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108 IGUANA  •  VOLUME 13, NUMBER 2  •  JUNE 2006 GUTSCHE

Ctenosaura bakeri basking in Utila’s mangroves.

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Field studies occurred mainly in two phases, from 27 June
1999 until 17 July 2000 and 1 January to 31 December

2001, allowing examination of distribution, abundance, and
population structures within a complete annual cycle.

Population Structure: Biometric Data
Data for Ctenosaura bakeri captured during the study at various
sites and for which sex could be clearly determined are included.
As in most iguanas, males were longer and heavier than females.
The total length (TL) of the largest animal captured, a male with
a partial tail, was 781 mm. Adding the snout-vent length (SVL,
315 mm) and the mean tail length/SVL suggest a possible TL of
about 828 mm. Males attain a larger size (mean 230.9 mm and
460.6 g) than females (mean 188.0 mm and 237.4 g), with females
averaging 81.4% of SVL and 51.5% of body weight of males.
Mean tail length for males (337.0 mm) was 11.9% longer than
that of females (296.9 mm). Differences in relative tail length (tail
length/SVL) of both sexes did not differ significantly.

Growth and Development
I examined growth of captive juveniles maintained in outdoor
enclosures for periods usually lasting one year, at which point the
sex of the animals could not yet be clearly determined. Some
individuals were raised and measured for up to 2.3 years,

enabling collection of sex-specific growth data. Increases in length
were predominantly linear, whereas weight increased exponen-
tially. Juveniles (sex unknown) attained 1.7 times initial SVL and
4.9 times initial weight at 300 days. Development was sex-spe-
cific. Males were 5.1% larger and 8.6% heavier than females at
hatching, 17.9% larger and 48.6% heavier at 300 days, and
9.1% larger and 27.9% heavier at 650 days. No comparisons
were made beyond 800 days as only one (small) male remained.
When I compared growth rates of captive-held and wild-caught
animals, rates of growth were essentially similar for captive and
wild caught animals of both sexes. 

The proportional growth of SVL and tail length was exam-
ined for 145 wild caught animals with intact tails. Relative tail
length decreased with increasing SVL, from longer tails in juve-
niles to relatively shorter tails in adults. The relative tail length of
juveniles was significantly greater than that of adults. Sex-specific
differences for relative tail length were not evident, although
males had longer tails than females.

Population Structure and Reproduction in
Ctenosaura bakeri on Isla de Utila

1

Alexander Gutsche

Institute of Biology, Department of Sensory Biology, Humboldt University, Berlin, Germany

Photographs by the author except where indicated.

IGUANA  •  VOLUME 13, NUMBER 2  •  JUNE 2006 109CTENOSAURA BAKERI ON ISLA DE UTILA

�

Relationship between mass (g) and snout-vent length (SVL, mm) of
adult Ctenosaura bakeri. Because of a suspected allometric relationship,
loge-transformed variables were used.

Biometric values for male and female Ctenosaura bakeri from Isla de
Utila (mean values ± 1 standard deviation, range, sample size). Lengths
are in mm and mass in g.

Character Males Females Total

SVL (mm) 230.9 ± 42.2 188.0 ± 18.6 208.9 ± 38.7
145–315 151–229 145–315
n = 76 n = 80 n = 156

Tail length/SVL 1.60 ± 0.09 1.57 ± 0.09 1.58 ± 0.09
1.40–1.74 1.32–1.79 1.32–1.79

n = 38 n = 48 n = 86

M (g) 460.6 ± 231.5 237.4 ± 79.9 339.9 ± 200.6
100–923 105–424 105–923
n = 76 n = 80 n = 156

1 This is a continuation of a series that began in Iguana 12(3):142. See
that article for an introduction to the series and for maps and
detailed descriptions of the study sites. All articles are based on the
dissertation research of the author. Translation by AJ Gutman.

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Sexual Maturity
In order to clarify the issue of age and size of C. bakeri at onset
of sexual maturity, I compared body weights of 87 wild-caught
gravid females with those of individually marked iguanas that
were raised under controlled conditions. The smallest wild-
caught gravid female had an SVL of 150 mm, likely the lower
limit of sexual maturity for this species. Among captive-raised
animals, three of five females had attained an SVL of 150–154
mm at an age of about 650 days, and at an age of about 840 days,
all females had attained an SVL > 150 mm.

By the end of 2001, mating activity had not been observed
among the captive-raised animals. Only in the following spring
(2002), during the regular mating season, was mating activity
first noted in this group (S. Knapinski, pers. comm., 2002). At
that time, animals were approximately 2.5 years old, likely the
earliest age at which mating would be initiated in the wild.

Population and Age Structure
The following data are based on the capture-recapture study.
Based on growth data from captive animals, I roughly evaluated
age structure. When comparing the individual study sites, the
proportion of subadult iguanas (< 150 mm SVL) in Iron Bound
was higher than that at Blue Bayou and Big Bight Pond. At Big
Bight Pond and Iron Bound, the proportion of both adult iguanas
and large iguanas was higher than at Blue Bayou.

Of the 24 iguanas captured at Blue Bayou, four animals
(16.7%) were subadults and 20 animals (83.3%) were adults.
Subadults (50–130 mm SVL) ranged in age from about 0.5–1.5

110 IGUANA  •  VOLUME 13, NUMBER 2  •  JUNE 2006 GUTSCHE

Snout-vent length (SVL) and mass of captive Ctenosaura bakeri.

Nesting area at Iron Bound.

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IGUANA  •  VOLUME 13, NUMBER 2  •  JUNE 2006 111CTENOSAURA BAKERI ON ISLA DE UTILA

years. Five females (150–170 mm SVL) were about 2.5 years old,
and four females (190 mm SVL) were more than 2.5 years old.
Four males (150–170 mm SVL) were about 2–2.5 years old, and
seven males (190–230 mm SVL) were more than 2.5 years old.

Of 40 iguanas taken at Big Bight Pond, two animals (5%)
were subadults and 35 animals (95%) were adults. The subadults
(50, 70 mm SVL) were about 0.5 years old. Of the 18 females,
one female (150 mm SVL) was about 2.5 years old and 17
(190–210 mm SVL) were older than 2.5 years. Five males
(150–170 mm SVL) were about 2–2.5 years old and 15 males
(190–310 mm) were more than 2.5 years old.

Of the 107 captured iguanas collected at Iron Bound, 40
(37.3%) were subadults and 67 (62.7%) were adults. Subadults
(50–130 mm SVL) were about 0.5–1.5 years old. Thirty females
(150–170 mm SVL) were about 2.5 years old, and 11 females
(190–210 mm) were older. Of the 26 males, four males were
about 2.5 years old, and 22 males (190–270 mm SVL) were
more than 2.5 years old.

The ratios of males to females at Blue Bayou (11:9) and Big
Bight Pond (20:18) were closely balanced, but that at Iron Bound
(26:41) was conspicuously female-biased. The latter probably
results from the polygynous mating system in the species, rather
than a real demographic skew.

Reproduction
The reproductive cycle occurs during the dry season, roughly
from mid-January to early August. Mating begins when both the
quantity and frequency of precipitation decreases in comparison
with the heavy rainfall of previous months. In 2000, the first
mating was observed on 17 January. Mating activity reached the
highest intensity by about mid-February; the last mating of the
year was observed on 24 March. Egg-laying began about mid-
March at all study sites and during all three years of the study.
The most intense nesting activity was observed from the begin-
ning until the middle of April. Egg-laying ended in early May at
all sites. Hatching began during the latter half of June, with the
earliest observed hatch occurring on 20 June 2000 at Iron
Bound. The latest observed hatching date was 10 August 2001.

Frequency distribution of Ctenosaura bakeri by snout-vent length (SVL)
for various size classes, differentiated by sex and study site. Data are
based on the capture-recapture study.

Ctenosaura bakeri in copula.

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112 IGUANA  •  VOLUME 13, NUMBER 2  •  JUNE 2006 GUTSCHE

Female digging a nest burrow.

Reproductive cycle of Ctenosaura bakeri comparing the amount of precipitation (bars) and air temperature (line) throughout the year. Climatic data
represent mean monthly values from 1999–2001 (at Big Bight Pond); the reproductive phases are averaged from observational data.

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IGUANA  •  VOLUME 13, NUMBER 2  •  JUNE 2006 113CTENOSAURA BAKERI ON ISLA DE UTILA

No reproductive activities occurred outside this time period in
this or other years.

Nesting Areas
Female Ctenosaura bakeri exclusively use a small number of sandy
coastal areas as nesting grounds. All are adjacent to the mangrove
areas where iguanas spend the bulk of their time. More distant
sandy coastal areas (e.g., at Pumpkin Hill on the northeastern
coast or between Big Rock and David Beach on the southern
coast) were not used, nor were inland sandy areas bordering man-
groves. The total extent of the nesting areas amounted to roughly
109 ha. Vegetation consists largely of plants characteristic of
Caribbean coasts (e.g., Ipomoea pres-caprae, Cocos nucifera,
Coccoloba uvifera). Nesting burrows were invariably located in the
vegetation-free areas. Iguanas avoided even those open areas
where the surface of the ground was covered with fallen leaves.

Erstwhile vegetation-free nesting sites used for many years
at Iron Bound and Rock Harbor were gradually colonized after
2001 by an invasive creeping vine. In 2002, nest burrows were
restricted to the remaining open areas; by 2003, these areas had
also become overgrown. Within a few days of the growth being
manually removed, the first test excavations were observed.

I noted a clear preference for sunny locations. Of 110 exca-
vations, 55.5% (test and nest burrows) were situated where they
received full sunshine throughout the day or were subject to light
shade for only a few minutes. Such conditions were present pri-
marily in sandy areas where short periods of shade occurred only
in the vicinity of isolated trees. Partly sunny locations subject to
shade at particular times of the day were also utilized (49 excava-
tions; 44.6%). These locations were close to or within adjacent
beach forest.

Burrows were preferentially selected in areas with sandy sub-
strate and few coarse coral fragments. Larger coral fragments or
pieces of driftwood were often (61.8%) incorporated into nest-
ing burrows, with entrances located directly beneath these sur-
face structures.

Depths of nest chambers were occasionally influenced by
various environmental factors. Chambers in areas with well-
developed root networks were directly beneath the root system.
These nests were generally deeper (mean 345.0 mm) than those
in sandy areas without root networks (mean 242.9 mm). Surface

substrates (to 170 mm) were dry. Nesting chambers, however,
were discernibly damp.

Temperature loggers buried at depths of 200 and 400 mm
(representing upper and lower limits where nests were typically
found) recorded mean temperatures between 29.7 °C (200 mm)
and 30.3 °C (400 mm).

Migration of Females to Nesting Areas
Five females marked in the mangroves were recaptured in vari-
ous nesting areas. Two were from Blue Bayou and the other three
from Iron Bound. Distances between the original capture site and
the nesting areas were 158.1–880.0 m. Four of the females were
recaptured in nesting areas bordering their home mangrove areas.
The fifth female was marked at Iron Bound and recaptured 880
m away on the beach at Rock Harbor. The return journey was
documented for two of the females at Iron Bound, with both
recaptured at their home trees. 

Nesting Burrows
Females dig individual nesting burrows in sandy beach substrate
in order to deposit eggs. Burrows are dug anew each breeding sea-
son. Females will usually dig a number of test burrows in various
locations before selecting a final nesting site. Test burrows are
abandoned in various stages of completion. The actual nest bur-
row is backfilled with substrate following successful oviposition.Beginning of a nest excavation (test dig).

Opened nesting chamber with temperature logger.

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Nest burrows consist of an entrance, a tunnel, and a connect-
ing egg-laying chamber. Openings were uniformly oval. The small-
est measured entrance was 60 x 30 mm, the largest 100 x 70 mm.
Tunnels were roughly circular in cross-section with diameters of
50–70 mm and lengths of 510–1240 mm from the entrance to the
end of the nest chamber. Nest chambers were fist-shapes, measur-
ing on average 112 x 73 mm (width x height). Depths, measured
from the surface of the ground to the bottom of the nest, were about
300 mm (160–450 mm). Of 22 nest chambers excavated, five had
approximately1-cm air pockets above the eggs, all others were filled
with loose sand. 

Incubation
Data on natural incubation temperatures were collected for four
clutches in 2000 and four clutches in 2001. Data loggers were
placed directly next to the eggs either on the day they were
deposited or on the day following and removed 95 days (13
weeks) later.Temperature fluctuations were generally small and
varied, despite differing locations, times of day, and depths, on
average between 29.1–30.8 °C with a total mean of 30.1 ± 0.2
°C. Deeper nests varied less (only by about 1.0 °C around the
mean value) than shallower nests. Observations on last oviposi-
tion and first hatch indicated incubation periods in the wild that

114 IGUANA  •  VOLUME 13, NUMBER 2  •  JUNE 2006 GUTSCHE

Incubation periods for Ctenosaura bakeri at various temperatures under
artificial conditions.

Distribution of gravid Ctenosaura bakeri females by size class. Numbers
above the bars are mean clutch size for that size class.

Opened nest burrow; the knife marks the entrance.

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IGUANA  •  VOLUME 13, NUMBER 2  •  JUNE 2006 115CTENOSAURA BAKERI ON ISLA DE UTILA

ranged from 91–99 days. Also, all eggs in nests (n = 22) excavated
after 95 days had hatched. Under artificial conditions, incuba-
tion period decreased with increasing temperature.

Hatching Rates
Seventeen clutches from 2000 consisted of 204 eggs with clutch
sizes of 6–16 eggs. Hatching rate was 94.1%. One clutch of 12
eggs all died. One day following oviposition, an inspection of the
nest, which had not been sealed by the female, revealed that hard
root tips had pierced the shells of two eggs and the yolk had run
out. The nest was full of tiny black ants feeding on the damaged
as well as undamaged eggs. All eggs had been fertile.

Five clutches from 2001 contained a total of 57 eggs with
clutch sizes of 9–14 eggs. Hatching rate was 86.0%. One clutch
of 14 eggs produced only six hatchlings; the other eight eggs died.
Of these, seven contained dark, hardened yolk remnants, and one
egg contained remnants of vertebrae. Fly pupae (family
Sarcophagidae) were on and in all of the dead eggs.

Clutch Data
Clutch data were obtained from free-living females (n = 87) that
were caught at various nesting sites and others housed in outdoor
enclosures at the “Iguana Station” until oviposition. The SVL of

captured gravid females ranged from 150–229 mm. Mean body
weight before egg-laying (body weight including clutch) was
224.5 g (118–435 g). Whereas clutches averaged 27.5% of
female body weight before egg-laying, the average loss of body
weight due to egg-laying was about 33%. The difference of
roughly 5.5% might reflect water released during ovipositioning.
Mean clutch weight was 55.3 g (30.6–107.3 g) and varied signif-
icantly according to the number of eggs per clutch. Mean egg
weight was 6.2 g (4.8–8.4 g). The mean number of eggs laid was
9.8 (5–16). Mean egg dimensions were 30.5 x 18.7 mm (27–35
x 18–19 mm). 

Larger females produced significantly more eggs per clutch,
but only very slightly larger eggs. The relationship of body weight
before and after egg-laying was strongly linear. Egg and hatchling
masses were significantly correlated, but clutch size had no appar-
ent effect on hatchling mass.

Using number of adult females captured, mean clutch sizes
for each size class, and the natural hatching rate (92.3%), I cal-
culated birth rates for females at each study site for the year 2000.
Using census and distribution data for each area, hypothetical
birth rates were 79.9 (Blue Bayou), 184.0 (Big Bight Pond), and
346.0 (Iron Bound) hatchlings per hectare.Female with collapsed flanks following oviposition.

Gravid female on the trunk of a Coconut Palm. Eggs are clearly visible
along the flanks.

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