San Francisco State University
Geography 316: Biogeography
The Biogeography of Marine Iguana (Amblyrhynchus cristatus)
Source: Martin Wikelski
by Elisa Gill, a student in Geography 316, Fall 1999
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Family: Iguanidae
Genus: Amblyrhynchus
Species: Amblyrhynchus cristatus
Introduction
The marine iguana is the only sea-going lizard. The marine iguana (Amblyrhynchus cristatus) is an endemic species to the Galapagos Islands, an archipelago of approximately 25,000 square miles located 600 to 700 miles west of Ecuador in equatorial waters (Carpenter 1966). They are related to the land iguana, which also lives on t he Galapagos Islands (Carpenter 1966). The geographical range of the marine iguana is in the coastal areas of all of the islands, islets, and sea rocks of the Galapagos Islands. There are probably more marine iguanas on the Galapagos Islands than any other creature.
Description of Species
The marine iguana body is elongate and fusiform. The average
length of an adult male is approximately 1.3 meters and an adult female is approximately
0.6 meters in length. The weight of these iguanas range between 0.5 kilograms and
1.5 kilograms (Berry 1984). They are either black or dark grey in color except when
males are near mating seasons when they turn shades of red and green. The dark
pigment in the skin helps shield the animals from the effects of ultraviolet rays.
It is also useful in warming the body. The tail is compressed into a propelling
structure which efficiently moves this lizard along the surface or beneath the
water. The long, sharp, recurved claws permit the lizard to hold fast to the lava in
heavy seas or when submerged. The snout of the iguana is short and blunt and this
enables it to get its jaws into close contact with the substrate. The tricuspate
teeth are flattened laterally and lie in single rows along the sides of the jaws
immediately inside of the labial scales (Carpenter 1966). Marine iguanas can live up
to thirty years (Berry 1984).
They are adept at both swimming and diving. Diving times have a
duration of usually a few minutes, but can be extended beyond half an hour
(Eibl-Eibesfeldt 1984). Large iguanas can dive to depths of 15 meters.
(Eibl-Eibesfeldt 1964). Marine iguanas swim by using their tails to propel them and
hold their legs tightly behind. An interesting adaptation to marine life is the salt
glands which allow the excretion of surplus salt taken into the body. These glands
open into the nose and their secretion is ejected with a jet of air through the nostrils
(Dunson 1969).
These iguanas are found in aggregations of 10 to over 1,000
individuals, usually within 30 meters of the sea (Boersma 1983).
Before feeding the iguanas must raise their body temperatures to
approximately 36 degrees Celsius. These iguanas are ectotherms and can lose up to 10
degrees Celsius when in the ocean. To regulate their body temperature they must bask
in the sun for long periods of time. Direct solar radiation infiltrates the iguana
when it basks as well as obtaining heat from siting on lava rocks (basalt) that have
absorbed heat. This acquired heat is important to the iguana because to obtain food
the iguana must go into the water. The waters off the Galapagos Islands is
controlled by the Humboldt Current. The Humboldt Current is a cold water current and
causes the iguanas to lose heat rapidly.
Habitat
Marine iguanas live in colonies where shallow reefs occur with an
extensive intertidal zone and a rocky coastline approximately 2 to 5 meters above sea
level on the Galapagos Islands. Softer substrate is needed for egg laying.
There is a clear preference for southern shorelines, which are more exposed to wave action
and where algae appear to grow more prolifically (Eibl-Eibesfeldt 1984). They sleep
either exposed on the coastline or even up in mangrove trees. Young iguanas are
found among the adults in most basking aggregations. Unlike adults, they spend a
great amount of time in cracks and crevices. They are never far away from a hiding
place.
This endemic species has sparked a great deal of academic conversation
about its’ origin. There are land iguanas scattered throughout the world and
even in the Galapagos Islands. The marine iguana is the only iguana that feeds
exclusively underwater. As there is food available on the islands this has made
zoologists ponder the origins of this adaptation.
Source: National Geographic
Diet
Marine iguanas are herbivorous. They feed almost exclusively
on marine algae. Nine species of algae have been identified as food sources.
Tylotus ecuadorianus, Bryopsis indica triseriata, Plocamium pacificum, Priontis
abbreviata, Glossophore galapagensis, Loposiponia villum, Ptersosiphonia paucicorticata,
Blossevillea galapagensis and Gelidium. Occasionally they will also eat
grasshoppers, crustaceans or even sea lion afterbirth (Carpenter 1966). They are
adapted physiologically to living in the coastal areas of the Galapagos Islands.
Male iguanas prefer to feed on submerged algae or at greater distance from the shore,
whereas females generally forage intertidally. Young iguanas forage only at low tide
(Boersma 1983). Most iguanas feed only once a day.
After feeding a marine iguana must reset its body temperature and heart
rate (from 30 to 100 beats per minute), because the digestive system works best at a
constant, relatively high temperature (Wikelski 1999). By flattening their bodies
again the lava rocks, the lizards expose as much skin surface as possible to the
sun. The flow of heat is regulated by vessels in the chest which close and open to
regulate body temperature.
Reproduction
Source: Martin Wikelski
Females reach sexual maturity when 3 to 5 years old. Males
when they are approximately 6 to 8 years old (Dellinger, et.al. 1990). Breeding
season begins in December and extends through March. During mating season the male iguana
will turn very vibrant colors. In some animals this would be a means of attracting
females, however studies can show no correlation between degree of color in males and
reproductive success (Rauch 1988). Females mate once per season. A courting
male will approach a female with his head down. As he moves forward he will nod his
head rapidly. As he continues to approach he will circle the female or move up
sideways into contact with her, moving his head along her tail. Then he will through
his front leg over the female, moving onto her back. From this position, he will
attempt to bit the skin of the neck or shoulder. It is common for the female to move
away at any stage of the courtship. After obtaining a sufficient hold on the female
the male will twist his tail while at the same time force it under the tail of the female
to bring the cloacal regions together. Copulation lasts approximately three to four
minutes (Carpenter 1966).
Approximately four weeks after mating females will leave the colony and
roam between 20 meters to 3 kilometers to sandy areas (Rauch 1988). Females will
compete for sites by rhythmic nodding which is not unlike the behavior exhibited by males
when courting. Choosing the site usually takes between 2 to 3 days. There the
female will dig a hole in which to deposit her eggs. Digging can take approximately
half a day. The female marine iguana can lay between one and six eggs. The eggs can
weigh up to a quarter of the weight of the female(Eibl-Eibesfeldt, 1966). The female
will then guard the nest up to sixteen days. Incubation of the eggs takes between 89
to 120 days. When the infants hatch the iguana can weigh about 50 to 60 grams.
Upon hatching the infants tend to look around and then immediately run, often bipedally,
to the nearest cover (Eibl-Eibesfeldt 1984).
Natural History
The marine iguana has adapted to feeding under water by developing a pair of salt glands beneath the skin between the eye and the nostril on each side of the head which periodically ejects forcibly a fine spray of highly saline secretion. The marine iguana can project this spray to over a foot in distance (Pough, et.al. 1989).Marine iguanas spend a great deal of their time basking on basalt to help absorb the heat. Because the marine iguana is ecothermic the ocean is cold enough to immobilize the lizard if it is immersed too long. This unique species has adapted to very different temperature variations.
The marine iguana is an endemic species to the Galapagos Islands. These islands are
located between 600 and 700 miles west of Ecuador in equatorial waters. They are
related to the land iguana, which also lives on the Galapagos Islands, as well as the
green iguana which inhabits the mainland of South America. There are probably more
marine iguanas on the Galapagos Islands than any other creature. Estimates for their
population are between 200,000 and 300,000 (Thornton 1971).
Predators
Potential predator of the marine iguana include
Galapagos hawk (Buteo galapagoensis), Lava Heron (Butorides sandevalli), Striated Heron
(Butorides striatus), Great Blue Heron (Ardea herodias), Frigate Birds (Fergata minor and
Fergata mgnificens), and Short-eared Owls (Asio flammeus). Researchers believe that
none of the native predators relied greatly on the marine iguana for food. This
hypothesis comes from the fact that marine iguanas are very docile and don't usually leave
an area when molested. There is also speculation that the meat of adult marine
iguanas may be too saline (Thornton 1971). They give every indication that they have
no predators. Marine iguanas are most vulnerable before birth or when
juvenile. However introduced cats, rats, pigs, dogs, and humans also prey upon
iguanas of any age.
The greatest impact on marine iguana populations has been El
Nino. The warm water that arises off the Galapagos Islands from El Nino causes the
algae that the iguanas eat to die. They are left with only brown algae to eat.
Brown algae is toxic to marine iguanas. In the second to last El Nino event
estimates are that 40 to 50 percent of the population died (Sayre 1984).
Additional threats to the marine iguana include oil spills and other
marine pollution. These could destroy the algae on which the iguana depend as well
as their coastal habitat.
Evolution
This endemic species has sparked a great deal of academic
conversation about its' origin. There are land iguanas scattered throughout the
world and even in the Galapagos Islands. However the marine iguana is the only
iguana that feeds exclusively underwater. As there is food available on the islands,
this has made zoologists ponder the origins of this adaptation.
Among Galapagos herpetologists there exist some differences of opinion
as to the development of the native iguanas. The Galapagos iguanas, the land iguana
(Conolophus subcristatus and Conolophus pallidus) and the marine iguana (Amblyrhynchus
cristatus), are theorized to have diverged between fifteen and twenty million years ago
(Wyles and Sarich 1983). This divergence of species predates the age of the
Galapagos Islands, indeed these islands were created by hot spots, approximately 5 million
years ago, and have never had contact with the South American continent. Regardless
these two iguanas are more similar in lineage than either is to any continental iguana.
Immunological studies show high similarity of genetic differentiation
in both genera (Higgins and Rand 1974). There are two suggested theories for this
evolution. (1) Divergence of the two taxa from a common ancestor with subsequent
radiation, or (2) two separate ancestors from different iguana stocks having colonized the
Galapagos recently (Rassman 1995). There is not an abundance of paleontological
information on these taxa so the two theories are continuing to be tested. These two
iguanas are largely considered to be sister taxa. The lack of resolution of the
iguanid phylogenetic relationships is surprising given the multitude of morphological and
biochemical studies of this group (Avery and Tanner 1971; Higgins and Rand 1975; Wyles and
Barich 1983; Etheridge and de Queiroz 1988).
Distribution
A complete population survey of the marine iguana has never been
accomplished. It is unlikely to ever be undertaken because of the nature of the
coast lines which these iguanas inhabit. However estimates of the population
approach hundreds of thousands (Boersma 1984). Visual counts on Santa Cruz Island
showed between 1,800 to 2,000 iguanas. Fernandina Island has an estimated 7,000 to
10,000 iguanas. Fernandina Island estimates approach 385,000 individuals. San
Cristobal Island estimates 80 iguanas. Published descriptions of marine iguana
abundance (Darwin 1845; Beebe 1924; Pinchot 1931; Slevin 1959; Eibl-Eibesfeldt 1960)
indicate that the marine iguana has been abundant over the past 150 years and has
abundantly inhabited Fernandina Island since at least 1906 (Boersma 1983).
Map of Distribution:
Other interesting issues
This species is completely protected by
national legislation (Ecuador). They are listed under C.I.T.E.S. Appendix II (nearly
threatened with extinction).
The marine iguana is a very interesting creature to study. There
does not exist a great deal of background information on this organism and further study
seems warranted. The future of the marine iguanas in unclear. Extinction of
this organism seems unlikely unless El Nino years proceed more frequently and cause their
food source to die out. One adaptation the marine iguana has made to combat the
recurrent El Nino is to increase mating after El Nino events. Populations of marine
iguanas have regained number shortly after this event. One cause for concern about
the marine iguana is that the human population of the Galapagos Islands has risen from
4,000 people just 15 years ago to approximately 12,000 people today. Population
pressure on the islands may have adverse effects for the iguana. Additionally, with
the introduction of more people on the island that will mean more pets and domestic
livestock. Dogs, cats and pigs have impacted upon the population of marine iguanas.
Bibliography
Berry, R.J. A Natural History of the Galapagos. Academic Press, London, England 1984
Boersma, P. Patterns of Evolution in Galapagos Organisms. Pacific Division, AAAS. San Francisco, CA. 1983.
Carpenter, C. 1996 "The Marine Iguana of the Galapagos Islands, Its Behavior and Ecology"; Proceedings of the California Academy of Sciences. Vol. 34 (6) pgs.
Cordero, L. "Recovery of the Marine Iguana Population after the El Nino Catastrope"; Noticias de Galapagos 1986 Vol 44 (3); pgs. 4 - 5.
Dawson, W. et. al. "A Reappraisal of the Aquatic Specializations of the Galapagos Marine Iguana"; Evolution Vol 31 (1) 1997; pgs. 891 - 897.
Dellinger, T. 1990 "Sex Identification through Cloacal Probing in Juvenile Marine Iguanas"; Journal of Herpetology. 24 (4), pgs. 424-426.
Eibl-Eibesfeldt, I. Galapagos. Pergamon Press, Oxford, 1984.
Pough, F., et. al. Vertebrate Life. Macmillan Publishing, New York, New York 1989.
Quammer, D. Song of the Dodo. Scriber, New York, New York 1996.
Rassman, K. 1996 "Molecular Genetics and Conservation in the Galapagos" Noticias de Galapagos. 56 (3), pgs. 22 - 25
Rauch, N. "Competition of Marine Iguana Females for Egg-Laying Sites"; Behavior Vol 107 (1-2) 1988; pgs. 91 - 104.
Sayre, R. "Creatures"; Audubon 1984 (1) pg. 98.
Thornton, I. A Natural History of the Galapagos. The Natural History Press, Garden City, New York 1971.
Widelski, M. 1999 "Diving Dragons of the Galapagos"; [Online]. 11/10/99
Available:
http://www.life.uiuc.edu/wikelski/iguana2.html
Widelski, M. & Trillmich, F. "Foraging Strategies of the Galapagos Marine Iguana: Adapting Behavioral Rules to Ontogenetic Size Change"; Behavior Vol 128 (3-4) 1994; pgs. 225 - 279.
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