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190 IGUANA  •  VOLUME 11, NUMBER 4  •  DECEMBER 2004 WIKELSKI AND NELSON

Adult Marine Iguana (Amblyrhynchus cristatus) from San Cristobal Island in the Galápagos Archipelago. Photograph by Colette Adams.



Introduction

When the Galápagos’ most famous visitor, Charles Darwin,
arrived on the rocky lava shores, he likened the islands to

the entrance of Hell. He found myriads of “dirty black” Marine
Iguanas (Darwin 1883). Young Charles found them “hideous in
appearance, sluggish, stupid, and ugly.” Nevertheless, like every
modern visitor, he was fascinated by their sociality and their
marine foraging style — Marine Iguanas are the only lizards
known to feed exclusively on algae. Marine Iguanas live in dense
clusters of up to 8,000 animals per kilometer of coastline.
However, iguana colonies are distributed very patchily, and tend
to occur only along the southwestern shores of the islands.

Amazingly, little has changed for Marine Iguanas on the
uninhabited islands since Darwin’s visit more than 170 years
ago. However, although Marine Iguanas still occur in healthy
population densities on the uninhabited islands, they face poten-
tially serious threats on several inhabited islands.

Marine Iguana Natural History
Marine Iguanas are endemic to the Galápagos Archipelago,
which belongs to the Republic of Ecuador. They feed exclusively
on marine algae in the rocky intertidal zone (Darwin 1883,

Carpenter 1966, Trillmich and Trillmich 1986, Wikelski et al.
1993, Wikelski and Hau 1995, Drent et al. 1999). Marine
Iguanas possess an internal biological clock that is synchronized
to the tides. This clock cues them to walk to the intertidal zone
every day at low tide, when the algae are exposed (Wikelski and

Conservation of Galápagos Marine Iguanas
(Amblyrhynchus cristatus)

Martin Wikelski1 and Karin Nelson2

1Department of Ecology and Evolutionary Biology, Guyot Hall 303, Princeton University, Princeton, NJ 08544-0001, USA (wikelski@princeton.edu)
2Department of Biology, Triton College, 2000 Fifth Avenue, River Grove, IL 60171, USA (knnelsonn@yahoo.com)

Photographs by Martin Wikelski except where indicated.

Abstract.—Galápagos Marine Iguanas are highly abundant along many of the archipelago’s shorelines. Total esti-
mated population size varies between 37,000 and 280,000 individuals. Marine Iguanas have evolved in the
absence of major predators, and their populations are regulated by cyclically recurring famine (El Niño) and feast
(La Niña) events. Population declines are strongly density-dependent: the higher the population density, the higher
the mortalities during El Niños (from 10–90%). Recovery after El Niños is rapid, as females compensate by repro-
ducing younger and laying more eggs. Marine Iguana morphology differs between islands. Seven subspecies have
been proposed, although only three major clades can be distinguished genetically. Twelve populations (approxi-
mately 74% of all Marine Iguanas) still live in pristine environments, whereas five populations (26% of all Marine
Iguanas) suffer from anthropogenic influences. Major conservation problems arise from introduced predators
(cats, dogs, rats, and pigs) and from combinations of natural events (El Niño) and anthropogenic disasters such
as oil spills. The most recent oil spill in 2001 killed 62% of all Marine Iguanas on Santa Fe Island. Management
requirements for the future include: (i) investigating population trends in Western Isabela and San Cristobal
islands, (ii) investigating whether harbor areas are population sinks because of environmental contaminants, (iii)
establishing a recovery program for oil-contaminated iguanas, especially their reinoculation with hindgut
microsymbionts, and (iv) developing husbandry techniques and establishing a captive propagation program as a
population backup plan (Marine Iguanas have not been bred in captivity).

Key Words: Marine Iguanas, Amblyrhynchus cristatus, South America, Galápagos Archipelago, Conservation, Oil
Spill, Feral Predators

IGUANA  •  VOLUME 11, NUMBER 4  •  DECEMBER 2004 191GALÁPAGOS MARINE IGUANAS

�

Male Marine Iguana on Seymour Norte Island eating Saltwort (Batis mar-
itima) on land. Some individuals supplement their food with land plants.



Hau 1995). The largest iguanas of each island population also
dive for algae (2–30 m depth; Buttemer and Dawson 1993). On
Genovesa, males with body mass >500 g are seen diving,
whereas on Fernandina usually only males >3500 g dive for food
(Wikelski and Trillmich 1994). A few individuals supplement
their food with land plants, in particular on Seymour Norte
Island. Only highly salty land plants are ingested (primarily
Saltwort, Batis maritima, but also other coastal succulents such
as Sesuvium portulacastrum), presumably because Marine
Iguanas possess very specialized hindgut micro-symbionts that

help them digest their food and effectively break up cell walls
(Mackie et al. 2003).

Marine Iguanas reproduce once a year during a month-long
mating season. The precise timing of the mating season coin-
cides with the highest abundance and best quality of food
(Rubenstein and Wikelski, in preparation). Because the nutri-
ent-rich upwelling from the Cromwell current affects all islands
in the archipelago differently, mating seasons occur at different
times (e.g., December on Santa Fe and Genovesa, January on
Santa Cruz, February/March on Española). During the mating
season, males defend small territories that contain no resources
other than the males themselves, which prompted the descrip-
tion of the mating system as a lek, or mating arena (Trillmich
1983, Wikelski et al. 1996). Male Marine Iguanas use three dif-
ferent mating strategies. The largest males defend territories and
court females using a slow, stereotyped head-bob courtship
behavior. Male mating success is highly skewed and depends on
body size, condition, and display rate (Wikelski et al. 1996,
2001). Male territories are generally clustered, but single terri-
tories also occur. Satellite males are smaller than territorial males
and roam around territories, attempting to (forcibly) mate with
females. “Sneaker” males are the smallest males, physically indis-
tinguishable from females. Sneakers try to copulate with females
“in secret” on territories of large males (Wikelski and Bäurle
1996). These three mating tactics appear to be partially regu-
lated by plasma levels of testosterone, and can be manipulated
by hormone administration (Wikelski et al., submitted).

Receptive females generally copulate only once after they
have selected a specific male, which they do after long periods
of mate choice. Mate choice is apparently costly for Marine
Iguanas, as indicated by mass loss of females that visit many
males, compared to those that visit only a few males or mate in
low-density areas (Wikelski et al. 2001). Females leave the mat-
ing area shortly after copulation to lay one to six eggs in deep
burrows in sandy areas. Eggs incubate for three months (Laurie
1990; Laurie and Brown 1990a, 1990b). Some females guard
their nests for a few days after egg-laying, mostly to defend
against other females that try to dig at the same spot. The entire
clutch amounts to about 20–28% of a female’s body mass. Both
males and females typically reproduce every other year, replen-
ishing their energy reserves in the year they do not reproduce.
However, during periods of food abundance, females may repro-
duce annually (Laurie 1990).

Marine Iguanas have only one natural predator, the
Galápagos Hawk (Buteo galapagoensis), which is generally unable
to prey on healthy adults. However, hawks can capture weak-
ened adult iguanas, such as starving individuals or females
exhausted by nesting. They also can learn to capture juveniles or
hatchlings close to the shoreline (Boersma 1983; personal obser-
vation, Santa Fe Island).

Natural Population Regulation via El Niño Events
Unpredictably recurring El Niño events can dramatically reduce
the abundance and diversity of marine algae — Marine Iguanas’
only food source — and can cause mass starvation (Laurie and
Brown 1990b). During El Niño events, the cold, nutrient-rich
upwelling ceases and warm water from the Gulf of Panama flows
toward the Galápagos. The normal food algae (red Gelidium and

192 IGUANA  •  VOLUME 11, NUMBER 4  •  DECEMBER 2004 WIKELSKI AND NELSON

Marine Iguanas in the intertidal zone on Genovesa Island during the
low tide, grazing on green algae (mostly Ulva sp.). This population is
the smallest in body size and males with body mass as little as 500 g
are seen diving.

Two sneaker male Marine Iguanas on Genovesa Island attempt to
forcefully copulate with a female outside of a territory (on sand). The
territorial male (right) left his territory and interrupted the copulation
attempt.

A satellite male Marine Iguana forcefully approaches a female to
attempt copulation.



IGUANA  •  VOLUME 11, NUMBER 4  •  DECEMBER 2004 193GALÁPAGOS MARINE IGUANAS

Centroseras species or green Ulva species) disappear when water
temperatures become too high and are replaced by brown algae.
However, brown algae are not as easily digested by the Marine
Iguanas’ hindgut bacteria, and they may also be toxic. The result
is widespread starvation of Marine Iguanas throughout the
archipelago. Individual animals are affected differently — the
largest animals starve first (Wikelski and Trillmich 1997,
Wikelski et al. 1997), presumably because they have the highest
absolute calorie requirements. Therefore, natural selection favors
smaller animals during food shortages. Interestingly, Marine
Iguanas can shrink their body size during El Niño events and
survive such conditions better (Wikelski and Thom 2000). How
such shrinkage is achieved physiologically or whether and to
what degree bone loss is involved is not clear.

Marine Iguana populations can crash dramatically. During
the El Niño of 1997–1998, about 90% of all Marine Iguanas on
Seymour Norte Island disappeared, thus reducing population size
on this island to less than 150 individuals (Wikelski and Wrege
2000, Romero and Wikelski 2001). A similar situation occurred
on Genovesa Island in 1991–1994, reducing the total population
size from about 15,000 to approximately 900 individuals.
However, Marine Iguanas have survived such dramatic natural
selection events throughout their evolutionary history and appar-
ently adjust to such situations. As soon as nutritious red and
green algae reappear after an El Niño ceases, individuals face plen-

tiful intertidal and subtidal foraging grounds. Iguanas quickly
replenish their fat reserves and return to good body condition.
They reproduce more frequently (every year), at a younger age
(mostly females), and lay larger clutches (e.g., three instead of two
eggs). The “rules” by which Marine Iguanas determine whether
to breed and how many eggs to lay are still unclear.

These adaptations allow Marine Iguanas to increase their
numbers after dramatic population declines, such that mortal-
ity rates of 30–50% after an El Niño event can be compensated
within four years. Even after enormously strong population
declines (90%), Marine Iguanas congregate along the shoreline
in small groups to reproduce (Wikelski et al. 1996). This gre-
gariousness helps them to find each other after dramatic popu-
lation declines that could otherwise cause individuals to scatter
along the long lava shores of the islands.

Island Populations and Threats
Early naturalists discovered that not all Marine Iguana populations
are alike (Fig. 1). For example, iguanas on Genovesa only grow up
to a maximum of 900 g (subspecies: A. c. nanus, “the small ones”).
Animals on Española are the most brilliant, at least during the
mating season, when they display a bright red and green col-
oration (A. c. venustissimus). Fernandina iguanas have especially
elaborate spines (A. c. cristatus subspecies). Although these popu-
lations differ very obviously on the phenotypic level, genetic dis-

Table 1. Rough estimate of Marine Iguana population sizes on the Galápagos Archipelago. Data are based on our own surveys,
accounts by Galápagos guides, and data by Andrew Laurie (unpublished report to the Darwin Foundation, 1981). Minimum num-
bers indicate estimates for total island numbers after a strong El Niño famine. Maximum numbers indicate total numbers after 
several years of La Niña (cold, nutrient-rich) conditions. Maximum density estimates relate iguana numbers to the total size of the
island. Please note that these are only very rough estimates.

ISLAND SUBSPECIES MINIMUM MAXIMUM MAXIMUM THREATS
NUMBER NUMBER DENSITY

(n/km2)
Fernandina cristatus 15,000 120,000 187 Oil spill
Isabela albemarlensis 5,000 40,000 9 Oil spill, dogs, cats, rats, pigs
Santa Fe 3,000 16,000 667 Oil spill
Floreana 2,000 16,000 92 Oil spill, cats, rats, pigs
Santa Cruz hassi 2,000 13,000 13 Oil spill, dogs, cats, rats, pigs
Española venustissimus 1,700 21,000 350 Oil spill
Genovesa nanus 900 15,000 1071 Oil spill
Marchena 1,000 10,000 77 Oil spill
Pinta sielmanni 800 6,000 100 Oil spill
Santiago mertensi 450 4,000 7 Oil spill, cats, rats
Wolf 400 1,500 1154 Oil spill
Darwin 200 800 727 Oil spill
Pinzon 200 900 50 Oil spill
Rabida 200 2,000 408 Oil spill
Seymour Norte 100 1,500 789 Oil spill
San Cristobal 50 400 1 Oil spill, cats, rats, pigs
Remaining islands 2,000 10,000 NA Oil spill
TOTAL ~37,000 ~280,000



tinction of “subspecies” is less clear (Fig. 1; Rassmann 1997,
Rassmann et al. 1997). Marine Iguanas reached the Galápagos
archipelago as early as 10–15 million years ago. They presumably
arrived by “leap-frogging” across now-sunken islands that are only
detectable as sea mounts between the present-day Galápagos
Islands and the South American mainland. From those early
Galápagos Islands, Marine Iguanas dispersed to the present islands
and split into northwestern, central, and southeastern clades.

Interestingly, the northwestern clade includes both the
largest (on southwestern Isabela) but also fairly small iguanas (on
Pinta), indicating that strong selection on body size is present,

although not apparent in the genetic markers that were investi-
gated (Rassmann et al. 1997). These distinct inter-island differ-
ences obviously indicate that all island populations should be
conserved, although no immediate threat exists to most island
populations. However, even the most pristine populations may
suffer immediately and heavily from environmental disasters
such as an oil spill, even if the contamination levels are very
minor (Wikelski et al., in preparation; Charles Darwin Research
Station, unpublished report on the January 2001 oil spill). On
Santa Fe, approximately 60% of the entire Marine Iguana pop-
ulation disappeared as a consequence of low-level oiling when

194 IGUANA  •  VOLUME 11, NUMBER 4  •  DECEMBER 2004 WIKELSKI AND NELSON

Schematic map of the Galápagos Archipelago and the different subspecies of Marine Iguanas as described by Eibl-Eibesfeldt (1962; inset subspecies
names). Superimposed (ovals) are the three genetically similar clades, based on nuclear and mitochondrial genetic analysis (Rassmann et al. 1997).
Genetic and morphological information is not entirely compatible, indicated, for example, by the fact that the Amblyrhynchus cristatus mertensi
subspecies is found in two genetically distinct clades (on San Cristobal and Santiago islands).



IGUANA  •  VOLUME 11, NUMBER 4  •  DECEMBER 2004 195GALÁPAGOS MARINE IGUANAS

the oil tanker “Jessica” ran ashore on nearby San Cristobal Island
and spilled about 750,000 gallons of diesel and bunker oil.
Consequently, oil contamination could pose a serious threat to
all populations of Galápagos Marine Iguanas, even those that
live far from human settlements and would otherwise be con-
sidered pristine. As to the specific mechanism of mortality, we
suggest that hindgut endosymbionts are highly susceptible to oil
in their digestive substrate. Marine Iguanas need these bacteria
to digest algae fully, a process that can take up to two weeks.
These specialized and highly effective gut bacteria probably suf-
fered heavily due to the oil spill, which in turn caused failed
digestion and widespread starvation among Marine Iguanas.

On at least five islands, Marine Iguana populations also are
heavily impacted by human-introduced predators (Cayot et al.
1994). The most important predators are feral cats, dogs, rats,
and pigs. We are currently uncertain whether those Marine
Iguana populations already affected are jeopardized to an extent
that their long-term survival is in question. Some populations
on certain parts of western Isabela Island appear to be in imme-
diate danger of extinction. During several research expeditions,
we only found adult iguanas and very few young or hatchlings,
indicating almost no recruitment, at least not along the shore-
line of the main island. Nevertheless, some hatchlings and year-
lings were seen on small offshore islets. A similar situation
applies to Marine Iguanas on San Cristobal Island, where only
several hundred iguanas survive. Thus, we would classify these
populations as highly endangered, and recommend that imme-
diate action be taken.

Management Needs for the Future
Four major problems exist for Marine Iguana populations, and
we recommend the following conservation-related research proj-
ects to determine the causes of the threats and to elaborate pos-
sible solutions:

(i) We need to study population trends and identify threats
to Marine Iguanas at sites where iguanas show no apparent
recruitment. Those sites include western Isabela and San
Cristobal Islands as mentioned above. We presently do not
know where and when reproduction occurs, which predators are
preventing recruitment of hatchlings, and to what degree big
mammalian predators are responsible for the observed lack of
recruitment. The most desirable conservation project would be
to habituate feral cats to the presence of a researcher and follow
them along their daily activities (H. Snell, personal communi-
cation). Such data would reveal the dangers that feral cats pose
to Marine Iguanas and would allow us to study the impact of
these fierce predators on other endangered coastal species. Dog
predation could be a problem on other islands like San
Cristobal, and this also needs further investigation (Kruuk and
Snell 1981).

(ii) The second important project is to study the impact of
human habitation and low-level environmental contamination
on Marine Iguanas. We are particularly concerned about oil
residues from boats around human settlements (e.g., the towns
of Puerto Ayora on Santa Cruz Island and Villamil on Isabela
Island). We currently do not know if areas around human set-
tlements should be considered population sinks for Marine
Iguanas or whether such areas still produce sufficient recruits to
be considered viable populations. Based on the current infor-
mation about the effects of a low-level oil spill in 2001, Marine
Iguanas in oil-impacted areas conceivably will suffer long-term
effects. Furthermore, our own observations in and around
Puerto Ayora (the main harbor town on Santa Cruz Island) sug-
gest that many of the hatchlings that pass through the town after
hatching disappear from the harbor area, and may die due to
human (or feral animal) actions. A long-term mark/recapture
program could resolve such questions.

(iii) A third area of high interest for the management of
Marine Iguanas is the planning and establishment of a program
for rehabilitation of Marine Iguanas. This would include indi-

Adult female Marine Iguana from Genovesa Island; this single egg was
laid about two hours prior to taking this picture.

Galápagos Hawk killing a hatchling Marine Iguana on Santa Fe Island.



viduals that suffered low levels of environmental contamination.
Hindgut microbes probably die during anthropogenic disasters
such as oil spills, and Marine Iguanas subsequently starve. Such
a situation is comparable to what happens to other vertebrates
when endogenous gut symbionts are eliminated, for example,
by chemical contamination or antibiotic medication. In such sit-
uations, the gut fauna can be reinoculated using pills containing
spores of endogenous gut symbionts. The horizontal transfer of
gut symbionts from unaffected to affected individuals also is pos-
sible in Marine Iguanas. Thus, once we have established how
many and what kind of hindgut endosymbionts Marine Iguanas
possess, and how such symbionts work, researchers presumably
could collect gut material from healthy individuals and treat suf-
fering individuals.

(iv) The fourth and potentially most difficult research activ-
ity for the conservation of Marine Iguanas would be to try to
breed animals in captivity. Astonishingly, this has never been
attempted, and we are unsure whether it is even possible. The
challenges for holding and breeding Marine Iguanas in captivity
are manifold. First, Marine Iguanas have very specific dietary
requirements. Only certain types of seaweed are preferred and
eaten — as soon as the natural algae composition changes, we
find dramatic mortality rates in the wild. Second, although early
naturalists removed several Marine Iguanas from the Galápagos
Islands and brought them into zoos, none of those iguanas ever

tried to reproduce. Nevertheless, several Marine Iguanas were
maintained for more than ten years. Marine Iguanas have a dif-
ferent breeding system than most other iguanids, and a fairly
large colony may be required in order to stimulate mating activ-
ities. At the same time, we do not know how hatchling Marine
Iguanas survive their first months, how they acquire their
endogenous hindgut microsymbionts, and how they become
recruited into the population. Marine Iguanas are the only
Galápagos animal about which we are uncertain whether they
can be kept and bred in captivity, should such a need arise. We
would like to remind the reader critical of any effort to remove
individuals from the wild that a combination of a strong El Niño
event and even small-scale anthropogenic disasters (like a low-
level environmental contamination) could effectively wipe out an
entire island population. If our aim is to conserve each island
population of Marine Iguanas because of their distinctive differ-
ences, we should seriously consider developing the husbandry
practices necessary to rear Marine Iguanas in a captive setting.

Acknowledgments
We are grateful to all friends, scientists, and field assistants for
their help. Work in the Galápagos is supported by the Galápagos
National Park Service and the Charles Darwin Research Station.
This is contribution #392 to the Charles Darwin Foundation.
This work was supported by the National Science Foundation
under grant IBN-0118069.

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Author Biographies
Martin C. Wikelski is an assistant professor in the
Department of Ecology and Evolutionary Biology at
Princeton University. He completed his Masters and Ph.D.
at the German Max Planck Institute and the University of
Bielefeld before he came to the United States as a postdoc-
toral researcher working at the Smithsonian Tropical
Research Institute and the University of Washington. Martin
then held an assistant professorship at the University of
Illinois at Urbana-Champaign for two years. His research
interest is ecological physiology of lizards and birds. He has
traveled extensively through South America, publishing a
nature travel guide to Venezuela and Brazil. He continues
work in Panamá and the Galápagos Islands, Ecuador.

Karin N. Nelson completed her Ph.D. in 2003 in the
Department of Animal Biology at the University of Illinois
at Urbana-Champaign. Before returning to academia, she
served as editor at Chicago’s Brookfield Zoo for eight years.
During her graduate studies, she investigated the hormonal
mechanisms of alternative mating strategies in Marine
Iguanas.

IGUANA  •  VOLUME 11, NUMBER 4  •  DECEMBER 2004 197GALÁPAGOS MARINE IGUANAS

Martin Wikelski holding a large male Marine Iguana on western
Isabela Island.

Dead Marine Iguana on Fernandina Island during the 1998–99 El
Niño period. The likely cause of death was starvation.




