unquestionable, one of the finest examples of adaptation on the islands is the

marine iguana. I chose to study the niche of the marine iguana because it is the only species of

iguana on the planet to adapt to an ocean environment and gain the ability to swim. Charles

Darwin described the marine iguana as a “hideous-looking creature, of dirty black coulour,

stupid and sluggish in its movements” when he saw them on the islands, but through by

observational studies, I hoped to discover the true reasons for this reptile’s bizarre appearance

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and behavior (Constant, 2002). I was particularly interested in studying the physical

characteristics, coloration, development and behaviors of the iguanas. The story of its existence

is remarkable because it seems improbable that an exothermic creature like the marine iguana

would be able to survive in the cold seas surrounding the Galapagos, so I was also interested in

learning about their mechanisms for maintaining homeostasis. The marine iguana is an

important species to the field of biology because of its adaptations and modifications that are

clearly comparable with land iguanas in the archipelago as well as ancestors from the mainland.

“The marine and land iguanas of the Galapagos are more closely related to each other than either

is to mainland relatives; Dr. Sarich said, ‘They have evolved in very different ways.’” (Brown,

1992).

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Method

Materials

Materials used to gather observational data include waterproof field notebook, field

guides, course reader, snorkeling gear (fins, mask, snorkel, wetsuit), camera, tennis shoes, M/Y

Yolita yacht, pangas, Level 3 Naturalist Guide Rommel Daniel Saa.

Methods

My journey through the Galapagos Islands to conduct observational studies allowed me

to explore different islands each day and record data about the marine iguana in its natural

environment (Figure 1.1). Each island in the Galapagos has different geographic, geological and

topographical components. I want to highlight the various differences from island to island

while tracking the behaviors of the marine iguana.

August 17, 2009: We began at South Plaza Island at 8:15am (Figure 1.2). It was a sunny

morning (74°F) at low tide and we first walked along the shoreline, known as the coastal or

littoral zone of the island to observe various species (Chupp, personal communication, August 9,

2009). We saw three marine iguanas basking on the lava rocks. We then moved to the other

side of the island which was around 60-70 feet in elevation, where I was surprised to spot marine

iguanas (approximately six) on the top of the cliff. This area, known as the arid zone, features

the marine iguanas as they had climbed the cliff from the coastal zone in order to sun-bathe (Saa,

personal communications, August 17, 2009). This highlighted flora of this island was the red

carpetweed that covers the island from zone to zone. This Sesuvium edmonstonei carpetweed

hosts the land iguana and can sometimes be the second food of choice for iguanas (Saa, personal

communications, August 18, 2009). Many cacti were seen also on this small island. We were

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also able to observe the iguanas sneeze out excess salts as they were sunning. In the late

afternoon to early evening (2:30-5:00pm) we also took a walk on Santa Fe at Barrington Bay. I

did not observe any marine iguanas while on Santa Fe Island because the beach we visited was

mainly inhabited by sea lions.

August 18, 2009: For our planned three hour walk on Espanola at Punta Suarez we began at

8:15am (Figure 1.3). The weather was slightly cooler (67°F) and more overcast (typical of

Galapagos in its early winter months) than the previous day at South Plaza and Santa Fe. The

island was inhabited by large numbers of marine iguanas to the point where taking a population

count became difficult because of the sheer numbers grouped together on the lava rocks. This

clumped dispersion is advantageous for the marine iguanas for warmth and protection. We

observed the marine iguanas up close on the beach as well as from a distance when we walked

up to the top of the cliff, known as Albatross airport, to see the blowhole. To the best of our

ability, we averaged a total count of 729 individuals with an approximate ratio of adults to

juveniles of 3:1. In the afternoon we went snorkeling around Gardner Bay but I did not see any

iguanas either on the beach or in the water.

August 19, 2009: We started our walk on Floreana at Punta Cormorant at 7:45am and the

weather was similar to the previous day on Espanola with a temperature of 68°F and overcast

skies. The island was different from South Plaza because it has some larger hills of about 1500-

2000 feet elevation, called tuff cones. The first beach that we visited was a green-sand beach

(olivine in the sand) and I did not observe any marine iguanas (Figure 1.4). Next, we walked

inland to view flamingos in a lagoon. The second beach we explored, opposite of olivine beach,

consisted of very fine white sand, which is sometimes called the flour sand beach (Fitter, Fitter,

Hosking, 2000). The surf line was overrun by stingrays making beach access difficult for many

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species including marine iguanas and sea lions, thus I did not observe any marine iguanas. In the

afternoon we also visited Post Office Bay to play soccer, drop off letters and climb down a lava

tube; however, I did not find any iguanas on that beach either.

August 20, 2009: We had a dry landing in Puerto Ayora on the island of Santa Cruz where we

then traveled by bus into the highlands of the islands. This excursion took us up into the

scalesia, brown, miconia and pampa zones (Figure 1.5). We arrived in the highlands at 9am,

where it was 66°F and more humid than at the coastal zone. We were in the highlands to observe

the giant tortoises and I did not see any marine iguanas because they inhabit the coastal/littoral

zone and not the greener vegetative areas.

August 21, 2009: In the morning we rode in pangas around Rabida before taking our walk on

the island (Figure 1.6). While in the pangas we spotted approximately 68 marine iguanas on the

rocky shoreline surrounding the island. After viewing the island from the pangas, we had a wet

landing on the red-sand beach (red sand from high iron content) at 9:15am. The day was warmer

at 72°F and partly cloudy. In the afternoon, after a navigation, we snorkeled around Sombrero

Chino or ‘Chinese Hat’ and I sighted approximately ten marine iguanas on the rocky

outcroppings as well as one marine iguana swimming in the water. This was the first time I had

observed one of the reptiles in the water and was surprised to watch its ease of movement as it

whipped its tail side to side.

August 22, 2009: In the morning we climbed the stairs on Bartolome to the lookout point of

Pinnacle Rock (Figure 1.7). We did not spot many organisms, in general, because the walk was

mainly to learn about the geology and unique volcanic properties on the islands; however, we did

find six marine iguanas on the lava rocks surrounding the landing site for the pangas. We also

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used this day to get additional information about our organism of focus from the naturalist,

Danny Saa, which helped fill in any questions we still had about our research

August 23, 2009: For our final walk in the Galapagos, we visited North Seymour at 7:00am. It

was a partly cloudy morning, at high tide, with a cool temperature of 67°F. I observed marine

iguanas on the lava rocks adjacent to the beach and can approximate the number counted to be

41. While on the beach, I also found the carcass of a dead iguana lying on the sand. I could not

determine the cause of death, but it was interesting because it was the first deceased specimen I

had come across.

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Results

As a species, marine iguanas are well suited to their surroundings, thanks to adaptations

as the reptiles evolved from their South American ancestors. “Geography is life’s limiting

factor…Speciation and life itself-is ultimately a matter of warm and cool currents, rich and bare

soils, deserts and forests, fresh and salt waters, deltas and jungles and plains” (Dillard, 1988).

With the environment that the archipelago provides, these iguanas have filled a niche uniquely

their own.

Physical Characteristics: The marine iguana has unique physical characteristics that make it

well-adapted to its niche and aquatic lifestyle. The reptiles have flattened and rounded faces

which allow them to reach algae on submerged lava rocks (Figure 1.8). If marine iguanas had

long snouts like their mainland ancestors or even the land iguanas on the islands, they would be

unable to feed on the marine algae because it grows low on the rocks. The iguanas also have

longer claws that improve their ability to grip slippery algae-covered rocks (Saa, personal

communications, August 18, 2009). These longer claws aid the iguanas when feeding because

they can remain on the rocks even when there are strong tidal currents and surge. Another

physical characteristic of marine iguanas is their flattened tails which provide greater propulsion

and ease of movement in the water.

Swimming: Not only is the marine iguana the only lizard species to adapt to an oceanic

environment, it has also gained a remarkable ability to swim. “When in the water this lizard

swims with perfect ease and quickness, by a serpentine movement of its body and flattened tail-

the legs being motionless and closely collapsed on its sides” (Darwin, 2001). The iguanas are

capable of breathing under water and are able to hold their breath for up to sixty minutes while

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submerged (Saa, personal communications, August 22, 2009). Generally it is the larger males

that dive at high tide (usually ten minutes) because reptiles lose body heat quickly and must then

return to the surface to sun-bathe (Saa, personal communications, August 18, 2009). The smaller

juveniles and females tend to feed at low tide on the exposed tidal rocks.

Food source: Marine iguanas are herbivores that feed almost solely on the Ulva red and green

algae (Constant, 2007). However, they have also been known to feed on their own feces and that

of sea lions and crabs (Fitter, Fitter, Hosking, 2006). The unique convergence of currents

surrounding the Galapagos makes marine algae abundant because upwelling churns nutrients to

the ocean’s surface (Chupp, personal communications, August 10, 2009). While adapting to its

nice, the marine iguana took advantage of a plentiful resource where little competition existed.

In years where El Niño has caused a scarcity of algae, the iguanas have been sighted eating

Sesuvium (Saa, personal communications, August 23, 2009). Scientists have also discovered that

during times of famine lizards lose weight as well as length. They are capable of shrinking their

vertebrae, disguising portions of their bones, and then regrowing when food sources return

(What You Will See, 2008). This is a unique and incredible adaptation that benefits the iguanas

during adverse conditions.

Salt expulsion-Marine iguanas ingest an abundance of salt when they eat marine algae so they

must get rid of excess salts. The reptiles have an uropigyal gland for filtering salts from the

bloodstream which are then sneezed out in a brine solution ((Saa, personal communications,

August 17, 2009). The iguanas can regularly be seen sneezing, which is the only noise that the

lizards make. This behavior also contributes to the iguana’s appearance because it leaves salty

crust on its face. The salt spray is ejected through the nostrils about two feet and it is thought the

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behavior could even be an adaptation to warn away or scare predators (Saa, personal

communications, August 18, 2009).

Habitat: The marine iguana has limited competition for resources within its niche because its

habitat is the coastal/littoral zone (Chupp, personal communications, August 9, 2009). The

iguanas live on land and feed inshore and in the intertidal zone as well as often being found

sunning on the sand or nearby lava rocks. The number of individuals on each beach varies island

to island (Table 1.1).

Intraspecific competition: These reptiles exhibit gregarious or thigmotatic behavior, meaning

that they benefit from touching other individuals. The marine iguanas often herd together in

groups and even lie on top of each other to stay warm (Saa, personal communications, August

18, 2009). For this reason, they are in clumped dispersion around the islands and generally the

clumps are high density around lava rocks where Ulva algae are more abundant (Figure 1.10).

Coloration: Juvenile marine iguanas begin life almost completely black in color. This allows

the young reptiles to be camouflaged against the lava rock and avoid predation by the Galapagos

hawk. As the iguanas mature they gain reddish patches (males are more colorful than females)

along the length of their body but retain much of their black color. Being dark in color is

beneficial to marine iguanas because it allows them to store heat more efficiently-which is

critical for this cold-blooded reptile which forages for food in the cold ocean waters and must

rely on the sun to maintain body temperature.

Homeostasis: The marine iguana is poikilothermic, meaning that it can adapt to wide

temperature variations, so even though it’s optimal body temperature is 95-98°F, it can withstand

extremes such as 104°F and go as low as 75.2°F (Constant, 2007). With their dark body color,

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the iguanas can store heat as well as being able to warm themselves quickly by sunning. To

avoid over-heating, the lizards are often seen switching their direction toward the sun and lifting

themselves above the ground to allow air to circulate (Saa, personal communications, August 18,

2009).

Development/Maturation: Juveniles are smaller versions of adult iguanas and can be

recognized by their darker colorations (almost completely black) as well as their smaller size of

about ten centimeters (Figure 1.11). “The males are larger and more brightly and distinctly

colored than the females” (Fitter, Fitter and Hosking, 2000). Full-grown adults are usually three

feet in length but can be as long as four feet because the size of adult of iguanas varies island-to-

island (Darwin, 2001). After becoming full-grown, the lizards reach sexual maturity at 5-6 years

of age. The lifespan of a marine iguana is 25-30 years (Saa, personal communications, August

18, 2009).

Mating: These generally nonaggressive lizards become territorial during the mating season

(February) because males want to protect their group of females (Saa, personal communications,

August 18, 2009). Males create a territory (approximately 100 square feet) and will drag females

back if they try to leave. During this season, makes become redder in color and often a green

color is also present on the back, forelegs and hind legs (Saa, personal communications, August

18, 2009). This type of sexual selection allows the female marine iguana to choose her mate best

on the best male colors. Typically, the more red on the male, the more likely a female thinks that

male is worth reproducing with. The male iguana will also lift its head up and down, like

shaking his head in a “yes” motion during mating season to attract a female for mating. After

mating, females need sandy beaches for nesting and compete for space so they can lay their eggs.

“The females excavates a nest in the sand, a burrow up to one meter long, then lays a clutch of

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up to four leathery elongated eggs, which take between three and four months to incubate”

(Fitter, Fitter Hosking, 2009).

Interspecific relationships: Marine iguanas are most at risk for predation when they are newly

hatched juveniles. They are often the prey of Galapagos hawks, frigate birds, lava herons,

snakes, moray eels, sharks and other water predators (Saa, personal communications, August 18,

2009). As the iguanas mature into adulthood, their main natural predator is the Galapagos hawk.

In general, though, marine iguanas have few natural predators so therefore have few defenses

which make them vulnerable to predation by introduced species like feral cats, feral dogs and

black rats. Tending to be nonaggressive, marine iguanas peacefully coexist with other organisms

such as the Sally Lightfoot crabs, sea lions, blue-footed boobies, lava lizards, and other species

that inhabit the coastal zone (Figure 1.12). One of the iguana’s only competitors for food is the

Sally Lightfoot crab, which also feed on marine algae. The algae, however, is generally

abundant because the reptiles swim to find algae underwater (Figure 1.13). Resource

partitioning minimizes competition between these herbivores (Raven, Berg, 2006).

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Discussion

After the research trip in the Galapagos Islands, I felt much more knowledgeable about

the niche and habits of the marine iguanas because of my observations. However, like most

research experiments, my methods could have been improved to collect more accurate data. For

example, there were too many variables involved: we performed our population studies at

different times of the day, with various tide heights, and a range of temperatures on different

islands. All of these factors may have contributed to the number of individuals that we observed

on each island. We were under the limitations of doing observational research so much of our

data is qualitative instead of quantitative.

After returning from the Galapagos, I also considered how the ecosystem would differ if

the marine iguana were to become extinct. The coastal environment would look different

because of the abundance of red and green algae that the marine iguanas would not be eating in

the food chain. Also, there would be an increase in the population of Sally Lightfoot crabs

because their competitor would no longer be present.

I still feel that to better understand the marine iguana, additional research would be

necessary. If I were to continue my studies, I would be interested to see how the iguanas’

behavior differs at night. Generally we viewed the lizards only in the morning, so observing

them at night would provide insight into how and where they sleep, night defenses, and

temperature control. To improve my ability to observe the marine iguanas at night, setting up

multiple infrared cameras around the beaches would allow their behaviors to be recorded. The

recordings might also provide additional insight to interspecies interactions.

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A recent study regarding the marine iguana was carried out by German researchers Jesko

Partecke and Arndt von Haeseler and a Princeton Professor of Biology, Martin Wikelski. The

three researched hypotheses to explain why males group territories in one area instead of being

more spread out. This behavior is called lekking-“leks are aggregations of males that defend

small territories containing no resources besides the males themselves” (Wiley, 1991). The

scientists linked the clusters of males to the presence of clusters of females, which made me

curious about Espanola and the grouped behavior that the iguanas exhibited because the data

exhibited by these scientists could clarify why it is populated in that manner.

Looking back on my collected population data, it is interesting to see such a large

variation in the number of individuals observed on each island (Table 1.1). The island Espanola

particularly stands out because 729 marine iguanas were viewed. I have wondered why there

were so many more iguanas surrounding Punta Suarez and would be interested to learn what

factors contribute to the high population density, such as available resources, geography, and

nesting grounds.

During my research I learned about the marine iguana’s unique swimming abilities, but

my sources had differing opinions about how long individuals actually remained on a dive. The

naturalist, Danny Saa, also mentioned that generally only larger males dive. I would be curious

to learn how large diving iguanas are and if there is an average weight or length that they begin.

To test this experiment, I would tag 25 iguanas ranging from juveniles to adults with miniature

dive computers. The computers, similar to recreational dive computers, would begin recording

data once submerged-taking depth readings as well as the length of each dive. The computers

can also store multiple dives, so they could be removed and collected after 2-3 days. To use the

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data from this experiment, it would also be important to keep record of the length, weight, and

gender of each tagged iguana.

Ultimately, my observations show that the marine iguana is well adapted to and matches

its niche-both its fundamental and realized niche. The juvenile’s black coloration allows for

camouflage on the lava rock and increases its chances of survival into adulthood as well as

allows it to maintain its body temperature by absorbing the sun’s rays. The uropigyal gland for

filtering salts allows the marine iguanas to dive for and eat algae with being affected by the

salinity of the ocean. Their gregarious behavior has helped the marine iguana population succeed

as this clumping mechanism regulates body temperature in the cold-blooded lizards. Lastly,

their ability to dive for algae gives them a unique advantage over any other species of iguana

(Figure 1.14). My final conclusions lead me to say that the marine iguanas are not hideous and

sluggish as Darwin suggested; rather they have physical characteristics and move in a way that

allows for their overall survival in their niche (Table 1.2).

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References

Constant, P. 2002. Marine Life of the Galapagos. Odyssey Guides, New York, 307pp.

Darwin, C. 2001. The Voyage of the Beagle. New York: The Modern Library Paperback, New

York, pp332-358.

Dillard, A. and Trimble, S. ed 1988. Words from the Land. Peragine Smith, Salt Lake City, pp

34-35.

Fitter, J., Fitter, D., and Hosking, D. 2000. Wildlife of the Galapagos. Princeton University

Press, Princeton and Oxford, 254pp.

Graphic Maps. (2010). World Atlas Maps: Galapagos Islands. Retrieved September 2, 2010

from http://www.worldatlas.com/webimage/countrys/samerica/lgcolor/gpcolor.htm

Partecke, J., von Haeseler, A., and Wikelski, M. 2004. Territory establishment in lekking

marine iguanas. Behavioral Ecology and Sociobiology. Springer Berlin. Heidelberg,

pp579-587.

Raven, P.H. & Berg, L. R. 2006. Environment. Wiley and Sons, Inc., Danvers, p70-78.

What You Will See. 2008. Galapagos Conservancy. http://www.galapagos.org/2008/index.

Php?id=79>.

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Appendix

Figure 1.1. Map of Galapagos Islands. This map shows major islands visited during

observational studies from August 17-23, 2009. Credit: GraphicMaps.

Figure 1.2. South Plaza Island. The brightly red-colored carpetweed, known as Sesuvium edmonstonei. Credit: Corinne Sathoff.

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Figure 1.3. Espanola Island.. Credit: Corinne Sathoff.

Figure 1.4. Floreana Island. Green sand beach with olivine. Credit: Corinne Sathoff.

Figure 1.5. Santa Cruz highlands. Transition to green vegetative zones. Credit: Corinne Sathoff.

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Figure 1.6. Panga ride. Observation of Rabida from panga to observe marine iguanas. Credit: Corrine Sathoff.

Figure 1.7. Bartolome Island. View from Pinnacle rock. Credit: Corinne Sathoff.

Figure 1.8. Marine Iguana. The anatomy of a marine iguana’s head allows for feeding on submerged algae. Credit: Corinne Sathoff.

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Figure 1.9. Uropigyal gland. The marine iguana’s sneeze. Credit: Corinne Sathoff.

Table 1.1. Island-to-Island Population Estimates of Marine Iguanas.

Island Juveniles Viewed Adults Viewed Observation Period

South Plaza 9 0 1 hr 45 minSanta Fe 0 0 2 hrs 30 minEspanola 182 564 3 hrsFloreana 0 0 2 hrsSanta Cruz 0 0 2 hrs 45 minRabida 17 51 2 hrsSombrero Chino 0 11 45 minBartolome 2 4 1 hrNorth Seymour 9 32 1 hr 30 min

Figure 1.10 Clumped dispersion. Marine iguana’s gregarious behavior is advantageous to their survival. Credit: Corinne Sathoff.

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Figure 1.11. Size comparison. Relative size difference between juvenile and adults. Credit: Corinne Sathoff.

Figure 1.12. Coastal zone. Species find in the littoral zone that marine iguanas interact with. Credit: Corinne Sathoff.

Figure 1.13. Ulva algae. Marine iguana’s primary food source. Credit: Corinne Sathoff.

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Figure 1.14. Diving. Marine iguana diving for algae. Credit: Corinne Sathoff.

Table 1.2. Island-to Island Behavioral Observations Summary.

Estimated Organism Count: Population Dispersion:

Vegetative Zone:Characteristics Fe eding Reproduction Habitat Description

Communication Mating Adaptations Defense

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