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98 IGUANA  •  VOLUME 11, NUMBER 2  •  JUNE 2004 BENDON

The Mona Island Rhinoceros Iguana, Cyclura cornuta stejnegeri.
Illustration by John Bendon.

Juvenile Rhinoceros Iguana at the headstarting facility on Mona.
Photograph by Alberto Alvarez.

Mona Island lies between Puerto Rico and the Dominican
Republic.  Illustration by John Binns.



IGUANA  •  VOLUME 11, NUMBER 2  •  JUNE 2004 99

Isla Mona is paradise!
When God created Eden, 

He probably thought of Mona.
If you go to Mona — 

it will change your life!
— Rafael Joglar, 2003

Istepped off the plane exhausted and excited, about 7000 miles
and 31 hours away from where my journey began. The time

in Puerto Rico was 9:30 pm, but my body told me that it was
really 2:30 am the next morning. This trip was very special; I had
volunteered to be a field assistant on a research project currently
underway on Mona Island, the home of Cyclura cornuta stej-
negeri, the Mona Island Rhinoceros Iguana. This magnificent
creature has lived untouched on this island, separated from other
large iguanines, for thousands of years, although an alternative
school of thought suggests that the lack of substantial genetic dif-
ferentiation from Cyclura cornuta of Hispaniola speaks to a
recent, possibly even human-mediated, arrival on the island.

Magnificent Iguanas of Isla Mona
John Bendon

Bath, England

Photographs by the author except where indicated.

IGUANAS OF ISLA MONA

�

Aerial view of Isla Mona. Photograph by Alberto Alvarez.



100 IGUANA  •  VOLUME 11, NUMBER 2  •  JUNE 2004

Alighting from the aircraft, I was immediately struck by the
grand tropical aspect of the place, the cacti growing at the airstrip,
unusual birds, and strange noises. That night, I was amazed by
the innumerable stars bright against the black sky, and by an
immense silence that covered us like a cloak and was interrupted
only occasionally by the bark of a Night Heron. Lying in bed,
knowing I was on an island in the middle of nowhere, no one
walking down the street, and hearing no cars or other sounds of
a modern metropolis was a strange and wonderful sensation.

Mona Island is a unique ecosystem, with considerable
endemism. In addition to the iguana, the island hosts an endemic
frog (Eleutherodactylus monensis), three endemic lizards (Anolis
monensis, Sphaerodactylus monensis, and Ameiva alboguttata), three
endemic snakes (Typhlops monensis, Epicrates monensis monensis,
and Alsophis portoricensis variegatus), and an endemic, albeit
extinct, tortoise (Geochelone monensis). Non-endemic reptiles
include widely distributed skinks in the Mabuya sloanii complex,
the introduced gecko, Hemidactylus mabouia, and three species
of sea turtles.

In October, hundreds of Careys (Hawksbill Turtles,
Eretmochelys imbricata) hatch on the beaches. They are gathered
by the researchers, duly processed, and returned to the beach for
their crawl to the sea. Although clutches may contain as many as
120 eggs, the chance of survival to adulthood is only one in
10,000. Their lives are fraught with danger; en route to the water
they are eaten by hawks and herons; once in the water, they
become prey for fish; and, if any survive, humans hunt them for
the rest of their lives. A 200-lb. turtle takes fifty years to grow, one
minute to kill, an hour to cook, and thirty minutes to eat. I am
genuinely surprised that any are left.

The climate on Mona is hot. The island consists mainly of
an elevated plateau. Beaches are scarce and the only substantive
one is on the southwestern shore. The unique vegetation includes
84% plateau forest, 8.7% shrub forest, 0.6% cliff forest, and
6.7% coastal plain. Eleven varieties of cacti and 29 different
species of Euphorbia have been identified. Two of these (E. peti-
olaris and E. mancinella) are poisonous to humans, although
iguanas eat fruits of the latter.

At any time, only about 12 persons, including two rangers,
inhabit the island. Facilities for the human population are lim-
ited to a few houses, a communal kitchen, a couple of workshops,

a hurricane-destroyed theater/museum, and a new shower and
toilet facility. Two vehicles, plus a digger, several bikes, and a boat
provide transportation. Nine years ago, the only two cars on the
island smashed into each other in a freak accident (don’t ask!).

What I find most appealing about Mona is that the iguanas
don’t just frequent the wilder, more distant parts of the island.
Finding one right there on the doorstep was not unusual. Mostly,
they would ignore me completely, looking to the left and right,
perhaps munching on a couple of fallen fruits or a leaf or two as
they moved about. Their lives seem quite tranquil — but, after
all, they’ve been living here since long before the research station
was built, walking the same routes, soaking up the sun’s rays, and
just being iguanas.

Returning from the kitchen to the bunkhouse, coffee in hand, the
early morning sun already hot, I see a big, calm, gray iguana. He

seems to be staring at the trees, maybe wondering what’s for
breakfast.

Two iguana-related projects are ongoing. Alberto Alvarez,
Department of Natural and Environmental Resources (DNER),
heads an Iguana Headstarting Program, and Nestor Pérez, a
Colombian post-graduate at the University of Puerto Rico–Río
Piedras (UPR), is conducting research on reproductive and terri-
torial behavior. The latter project, funded by UPR and DNER,
is currently in the third of a projected five years. The only sub-
stantive prior research on Mona Island Iguanas was the basis of
Thomas Wiewandt’s 1977 doctoral thesis.

Nestor’s study uses nest monitoring, radiotracking, and indi-
vidual identity numbers in an effort to determine sex ratios,
paternity information, size of home ranges and territories, move-
ments, survival rates, and total population size. Mona Island cov-
ers about 65 km2 and much of it is very difficult terrain. Nesting
sites are scattered widely and some are almost inaccessible. Twelve
nests have been carefully monitored and surrounded by zinc
fences. These are visited daily during the expected time of hatch-
ing. Hatchlings are trapped inside the fences, collected, and taken
to the research station for processing. Each is measured and
weighed, blood samples are taken, and transponders inserted.
This year, 121 hatchlings were processed. At about 30 minutes
per lizard, this entailed 60 hours of work for each of two people.

BENDON

The entire human population of Mona Island taking a lunch break.

Twelve carefully monitored nests were surrounded by zinc fences.



IGUANA  •  VOLUME 11, NUMBER 2  •  JUNE 2004 101IGUANAS OF ISLA MONA

About half of the lizards are released in designated areas and the
rest are held in headstarting cages. Some of those remain for sev-
eral years and attain sizes sufficient to enhance chances for sur-
vival after release. Others may be released earlier, according to
their progress, and the capacity of the headstarting facility.

Three different radio transmitters are used. One, for the
larger animals, is enclosed in a pouch attached to a collar made
out of plastic tubing, and hangs loosely around the neck. These
present some risk of strangulation, and radios attached directly
to iguanas without straps are generally preferred. The second type
of radio is much smaller and is used for juveniles and even occa-
sional hatchlings. Because young animals grow quickly, these
transmitters are tied around the lower part of the body above the
pelvis, swing loosely, and usually fall off after some time. Not only
does this prevent the ties becoming too tight over time, but also
the transmitters can be recovered and re-used. The third type of

Radio transmitters used for the larger animals are enclosed in a pouch
attached to a collar made out of plastic tubing.

Hatchlings are collected from the nesting sites and processed at the field station. About half of them are returned to the wild at the release sites and
the remainder are placed in headstarting cages. 



transmitter, about the size of an AA battery, is implanted in the
body cavity of larger animals. Although a longer recovery period
might be preferable, animals are usually released after three days
due to space and time constraints. Once transmitters are fitted,
animals are individually tracked using a VHF receiver. Radio
transmitters usually last three months to three years, depending
on type.

My own experiences tracking iguanas were mixtures of
struggle and elation. One day, in the very dense, almost “jungle-
like” part of Mona, I set out with the radio receiver to find iguana
#22. The antenna was unwieldy and kept getting caught on
branches. I kept getting tangled in vines, and had a difficult time
determining in which direction I was going and where I had
been. Finding an iguana was not so easy. I would turn the
antenna until the signal was stronger and then walk in that direc-
tion. Then the signal would fade and I would turn, lose the sig-
nal, lose myself, trip a lot, but eventually catch the signal again.
I followed it until I reached a sheer rock wall. Shouldering the
radio equipment, my tote bag, and a camera, I clambered up
more easily than I had suspected I could — only to find, sitting
on the top, a few feet away from the edge, staring at me, a beau-
tiful female iguana with #22 painted in red latex on her side. The
experience of stumbling through the jungle, scratched, badly bit-
ten, and very dirty, was well worth the reward of seeing her nod
“hello.” The sight of number twenty two at the end of all that
will stay with me forever.

Adventures with Iguanas
Ciudad Lagarto.—A garbage dump near the headstarting facil-
ity has grown steadily over the years. I named it Ciudad Lagarto
(“Lizard City”) and erected a large sign to that effect. Many of
the released headstarters don’t stray very far and several of them
have taken up residence, along with wild animals, in the dump,
which supports a total population of about 20 lizards. If I
approached slowly and stayed low, I could easily observe them,
and I noted that each one had its own place. Once I stood up,
however, most of them quickly disappeared.

A fairly large iguana lived near the front by a pile of old
doors. She was beaded and tagged and had made a “nest” of dead
grass under one of the doors. She was the largest and obviously

the “alpha” animal in the area, even though a female. Quite
assertive and very much the “home-body”, she was close and easy
to recapture, and had been weighed and measured more often
than most of the other iguanas, providing us with important
information on growth in large adults.

Other individuals, several of them similarly prone to recap-
ture, lived inside refrigerators. Resultant data demonstrate that,
in 18 months, between the ages of about two to three years,
weight gain ranged from a quite satisfactory 500 g to as much as
one kilo.

I visited Ciudad Lagarto and the inhabitants of the enclosure
nearly every day that I was on the island. I very much enjoyed sit-
ting inside the cages and watching them. If I was still and did not
attempt to grab them, they would go on about their business and
ignore me. I only did this at the beginning and end of the day,
since the animals go “mad” when disturbed during the heat of
the day and rush headlong into the wire mesh sides of the cages.
But, in the evening, when the last rays of sunshine dip into the
ocean, whole new hordes of life forms awaken, namely the mos-
quitoes and sand flies. That was my signal to get out as quickly
as possible and slap on the repellent (one can’t wear it near the
reptiles, as it may harm them).

102 IGUANA  •  VOLUME 11, NUMBER 2  •  JUNE 2004 BENDON

This large male was about to shed its skin.

Headstarted iguanas spend the first months of their lives in one of ten
contiguous units.

This occupant of Ciudad Lagarto monopolizes a prized basking site.
Photograph by Alberto Alvarez.



IGUANA  •  VOLUME 11, NUMBER 2  •  JUNE 2004 103IGUANAS OF ISLA MONA

Bathroom Hatchlings.—Gravid iguanas enter the nesting bur-
rows and emerge looking spent and emaciated. One nest site was
in an open area where the station’s massive grass cutter would pass
over it. Fearing that the nest or emerging hatchlings might be
damaged, the eggs were removed and reburied elsewhere. The
reburied eggs were late in hatching; so, after some discussion, we
dug them up and placed them in a container filled with soil from
the site and set it under the house.

Although quite warm by temperate standards, October tem-
peratures drop each day — and iguana eggs need temperatures
of about 30 °C to hatch. Nothing was happening. We peered
into the container each day and checked the temperature, which
was hovering around 26 °C. Finally, we decided to bring them
into the old wooden house and, more specifically, into the bath-
room, which seemed to be the warmest place, especially when we
added a light. A little water stabilized the humidity and the tem-
perature settled at 31 °C under the sand and at 35 °C on the sur-
face. Now, all we had to do was continue waiting.

The very next morning, I was in the bathroom brushing my
teeth when, as an afterthought, I removed the light to see what
was going on in our incubator. There, poking up like dark, wet
pebbles on a beach, were four heads. The first hatchlings had
started to come out of their eggs. This continued for four days.
Every so often, I would go into the bathroom and another head
would have popped up like a crocus in spring. I kept them warm
and watered them, just as one would with delicate flowers — and

they responded by growing larger in front of my eyes (they really
did grow, as they were absorbing their yolk sacs). Thirteen little
iguanas eventually sprang to life. It was like magic. Even the sea-
soned researchers were excited and appeared from time to time
with their cameras. Hatching is typically a very private enterprise,
as the eggs are normally underground.

What we learned from this exercise is that we could not
process these babies straight away. When hatchlings appear above
ground for the first time, they may be anywhere from a few days
to well over a week in age. At that age, they could be safely han-
dled, if the yolk sac has shrunken and dried and the skin has
healed. If we tried to handle them at a younger age, their moist
and slippery skin wouldn’t take the paint with which we wrote
their numbers, nor would it cut cleanly for the insertion of trans-
mitters. So, the bathroom hatchlings taught us to use the umbil-
ical scar as an indicator of when processing could begin.

Two for the Price of One.—The fences placed around iguana
nests sometimes did not work. In one instance, two fences next
to each other were supposed to yield three sets and one set of
hatchlings, respectively. Instead, the former yielded only two and
the latter two as well. We think that the third set from the first
fence tunneled to the surface at an angle, only to emerge within
the second fence — mystery solved. Unfortunately, when the
time came to collect the second set of hatchlings, the heat of the
day was upon us. The little lizards running around the circular
wall of their enclosure were far too fast to catch, and, if we came
close to succeeding, they disappeared into their own hatching
tunnel. As we needed to process them the next morning, I sug-
gested that we go out at night with headlamps and grab them
while they were sleeping. The result was a comic farce.

Most were hidden under some pipes that had been cut
down the middle. I suggested that we hold a pillowcase at one
end and scare them into it. It didn’t work — about nine iguanas
ran out all at once, and the pillowcase caught only two. The rest
ran round and round the circular fence as we turned on our heels
trying to follow them, the bouncing lights on our heads only
adding to the confusion. Somehow, we discovered the rest of the
hatchlings, all buried in the sand, jumbled together in a mis-
shapen ball. Neither of us had ever seen this kind of behavior, but
we quickly took advantage of it and bagged some more of the
lizards. Eventually, we caught all fifteen, but we were pretty sure
that we had played the fools while the little iguanas outsmarted
us. We were laughing out loud, but silently pleased that no one
had been watching.

The Legend of 98.—The day iguana #98 escaped, he just
jumped out of my hand, ran up the mosquito screen, found a
small hole, and was suddenly outside. I ran down the stairs and
looked up at him perched on the window ledge. He jumped,
landed beside me, and started looking around. However, as I bent
to pick him up, he took off, running about 3 m. I just watched,
knowing it would be futile to try to chase him. By then, he had
spotted the outside world for the first time. The sun shone down
on him and, when he looked up, he didn’t see me any more.
Stepping lightly across the grass, stopping and nodding, then
running, he fled into the brush and was gone. Unfortunately his
number will have shed with his skin by the next time I visitThis individual is one of the largest iguanas on the island.



Mona; I will never know if he survived. If this was a novel, I
would say that, “the legend of 98 lives on.”

Battles.—I had seen iguanas fighting in photographs only. It
looked pretty vicious, but didn’t prepare me at all for what I saw
on Mona. These iguanas are among the largest and toughest
species of Cyclura. They scramble over sharp rocks and cactus,
they lose tails and fingers, and they’re not fazed by harsh condi-
tions — except possibly for cold weather, which forces them into
a cozy hole somewhere. They are stocky and muscular, many
have battle scars, and their horns and spikes often are worn or
broken as a consequence of fighting or the mere wear and tear of
living in a harsh environment.

A couple of hardened old hands lived near the research sta-
tion. One was iguana #21, who I called “Tough Guy 21,” and
the other was just “Tough Guy.” Both were aggressive. Tough
Guy 21 had been around for a long time and was the uncrowned
king of the area. He would swagger around and chase lesser males

away. He always got the girl. One day during lunch, someone
yelled: “Hey, iguanas are fighting over here.” We dropped our rice
and beans and dashed over, cameras at the ready, to find #21
fighting with Tough Guy. The new arrival was larger than our
#21 and Nestor told me he had never seen him before. He was
identifiable by a white spot of detached skin just behind his right
shoulder (which we duly noted and recorded). The fight
appeared to be a stalemate, either that or both combatants lost
interest. After several minutes, each went his own way, nodding
furiously and gaping so widely that it seemed like one could park
a car in the cavity. 

We didn’t think much more about that encounter until a
few days later. Female #27 seemed to be minding her own busi-
ness under the watchful gaze of Tough Guy 21, when, out of
nowhere, came the new Tough Guy. He sidled up to her and,
from where we stood, appeared to be chatting to her amiably.
Tough Guy 21 approached, the female backed off, and, like
jousting knights, the two males rose up on their haunches, tipped

104 IGUANA  •  VOLUME 11, NUMBER 2  •  JUNE 2004 BENDON

Many of the headstarted iguanas don’t stray very far after they are
released. Several have taken up residence, along with wild animals, in
the dump, which supports a total population of about 20 lizards.
Photograph by Alberto Alverez.

A young Rhinoceros Iguana peers out from the safety of a PVC pipe
within the headstarting cages. Photograph by Alberto Alverez.

The fences placed around iguana nests sometimes did not work as
intended.

Fights between adult males can be vicious.



their heads and bodies slightly to the side, and began trotting
around each other. All at once, #21 pounced forward and spat
with a kind of hissing roar. This seemed to inflame the new-
comer, who opened his mouth and went for 21. Now, with both
their mouths wide open, they scraped alongside each other’s faces,
turning in a circle. They did not lock jaws, but continued turn-
ing and scraping and drawing blood all the while. This went on
for about ten minutes, during which they ignored us as we
approached rather closely to get photographs. Suddenly, they
stopped, seemingly taking a break — or was the battle over?

Apparently, the new Tough Guy had won. The next day we
saw all three animals again. The new guy and the female wan-
dered off together, while #21 skulked in the background. This
behavior was quite unusual for that time of year. It was like mat-
ing behavior, but occurred during the hatching season.

The third fight I saw was so vicious that I got a bucket of
water and threw it over the two combatants like they were dogs,
after which they dashed off in opposite directions. I was admon-
ished by the biologists for “interfering with nature,” which I
should have let take its course. I argued that the cat we caught
had just killed and eaten three hatchling iguanas and also had
interfered with nature, as it was not supposed to be on Mona at

all. I felt that I had merely tipped the balance a little by ensuring
two big, healthy iguanas would survive. I can see the other side
of the argument, but I still believe I did the right thing. I was not
going to watch two endangered lizards kill each other.1

Epilogue
Two nests, each surrounded by aluminum flashing, are on Pajaros
Beach. Although 9 km from the station, we check them daily.
This day, nothing. Perhaps, if it thunders again tonight, events
will move along and tomorrow, early, the hatchlings may emerge.
But now, I walk along the beach, watch the birds, move inland
to a Seagrape tree, and find a place to sit in the shade. Alerted by
a rustle in the leaves, I see an enormous iguana amble toward me.
Unbelievably, he sits and rests beside me, right beside me, nod-
ding. I nod back. I stare at his big black eye, sunlight glinting off
the blackness. All at once we are friends, sitting there together.

IGUANA  •  VOLUME 11, NUMBER 2  •  JUNE 2004 105IGUANAS OF ISLA MONA

1 Editors’ note: Although male iguanas sometimes die as a conse-
quence of complications from combat-related wounds, most conflicts
in nature cease before fatal injuries are inflicted — as long as the
“loser” has an opportunity to escape.

Mash is placed daily in split-bamboo feeding “stations” for young Mona Island Iguanas (Cyclura cornuta stejnegeri) at the headstarting facility.
Photograph by Alberto Alverez.



The magic of Mona was working again; the place weaves spells
around those who visit, and I know I shall return again and
again, if only I can. Presently, the lizard rose, glanced at me, and
walked away. As he did, he looked back one last time before dis-
appearing into the brush. I’ve had many experiences during my
travels, but this beat them all. From the back streets of Cairo to
the caves of Kandahar, travel is enlightening, but sharing the
shade of a tree on a blazing afternoon with my lizard companion
is a memory that will remain with me always.

Acknowledgments
I thank Thomas Wiewandt, Nestor Pérez, Robert W. Ehrig,
M.P.C. Lawton, Alberto Alvarez, and Charles Knapp for inspi-
ration, information, and guidance.

Reference
Joglar, R.L. 2003. An island enchanted by frogs, pp. 203–211. In: R.W.

Henderson and R. Powell (eds.), Islands and the Sea: Essays on Herpetological
Exploration in the West Indies. Society for the Study of Amphibians and
Reptiles Contributions in Herpetology, vol. 20. Ithaca, New York.

106 IGUANA  •  VOLUME 11, NUMBER 2  •  JUNE 2004 BENDON

The iguana headstarting program began in October
1999 with the construction of an enclosure on Playa

Sardinera (in the west of Mona Island) where the DNER
facilities are located. The enclosure is composed of ten
contiguous units, each 3 m x 3 m x 2.3 m. Landscape
arrays simulate a natural environment. Rocks and logs pro-
vide perches, plants provide shade, hiding places, and
water reservoirs, PVC pipes (4" diameter, 1 m long),
approximately 6.5' above the ground, provide climbing
opportunities and more hiding and basking places. Several
units contain smaller pipes (3") and bamboo sticks for
hatchlings, an average of eight of which are kept in each
unit, with the number of individuals in each enclosure
depending on landscape characteristics, food resources,
aggressiveness, and individual needs. 

In 1999–2000, 47 hatchlings were collected at
Sardinera and 4 at Playa de Pájaros; in 2000–2001, the
numbers were 25 and 0; in 2001–2002, 1 and 5; and, in
2002–2003, 3 and 34, respectively.

Iguanas are fed daily or on alternate days with a mix-
ture of moisturized prepared food (Zeigler® Iguana Mash)
and wild fruits, flowers, and, less abundantly, leaves. Fruits
and flowers have higher nutritional value. In general, about
30% of iguana food intake consists of collected natural
food items. Mash (5–10 g for hatchlings, 10–15 g for
older animals) is placed daily in split bamboo feeding “sta-
tions,” whereas collected foods are spread randomly in
each unit. Late in the afternoon, food stations are removed
and cleaned. Regular cleaning is important to reduce infes-
tation by fire ants, centipedes, scorpions, land crabs, and
rodents. Vegetation inside the enclosure is watered as
needed.

By the end of November 2002, all the signals from
transmitters implanted in August 2002 were lost. Two sig-
nals from transmitters implanted in April 2002 were still
detectable, but one was lost in December and the other

one in January 2003. We discount animal predation, and
believe that a combination of out-of-range signals and
transmitter malfunction may be responsible. Collected
data reveal that the fastest and farthest any one individual
had moved was about 1 km in 3 days. The remaining ani-
mals were lost no more than 400 m from the release site.
Animals with transmitters implanted in April 2002 were
less than 800 m from the release point when the signal was
lost.

In December 2002, seven juvenile iguanas hatched in
2000 and weighing more than 1 kg, were released in the
Sardinera and Playa Mujeres nesting sites. These were
dominant individuals and responsible for considerable
aggression in some enclosure units.

Six released iguanas have been recaptured in June and
July 2003, including one with a disabled transmitter that
had been implanted in April 2002. The remaining animals
were from August 2002 and one from December 2002
releases.

H E A D S T A R T I N G
Adapted from a report by Alberto Alvarez and used with permission.

Hatchling iguanas at the headstart facility in 2003. Photograph by
John Bendon.


