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The benefits of sociality have been
widely discussed. Because the probability of
detecting an approaching predator increases
with the number of guarding eyes, it has been
proposed that animals gain protection against
predators by living in groups (Brown and Brown,
1987; Da Silva and Therhune, 1988; Yáber and
Herrera, 1994). Other benefits to sociality in-
clude decreased likelihood of predation through
the selfish-herd effect (Hamilton, 1971), active
deterrence of predators (Gross and MacMillan,
1981), and confusion of predators through a
perceptual bottleneck that leads to lower capture
efficiencies (Krakauer, 1995).
Studies of antipredator behavior in gregarious
reptiles have not been thorough in any species
(Greene, 1988). In particular, studies of social
behavior in iguanas have focused on territorial
interactions and mating behavior (Rand and
Rand, 1976; Alberts et al., 1992a; Rodda, 1992;
Phillips et al., 1993; Pratt et al., 1994). Although
some attention has been given to the benefits of
sociality in predator avoidance among green
iguanas (Burghardt, 1977b; Burghardt et al., 1977;
Greene et al., 1978; Burghardt, this volume), these
studies have been observational rather than
experimental. Most carnivorous species are ex-
pected to show the types of territoriality and intra-
specific aggression that is widely documented
among lizards (e.g., Stamps, 1983). However, as a
result of their strictly herbivorous diets, iguanas
might be expected to differ from other lizards due
to decreased competition for food resources.
Cooperation among relatives has been re-
ported in many species of social insects as a
mechanism to increase fitness by increasing
the reproductive output of related individuals
(Hamilton, 1964). For example, in honeybees,
there are a variety of social behaviors in which
some siblings care for and brood younger ones
(Wilson, 1971). Some vertebrates have been re-
ported to show analogous behaviors (Alexander
et al., 1991). To date, there have been no reports
of any reptile performing similar altruistic acts.
Indeed, we know of no vertebrate in which sib-
lings protect other siblings of the same age.
From 1988 to 1991, we hatched eggs from
both natural and artificially incubated nests of
119
9
Sexually Dimorphic Antipredator Behavior
in Juvenile Green Iguanas
KIN SELECTION IN THE FORM OF FRATERNAL CARE
?
Jesús A. Rivas and Luis E. Levín
green iguanas (Iguana iguana) at Hato Masa-
guaral, a cattle ranch and biological field station
located in Estado Guárico, Venezuela (8°34′N,
67°35′W). We repeatedly observed that when a
researcher approached a naturally hatching nest,
some animals remained immobile at the en-
trance to the nest, while one to several others fled.
Such escape attempts were not usually directed
toward cover, but rather toward the observer or
into relatively open space. The animals typically
ran along a straight trajectory with their tails
raised.
When we handled iguanas incubated in ar-
tificial nests, they frequently ascended our
arms, in an apparent escape maneuver potentially
related to their natural tendency to escape by
climbing trees. However, similar to the behavior
we observed in the field, other individuals either
froze or hid in the bottom of the enclosure. Their
subsequent behavior depended on the observer’s/
intruder’s behavior. Most iguanas remained im-
mobile or hidden if we remained motionless, but
if we chased the fugitive, then five or six addi-
tional individuals would flush from the nest. In
the seven instances where we were able to cap-
ture the fugitive, it was a male. A similar behav-
ior was observed in ten-month-old animals.
Given that the sex ratio at birth is 1:1 in green
iguanas (J. Rivas, unpubl. data), we would have
expected to find females among the fugitives if
the likelihood of males and females exhibiting
these behaviors were the same. That we did not
prompted us to carry out two pilot studies of sex-
ually dimorphic antipredator behavior in juvenile
green iguanas. Such behavior has been reported
in other juvenile squamata (Greene, 1988; Her-
zog et al., 1989). Here we present our prelimi-
nary observations and discuss their possible
implications.
SIMULATED PREDATOR RESPONSES
Our initial field observations suggested that
males reacted more actively than females to po-
tential predators. However, because we did not
know the sex ratio of the animals in the nest at
the time we found them, we were unable to draw
solid conclusions. To further explore this phe-
nomenon, we observed the reactions of juvenile
males and females to a simulated predator un-
der controlled conditions. For five months, ani-
mals from different artificially incubated clutches
were kept in separate outdoor enclosures meas-
uring 60 ×60 ×80 cm and fed daily with a
mixture of papaya and dog food supplemented
with vitamins and minerals. Twelve groups were
formed, each composed of five females and five
males from the same clutch. Sex was determined
through visual cloacal examination (Rivas and
Ávila, 1996).
The experimental arena was a rectangular
opaque plastic enclosure measuring 180 ×30 ×
30 cm. Dimensions were chosen to limit the
direction in which the animals could flee, thus
making it easier to score the behaviors. The bot-
tom of the arena was lined with foam rubber to
provide traction, with a 10 ×10-cm refuge (con-
sisting of an opaque cover supported by four 2-cm
legs at its corners) placed at the center of the
arena. The refuge was encircled by a removable
25 ×25 ×30-cm transparent plastic corral, which
at the beginning of the trial enclosed the iguanas
(figure 9.1). The trial was initiated by remotely
lifting the corral to avoid disturbing the animals.
The arena was illuminated by eight-reflector hood
lights hanging from the room ceiling.
The simulated predator consisted of a model
of a hawk species (Falco femoralis)known to prey
on juvenile iguanas (Rivas et al., 1998). The body
of the model was constructed of wood and the
wings of cardboard (39 cm long ×52 cm wide).
The shape, color, and body pattern were based
on a descriptive illustration of the bird (Phelps
and De Schauense, 1978; figure 9.2). Eyes were
simulated with two black dots (Gallup, 1973;
Burger et al., 1991). To add a mechanical com-
ponent to the stimulus, a celluloid sheet was
hung below the model, which, when in contact
with the wall of the enclosure, produced noise
and vibration. Air movement, contact by the cel-
luloid sheet (simulating the bird’s feathers) and
the shadow of the model (Prestude and Crawford,
1970) were additional components of the stim-
ulus. The model was fixed to one end of a pen-
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dulum. The other end was articulated at a hinge
joint on the ceiling above the refuge. The pen-
dulum was held horizontally by an electro-
magnet fixed to the ceiling of the room. When
the electromagnet was turned off in an adjacent
observation room, the model swept down over
the arena.
All observations were carried out during the
normal daily activity period. For ease of recog-
nition during experimental trials, females and
males were labeled with black and white tape,
respectively, on their backs. After two hours of
acclimatization to the test arena, the plastic cor-
ral was removed, and five minutes later, the
stimulus was presented. We let the model pass
over the arena forward and backward three times
and recorded the first movement performed by
each animal during the three passages. After
the trials, all animals were returned to the field
and released at the site where the eggs were
collected.
The iguanas’ behavior was recorded with a
video camera placed 2 m above the refuge. Dur-
ing preliminary trials, some iguanas concen-
trated at the end of the enclosure, and a second
camera was directed at this area. Recording al-
ternated between the two cameras by an elec-
tronic switch operating at one-second intervals.
Trials were analyzed at one-fifth of actual speed,
and frame by frame, where necessary. We scored
the following mutually exclusive behaviors dur-
ing each passage of the pendulum: moving ahead
of the model running in the same direction as the
model, moving in the opposite direction of the
model, hiding under the refuge, and appearing
from under the refuge and exposing either part or
all of the body. We also observed an unexpected
behavior that consisted of one animal climbing
onto another animal and covering it with its
body at the moment when the model was start-
ing its downward movement.
Risky behaviors, such as running in front of
the hawk, appearing from under the cover, and
covering another iguana were performed most
often by males (table 9.1). Females more often
performed behaviors that increased safety, in-
cluding hiding, immobility, and running in the
opposite direction of the model. Only males
(seven of twelve trials) showed the behavior of
covering another iguana, and it was always di-
rected toward females.
Both male and female iguanas responded
more strongly when the model predator passed
in a forward direction. This result suggests that
the iguanas discriminated the shape of the
model, responding more actively to the head-
forward movement of the hawk model, as found
by Tinbergen (1948) in gray geese. The forward
movement of the hawk also might have pre-
sented additional predator cues (e.g., eyes) to
the iguanas (Gallup, 1973; Burger et al., 1991).
In evolutionary terms, the higher responsive-
ness of the males to the simulated predator could
have two opposing but not mutually exclusive
explanations, one selfish and the other altruis-
tic. First, this rapid response might surprise a
searching predator and give the iguana more
time to escape at the expense of the remaining
SEXUALLY DIMORPHIC ANTIPREDATOR BEHAVIOR
121
FIGURE 9.1. Test arena for simulated predator presenta-
tions to juvenile green iguanas.
animals. Alternatively, it might serve to divert
the predator away from others, giving his rela-
tives the opportunity to escape. If the fugitive
escapes, he accomplishes the double goal of
surviving and helping his clutchmates to escape.
If he fails to escape, he may still provide the op-
portunity for his siblings to escape.
The selfish explanation requires that the
escapee start the escape early, when the proba-
bility of escaping successfully is high. Con-
versely, the altruistic explanation requires that
the escapee wait until the attack on the group is
imminent, and assumes that the predator does
not know that additional animals are present.
Our preliminary observations do not support
one hypothesis over the other. However, in ob-
servations of natural nests, escapes took place
after digging and harassing the animals for
some time, which does not support the selfish
hypothesis. The high synchrony of hatching, in
which several hundred iguanas may emerge
from a single nest site over approximately two
or three weeks (Burghardt, 1977b; Burghardt et
al., 1977; Rivas et al., 1998), may limit oppor-
tunities for predator learning. In fact, predators
cueing on mass hatching events could have been
an evolutionary force leading to such synchrony,
as has been documented in tadpoles of Bufo
boreas that metamorphose synchronously to
decrease predation by garter snakes (Devito et
al., 1998).
Our observations suggested that females stay
motionless more often than males (table 9.1).
Avoiding detection by predators is crucial for
iguanas, given that a small iguana probably can-
not repel a relatively large bird (Greene et al.,
1978); one strategy to avoid being detected is to
remain immobile (Prestude and Crawford, 1970).
Greene et al. (1978) reported that a young iguana
avoided being discovered by a hunting coati (Na-
sua sp.) by freezing.
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TABLE 9.1
Responses by Male and Female Juvenile Green Iguanas
to the Forward Passage of a Simulated Hawk Model
number
action females males
Move ahead of the model 20 35
Move in the opposite 5 2
direction
Hide 6 3
Appear 1 4
Cover another 0 7
Remain stationary 28 9
Notes: Only the first movement performed by each animal
during the three passages of the model was scored. Moving
ahead of the model, appearing from under the refuge, and
covering another animal were lumped together as risky
behaviors. Hiding and moving in the opposite direction of
the model were labeled as risk-averse behaviors. The differ-
ence in the first behavior performed by individual females
and males in response to the model was highly significant
(χ
2
1
=24.43, P<0.001), with males showing more risky be-
havior than did females. The hypothesis of equal probability
for both sexes of covering another was rejected by a two-
tailed binomial test (P<0.02).
FIGURE 9.2. Frontal view of
the hawk model presented to
juvenile green iguanas.
Given the relatively high speed of an ap-
proaching falcon, to run in front of the predator,
a behavior seen more frequently among males,
is arguably more likely to attract the hawk’s at-
tention than to facilitate escape. Indeed, for an
iguana attempting to escape by running, the
best direction to run would be in the opposite
direction of the flying hawk. Movements in the
opposite direction of the model, which could
have avoidance advantages, were more frequent
in females than in males (table 9.1).
Covering behavior is particularly striking. In
all likelihood, a male that covers a female with
his body increases his risk of being predated
while decreasing hers. Our observations strongly
suggest the possibility of an altruistic behavior
in which a male assumes the predation risk of
his female clutchmates. That females never
climbed on any other animal and males climbed
only on females leads us to speculate that this
behavior could be performed to protect the fe-
males that are being covered. Because males
never climbed on each other or on the refuge,
we find it unlikely that the observed instances of
covering another may be misdirected climbing
behavior in the presence of a stressful situation.
Earlier studies have reported juvenile iguanas
perching and sleeping in physical contact with
each other and even on the top of each other
(Burghardt, 1977b; Burghardt et al., 1977; per-
sonal observation), indicating that covering
behavior occurs in other natural contexts. Un-
fortunately, inability to determine the sex of ju-
veniles in these studies prevented collection of
the information needed to document behavior
in the wild. Future studies should address this
issue.
NATURAL PREDATION
Responses to the simulated predator suggested
that males may be more risk-prone than females
in their antipredator behavior, a response pat-
tern that could have opposing consequences for
male survival. Males could surprise the predator,
allowing them to escape more often than females
who respond less actively. Another possibility is
that the behavior of males attracts the attention
of predators, facilitating higher survival of their
female clutchmates. To see how our observations
in captivity may be related to the wild, we exam-
ined survival probabilities of males and females
facing free-ranging, natural predators.
We excavated eight nests from the commu-
nal nest at Hato Masaguaral (Rodda and Grajal,
1990) and incubated the eggs until hatching. A
total of ten groups was used, each composed of
seven females and seven males, up to two weeks
old, randomly chosen from the same clutch. An-
imals were identified with a number drawn with
ink on the ventral side. Snout-vent length (SVL),
total length, and mass were measured for each
animal. A 3 ×3-m outdoor escape-proof enclo-
sure was constructed with 60-cm-wide zinc sheet-
ing. The enclosure contained a shelter made from
a 40 ×40-cm wood board on two cinder blocks,
under which food and water were placed. Sev-
eral 40-cm natural bushes were included in the
enclosure to provide natural perches and hide-
outs for the animals. Animals were released into
the enclosure at 0600 and exposed to natural
predators until 1800 (twelve trials) or released
at 1800 and exposed to natural predators until
0600 (six trials). At the end of each trial, we
recorded which animals were present or absent.
For animals that were present, we noted which
were missing a piece of the tail, as evidence of
attack. Absent animals were scored as predated.
During diurnal trials, we saw some avian
predators flying nearby or perching next to the
enclosure, including savanna hawks (Hetero-
spizias meridionalis), crane hawks (Geranospizias
caerulens), and great kiskadees (Pitangus sulphu-
ratus). A snake was also seen in the area (Chiro-
nius charinatus). All of these animals are known
to prey on juvenile iguanas (Rivas et al., 1998).
Nocturnal predators seen included opossums
(Didelphis marsupialis) and an unidentified ro-
dent that entered the enclosure. Actual predation
events could not be documented, as our prox-
imity would have deterred predators from ap-
proaching the enclosure.
Of the 140 animals tested, twenty-one were
predated and 71% of these were males (χ
1
2
=3.86,
SEXUALLY DIMORPHIC ANTIPREDATOR BEHAVIOR
123
P=0.05; table 9.2). We compared the mean
SVL of males (74.89 ±3.04 cm) with that of
females (74.59 ±2.57 cm) and found no signifi-
cant difference (t
138
=0.63, P=0.53). Nor was
there a difference between mean mass of males
(12.44 ±2.29 g) and females (12.45 ±2.26 g;
t
138
=0.04, P=0.97). Neonate green iguanas
are not sexually dimorphic; therefore, selective
capture of larger animals by predators cannot ex-
plain the observed differences in predation rate
between the sexes. We also compared the mean
SVL and mass of animals that survived (74.89 ±
2.72 cm; 12.62 ±2.26 g) with those that were
predated (74.61 ±2.87 cm; 11.93 ±1.99 g). No
significant effects were found for either SVL
(t
138
=0.41, P=0.68) or mass (t
138
=1.32, P=
0.19).
Among the surviving animals, seven (five fe-
males and two males) were missing a piece of
tail, presumed evidence that they had sustained
an attack. Our findings suggest that the high re-
sponsiveness of male juvenile green iguanas to
predators does not contribute to their individual
survival. Males were predated more often than
females, lending little support to the hypothesis
that the males enhance their probability of suc-
cessful escape by surprising predators. Rather,
the risky behavior of males seems to attract the
attention of natural predators.
The refuge provided within the enclosure
was large and protective enough for the iguanas
to escape beneath it and avoid detection. Thus,
the animals that were predated had the option of
either hiding or being exposed. The larger num-
ber of males predated cannot be explained by
sexual dimorphism in body size, as we did not
detect any differences between the sexes in SVL
or mass. We presume that the higher number of
males predated is the result of behavioral differ-
ences, a conclusion supported by the observed
differences in behavior between the sexes in their
responses to the simulated predator. The larger
number of females missing part of the tail sug-
gests that females do get attacked by predators,
but that they manage to escape predation more
often than males. It is possible that covering be-
havior by males toward females explains these
findings, but field experimentation is needed to
document the extent to which these patterns oc-
cur in nature.
KIN SELECTION IN THE
FORM OF FRATERNAL CARE?
We observed that male iguanas seem to show
more risk-prone behavior than females when
presented with a model predator and were pre-
dated more often than females by natural pred-
ators. Here we offer two nonmutually exclusive
explanations that may help explain our obser-
vations, one involving mechanisms of control,
and one involving adaptive function.
If it occurs in the wild, the risk-prone behav-
ior of males that we saw in response to the sim-
ulated predator could attract attention, resulting
in the higher mortality we observed in the field
enclosure. One possible explanation for this
behavior in males is a consequence of higher
androgen levels, important for social dominance
124
JESÚS A
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TABLE 9.2
Number of Juvenile Iguanas of Each Sex
That Were Predated Naturally in
Outdoor Experimental Enclosures
trial females males
102
222
301
402
510
611
701
8 02
913
10 1 1
Total 6 15
Note: Each trial was based on fourteen siblings
(seven males and seven females).
in early stages of maturation (Phillips et al., 1993;
Pratt et al., 1994). To be dominant early may
produce a larger payoff later in life that out-
weighs the cost of increased risk of predation.
Thus, the higher risk incurred by males may be
a by-product of the social system of green igua-
nas, in which dominant males perform the vast
majority of mating as adults (Dugan 1982a;
Rodda 1992). However, higher androgen levels
do not explain the difference in the direction
of runs performed in response to the predator
model, in which males ran in front of the model
more often, while females tended to run in the
opposite direction. Nor do they explain the
covering behavior exhibited by males directed
toward females. In addition, this hypothesis re-
quires that the benefits of high androgen levels
outweigh the increased predation risk. Neonate
green iguanas suffer extremely high predation
pressure by a large variety of predators (Rivas
et al., 1998), meaning that the benefits of in-
creased androgens would have to be extremely
high for this explanation to be tenable.
Another explanation for our results suggests
adaptive reasons for the observed differences in
behavior of males and females. Males react more
actively than females, which may attract the at-
tention of a predator and increase the chance for
clutchmates to escape. This apparently altruistic
behavior can be explained in terms of kin selec-
tion (Hamilton, 1964). Because they are from
the same nest, the probability that such males
and the individuals they assist are siblings is
high. Hence, an individual could potentially in-
crease its indirect fitness by increasing the sur-
vival probability of his peers. It has been reported
that green iguana hatchlings show a tendency to
remain in groups in the wild for many months
(Burghardt, 1977b; Burghardt et al., 1977; Drum-
mond and Burghardt, 1982; Burghardt and Rand,
1985) and that individuals recognize and prefer
to group with their kin (Werner et al., 1987).
Female green iguanas perform seasonal mi-
grations to lay eggs in communal aggregations,
showing a high degree of philopatry (Bock et al.,
1985; Rodda and Grajal, 1990). These nesting
aggregations are isolated from one another, lead-
ing to low levels of heterozygosity (Bock and Mc-
Craken, 1988). This pattern suggests the possi-
bility of breeding with relatives (Waldman and
McKinnon, 1993) such that relatedness among
the hatchlings would be higher than the ex-
pected 0.5, conditions under which cooperative
behaviors are more likely to evolve (Michod,
1993). Thus, a male that attracts a predator to-
ward himself and saves several clutchmates
might be increasing his indirect fitness. Such
fraternal care could help account for the main-
tenance of sociality in juvenile green iguanas.
Because only animals within a cohesive social
group will benefit from such risk-prone behavior,
it benefits clutchmates to stay together.
A remaining question is why males should
direct their altruistic behavior differentially to-
ward females. One possible explanation derives
from differential variability in the reproductive
success of males and females. Iguanas breed in
harems that are vigorously defended by domi-
nant males (Dugan, 1982a; Rodda, 1992). A male
cannot gain control of a harem until he reaches
an appropriate size to fight and win contests.
Dugan (1982a) suggests that a male needs six or
seven years to reach the size at which he can de-
fend a territory, and even then, only a fraction of
males can successfully control a harem. Females,
on the other hand, virtually all breed by their
third year, with some breeding as early as 1.5
years. Once they reach maturity, females breed
annually (Werner, 1991; Rand and Bock 1992).
Due to high rates of predation in the wild and
strong intrasexual competition, the probability
of a male reaching breeding size and controlling
a harem is very low. Given that variance in breed-
ing success is so much lower for females than
males, a male that protects his female clutch-
mates might be significantly increasing his
inclusive fitness.
Future studies are needed to determine if the
risk-prone behavior we observed in males oc-
curs in the wild, and if so, whether it is main-
tained into adulthood. Dominance relations are
established early in life among male green igua-
nas (Phillips at al., 1993), but their significance
for adult mating success can only be speculated
SEXUALLY DIMORPHIC ANTIPREDATOR BEHAVIOR
125
upon. One might expect risk-prone behaviors to
be most beneficial to young, subordinate males
that have a low probability of reproducing suc-
cessfully as adults. For such males, the best op-
tion may be to enhance survival of their female
clutchmates, for whom variance in adult repro-
ductive success is considerably lower. Further
testing would be helpful in determining if altru-
istic risk-prone behavior is more prevalent among
males of lower competitive ability.
Energetic constraints associated with her-
bivory may be an important evolutionary force
in maintaining altruistic behavior in green igua-
nas. In this species, variance in male reproduc-
tive success may be due in part to the very long
period of time it takes for dominant males attain
the large body size needed for successful terri-
tory defense and harem control. Larger size has
been related to the evolution of increased colon
complexity, necessary for high efficiency in di-
gesting the energy-poor diet that characterizes
herbivory (Iverson, 1982). That males with slow
growth rates as a consequence of the low nutri-
tional value of plant matter will remain small for
extended periods with a low probability of breed-
ing compared with larger, older males (Pough,
1973; Rand, 1978) may have led to alternative
strategies for increasing individual fitness.
This is the first report of possible altruistic
behavior in any reptile, excluding parental care.
However, the traits that favor its evolution are
not unique to green iguanas. All iguanines are
folivorous and most have similar hierarchical
social structures and mating systems. Therefore,
the potential exists for altruism to be present in
other related taxa. However, the evolution of
altruistic behavior might not be favored or might
be constrained in some groups. Taxa living on
islands (e.g., Amblyrhynchus, Brachylopus, Cono-
lophus, Cyclura) with lower predation pressure
and smaller clutch size would not be predicted
to exhibit the altruistic behaviors reported here.
Similarly, iguanines living on the mainland that
have an insectivorous stage in their life cycle
(Ctenosaura) may be less likely to evolve altruis-
tic behavior because the cost of sociality in juve-
niles might be much higher due to competition
for limited food resources. Mainland species that
are herbivorous throughout their lives would
be good species to examine for the behaviors
described here. In particular, the genus Sauro-
malus meets these conditions and is genetically
closely related to the green iguana (Sites et al.,
1996). The insular species I. delicatissima would
be another interesting taxon to consider because
it is very closely related to I. Iguana, but does not
experience the putative environmental condi-
tions expected to lead to altruism. More details
on the phylogeny, ontogeny, consistency, and
variance of these altruistic behaviors, as well as
on their relationship to ultimate reproductive
success of those that perform and benefit from
them are needed to understand fully the poten-
tial role of altruism in iguanine lizards. We have
shown how antipredator strategies of green
iguanas, like other aspects of their behavior (Burg-
hardt, 1977b), seem to be far more complex than
previously believed. Our observations are the first
report of possible fraternal care in a reptile, and
suggest intriguing avenues for future research.
Behavior of reptiles has been considered as prim-
itive and simple, but may actually involve a level
of intricacy not previously appreciated.
ACKNOWLEDGMENTS
We thank Tomás Blohm for his hospitality
and interest in this research. We are also grate-
ful to Gordon Burghardt, Samantha Messier,
Juhani Ojasti, Stanley Rand, and Mark Waters
for their helpful comments on the manuscript.
This research was supported by Concejo de De-
sarrollo Cientifico y Humanístico (CDCH) grant
C.03.10.1981/92, Concejo Nacional de Ciencia y
Technología (CONICIT) grant S1.95-000-726,
and the Wildlife Conservation Society.
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