Spatial Ecology of the Endangered Mona Island Iguana Cyclura
cornuta stejnegeri: Does Territorial Behavior Regulate Density?
Author(s): Néstor Pérez-Buitrago, Alberto M. Sabat, and W. Owen McMillan
Source: Herpetological Monographs, Number 24:86-110. 2010.
Published By: The Herpetologists' League
DOI: http://dx.doi.org/10.1655/HERPMONOGRAPHS-D-09-039.1
URL: http://www.bioone.org/doi/full/10.1655/HERPMONOGRAPHSD-09-039.1
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Herpetological Monographs, 24, 2010, 86–110
E 2010 by The Herpetologists’ League, Inc.
SPATIAL ECOLOGY OF THE ENDANGERED MONA ISLAND IGUANA
CYCLURA CORNUTA STEJNEGERI: DOES TERRITORIAL BEHAVIOR
REGULATE DENSITY?
NÉSTOR PÉREZ-BUITRAGO1,3,4, ALBERTO M. SABAT1,
AND
W. OWEN MCMILLAN2
1
Department of Biology, University of Puerto Rico, Rı́o Piedras, San Juan, PR 00931, USA
Smithsonian Tropical Research Institute, Apartado 0843-03092, Panama, Republic of Panama
2
ABSTRACT: The endangered iguana Cyclura cornuta stejnegeri is endemic to Mona Island, Puerto Rico.
Factors accounting for its protected status include low adult densities and relatively low abundance of
juveniles. We studied the spatial ecology, territoriality, and philopatric behavior along a gradient of humandisturbed areas to increase our understanding of its unusual demography. We used the minimum convex
polygon (MCP) method to estimate the home range of radiomarked iguanas, the degree of inter- and intrasex
home-range overlap, and the temporal patterns of space use by 42 iguanas (19 males and 23 females,
including 4 juvenile individuals) during the nonbreeding (October–November) and mating season (June) at
three Mona Island localities with different levels of human disturbance. Juvenile iguanas had larger home
ranges and move across the home ranges of several adults. Adult males had larger home ranges than females
and no significant temporal differences in size or pattern of usage (Cole index) were detected between the
study periods. Females, despite reducing their home-range size during the mating season, changed the usage
patterns, resulting in increased interactions with neighboring males. Home-range overlap was minimal
between males, followed by female–female overlap and maximal between males and females. Our results
suggest that Mona Island iguanas are highly territorial throughout the year; particularly males in the mid- and
undisturbed areas where almost all home-range areas appear to be equivalent to the defended territory. In
the most disturbed area, where supplemental feeding by humans exists, home ranges are smaller and there is
larger intersex home-range overlap. Previously, the low density of the population was attributed to lack of
recruitment into adult stages due to predation of juveniles by nonnative mammals. However, the high levels
of territoriality documented in this study may be an additional factor explaining the low densities exhibited by
this population. Our data suggest that the three study sites may be at or close to carrying capacities for males,
because there appear to be no vacant areas for additional males. Furthermore, the large and highly
overlapping home ranges exhibited by four juvenile iguanas suggest that they are or will become floaters in
search of unoccupied space.
Key words: Cyclura; Home range; Iguana; Telemetry; Territorial behavior; Territoriality
and Buechner, 1985). This change in behavior
has been observed in Cyclura iguana populations inhabiting Caribbean islands of different
sizes. Cyclura species living on small islands in
high densities tend to be nonterritorial or to
show hierarchical social systems (Goodyear
and Lazell, 1994; Knapp, 2000a,b). In addition,
populations at high densities are also characterized by having pyramidal size class distribution, a trait that is considered diagnostic of
healthy Cyclura populations (Alberts, 2000). In
contrast, low relative abundance of juveniles
and low density of adults have been associated
with human-induced disturbances such as
habitat degradation and development and
introduction of exotic species, and are considered symptoms of nonsustainable levels of
recruitment that can compromise the longterm persistence of the species (Iverson, 1979;
Wiewandt, 1977). Besides the negative impact
on Cyclura populations that humans can cause
A GENERAL pattern across many taxa is that
territorial mainland species become nonterritorial or hierarchical when they colonize
islands, thus reaching higher densities than
mainland counterparts (Carey, 1975; Stamps
and Buechner, 1985). This pattern may be
influenced by low levels of predation and/or
competition on islands, but also by anthropogenic factors such as supplemental feeding
(Iverson et al., 2006; Knapp, 2000a; Lacy and
Martins, 2003). High densities on islands can
induce lower levels of aggressiveness that result
in reduced territory size, increased homerange overlap (Brown et al., 1995; Ferner,
1974; Maher and Lotta, 2000; Tinkle, 1967)
and abandonment of territory defense (Stamps
3
PRESENT ADDRESS: Universidad Nacional de Colombia, Sede Orinoquı́a, km 9 vı́a Tame, Arauca, Arauca,
Colombia
4
CORRESPONDENCE: e-mail, [email protected]
86
2010]
HERPETOLOGICAL MONOGRAPHS
(i.e., habitat reduction/degradation, introduction of exotic species, illegal trade and poaching), the limited geographic distribution of
Cyclura species and the susceptibility to
natural catastrophes are reasons justifying the
current threatened status of most Caribbean
iguanas by the International Union for the
Conservation of Nature (IUCN, 2006).
The Mona Island iguana (Cyclura cornuta
stejnegeri) is considered endangered according to the IUCN. The species is characterized
by a very low adult density and a scarcity of
individuals in juvenile stages when compared
with other Cyclura species (Garcı́a et al.,
2000; Pérez-Buitrago and Sabat, 2000; Wiewandt and Garcı́a, 2000). The introduction of
exotic species (feral cats, rats, pigs, and goats)
and their potential negative effects on iguana
demography (such as nest destruction and
increased juvenile predation) are considered
to be the main culprits for the current
‘‘critical’’ situation of the Mona Island iguana
population (Wiewandt, 1977; Wiewandt and
Garcı́a, 2000). However, there could be other
explanations for the observed demographic
traits of the Mona Island iguana population.
Recently, Pérez-Buitrago et al. (2007) documented the home range of nine iguanas (five
females and four males) and the spatial
distribution of other resident iguanas in an
undisturbed Mona Island environment during
the nonreproductive season. They found that
males expressed high levels of territoriality
(i.e., minimum overlap among abutted individuals) and a regular spacing pattern. In
addition, size of home ranges were intermediate (0.47 6 0.25 ha; females 5 0.28 ha and
males 5 0.6 ha) when compared with other
Cyclura species. Further evidence of a
relationship between density and spacing
behavior in Cyclura is the significant negative
relationship that results when one plots
density and home-range size (Pérez-Buitrago
et al., 2007). Although suggestive, these data
were obtained from a single site, and are not
sufficient to understand how the iguana’s
spatial organization may be contributing to
the variation in densities. Moreover, the study
did not address how human disturbance
affects use of space, nor to what extent males
and females change space use between the
breeding and nonbreeding seasons.
87
In this study we describe the spatial ecology
of the Mona Island iguana during the
reproductive and nonreproductive season at
three Mona Island environments with different landscape features and levels of human
disturbance. Our primary objective was to
understand how patterns of space use and the
social system of this iguana might be affecting
its demographic traits. We hypothesized that
(1) lack of supplemental feeding in an
undisturbed environment will result in larger
home ranges for both sexes because animals
would be forced to expand their foraging area,
(2) larger home-range overlaps will be found
in areas where human disturbance is high and
supplemental feeding occurs because this
creates small areas of superabundance of
resources, (3) males will increase their home
ranges during the mating season in order to
increase their chances of finding mates, and
(4) smaller home-range sizes and larger homerange overlap would be expected in areas of
higher iguana density because individuals
would try to reduce agonistic interactions
and would be unable to control an specific
space due to crowdedness.
METHODS
Study Sites
Mona Island (18u 039 080 N, 67u 519 570 W)
is a natural reserve, part of the national
territory of the Commonwealth of Puerto
Rico. It is a 5550-ha, heart-shaped limestone
island situated midway in the Mona Passage,
68 km west of Punta Higüera, Puerto Rico,
and 60 km east of Punta Espada, Hispaniola.
Essentially a tableland, Mona has a perimeter
defined by 45–90-m near-vertical cliffs, except
along the southern coast, where a narrow belt
of coastal terrace stretches from Pájaros
Beach west to Punta Oeste (Frank et al.,
1998). On the plateau, caves and sinkholes
dominate the topography, and soil is sparse
except in large depressions and on the coastal
terrace. These soil sites on the plateau and the
sandy coastal beaches provide most areas with
appropriate environmental conditions for
nesting iguanas. The plant community on
Mona consists of approximately 400 species
(Cintrón and Rogers, 1991). The plateau
vegetation has not been greatly impacted by
88
HERPETOLOGICAL MONOGRAPHS
humans (Wadsworth and Gilormini, 1945)
and it is dominated by Coccoloba microstachya, Bursera simaruba, Tabebuia heterophyla, and Plumeria obtusa (Cintrón and
Rogers, 1991). On the coastal plain, the
dominant species are Guaiacum sactum,
Amyris elemifera, Bourreria suculenta, and
Exostema caribeum. A large proportion of the
coastal plain vegetation on the west side of the
island has been replaced with introduced
Casuarina equisetifolia and Swietenia mahogani (Cintrón and Rogers, 1991; Diaz, 1984).
Average rainfall is approximately 800 mm/
year, peaking during October and November,
with February and March being the driest
months (Wiewandt, 1977). Mean annual
temperature is 25 uC and the humidity
fluctuates from 40 to 64% during the day to
89 to 100% at night. Based on the categories
proposed by Holdridge, the climate of Mona
is subtropical dry forest (Ewel and Whitmore,
1973).
The study was conducted at three sites:
Sardinera Beach, Pájaros Beach, and the
Lighthouse area (Fig. 1). The three areas
differ in vegetation structure and composition,
levels of human disturbance, and landscape
features. The Sardinera area is located on the
west part of the island and consists of a highly
disturbed sandy beach where the facilities of
the Department of Natural and Environmental Resources (DNRA) are located. Human
presence is constant, but fluctuates through
the year depending on tourism. This study site
includes areas where the native vegetation was
replaced or colonized by the exotic species
Casuarina equisetifolia and Triphasia trifolia.
It also has areas with nondisturbed vegetation
with a continuous canopy (6-m height) associated with a 45u slope that connects the
coastal region with the plateau environment.
Pájaros Beach is located on the southeastern part of the island and lacks permanent
human settlements. This area, however, is
occupied by tourists who feed iguanas approximately 30% of the year but their presence is
restricted to a few camping areas. Vegetation
at the camping facilities consists of exotic
grasses, but the area also has native vegetation
with a continuous canopy between 4 and 9 m
in height. In contrast to Sardinera, Pájaros
Beach is delimited by vertical cliffs that
[No. 24
provide some degree of isolation from the
plateau environment. The Lighthouse area is
located on the plateau on the east side of
Mona Island, and there is no human presence.
This is a pristine limestone area with very few
sites of soil accumulation. The vegetation is
elfin and ranges in heights from 10 cm to 3 m
because of soil scarcity and constant winds
(Cintrón and Rogers, 1991). The area has
many crevices and deep sink holes that are
used by iguanas as retreats.
The limits of each study area were defined
by the outer locations of radiomarked individuals and/or the capture locations of nonradiomarked individuals with territories that
abutted with radiomarked iguanas. Under this
criterion the size of the Sardinera study area
was 5.85 ha, Pájaros 6.1 ha, and Lighthouse
8.0 ha. In all three sites and the two seasons
we measured home-range size (minimum
convex polygon), the degree of overlap
between individuals, and a season stability
index (i.e., Cole index) as a measure of the
degree of philopatry in the species, by
comparing how much overlap exists between
the home-range areas of an individual in two
discrete time periods (Knapp and Owens,
2005).
Capture Protocol and Transmitter Attachment
We initiated iguana captures at Sardinera
and Pájaros in April 2003 and added the
Lighthouse area in 2004. Recaptures were
made in March–April, June–July, and October–November with the use of nets and cat
traps in the three study areas. At the end of
2005, a total of 90 iguanas were captured.
From each captured animal we recorded the
snout–vent length (SVL) to the nearest 1 mm,
body mass (BM) to the nearest 5 g, tail length
(LT), and tail breaks to the nearest 1 mm. Sex
was determined visually in adult animals and
via probing for hemipenes in juveniles (Dellinger and von Hegel, 1990; Schaeffer, 1934).
Iguanas were marked externally with a unique
combination of color beads attached to the
dorsal crest (Rodda et al., 1988) and permanently marked with a passive integrated
transponder (PIT tags, AVIDH).
To measure home-range sizes, a subset of
42 iguanas (Sardinera, 9 females, 6 males, 3
juvenile iguanas, ,35 cm SVL, based on
2010]
HERPETOLOGICAL MONOGRAPHS
89
FIG. 1.—Schematic maps of Mona Island and the three study areas (a) Sardinera (Department of Natural and
Environmental Resources [DRNA] facilities), (b) Pájaros, and (c) the Lighthouse, with major vegetation/terrain features
shown for each.
Pérez-Buitrago et al., 2008; Pájaros, 8 females,
6 males, and 1 young female; Lighthouse, 4
females and 5 males) were radiotracked
during 2003 and 2004 and up until November
of 2005, unless the iguana lost its radio or the
radio signal stopped. Initially, collar radiotransmitters were used on adults, but in April
2004 we also used four internal radiotrans-
90
HERPETOLOGICAL MONOGRAPHS
mitters (model R1-2D, 15 g, and S1-2, 20 g;
Holohil Systems, Ltd., Ontario, Canada,
implanted by Toledo Zoo veterinarians) on
three juvenile iguanas and one adult female
captured on the Sardinera and Pájaros beaches.
Collar radiotransmitters were outfitted initially as recommended by the manufacturer;
however, some animals lost their radios 1–
4 weeks after attachment. We thus modified
the attachment method by wrapping the radio
with a nylon bag to increase its ability to
withstand the physical rigors that iguanas can
generate when in the wild (Goodman et al.,
2009). This modification minimized radio loss
and allowed us to monitor iguanas from 4 to
22.8 months. No evidence of injury due to the
collar transmitter was found during the study.
We did not attach the radiotransmitters to the
dorsal crest as described by Goodman et al.
(2009) or other methods because Mona Island
iguana males bite females in the crest during
copulation, and because iguanas use tight
karst crevices that can cause radiotransmitters
to be stripped off.
Monitoring Protocol
We monitored radiocollared iguanas from
2003 to 2005 throughout the wet seasons
(October–November) and during the mating
(June) and nesting seasons (July). The monitoring period varied depending on the time
when iguanas were initially outfitted with
radios, or in some cases, the time when they
lost their radiotransmitters. We tracked iguanas with the use of hand-held three-element
Yagi antennas and receivers (Habit Research
Inc., Victoria, Canada, model OSPREY
HR2600). Iguanas or the sinkholes in which
they were hiding were detected visually 95%
of the time. In a few cases, iguana positions
were estimated by triangulation using from
two to four bearings from fixed localities.
Iguana localities were recorded using Magellan WAAS-enabled GPS units.
We were also able to identify individually
(based on sex, and external morphological
traits) noncaptured, or individuals captured
late in the study, that lived in the study areas.
For these animals, we recorded their locations, which allowed us to approximate the
potential interactions with radiocollared igua-
[No. 24
nas. All recorded locations were confirmed by
using recognizable terrain features in satellite
images of each study site (Ikonos images from
2002 with 2-m pixel resolution). In addition to
iguana position, we recorded their activity,
sun exposition, and habitat association (vegetation type). Iguanas were tracked once every
1 or 2 days during study periods and time of
tracking was balanced throughout each period.
Home-Range Data Analysis
We defined home range as the area
traversed during the normal daily activities
of food gathering, thermoregulation, sheltering, and mating (Burt, 1943). We define
territory as the area aggressively defended
for exclusive use by an individual (Brown and
Orians, 1970). The minimum convex polygon
(MCP; Jennrich and Turner, 1969) was used
to estimate home range because this method
works particularly well for studies of territoriality and home-range overlap. The use of
MCP allowed us to establish well-delimited
home-range boundaries and to reduce the
uncertainty of boundaries that result when
Kernel home-range estimators are used (Haenel et al., 2003; Millspaugh et al., 2004). To
determine the appropriate number of locations per animal for obtaining accurate
estimates of MCP home ranges, we generated
graphs of the percent of the MCP home range
accumulated versus the number of locations
(Fig. 2; Stone and Baird, 2002). Sixteen
locations per animal per period were determined to be reliable for estimating the MCP
with an accuracy of 80% (Fig. 2). Thus, for all
analyses we performed we used only radiomarked iguanas that fulfill that criteria per
monitoring period (see below).
We calculated the MCP during the breeding (June) and the nonbreeding season
(October–November) in 2003–2004. According to Wiewandt (1977, 1982) the mating
season of Mona Island iguanas occurs during
the second half of June, with nesting late in
July. However, our observations indicate that
females may start laying eggs during the first
week of July. Thus, for processing the female’s
information concerning home range during
what we call the ‘‘mating season,’’ we only
included the information recorded until the
2010]
HERPETOLOGICAL MONOGRAPHS
91
FIG. 2.—Relationship between number of sightings and mean (6SE) percent of total minimum convex polygon
(MCP) home range for adult Mona Island iguanas. Based on this analysis it was established that 16 locations were
sufficient to calculate the MCP home ranges with an accuracy of 80%.
end of June. This allowed us to avoid
including locations that could represent a
female’s movements associated with searching
for nesting sites. In contrast, males appeared
to reside year-round in the same area. Thus,
to calculate male breeding home ranges, we
pooled locations collected in June and July,
because no differences were detected between the home ranges for these 2 months
(paired t-test, t 5 0.32, df 5 17, P 5 0.75). In
addition, and because some iguanas were
radiomarked in 2003 and others in 2004, we
first performed an analysis to determine if
there were effects due to year in MCP home
ranges. That analysis revealed no differences
associated with these variables (F1,36 5 0.24,
P 5 0.62) and thus for posterior analysis we
made no distinction with respect to year
effects on MCP home ranges.
The degree of overlap among individuals
has been widely used to describe social
systems of lizards (Guarino, 2002; Morrison
et al., 2002; Stone and Baird, 2002). We
calculated the degree of overlap between
iguanas as the proportion of MCP ranges
shared between individuals (Knapp, 2000a;
Millspaugh et al., 2004). In addition, we
calculated the home-range season stability
index between the breeding and nonbreeding
season by using the Cole index (CI) that
estimates the overlap in home-range area used
by each individual in two discrete time periods
(Knapp and Owens, 2005). CI may vary from
0% (no coincidence in home ranges between
periods) to 100% (complete coincidence in
home ranges between periods) and can be
interpreted as a measure of philopatry between two study periods (Knapp, 2000a).
Statistical Analyses
Factors including loss of transmitters or
signals prevented us from attaining 16 relocations from every individual. We therefore
excluded from statistical analysis individuals
with less than 16 relocations. In addition, for
analysis concerning iguana morphometrics we
used the data from the first capture, including
the distinction between adult and juveniles.
We also excluded the data of juvenile iguanas
for most of the analyses, unless stated. For
spatial data analysis we used ArcView 3.2 and
Animal Movement extensions (Hooge et al.,
1999) to calculate the MCP home range,
overlaps, and to perform the nearest-neighbor
analysis with the use of the first capture
location of all iguanas (radiomarked and nonradiomarked) to determine the spatial arrangements of iguanas at each study site. We
performed a three-way analysis of variance
(ANOVA) to test for differences in MCP
92
HERPETOLOGICAL MONOGRAPHS
home ranges among sites, seasons, and sex.
For testing variation in the Cole index across
seasons, we used a two-way ANOVA with sex
and site as factors. A three-way ANOVA to
explore the interaction between site, season,
and sex on home-range overlap was not
performed, because female–female interactions were not quantified in the Lighthouse
area (see results). Thus, to test the effect of
site and sex on overlaps we used a two-way
ANOVA including only Sardinera and Pájaros
data. Post hoc comparisons were performed
with the use of the Tukey test. All statistical
analyses were performed with the use of
STATISTICA software (StatSoft, Inc., 2000).
All central tendency values are expressed as
the mean and standard deviation, unless
stated otherwise.
RESULTS
Demography in the Study Areas
A total of 90 iguanas were identified (i.e.,
captured, 87, or adults individually recognized
by external features, 3) as resident iguanas in
the three study sites. Forty-three were captured in Sardinera, 25 in Pájaros, and 22 in the
Lighthouse. Of these individuals, 14 were
immature (Sardinera, 12; Pájaros, 2; Lighthouse, 0) with SVL ranging from 16.5 to
35 cm. For all captured adult iguanas (SVL .
35 cm), males were larger (SVL) and heavier
than females (SVL, n 5 74, df 5 72, t 5 3.47,
P , 0.01; body mass, t 5 2.93, n 5 73, df 5
71, P , 0.05). Forty-six percent of the males
had broken tails, versus only 28% of the
females (chi-square 5 4.37, P , 0.04, df 5 1).
Densities in Sardinera were the highest
(7.35 iguanas/ha), followed by Pájaros (4.1
iguanas/ha) and the Lighthouse (2.75 iguanas/
ha). The adult male:female sex ratio for
Sardinera was 1:0.94, for Pájaros it was 1:1.3,
and it was 1:1.2 for the Lighthouse area, and
did not deviate statistically from a 1:1 sex ratio
for any site (all chi-square , 0.18, all P .
0.67, df 5 1). Nearest-neighbor analysis
indicated that adult males in Pájaros and the
Lighthouse followed a uniform dispersion
pattern (z 5 5.52, R 5 1.96; z 5 2.51 R 5
1.41, respectively), whereas at Sardinera adult
males were randomly dispersed (z 5 0.31, R
5 0.96). Adult females in Sardinera and
[No. 24
Pájaros were clumped (z 5 4.1, R 5 0.62;
and z 5 2.1, R 5 0.68, respectively) and
randomly distributed (z 5 0.1, R 5 1.02) at
the Lighthouse.
MCP Home Ranges
There was a significant effect of sex and site
on home-range size, whereas season had no
effect (Table 1a,b). Males had larger (0.41 6
0.22 ha) home ranges than females (0.26 6
0.18 ha), and iguanas at the Lighthouse
exhibited the largest home ranges (both sexes
pooled 0.46 6 0.18 ha; Fig. 3), followed by
Pájaros (0.28 6 0.17 ha; Fig. 4) and Sardinera
(0.29 6 0.21 ha; Fig. 5). In addition, there was
a significant interaction between sex and site
(Table 1b) in which post hoc comparisons
revealed that males from the Lighthouse and
Pájaros had larger home ranges than those
from Sardinera, or from females of the three
sites (Table 1b).
Paired home-range comparisons across all
sites during both study periods showed that
females reduced their home-range area during the mating season (0.21 6 0.15 ha) with
respect to the nonbreeding season (0.31 6
0.19 ha; paired t-test 5 2.24, df 5 19, P 5
0.04). In contrast, although not significant,
males tended to increase their home-range
area during the mating (0.43 6 0.23 ha)
season compared to the nonbreeding season
(0.35 6 0.21 ha; paired t-test 5 21.66, df 5
14, P 5 0.12).
We found a negative relationship for all
individuals across localities (including juvenile
iguanas) between home-range size and initial
SVL (r 5 20.38, n 5 46, P 5 0.00) and
between home-range size and body mass (r 5
20.34, n 5 46, df 5 44, P 5 0.01). However,
these relationships were not significant when
only adults of each sex were analyzed separately.
Season Stability Index
There were independent effects of sex
(F2,26 5 8.17, P 5 0.00) and site (F1,26 5
5.46, P 5 0.03) on the Cole index (CI, the
degree of interseasonal changes in area used)
during both the reproductive and the nonreproductive seasons. However, the sex–site
interaction was not significant (F2,26 5 0.08,
P 5 0.92). All individuals at the Lighthouse
Site
Season
Sex
Site 3 season
Site 3 sex
Season 3 sex
Site 3 season 3 sex
(b)
All
Females
Males
(a)
2
1
1
2
2
1
2
df effect
0.39 6 0.19
(n 5 6)
0.22 6 0.12
(n 5 8)
0.29 6 0.16
(n 514)
Sardinera
Breeding
57
57
57
57
57
57
57
df error
0.38 6 0.21
(n 5 5)
0.14 6 0.11
(n 5 8)
0.23 6 0.19
(n 5 13)
Pájaros
0.58
(n
0.34
(n
0.47
(n
6
5
6
5
6
5
6.058
0.085
13.035
0.94
4.28
2.934
0.276
F
0.23
5)
0.24
4)
0.25
9)
Lighthouse
0.0041*
0.7721
0.0006*
0.3966
0.0185*
0.0922
0.7598
P value
0.42 6 0.22
(n 5 16)
0.21 6 0.16
(n 5 20)
0.31 6 0.22
(n 5 36)
All
0.32 6 0.26
(n 5 5)
0.24 6 0.15
(n 5 10)
0.26 6 0.17
(n 5 15)
Sardinera
0.37 6 0.21
(n 5 15)
0.25 6 0.13
(n 5 22)
0.29 6 0.17
(n 5 37)
All
Lighthouse .. Sardinera 5 Pájaros
–
Males .. Females
–
Male Lighthouse .. Male and female other sites
–
–
Post hoc comparisons
0.48 6 0.2
(n 5 5)
0.28 6 0.1
(n 5 4)
0.39 6 0.18
(n 5 9)
Lighthouse
Nonbreeding
0.31 6 0.16
(n 5 5)
0.24 6 0.13
(n 5 8)
0.27 6 0.14
(n 5 13)
Pájaros
TABLE 1.—Minimum convex polygon (MCP) home ranges in hectares (6SD) for males and females at the three study sites during the breeding and nonbreeding seasons (a).
Numbers in parentheses represent the number of individuals radiotracked. (b) Three-way ANOVA analyzing the effect of site season and sex on home-range size. Interactions
between factors are shown.
2010]
HERPETOLOGICAL MONOGRAPHS
93
94
HERPETOLOGICAL MONOGRAPHS
[No. 24
FIG. 3.—Radiomonitored iguanas in the non–human-disturbed Lighthouse area during the nonbreeding (a) and
breeding seasons (b). Thick lines represent males and thin lines represent females.
had a significantly larger CI score (69 6
11.7%) than those in Pájaros (49.5 6 14.5%)
or Sardinera (40.7 6 14.7). This indicates that
changes in the space used by iguanas between
the breeding and nonbreeding seasons was
lowest at the Lighthouse (Fig. 3), followed by
Pájaros (Fig. 4) and Sardinera (Fig. 5). Also,
females tended to have a lower CI score (44.4
6 16.1%) than males (60.3 6 15.4), indicating
than they changed a larger proportion of their
home range between the breeding and
nonbreeding seasons. The lower CI of females
influenced the number of males with which
they interacted during the breeding (1.38)
versus the nonbreeding seasons (0.9).
Home-Range Overlaps
The territories of male iguanas overlapped
the least (6.2 6 8.5%), followed by female–
female overlap (21.4 6 20.8%), whereas
male–female overlap was greatest (44.1 6
29.7%; two-way ANOVA, F2,83 5 20.81, P ,
0.01). There was no effect of season on the
amount of overlap (mating, 25.2 6 24.7%;
nonbreeding, 25.9 6 29.6%; F1,83 5 0.06, P 5
0.8), nor was the interaction between sex and
season significant (F2,83 5 0.20, P 5 0.82). We
had no data for female–female overlap in the
Lighthouse, but data from Sardinera and
Pájaros suggest that there was no effect of
site on home-range overlaps (Pájaros, 26.2 6
29.9%; Sardinera, 22.0 6 18.1%; F2,70 5 0.12,
P 5 0.72).
A total of 48 interactions of all sex-pair
combinations were detected during the nonbreeding season and 57 during the breeding
season (chi-square 5 0.77, df 5 1, P , 0.38).
The increase in the number of interactions
was caused by small overlaps among males
that did not interact (i.e., no overlap) during
the nonbreeding season (Figs. 3–5), and by
the increase in number of males and females
with which some females interacted. MCP
maps show that there was an expansion in
2010]
HERPETOLOGICAL MONOGRAPHS
95
FIG. 4.—Radiomonitored iguanas in the intermediate human-disturbed Pájaros area during the nonbreeding (a) and
breeding seasons (b). Thick lines represent males and thin lines represent females.
male home-range size during the breeding
season that resulted in increases, but still very
little, male–male overlaps (Figs. 3–5). Nevertheless it seems that neighboring males
impose a limit to the amount of area by which
a male can increase its home range (Figs. 3–
5). In addition, there was an increase in the
number of male–female interactions during
the breeding season because females interacted with more males during that period
(Fig. 3–5; see season stability index discussion
section).
In the Lighthouse area, four out of five
radiocollared males had home ranges abutting
those of other males, and the degree of
overlap was minimal during both the breeding
and nonbreeding seasons (Table 2 and
Fig. 3). The increase in overlap during the
breeding season, although not significant
(Wilcoxon test, z 5 1.4, n 5 4, P 5 0.14),
was caused by the overlap of two males, one of
which ‘‘shared’’ 42.7% of his home range. For
females living in the Lighthouse area, we did
not detect overlap in any of the study periods,
because radiocollared females were not close
enough to each other to quantify this parameter. However, we believe that if overlaps
among females occur, they must be very small,
because during the field observations we
never detected other females in the home
ranges of the radiocollared individuals.
In Pájaros, we also found low levels of
home-range overlap between males in both
seasons (Fig. 4), and as in the Lighthouse, an
increase during the breeding season was
observed. In this site, the potential number
of male–male interactions among the five
radiocollared individuals was restricted to
two males that shared 15.8% of their home
ranges during the breeding season (Fig. 4b),
whereas during the nonbreeding season there
was no overlap at all (Fig. 4a). In general, in
both study periods, our observations of nonradiocollared (but captured) individuals living
in this study area supported our findings,
because these males were never seen in the
home ranges of radiocollared males. Females
from Pájaros showed a high degree of
variation in home-range overlap, reaching
the highest values in areas located near the
camping facilities, where sporadic feeding by
tourists occurs.
Sardinera, the most disturbed area, exhibited the highest levels of home-range overlap
96
HERPETOLOGICAL MONOGRAPHS
[No. 24
FIG. 5.—Radiomonitored iguanas in the high human-disturbed Sardinera area during the nonbreeding (a) and
breeding seasons (b).
(for both seasons) among males (Fig. 5,
Table 2). Here, we also observed an increase
in male–male overlap during the breeding
season with respect to the nonbreeding season
(Fig. 5b). In addition, as detected at Pájaros,
female–female overlap was the highest where
daily feeding by humans occurs. However, at
Sardinera, male–male overlap was also high
close to these artificial feeding sites, something that was not observed at Pájaros.
Juveniles and Unusual Adult Iguanas
Home ranges of juveniles (three females
and one male) were almost three times larger
than that of adults (mean 5 2.8 6 1.21 ha). In
addition, juvenile home ranges overlapped
23.7 6 23.4 (17)
39.6 6 30.2 (21)
2.8 6 7.3 (10)
None detected
57.6 6 29 (3)
1.4 6 1.5 (3)
20.3 6 18 (14)
25 6 28.2 (7) 20.4 6 20.8 (10)
35.4 6 25.0 (28) 36.5 6 32.4 (10) 31.7 6 25.8 (8)
8.4 6 8.8 (15)
0 6 0 (5)
11.8 6 16.3 (2)
None detected
36.3 6 28.2 (4)
6.8 6 9.8 (4)
15.9 6 10.1 (7)
30.4 6 21 (15)
13.9 6 8.7 (6)
24.7 6 23.5 (7)
41.6 6 29 (9)
3.1 6 4.3 (5)
Female/female
Female/male
Male/male
Lighthouse
Pájaros
Sardinera
Breeding
Lighthouse
All
Pájaros
Sardinera
Nonbreeding
All
HERPETOLOGICAL MONOGRAPHS
TABLE 2.—Percent (mean 6 standard deviation) of home-range overlap within and between the sexes during the breeding and nonbreeding season for the Mona iguana.
Numbers in parentheses correspond to the number of interactions detected (not considering adjacent males with no overlap). Only overlap among sex pair types was significant
(F2,83 5 20.81, P 5 0.0001) with male–male overlaps being the lowest, whereas male–female overlaps were the highest. All other factors (season, sites) and all interactions were
not significant (all F , 0.2, all P . 0.05).
2010]
97
completely with those of several adult individuals, and juveniles moved extensively
across the boundaries of several adult males
and females during both seasons (Fig. 6). It is
unclear if juveniles are tolerated by adults or
are excluded from adult home ranges. The
Cole index for juveniles was similar to the
ones exhibited by males, indicating that they
have a similar degree of home-range philopatry as adult males.
We detected two adult males that behaved
differently from the other adults and juveniles
in that they completely switched their home
range during the study period. An adult male
(ID-M42, SVL 5 52.5 cm, body mass 5 6.6 kg;
Fig. 7a) was initially captured at Pájaros and
stayed for a few days in one area but then
disappeared. Six weeks later, he was found
500 m to the northeast, where he stayed for
2 months. Later on, he returned to the first
study area and was observed having short
fights with the resident radiocollared males.
In these fights he was always defeated. At the
end of the study period, this male settled in an
empty section of the study area, which was not
part of the home range of any resident males
(Fig. 7a). Because of the unusual behavior of
this male, we excluded his data from all
analysis concerning home-range size and
overlaps.
Another male (M3) from Sardinera was
monitored from when he was relatively young
(SVL 5 44 cm, and BM 5 4 kg) in 2003 until
he reached a SVL of 55 cm (BM 5 8 kg) in
2005 (3 years and more that 120 fixes). We
calculated his home range twice, using his first
20 locations (MCP 5 1.2 ha) and his last 20
locations (MCP 5 1.1 ha). Despite the
similarity in home-range size, there was a
shift in the area (Fig. 7b) used at the
beginning with respect to the one used at
the end of the monitoring. Initially his home
range overlapped with that of several larger
adult males, whereas at the end of the
monitoring period he had settled into a vacant
area that did not belong to any resident male
and but was highly used by humans (Fig. 7b).
MCP maps for this male show the exclusivity
of the area where he settled at the end of the
monitoring period, and the low MCP overlap
with neighboring males that abutted his
territory (Fig. 7b).
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HERPETOLOGICAL MONOGRAPHS
[No. 24
FIG. 6.—Minimum convex polygon (MCP) home ranges of juvenile iguanas: (a) represents a young female (snout-tovent length [SVL] 5 33 cm; thick polygon) in Pájaros. The MCPs of the larger resident females (SVL . 45 cm) during
the nonbreeding season are represented by thinner lines. Map (b) shows three young iguanas (thick polygons; sex
specified on map) in the Sardinera area. Thinner polygons in (b) represent radiotracked resident males during the no
breeding season.
2010]
HERPETOLOGICAL MONOGRAPHS
99
FIG. 7.—Changes in home ranges of two males: (a) represents one male (snout-to-vent length [SVL] 5 52.8 at the
time of the first capture) in Pájaros beach. (b) Represents a male in the Sardinera area during the time it grew from
44 cm SVL to 55 cm SVL. Thin-line polygons represent males during the nonbreeding season and are shown
for reference.
100
HERPETOLOGICAL MONOGRAPHS
DISCUSSION
Demography in the Study
We consider the 90 monitored iguanas
(radiocollared and non-radiocollared) captured in the study areas as resident individuals, as they were observed repeatedly in these
areas. Therefore, the abundance and densities
reported here are reliable estimates of the
densities for each site. For the three sites,
juvenile iguanas (SVL , 35 cm) comprised
16% of the population. This value is much
higher than the 0.1% previously documented
(Pérez-Buitrago and Sabat, 2000), but still low
when compared with other Cyclura species
(Wiewandt and Garcı́a, 2000). Most of the
juveniles were detected in the Sardinera area
(85%), followed by Pájaros (15%), and none at
the Lighthouse study area. These large
differences in relative abundance of juvenile
iguanas across sites (Sardinera, 30%; Pájaros,
1%; and the Lighthouse, 0%) could be caused
by two nonexclusive factors: First, at the
highly disturbed Sardinera area, juvenile
iguanas may be more easily detected because
of the open areas associated with the camping
facilities and because we spent much more
time there than in the other two study sites.
Second, at Sardinera feral cats were trapped
periodically by the Department of Natural
and Environmental Resources of Puerto Rico
(DRNA) staff, perhaps lowering juvenile
mortality at this site. Thus, we believe that
our data of Pájaros and the Lighthouse do not
necessarily contradict previous results suggesting that the proportion of juvenile iguanas
in natural settings is remarkably low for the
Mona Island iguana population (Pérez-Buitrago and Sabat, 2000; Wiewandt, 1977).
Sardinera showed the highest density with
7.35 iguanas/ha, followed by Pájaros (4.12
iguanas/ha) and the Lighthouse (2.2 iguanas/
ha). Differences in iguana abundance and
densities were previously documented for
Mona and associated with availability of
resources such as shelter sites (Pérez-Buitrago
and Sabat, 2000; Wiewandt, 1977). The
density at Sardinera coupled with the abundance of juvenile iguanas appears to be
comparable to values that are considered
healthy for populations of Cyclura, such as
Cyclura nubila in Cuba (Alberts et al., 2002).
The lower densities recorded in Pájaros and
[No. 24
the Lighthouse cannot be attributed to shelter
limitation as previously suggested by PérezBuitrago and Sabat (2000), because shelters
appear readily available at these sites. Our
results show that iguana densities at Mona
vary positively with respect to the degree of
human presence among the three sites, a
phenomenon that has also been evidenced in
other impacted Cyclura populations and has
been attributed mainly to (1) supplemental
feeding provided to iguanas by humans
(Iverson et al., 2004; Iverson et al., 2006;
Lacy and Martins, 2003), (2) the effort to
remove feral cats when detected in Sardinera,
(3) differences in habitat productivity among
sites, and (4) the variation in territorial
behavior expressed by the species (see below).
Home Ranges
Studies describing space usage for other
Cyclura iguanas have reported very large
variation in MCP values. Some authors
attribute the large variation in Cyclura home
ranges to methodological limitations in the
older studies, when radiotelemetry techniques
were not completely reliable (Goodman, 2004;
Knapp and Owens, 2005). However, even if
one only considers recent studies that use
radiotelemetry as the primary method to
calculate home-range size, remarkable variation across species and sexes still remains.
Home-range size for males ranges from
0.04 ha in Cyclura rileyi rileyi (Hayes et al.,
2004) to 14.3 ha in Cyclura lewisi (Goodman
et al., 2005), and for females from 0.06 ha in C.
r. rileyi (Hayes et al., 2004) to 2.47 ha in
Cyclura cychlura cychlura (Knapp and Owens,
2005). MCP values for Mona Island iguanas
were between these estimates, with males at
0.41 6 0.22 ha and females at 0.26 6 0.18 ha.
Variation in lizard home-range size has
been related to lizard body size, climate
change, breeding behavior, habitat productivity, social behavior, human disturbance, density, and island size (Christian and Waldschmidt, 1984; Stone and Baird, 2002). Male
iguanas typically expand home ranges during
breeding seasons, presumably to increase
encounter rates with females (Alberts et al.,
2002; Goodman et al., 2005; Knapp and
Owens, 2005; Rose, 1982; Stamps, 1983).
Although not significant statistically, our data
2010]
HERPETOLOGICAL MONOGRAPHS
suggest that males tend to increase their home
ranges during the breeding season (Figs. 3–5;
Table 1). However, MCP expansion in male
Mona Island iguanas is constrained by the
territories of neighboring males (Figs. 3–5).
This constraint implies that male interactions
with females in both seasons are limited to the
females that live in proximity to or within the
male’s home range. It is noteworthy that all
males in this study had access to at least one
female year-round.
For many lizards, a positive relationship
between lizard size (SVL and body mass) and
home-range size has been reported (Christian
and Waldshmidt, 1984; Tinkle, 1967; but see
Civantos, 2000). This relationship has been
explained in terms of energetics, because
larger individuals require larger foraging areas
(Alberts et al., 2002; Christian and Waldschmidt, 1984; Knapp and Owens, 2005; Rose,
1982; Stamps, 1983; but see Perry and
Garland, 2002). Although Mona Island iguana
males were larger in size than females and had
larger home ranges, a positive relationship
between body size and MCP was not observed
when only adults of both sexes were pooled.
This result could be due to (1) the large
variation in MCP exhibited for each sex; (2)
the negligible differences in SVL among
individuals that prevented the detection of
significant correlations for each sex or both
sexes grouped (Christian and Waldschmidt,
1984); (3) the great seasonal variation in body
mass (adults captured repeatedly sometimes
varied more than 1.4 kg between captures); or
(4) the possibility that social factors are more
important determinants of home-range size
than energetic ones. However, when juvenile
iguanas were included in our analysis, there
was a significant negative relationship between body size and home range (Fig. 8).
Though unusual for lizards generally, Knapp
(2000a) also reported this relationship for
Cyclura cychlura inornata and suggested that
larger individuals have smaller but more
exclusive home ranges. This appears to be
true for the Mona Island iguana, as indicated
by the negative relationship between the
degree of overlap and the SVL for males and
females (Fig. 9; see overlap discussion below).
Our results indicate a strong effect of
human disturbance on home-range size.
101
MCPs were larger at the Lighthouse compared with Sardinera and Pájaros. The smaller
home ranges in Pájaros and Sardinera may be
due to the supplemental food supplied by
visitors that may reduce the foraging areas
necessary to meet the iguana’s energetic
needs in these sites. Supplemental feeding
by humans may promote high iguana densities
and disruption of their social systems. It can
result in tighter assemblages of individuals
(Knapp, 2000a), higher levels of aggression
and social interactions in general (Lacy and
Martins, 2003), and differences in survival
probabilities (Iverson et al., 2006). The former
two are consistent with our results. In the
Sardinera area, where there is high human
disturbance with constant supplemental feeding, the iguana density is the highest for any
locality recorded on Mona, implying a more
tightly packed assemblage of individuals and
more inter- and intrasex interactions (see
overlap discussion below).
Population density and island size have
been suggested as important variables influencing home-range size in Cyclura (Knapp
and Owens, 2005). Caribbean islands occupied by Cyclura have areas that range from
less than a hectare to tens of thousands of
square kilometers (Alberts, 2000; Knapp and
Owens, 2005), and iguanas can be found at
densities fluctuating from 0.3 to 128 iguanas
per ha in both natural and disturbed settings
(Goodman et al., 2005). On islands with low
population densities, large home ranges can
be found and territorial behavior may be
expressed (Knapp, 2000a). Alternatively, on
islands with large population densities, individuals may be forced to occupy small home
ranges, and hierarchical or nonterritorial
social systems tend to prevail. Because populations fluctuate over time, there may be
continuing alterations in the social system
when density reaches certain thresholds.
Alberts et al. (2002) reported small home
ranges for C. nubila in Guantánamo Bay
where the density was 7.8 iguanas/ha. Goodman et al. (2005) and Knapp and Owens
(2005) reported the largest home-range sizes
for any Cyclura in populations with densities
as low as 0.67 iguanas/ha for C. lewisi
(Goodman et al., 2005) and 0.5 iguanas/ha
for Cyclura cychlura (Knapp and Owens,
102
HERPETOLOGICAL MONOGRAPHS
[No. 24
FIG. 8.—Relationship between minimum convex polygon home range and iguana snout-to-vent length [SVL].
Relationships for females, juveniles or males are not significant (r2 5 0.24, P 5 0.49; r2 5 0.37, P 5 0.20; r2 5 0.01, P 5
0.67, respectively); however, a negative relationship exists (r2 5 0.26, P 5 0.0002) when all individuals are included.
2005). Goodman et al. (2005) suggested that
the large home ranges (14.3 ha) used by C.
lewisi could be the result of low population
density. Thus, it appears that high densities in
Cyclura may induce home-range compression. Our estimates of MCP are consistent
with this conjecture. The Lighthouse presented the lowest density (2.2 iguanas/ha) and the
largest home ranges for both sexes when
compared with the other two sites with higher
densities (Pájaros, 4.12; Sardinera, 7.35 iguanas/ha). Indeed, male home-range size is
negatively correlated with local density across
species of Cyclura (Pérez-Buitrago et al.,
2007). This suggests that density is an
important proximate correlate of home-range
size. However, because iguana populations
fluctuate over time because of natural causes
FIG. 9.—Relationship between snout-to-vent length (SVL) and the percent of minimum convex polygon (MCP) homerange area shared by male and female conspecifics during the breeding season. For males: (% overlap) 5 26.0504 (SVL
+ 342.25, r2 5 0.32, df 5 12, P 5 0.03); for females (% overlap) 5 25.6966 (SVL + 340.11, r2 5 0.58, df 5 9, P , 0.01).
2010]
HERPETOLOGICAL MONOGRAPHS
(e.g., hurricanes) and also from humaninduced causes (poaching for food or the pet
trade, introduction of exotic predators or
competitor species, and habitat degradation),
the social system is expected to be dynamic.
Although the significant density/home-range
size relationship we report here does not
establish causality, we hypothesize that density is an important factor allowing the existence
of the current territorial social system in Mona
Island iguanas. This conjecture is based on the
supposition that colonization of a new area
typically is accomplished by a few individuals,
and thus initial density will be very low. In this
scenario, a nonterritorial system would prevail
because of the abundance of food and shelter,
and large home ranges to search for mates are
expected. Once the population approaches
carrying capacity and competition for resources intensifies, territoriality should dominate. If
densities become extreme, individuals will not
be able to control an area effectively because
of extreme competition for resources (including mates). In this scenario, the optimal
strategy for individuals would be to adopt a
hierarchical social system, in which resource
allocation is linked to social rank. Our data
suggest that the Mona Island iguana population appears to maintain sufficiently low
densities to express territorial behavior. However, the reasons for this are not completely
clear. Indeed, human-induced factors such as
the introduction of feral cats and pigs or
environmental constraints as limited nesting
sites (i.e., less than 1% of Mona Island is
suitable for nesting) may be contributing to
keeping the population at the low density
values that allow the expression of territorial
behavior.
Cole Index (Interseason Stability Index)
We found significant differences between
sexes in the interseason stability index (CI),
and also among sites, with iguanas from the
Lighthouse exhibiting higher values of CI as
compared to iguanas from Sardinera and
Pájaros. The higher CI detected at the
Lighthouse implies that animals in this area
are more temporally consistent in their
pattern of space usage when compared to
those in Pájaros and Sardinera. Higher levels
of human presence (i.e., supplemental feed-
103
ing) at Sardinera and Pájaros may be contributing to this pattern because iguanas living
relatively far from feeding areas need to make
sporadic movements to visit these rich food
zones, creating a behavioral pattern that
would make them appear less philopatric than
at the Lighthouse. As the Lighthouse lacks
artificially food rich sites, the spatial data for
the iguanas living there is more likely a
reflection of space use in natural settings.
The significantly lower interseason stability
index in females indicates that females are less
philopatric than males, showing greater
changes in space use between the breeding
and nonbreeding seasons. The reduced philopatry of females appears to be the result of
increased visitation to territories of neighboring males during the mating season (Figs. 3–
5). This finding contrasts with studies of
Cyclura cychlura cychlura on Andros Island
(Knapp and Owens, 2005), where males are
less philopatric than females, and it is males
that seek females during the reproductive
period. Knapp and Owens (2005) suggested
that lower levels of philopatry by their males
could occur because males migrate during the
reproductive season to areas that have the
appropriate conditions for nesting, and thereby increase their probability of finding mates.
This is plausible in areas where iguana
populations have low densities and/or a
skewed male sex ratio. Low availability of
females may induce males to seek mates, and
hence to change their use of space (Goodman
et al., 2005; Knapp and Owens, 2005). Our
data differ from those of Knapp and Owens
(2005) in the following ways: First, despite the
existence of communal nesting areas at
Pájaros and Sardinera, male iguanas do not
visit these areas during the nesting season and
they show a higher degree of philopatric
behavior than females throughout the year.
Second, the high levels of territoriality in
Mona males (minimum MCP male–male
overlaps, see below) may be a major factor
precluding males from changing their space
use patterns throughout the year. Finally,
females do not appear to be a limiting
resource because sex ratios are close to 1:1,
and our data suggest that male Mona Island
iguanas have access to at least one female
year-round (Figs. 3–5). The higher level of
104
HERPETOLOGICAL MONOGRAPHS
male philopatry also implies that the number
of copulations that a male can attain may be
limited to the females that overlap his
territory, which is dependent on how females
use their home-range space during the mating
period.
In a broader context, the low Cole indices
for both sexes of the Mona Island iguana
indicate that this species has very stable home
ranges compared with other Cyclura (Goodman et al., 2005; Knapp and Owens, 2005).
This high seasonal stability in the use of space
by the Mona Island iguana has implications
for its social system. Most iguanids are
polygamous and male behavior during the
mating season appears to be influenced by the
need to search actively for females (Goodman
et al., 2005; Knapp and Owens, 2005).
However, our data indicate that on Mona, it
is females that modify their space use and
‘‘decide’’ with which males they will interact
and potentially mate. This is consistent with
Wiewandt’s (1977) observations of large variation in the number of copulations that Mona
females could attain. Indeed, although some
females were seen copulating many times with
different males (i.e., by moving among several
male territories), others were apparently
‘‘loyal’’ to a single male. For this reason,
Wiewandt (1977) stated that the Mona Island
iguana social system could be difficult to
categorize within established categories. Female-driven sexual selection has also been
described for marine iguanas (Amblyrhynchus
cristatus) in which females visit and mate with
many males (Wikelski et al., 2001).
In summary, a major finding of this study,
which is based on the movement patterns of
both sexes, is that polygamy in the Mona
Island iguana, if it exists, is driven by females
seeking multiple males. By being promiscuous, females could increase the genetic
variability of their offspring and/or the fertilization rate of their ovules (Wiewandt, 1977).
Future paternity studies will be necessary to
test this hypothesis. Although female mate
choice criteria are not clear in Mona Island
iguanas, male size and robustness appear to be
important for female choice in other iguana
species (Alberts et al., 2002; Wikelski et al.,
2001). In order to maximize the potential
benefits of promiscuity, it could be ‘‘optimal’’
[No. 24
for females to be the ones moving across male
territories, which are neighbor constrained
during the mating season (this study; Wiewandt, 1977). Paternity studies could also
elucidate the basis of mate choice.
Overlap and Territorial Behavior
For lizards, three types of social systems
have been described: nonterritorial, hierarchical, and territorial (Stone and Baird, 2002).
Non-territorial systems are characterized by
space sharing and no defense by either of the
sexes. Hierarchical systems are common in
populations with high densities, and are
characterized by large home-range overlap in
which access to resources is determined by
social status. In territorial systems, individuals
defend their home range or a part of it to
exclude consexual conspecifics (Stone and
Baird, 2002). Territoriality is characterized
by little to no home-range overlap, regular
spacing patterns (Gordon, 1997), and site
fidelity (Wyman and Hotaling, 1988). This
has been considered the most common social
system in iguanian species.
Wiewandt (1977) noted that the Mona
Island iguana appears to be extremely territorial based on its morphological traits (i.e.,
head ornamentation, well-developed jaw musculature, large dewlap), lack of submissive
displays, and frequent fighting encounters
throughout the year. Wiewandt (1977) also
described three types of territories for the
Mona Island iguana: mating territories (defended by males), retreat territories (defended
by females and hatchlings), and nesting
territories (defended by females during the
nesting season).
Consistent with what could be expected for
a territorial species, we found that the Mona
Island iguana showed high levels of site
fidelity, and a low degree of home-range
overlap among males, intermediate overlap
among females, and maximum overlap between the sexes (Table 2). Also consistent
with territorial species, males in sites without
or with only moderate levels of human
disturbance (i.e., the Lighthouse and Pájaros)
exhibit a regular spacing pattern (Gordon,
1997). Regular spacing can arise from competition with neighboring individuals defending all or most of their home range through
2010]
HERPETOLOGICAL MONOGRAPHS
direct aggression or advertisement behavior
(Stone and Baird, 2002). In Cyclura, a
quantifiable measure of aggression is tail
break frequency, which is high on Mona
(46% and 27.7% for males and females,
respectively) and similar to other territorial
Cyclura species (Knapp, 2000a).
The basis for territorial behavior is the
defense of an area for exclusive access to highquality resources such as food, shelter, and
mates (Martins, 1994). Many studies have
focused on determining the causes that may
induce a species to become territorial, because space by itself may not be the contested
resource, or the resource in conflict may be
different for each sex (Haenel et al., 2003).
However, when space is the contested resource, very well delimited territories with
minimum overlaps and a regular spacing
pattern are to be expected (Haenel et al.,
2003). For Mona Island iguana males, the lack
of male–male overlap suggests that space itself
may be the contested resource. From the
male’s perspective, female distribution may be
another factor promoting territorial behavior
and determining the quality of a particular
area (Haenel et al., 2003). By controlling an
area that, from the female’s perspective,
provides high-quality priority resources (i.e.,
abundance of food and/or nesting areas),
males can interact with more females than
other males living in areas where such
resources are less abundant in quality or
quantity (Werner, 1982). This explanation is
consistent with our data from Sardinera and
Pájaros, where nesting sites and artificial
feeding sites exist, and females aggregate
close to these enriched areas, making these
sites particularly attractive for males (Figs. 4a,
5a, 6a). Moreover, males controlling areas
where females were clumped were among the
largest and apparently most vigorous residents
in Sardinera and Pájaros. In the nondisturbed
area (Lighthouse), which lacks supplemental
feeding and nesting areas, females appear
randomly distributed and males apparently
have access to at least one female; thus space
(including the females using it) may be the
contested resource in this natural environment.
We believe that for the Mona Island iguana,
shelter is not a factor inducing territoriality,
105
because retreats are readily available, at least
in abundance though not necessarily in
quality. It has been demonstrated experimentally in other taxa that increases in food
abundance may result in shifts from territorial
to hierarchical behavior because high abundance of competitors can render the defense
of an area energetically impractical (Maher
and Lotta, 2000). Many Cyclura populations
are exposed to supplemental feeding by
tourists and there is evidence suggesting that
alterations in the social system may occur
under this scenario (Lacy and Martins, 2003).
Our study sites have different degrees of food
availability due to human visitation. In Sardinera, with high food supplementation, the
degree of male–male overlap is maximal, and
individuals live in a tight assemblage, (particularly near the feeding sites). In addition, the
number of potential interactions (i.e., larger
home-range overlap) increases with the level
of food supplementation among sites. To a
lesser extent, these patterns are found in
Pájaros, but not in the Lighthouse area, where
no supplemental feeding occurs. As proposed
by Wiewandt (1977) for undisturbed sites, like
the Lighthouse, food may not be a contested
resource given its patchy distribution in time
and space. Thus, the observed variation in
home-range size and overlap among our study
sites is consistent with what is known about
the impact of human presence on Cyclura
territorial behavior and demography. However, Knapp (2000a) compared the behavior of
two populations of C. cychlura cychlura with
and without supplemental feeding by tourists
and found that supplemental feeding was not
the only variable explaining the lack of
territoriality. He suggested that island size
(small islands induce nonterritorial systems
and larger ones induce territoriality) and
population density (high density induces
nonterritorial systems and low densities induce territoriality) could be important factors
affecting territoriality.
Population density is one of the best
predictors of territorial behavior across many
taxa (Maher and Lotta, 2000). It has been
predicted that at low densities, territorial
behavior is more likely to occur. This also
implies that at low densities, home range and
territory of an individual may be equivalent in
106
HERPETOLOGICAL MONOGRAPHS
size, but that at higher densities, territory size
should be smaller than home-range size
because of high costs of defense against
intruders. For Cyclura species on small and
nondisturbed islands, densities can be high,
and the predominant social system is nonterritorial or hierarchical (Knapp, 2000a,b).
Densities at our study sites were relatively low
when compared with other Cyclura populations (Goodman et al., 2005); thus density of
the Mona Island iguana at our study areas
seems to be at a level that allows the species to
express territoriality. The degree of intersex
overlap among males increases consistently
with iguana density in the three study sites
(Lighthouse 5 2.2 iguanas/ha; Pájaros 5 4.1
iguanas/ha; and Sardinera 5 7.3 iguanas/ha).
Indeed, if the degree of overlap is used as a
criterion to describe the degree of territoriality, iguanas living in the Lighthouse have the
lowest values of male–male overlap and
therefore are more territorial than those living
in the Pájaros and Sardinera areas (Table 2).
The overall low male–male overlap on
Mona suggests that home range and territory
are equivalent in size. For females, however,
their higher levels of overlap imply that
territories are smaller than home ranges and
that active defense (i.e., via agonistic behaviors) is restricted to smaller portions inside of
home ranges (e.g., shelter sites, personal
observation) as suggested by Wiewandt
(1977). The large degree of male–female
overlap indicates that territorial defense between sexes is insignificant, a fact that is
supported by observations of radiocollared
male–female pairs recurrently sharing the
same shelter in both study periods.
Another characteristic associated with territorial systems is the sharpness of MCP
boundaries (Pyke et al., 1996). Even though
we did not quantify this variable, our observations on aggressive displays indicate that
territory boundaries for Mona Island iguana
males may be sharp. Aggressive displays were
frequently observed at the edges of male
home ranges, and the few observed inside of
the home range resulted in the occupation of
the territory by the intruding male a few days
later. Cyclura iguanas are long-lived lizards,
and it is possible that long-term interactions
(including severe fights) result in the defini-
[No. 24
tion of well-delimited territory edges. Subsequent encounters are likely to be less intense
because of experience and the well-defined
boundaries of the territories of neighboring
males. Our observations of iguana fights
indicated that they can be mild (a few minutes
with no obvious injures for any of the
contenders) when individuals are established
neighbors, or in some cases very bloody and
long lasting (up to more than an hour,
Wiewandt, 1977; personal observation) when
a new male is trying to take over the territory
of another male. These observations are
consistent with McMann and Paterson
(2003), who suggested that aggressiveness
can be associated with specific locations and,
when establishing a territory, the dear-enemy
phenomenon and habituation may play a role
in subsequent encounters by reducing the
willingness to fight (Yang et al., 2001).
A feature of populations with territorial
systems is the occurrence of floaters, defined
as individuals that lack (or at least do not
defend) their own territories (Kokko and
Sutherland, 1998; Stapley and Keogh, 2005).
This was the case for only one of the
radiocollared adult males in our study (SVL
5 52.5 cm, body mass 5 6.6 kg). It has been
proposed that beyond a certain density, all
high-quality habitats become filled with dominant individuals, and the remaining lowranked individuals (floaters) have to occupy
low-quality areas or move across the territories of dominant individuals (Sutherland and
Norris, 2002). Evidence of this dynamic
process was obtained with C. nubila by
performing a temporary, dominant-male removal experiment in which the newly vacant
territories were occupied by four lowerranked males (based on hormone levels and
robustness) that had previously lived as
floaters in the area (Alberts et al., 2002).
Moreover, the shape and size of the territories
of the new owners were similar to those of the
original occupants. Floaters can be important
for long-term conservation of populations,
because their existence may play a role in
population dynamics and reproductive output
of a population (Hunt and Law, 2000).
Nevertheless, their importance may be limited
by their demographic and genetic contributions. On Mona, we only detected one floater
2010]
HERPETOLOGICAL MONOGRAPHS
out of 17 monitored adult males, suggesting a
low proportion of floaters in this population.
Moreover, paternity analysis of nine clutches
collected from 2003 to 2005 from five females
living in areas within or neighboring that
floater’s male home range showed that these
females were not fertilized by him, but instead
by the long-term male residents in the area
(unpublished data).
In summary, our results indicate that the
Mona Island iguana is highly territorial yearround, to an extent not previously reported for
any Cyclura species. This is supported by the
following results: (1) the regular spacing
pattern of males in the non- and middisturbed
areas, (2) the low levels of home-range overlap
between male–male and female–female pairs,
and (3) the negative relationship between
home-range size and body size that suggest
that juvenile iguanas behave as floaters
moving across well-delimited adult home
ranges. However, comparisons of territorial
behavior with other Cyclura species are
problematic. Very few studies have actually
measured the degree of home-range overlap
among individuals and none, to our knowledge, have used the spatial distribution
pattern of individuals as a measure of
territoriality. Further, the criteria used to
define a species as ‘‘territorial’’ vary among
researchers. For example, males of Cyclura
carinata can share up to 50% of their home
ranges, but are considered territorial (Iverson,
1979). The increase in the number of
radiotelemetry studies with Cyclura and other
iguana species will allow the testing of specific
hypotheses to determine the extent that
abiotic and biotic factors have on social
systems and territoriality in this endangered
genus.
MANAGEMENT IMPLICATIONS
The density and abundance data presented
in this mark/recapture and radiotelemetry
study suggest that previous censuses (PérezBuitrago and Sabat, 2000) of the population
may have underestimated actual population
size. For the Lighthouse area, the actual
density (2.2 iguanas/ha) was higher than
documented previously (1.38 iguanas/ha) with
the 16-m-width linear transects. Likewise,
actual density at Pájaros (4.12 iguanas/ha)
107
was higher than previously estimated, with16m-width line transects (3.5 iguanas/ha). The
discrepancies between linear transects and the
methods we used here (radiotelemetry and
mark–recapture) were attributed to previous
failures in using the proper linear transect
width for censuses. A major potential implication of these discrepancies is that the overall
population size for the Mona Island iguana
documented previously may be underestimated because of the difficulty of detecting
iguanas and/or using an incorrect transect
width. Thus, the actual population size would
be closer to the upper limit of 7500 iguanas
for the whole island, rather than the 5000
iguanas (Pérez-Buitrago and Sabat, 2000) or
the 4000 iguanas estimated by Wiewandt
(1977). However, the overall density of
iguanas for the whole island and across
different Mona Island localities still appear
relatively low when compared to other Cyclura populations, which can reach densities
ranging from 0.3 to 128 iguanas/ha (Alberts,
2000; Goodman, 2004). Previously, the overall
low densities of the Mona Island iguana along
with the low representation of young stages in
the population was considered abnormal,
making the persistence of the population a
conservation concern (Wiewandt and Garcı́a,
2000). The factor believed to be accounting
for these demographic traits was low survival
of juvenile iguanas. Some evidence of this was
obtained by Pérez-Buitrago and Sabat (2007),
who estimated a 22% survival in the first
5 months of life with the use of radiotelemetry. Despite high levels of juvenile mortality,
the actual impact of low recruitment of
juveniles into the adult stage is unclear.
Our telemetry and demographic data show
that the Mona Island iguana is highly territorial, and we believe that it may also be a factor
contributing to the observed low density on
Mona, given that territorial behavior is an
important factor limiting population size and
density (Brown, 1969). For male Mona Island
iguanas, the home-range maps suggest that
areas free of males also lack the permanent
presence of females. Furthermore, if the
male’s home-range size is used to calculate
how many males could occur in the study
areas, the obtained values are close to the
actual number of males in each study area (see
108
HERPETOLOGICAL MONOGRAPHS
above). This implies that the Mona Island
iguana population may be near carrying
capacity. It also implies that management
activities such as the head-starting program
implemented by the DRNA in 1999 may not
be as effective because the release of headstarted iguanas in areas already saturated by
adults may only contribute to adding floaters
to the population. However, because the
Mona Island iguana population seems to be
affected negatively by other factors, such as
introduced species preying on young stages, a
management strategy should include control
of these exotic predators in combination with
intermittent head-starting actions and sustained monitoring of the population in order
to prevent critical declines or crashes.
Finally, this is the first study assessing the
spatial ecology of any Cyclura species across
different environmental settings and during
both the reproductive and nonreproductive
season. Indeed, most previous Cyclura spatial
ecological studies have been conducted in
highly disturbed settings and only a few in
natural conditions (Knapp and Owens, 2005).
We found that some parameters (i.e., homerange size, the degree of home-range overlap,
and the spatial arrangement of individuals)
varied among the study sites and according to
levels of human disturbance. Moreover, the
differences in home-range size, the degree of
home-range overlap, and the spatial arrangement of individuals suggest that special
caution should be taken when inferences and
extrapolations are made at the population
level and/or management actions are implemented for a species from studies that were
only conducted in nonnatural settings.
Acknowledgments.—This research was funded by the
Center for Applied Tropical Ecology and Conservation
(CREST-CATEC; grant NSF HRD-0206200) at the
University of Puerto Rico, Rio Piedras Campus. Logistic
support was provided by the DRNA, and we are
particularly grateful to A. Alvarez, C. Diez, M. Garcı́a,
and the staff of the Endangered Species Division of the
DRNA. The DRNA Mona Island staff (rangers, biologists
and technicians) helped us in many ways while conducting
this project. Particularly, we are very grateful to G. Pons,
M. Nieves, M. Pérez, J. Jimenez, M. Bonet, and R. Pagan.
Many people helped us in the field, mainly A. Alvarez, J.
Delgado, J. Bendon, S. Boven, R. Menendez, M. J.
Andrade, J. Castro, C. Toledo, M. Zalamea, M. Rujas, M.
Correa, and K. Rosas. We also want to thank A. Alberts, J.
B. Iverson, S. M. Funk, and P. Angulo-Sandoval for
reviewing early versions of this manuscript. Finally, we
[No. 24
thank two anonymous reviewers for their comments to
improve this document.
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