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DIETARY COMPETITION BETWEEN THE BLACK CAIMAN
(Melanosuchus niger) AND THE SPECTACLED CAIMAN (Caiman
crocodilus) WITHIN THE LAGO PRETO RESERVE, PERU.
Caiman crocodilus
Melanosuchus niger
Nick Reynolds
BSc Wildlife Conservation
Durrell Institute of Conservation and Ecology
University of Kent
2007
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Abstract:
This research was undertaken to determine the diets of the Spectacled Caiman
(Caiman crocodilus) and Black Caiman (Melanosuchus niger) within the Lago
Preto Conservation Concession, Peru. This was done as dietary competition
may be one of the ecological factors affecting the lack of recovery of the Black
Caiman in this area. The diets of the two species were determined similar, using
the numerical and frequency occurrences of certain pre-determined prey
categories found in stomach samples.
A dietary overlap index showed the
overlap was biologically significant within both habitat types surveyed. Results
were compared to previous studies and the comparisons highlighted similarities.
A comparison to results from a different region of Peru showed differences in diet
preferences for the two species.
However, both studies highlighted dietary
competition.
This research underlined further study is required to collaborate caiman size and
diet of the two species within Lago Preto. Further comparative study between
different regions of South America is also highly recommended.
1.0 Introduction:
The Black Caiman (Melanosuchus niger) and the Spectacled Caiman (Caiman
crocodilus) are the two largest crocodilians in the Amazon basin, and both
species have a long history of human exploitation (Smith, 1980).
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The Black Caiman is the largest Neotropical predator. The species was formerly
abundant in South America and present in the entire Amazon basin. During the
last century the Black Caiman along with the Spectacled Caiman have faced
strong hunting pressure for the leather industry, and a high rate of habitat loss
(Smith, 1980 and Plotkin et al, 1983).
Although these pressures have reduced
greatly due to legal protection within most of their range, they seem to have had
stronger adverse effects on the Black Caiman. The Black Caiman’s life history
and ecological traits are believed to limit its recovery. The species is known to
have a sedentary diet and to be a habitat specialist found in slow-moving
freshwater rivers, lakes, wetlands, black water swamps, and seasonally flooded
areas of the Amazon (De Thoisy et al, 2006).
As a result, the total species population size of Black Caiman may have
decreased by 90%, and concomitantly has experienced a high level of
fragmentation (Ross, 1998).
The Black Caiman competes ecologically with the Spectacled Caiman (Caiman
crocodilus), which is believed to be a much more opportunistic species (Rebelo
and Magnusson, 1983, Herron, 1991, Herron, 1994 and Thorbjarnarson, 1991).
Spectacled Caiman population recovery rates have been measured at four times
higher than the Black Caiman (Farias et al, 2004 and De Thoisy et al, 2006). To
date, field surveys have been irregularly conducted all over both species range;
but available data reveal that the Black Caiman is locally extinct in many
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Amazonian areas, and occurs in reduced densities in many others (Rebelo and
Lugli, 2001).
On the other hand, field observations indicate that some populations may have
recovered (Rebelo 2001). Unfortunately the Black Caiman population within the
Lago Preto Reserve in the North-Eastern Peruvian Amazon is not one of these
fortunate populations. In this study the diets of the Black Caiman and Spectacled
Caiman were analysed.
This was achieved by taking stomach samples to
investigate the factor of diet within the ecological competition between the two
species.
This research will hopefully help take another step towards
understanding why the Lago Preto population of Black Caiman are struggling to
recover.
1.1 The Study Animal:
The Black Caiman (Melanosuchus niger)
Common names:
Black Caiman, Caiman, Caiman Negro, Caiman Noir, Lagarto Negro, Jacare
Acu, Jacare Assu, Jacare Acu, Jacare Uassu, Jarace Una, Yacare Assu.
Distribution:
Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Perú and Venezuela
(Unconfirmed) (See Fig 1)
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Fig 1
Habitat:
Found in various freshwater habitats (e.g. slow-moving rivers, streams, lakes and
flooded savannah and wetlands). Although overlapping with the range of other
caiman species in South America, it appears to occupy different habitat niches
(http://wwwflmnh.ufl.edu/cnhc/csp_mnig.htm).
Status:
CITES: Appendix 1,
IUCN: Red List: Low risk, conservation dependent
Estimated wild population: 25,000 to 50,000
Summary: Widely distributed, but historically heavily exploited. Most populations
appear to be recovering well (http://www.CITES.org).
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Diet:
The Black Caiman eats fish (including Piranha and Catfish) and aquatic
vertebrates, including large Capybara rodents (Hydrochaeris hydrochaeris). This
species shows more terrestrial hunting activity, particularly at night, having acute
sight and hearing.
Juveniles take crustaceans before moving onto larger
terrestrial prey. Larger adult caiman of this species have been reported to attack
domestic animals and humans (http://www.flmnh.ufl.edu/cnhc/csp_mnig.htm).
Group of Black Caiman hatchlings demonstrating the
safety in numbers strategy.
(http://www.flmnh.ufl.edu/cnhc/csp_mnig.htm)
Juvenile Black Caiman
(www.pbs.org/.../images/whos-blackcaiman.gif)
Conservation:
Historically the distribution of the Black Caiman has been widely distributed
throughout the Amazon basin and beyond. However, once populations of both
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Crocodylus acutus (American crocodile) and Crocodylus intermedius (Orinoco
crocodile) became severely depleted due to over-zealous commercial hunting,
attention was turned to those species with slightly smaller or lower-grade skins
(Best 1984).
The skin of the Black Caiman produces shiny, black leather.
Hunting was
directed very intensely towards the Black Caiman during the 1950’s. Some areas
were affected more severely than others, with hunting pressures continuing into
the 1970’s and beyond. However, within the 1970’s, a major shift in caiman
hunting occurred in the Western Brazilian Amazon (Amazonas state).
As
markets for skins disappeared, hunters began selling the meat of caiman instead.
By the early 1980’s, a trade in salted meat from the Amazonas to Para state in
Brazil to Colombia was reported, and this trade continues to flourish (Best 1984).
The Black Caiman is estimated to have been reduced in numbers by 90% in the
space of the last century.
Population recovery today is impeded both by
continued illegal hunting and through increased competition with the more
numerous Caiman crocodilus (Spectacled Caiman).
This latter species has
moved into areas once inhabited by the Black Caiman and proliferated due to its
increased reproductive capacity.
Hunters can take both species with ease.
Habitat destruction through deforestation and burning of swamplands (e.g.
French Guiana) continues the onslaught
(http://www.flmnh.ufl.edu/cnhc/csp_mnig.htm).
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Little information was available about this species until the 1980’s, when research
was carried out into both biology and population ecology. There is still much to
be learnt however. Although some data is available concerning interactions with
other South American Caiman species, the dramatic decline in populations of the
Black Caiman have obscured trends
(http://www.flmnh.ufl.edu/cnhc/csp_mnig.htm).
Population declines have been correlated with a decrease in fish production in
rivers.
This is believed to be due to the removal of the nutrient-recycling
component in the ecosystem as provided by apex predators such as Caiman.
Both Piranha and Capybara have benefited from the reduction of their main
predator. This had led to increased agricultural and livestock losses. Survey
data, which is available throughout most of the species range, reveals drastically
reduced populations. Further survey work is required to update this information.
The Black Caiman is severely depleted in over half of the countries in which it
occurs, and considered to be depleted in the rest. Only populations in isolated
locations remain stable (http://www.flmnh.ufl.edu/cnhc/csp_mnig.htm).
Management programs centre on the legal protection of remaining wild
populations, but these laws are difficult to enforce effectively. Captive breeding
and reintroduction was initiated in Bolivia in 1990. Both of these conservation
strategies need to be extended and implemented as effectively as possible in
other countries (http://www.flmnh.ufl.edu/cnhc/csp_mnig.htm).
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The Spectacled Caiman (Caiman crocodilus)
Common names:
Common Caiman, Spectacled Caiman, Tinga, Baba, Babilla, Babiche, Cachirre,
Caiman blanco, Caiman de Brasil, Cascarudo, Jacaretinga, Lagarto, Lagarto
Blanco, Yacare Blanco.
Subspecies:
 C.c.apaporiensis (Rio Apaporis Caiman)
 C.c. fuscus (Brown Caiman)
 C.c. yacare (Yacare Caiman)
(http://www.flmnh.ufl.edu/cnhc/csp_ccro.htm#dist)
Distribution:
Brasil, Colombia, Costa Rica, Cuba*, Ecuador, El Salvador, Guyana, French
Guiana, Guatemala, Honduras, México, Nicaragua, Panamá, Perú, Puerto Rico*,
Suriname, Tobago, Trinidad, United Status, and Venezuela. [*=Introduced –
C.c.fuscus in Cuba and Puerto Rico]. (See Fig. 2.0)
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Fig 2.0
(http://www.flmnh.ufl.edu/cnhc/cst_ccro_dh_map.htm)
Habitat:
The Spectacled Caiman is an extremely adaptable species found in virtually all
lowland wetland and riverine habitat types throughout its range, particularly as a
result of the now-diminished ranges of sympatric competitors (e.g. C.acutus,
C.intermedius, M.niger).
The Spectacled Caiman has the widest distribution of any species in the
Alligatoridae family, it can tolerate a reasonable degree of salinity and if
environmental conditions become too harsh it will burrow into mud and aestivate
(http://www.flmnh.ufl.edu/cnhc/csp_ccro.htm#dist).
.
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Status:
CITES: All subspecies Appendix 2, except C.c.apaporiensis (Appendix 1)
IUCN Red List: Low Risk, Least concern.
Estimated wild population: Over 1,000,000
Summary: The Spectacled Caiman is the most common of all crocodilian
species, although some populations are locally depleted (http://www.CITES.org).
Diet:
Juvenile Spectacled Caiman take a variety of aquatic invertebrates (insects,
crustaceans, molluscs). As they grow, various vertebrates make up a greater
percentage of the diet. These include fish, amphibians, reptiles and water birds.
Much larger, older animals are capable of taking larger mammals such as bush
pigs. Past observations have shown that the drier the conditions become the
less the caiman feed, cannibalism is common at this time
(http://www.flmnh.ufl.edu/cnhc/csp_ccro.htm#dist).
.
The ecological importance of this species has been demonstrated in terms of
nutrient recycling- its nitrogenous waste re-enters the ecosystem to the benefit of
other plants and animals. In areas where the Spectacled Caiman has become
depleted, fish populations have also been shown to decline. It has also been
reputed to control Piranha numbers; however there seems little evidence to
support this. In reality, it is likely that the Spectacled Caiman is very much a
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generalist
and
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adaptive
predator,
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given
its
ecological
success
(Silveira+Magnusson 1999).
Juvenile Spectacled Caiman (Caiman crocodilus)
(http://www.flmnh.ufl.edu/cnhc/csp_ccro.htm#dist)
Conservation:
This species has actually benefited from commercial utilization and over-hunting
of other species within its range (Crocodylus acutus, C.intermedius and
Melanosuchus niger), taking over habitat from which it would otherwise have
been out-competed by healthy populations. The skin of the Spectacled Caiman
is not ideally suited to tanning, as the ventral scales contain well-developed
osteoderms. Only the lateral flanks provide skin of an acceptable quality for
tanning.
Hunting pressures on this species remained relatively low until the
1950s. By this time populations of the sympatric crocodilian species became
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depleted and hunting of the Spectacled Caiman intensified. The numbers of
caiman harvested since then has been huge, and they currently supply the vast
majority of the hide market in America. Leather from this species is often passed
off as the American Alligator (Alligator mississippiensis) or other species
(http://www.flmnh.ufl.edu/cnhc/csp_ccro.htm#dist).
Despite pressure from hunting and also collection for the pet trade, existing
surveys suggest that populations are in relatively good condition in most areas.
This seems to reflect the adaptability of the species, its reproductive potential,
and the increase in available habitat through the removal of competing species.
Also an increase in man-made water bodies (e.g. Brazilian patanal) has also had
beneficial affects. However, it is these factors which make it difficult to determine
the overall status of the species, as populations are faring less well in other
areas- surveys reveal severe depletion in El Salvador. More up to date surveys
are required for clarification, and to examine the interactions between different
subspecies.
hunting.
The major threat to this species and others is currently illegal
Smuggling rings operating through Thailand and Singapore are
extremely damaging to individual populations, and greater controls and more
effective legislation are required
(http://www.flmnh.ufl.edu/cnhc/csp_ccro.htm#dist).
Sustainable use programs are well developed in several countries. Most of these
rely upon regular cropping of wild populations. The long term effects of this
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cropping need to be investigated.
The reproductive potential of this species
makes properly controlled sustainable yield programs look promising. Farming
and ranching programs, while they exist, may be uneconomical in the long run,
given the value of the hide and the number of animals which need to be culled in
order to produce a profitable amount of hide (Gorzula+Seijas 1989).
1.2 Identification of Species:
The Black Caiman (Melanosuchus niger):
(Wermuth + Fuchs 1978)
The Black Caiman is the largest species in the family Alligatoridae (males can
reach at least 4 metres and huge 6 metre specimens have been reported but not
confirmed).
General appearance is not dissimilar to the American Alligator
(Alligator mississippienis). As the common name suggests, they have a dark
colouration. The lower jaw has grey banding (brown in older animals), and pale
yellow or white bands are present across the flanks of the body; although these
are more prominent in juveniles. This banding fades only gradually as the animal
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matures. The Black Caiman is structurally dissimilar to other caiman species,
particularly in the shape of the skull. The Black Caiman has distinctly larger
eyes, and a relatively narrow snout. The bony ridge extending from above the
eyes down the snout, as seen in other caiman, is present.
(www.flmnh.ufl.edu/cnhc/csp_mnig.htm)
The Spectacled Caiman (Caiman crocodilus):
(Wermuth + Fuchs 1978)
The Spectacled Caiman is a relatively small to medium sized crocodilian (males
generally reach 2.0 m to 2.5m, with the largest specimens reported to approach
3m- but these are undoubtedly rare).
Females are smaller, reaching a mean
maximum size of 1.4m, and rare individuals may approach 2m.
Its common
name derives from a bony ridge which is present between the front of the eyes
(infra-orbital bridge), appearing to join the eyes like a pair of spectacles.
A
triangular ridge is present on the heavily- ossified upper eyelids, vaguely
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reminiscent of those on the dinosaur Allosaurus. Juveniles are yellow in colour
with black spots and bands on the body and tail. As they mature, they lose this
yellow colour and the markings become less distinct. The adults are dull olivegreen. The different subspecies vary in colour, size and skull shape.
(www.flmnh.ufl.edu/cnhc/csp_ccro.htm)
1.3 The Study Site:
The Lago Preto Conservation Concession:
The Lago Preto Conservation Concession (LPCC) was awarded to the Wildlife
Conservation Society (WCS) in August 2006, and the WCS manage it in
collaboration with the Durrell Institute of Conservation and Ecology (DICE). It is
located approximately 175km southeast of the city of Iquitos along the Yavari
River; the concession is 9926.19 ha in area.
The Yavari River forms the southern border of the concession area. The river,
winds through the Amazonian lowlands forming the border between Brazil and
northeast Peru. To the north and east the concession area is bordered by the
Iquitos-Yavari logging concessions. The concession area gains its name from
the local oxbow lake, which is one of a system of several oxbow lakes in the
floodplain forests on the Peruvian side of the Yavari River.
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Fig 3.0: The coordinates of the limits of the Lago Preto Conservation
Concession
Point
UTM
East
North
SW corner
857949
9503866
NW corner
857943
9509773
NE corner
872019
9509711
SE corner
872005
9504831
(Bowler 2007)
Fig 4.0: Map of the Lago Preto Conservation Concession
(http://www.iucn.org/themes/protectedareas-cmaretti.pdf)
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1.4 The Habitats:
The two types of habitats surveyed in this research project were lakes and rivers,
three oxbow lakes and two rivers were surveyed during the research period. The
three oxbow lakes were Lago Preto, Lake Tipischa and Lake Hipiranga. All these
lakes are part of a system of oxbow lakes that make up the floodplain forests on
the Peruvian side of the Yavari River. All these oxbow lakes are blackwater
lakes, which refers to the colour of the water as the contents of these lakes
drains from the surrounding floodplain forest and upland terraces. All the lakes
are surrounded by dense varzea forest, and one has to use a canoe to enter the
lakes during the wet season (Bowler 2007).
The two Rivers surveyed were the Yavari and the Yavari Miri . The Yavari River,
winds through the Amazonian lowlands forming the border between Brazil and
northeast Peru. It is over 500km long and is one of the larger tributaries of the
Amazon River (Bowler 2007). The Yavari Miri has its headwaters in the upland
formations that divide the Yavari and Amazon valleys.
Both rivers are
Whitewater Rivers, which consist of muddy water that has an abundance of fine
suspended inorganic material. Usually this suspended matter originates from
erosion of Andean slopes and consists of former marine sediments rich in
minerals, particularly calcium, magnesium and phosphorus (Furch 1984).
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1.5 Aims and Objectives:
This study was undertaken to answer the following questions:
(1) Which prey types are consumed by the Spectacled Caiman and the Black
Caiman within the Lago Preto Conservation Concession?
(2) Are the diets of the two species similar?
Both these questions are being answered to investigate if there is any dietary
competition between the species, to try and explain the Black Caimans lack of
recovery within the Lago Preto Conservation Concession. It is already known
that the Spectacled Caiman is a much more opportunistic predator than its larger
relative, and has definitely benefited from the demise of the Black Caiman
indicated by its population increase within this area. However, the reason behind
the lack of recovery by the Black Caiman within this area is still yet to be
established. The purpose of this study is to investigate the factor of dietary
competition, as it may be significant in the recovery of the Black Caiman in this
area of Peru.
2.0 Methods and Materials:
2.1 Sample Collection
For this study a total of 53 Caiman stomach samples were analysed. Within my
personal period of field work 24 caiman were caught during the onset of the
summer dry season within the Lago Preto Conservation Concession. The study
period was between the 25th May and the 14th June 2007. However, to contribute
to a more thorough comparative analysis, other studies data from the same area
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was used. Caiman dietary data from a field study straight after mine was used
(Smith 2007), as well as Black Caiman data from a study done in 2005 (Swan
2005).
Altogether the captures consisted of 40 Spectacled Caiman, and 13
Black. The data analysed within this study was compared to three other data
sets to highlight any similarities. Two of the comparisons were data collected
from Lago Preto (Swan 2005, Mountain 2005). The final data set was collected
from Pacaya Samiria National Reserve, Peru (Street 2003). This final data set
would allow a comparison of the two species diet in two different regions of Peru.
The caiman were caught in three oxbow lakes, Lake Hipiranga, Lake Tipischa
and Lago Preto, and two rivers, the Yavari and the Yavari Miri. 19 Caiman were
caught along the Yavari, 25 Caiman were caught along the Yavari Miri, 6 were
caught in Lake Tipischa and 1 individual was caught in Lago Preto, 2 individuals
were caught within Lake Hipiranga.
All animals were caught at night, between 20.00hr and 23.30hr.
Captures were
made either by hand or by a noose from two types of vessel.
The crew
numbered 5 which were two field guides, myself and another student researcher,
and a supervisor/coordinator.
The two vessels were a small wooden canoe,
required when entering the lakes as the entrances were still flooded areas of
floodplain varzea forest. For the wider river habitats a larger wooden boat was
used which had a canopy for cover. The Caiman were located for capture with
the use of a large spotlight. When the spotlight was shone on the Caimans eyes
an orange glow was reflected back to give away the animal’s location; within 80m
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of approach to the individual the engine was cut and the approach continued
silently to avoid a flight reaction and submersion from the individual.
A global positioning system (GPS) was used to measure the distance of each
transect completed, and the geographic location of each capture on the individual
transect.
2.2 Caiman handling and restraint:
Once the Caiman were caught and brought aboard the boat, the individual was
restrained using nylon rope. Firstly the jaws were tied shut, followed by both sets
of legs being tied together behind the animals back. Once this was done, when
the individual was actually held, a firm grip was placed just behind the animals
head and at the base of the tail. The grip at the base of the individual’s tail was
important because if the animal struggled all the individuals’ power would come
from this area.
2.3 Measurements:
Once the animal was restrained, various morphological measurements were
recorded these included:
1) Species
2) Weight (kg)
3) Total length (cm/m)
4) Snout-vent length (SVL)
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5) Head length (cm)
6) Muzzle-eye longitude (cm)
7) Sex
2.4 Collection of Dietary data:
With every individual captured the contents of their stomach was removed using
an adaptation of the hose-Heimlich technique after Fitzgerald (1989).
Once
secured, the caiman’s mouth was opened, by gently tapping the tip of the snout
and a PVC cylinder was tied in place between the jaws. The length and diameter
of the cylinder was chosen in accordance to head length of the particular
individual.
A lubricated Teflon tube (0.5-1.5cm diameter) was then carefully
inserted down the oesophagus into the stomach where it could be felt by
palpation of the flanks.
A trickle of water through the tubing served as a
lubricant, easing its passage down the oesophagus.
Water was then poured through a funnel down the tube. The caiman’s stomach
filled with water, and with the flow maintained, a second person gently massaged
the belly. The individual was then turned upside down resulting in the expulsion
of the water and stomach contents into a bucket. The process was repeated
three times until only water was expelled by the Caiman. With the animal still
secure, the morphological measurements were taken; the entire process lasted
approximately fifteen minutes and was repeated on all Caiman caught in the
study.
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2.5 Analysis of Stomach Contents:
After removal of the stomach contents, the samples were analysed, all items
were grouped into one of ten major classes: (1) Bird (Aves) (2) Mammal
(Mammalia) (3) Parasite (Nematoda) (4) Insect (Insecta) (5) Crustacean
(Crustacea) (6) Fish (Pisces) (7) Vegetation (8) Reptile (Reptilia) (9) Gastropods
(Gastropoda), and finally (10) Non-food items (consisting of items not considered
to be ingested for sustenance, e.g. stones, string, metal etc).
Whilst in the field the designated food categories were separated from each
sample and then dried for 12 hrs, then they were weighed in grams to obtain a
dry weight. However, due to the wide variation in degree of food digestion, data
is analysed as numerical and occurrence percentages, rather than volumetric
and weight measurements. The numerical method (N%) describes the number
of items in a given prey class, expressed as a percentage of the total number of
items across all the stomachs of the species. The frequency occurrence method
(O%) describes the number of stomachs containing a particular item expressed
as a percentage of all stomach samples of the species.
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Fig 5.0 Prey categories with description
Food category
Description/example
Crustacean
Freshwater crab
carapace etc.
Gastropod
Freshwater snails, fragments of shell
etc.
Fish
Whole fish, flesh, bones, scales etc
Insect
Bird
Whole insects, exoskeletons, wing
cases etc.
Feathers, flesh, claws, bones etc.
Mammal
Flesh, bones, fur etc
Vegetation
Leaves, sticks, bark etc
Parasites
Nematode
spp,
other
stomach
parasites.
Reptilian scales, claws, teeth etc.
Reptile
Non-food items
species,
claws,
Sticks, stones, metal etc (Items not
considered
to
be
ingested
for
sustenance).
2.6 Sexing of Caiman:
In some species of Crocodilian females may have narrower snouts, and a more
slender body, but such traits are highly unreliable indicators of sex. There’s only
one realistic way to be 100% certain-looking inside the vent. To be certain of a
crocodilian’s sex, you need to either feel or visually identify the penis (male) or
clitoris (female) (http://crocodilian.com/crocfaq/faq-8.html).
In this study the vent of the individual was carefully opened with a small
lubricated pair of tweezers, this managed to expose the sex organs be it a penis
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(male) or clitoris (female). The male has a single, very obvious penis with a
fleshy head and a cartilaginous shaft. It originates from the wall of the cloaca,
directly in front of the vent on the belly side, and curls backwards so the shaft and
head lies directly beneath the vent opening. Females have a clitoris in the same
location which is quite similar in shape to the male’s penis, but it is much smaller
and not cartilaginous (http://crocodilian.com/crocfaq/faq-8.html).
3.0 Statistics:
The first stage of the analysis was calculating the numerical frequencies (N%),
and frequency occurrences (O%) of each prey class for each species.
3.1 Numerical method:
N% = number of a prey class expressed as a percentage of total number of items
across all stomachs of the species.
No of a particular prey class
No of all prey items collected for that species
x 100 = N%
3.2 Frequency method:
O% = number of stomachs containing a particular item expressed as a
percentage of all stomachs of the species.
No of stomachs containing a particular prey class
No of total stomachs sampled for the species
x 100 = O%
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3.3 Quantifying diet overlap:
The next stage was to quantify diet overlap of the two species overall, and within
the two habitats (River + Lake).
This was done using the Schoener index
(Darvishi et al 2004), and it is calculated as equation:
PSIxy = 1- 0.5 (∑ |Pxi – Pyi | )
Where:
PSIxy = overlap index
Pxi = proportion of food category (i) in the diet of species (x)
And
Pyi = proportion of food category (i) in the diet of species (y)
The result of the index ranges from a value of 0.0 to 1.0 and is considered to be
biologically significant when the index exceeds 0.60.
4.0 Results:
4.1 Morphology of Caiman Sampled:
The largest M.niger sample captured was a male animal with a total length of
1.94m, its weight could not be measured as the scales we had were not of
suitable size or strength, its weight was estimated at >30kg. This individual was
captured in Lake Tipishca in the water alongside a vegetated bank. During this
particular capture, once the animal was noosed it had to be taken to a bare bank
to be beached, as it was too large to be brought into the canoe safely. The
smallest M.niger sample captured was a female animal with a total length of
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95.5cm; its weight was 6.5 kg. This individual was captured along the Yavari Miri
River, in amongst tree fall.
The largest C.crocodilus sample captured was a male animal with a total length
of 1.82m, with an estimated weight of >30kg as again the animal was too large to
accurately weigh with our equipment. This individual was captured along the
Yavari Miri river, on a vegetated bank.
The smallest C.crocodilus sample
captured was a female animal with a total length of 69.8cm, and a weight of 1kg.
This individual was captured along the Yavari River, in the water by a vegetated
bank.
4.2 Diets of Caiman from Lago Preto:
Table 1:
Summary of Prey Categories in the Diet of 40 Spectacled Caiman overall and
within each habitat type, expressed as percentage of number (N) and frequency
of occurrence (O).
Overall
N = 126
N = 40
Habitat type
River
N = 115
N = 36
N%
O%
N%
Food category
Bird
Mammal
Parasite
Insect
Crustacean
Fish
Vegetation
Reptile
Gastropod
Non-food items
0.79
1.6
7.1
16.7
16.7
20.6
22.2
3.2
3.2
7.1
2.5
5
22.5
52.5
52.5
65
70
12.5
10
22.5
O%
0.9
1.7
7.8
17.4
17.4
18.3
21
4.4
2.6
7.8
2.8
5.6
25
56
56
58.3
67
47.2
8.3
25
Habitat type
Lake
N = 11
N=4
N%
O%
0
0
0
9.1
9.1
36.4
36.4
0
9.1
0
0
0
0
25
25
100
100
0
25
0
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I analysed stomach contents from 40 Spectacled Caiman. Spectacled Caiman
ate a variety of prey in Lago Preto. Vegetation, fish, insects and crustaceans
were the categories of prey that occurred at the highest frequencies. Vegetation
made up the highest percentage of the Spectacled Caiman sample, with fish a
close second (Table 1).
Table 2:
Summary of Prey Categories in the diet of 13 Black Caiman overall and within
each habitat type, expressed as a percentage of number (N) and frequency of
occurrence (O).
Overall
Habitat type
River
Habitat type
Lake
Food category
Bird
Mammal
Parasite
Insect
Crustacean
Fish
Vegetation
Reptile
Gastropod
Non-food items
N=38
N=13
N= 26
N=8
N=12
N=5
N (%)
O (%)
N (%)
O (%)
N (%)
O (%)
0
2.6
11
23.6
11
16
29
0
2.6
5.3
0
7.7
30.8
69.2
30.8
46.2
84.6
0
7.7
15.4
0
0
15.3
26.9
7.6
15.3
23
0
3.8
7.6
0
0
50
87.5
25
50
75
0
12.5
25
0
8.3
0
16.6
16.6
16.6
41.6
0
0
0
0
20
0
40
40
40
100
0
0
0
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I analysed the stomach contents from 13 Black Caiman. Again a variety of prey
was ingested by the Black Caiman within Lago Preto. Vegetation, insect, fish
and crustacean were the categories of prey that occurred at the highest
frequencies. Vegetation was also recorded as the highest percentage of the
Black Caiman sample, with insect highlighted as the second highest (Table 2).
4.3 Schoener Index Quantifying Diet Overlap:
PSIxy = 1 - 0.5 (∑ |Pxi - Pyi | )
Habitat
Table 3:
Dietary overlap
River
0.8
Lake
0.71
Overall overlap
0.82
The result of the index ranges from a value of 0.0 to 1.0 and is considered to be
biologically significant when the index exceeds 0.60 (Darvishi et al 2004).
Using the above scale it can be seen that the diet overlap between the
Spectacled Caiman and the Black Caiman within Lago Preto is biologically
significant, and there is some level of competition for food resources between the
two species. In relation to habitat, it can be seen there is significant diet overlap
of the two species in both habitat types.
However, the diet overlap and
competition for food resources is slightly greater within the river habitat (Table 3).
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4.4 Previous Studies from Lago Preto:
Figure 6 gives the percentage occurrence of each food category based on mean
mass data for C.crocodilus and M.niger from a study done in Lago Preto in 2005
(Swan 2005).
Figure 6.0:
Percentages of Prey Categories of C.crocodilus+M.niger
C.crocodilus
er
Ot
h
sit
e
n
Pa
ra
ta
tio
al
Ve
ge
am
m
Av
es
M
h
se
ct
In
Fis
st
ro
M.niger
Ga
Cr
u
st
ac
%
40
35
30
25
20
15
10
5
0
Prey Category
The highest diet proportions consumed by C. crocodilus were insects (21.73%)
fish (19.70%), and particularly ‘other’ food components (27.73%). The most
notable components of the ‘other’ category were found in Lake Tipisca and
consisted of species of worm (in three individuals), and even the foreleg bone of
a caiman (species unidentified), possibly indicating cannibalism.
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The diet of M. niger was found to comprise mainly of insects (37.17%) and
vegetation (31.05%), also with relatively high amounts of fish (13.15%) and
crustacean (11.11%), (Swan 2005), (Figure 6.0).
Figure 7.0:
Figure 7 gives the percentage occurrence of each food category based on mean
mass data for C.crocodilus and M.niger from another study done in Lago Preto in
2005 (Mountain 2005).
Mean percent of diet in each prey category
35
30
25
20
Mean %
15
10
5
0
C.crocodilus
Ot
he
r
Ve
g
s
h
Bi
rd
Fis
al
s
am
m
M
pt
il
es
cs
Re
ol
lu
s
st
ac
M
Cr
u
In
se
ct
s
M.niger
Prey category
The highest diet proportions consumed by C.crocodilus were insects, vegetation
and crustaceans.
The highest diet proportions of M.niger were vegetation,
insects and crustaceans. For both species fish also made up a high proportion
(Figure 8.0) (Mountain 2005).
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4.5 Caiman diet in Pacaya Samiria National Reserve, Peru:
Figure 8 shows the overall mean masses (g) of prey categories in samples of
both species, from a study done within the Pacaya Samiria National Reserve in
Peru in 2003 (Street 2003). This allows a comparison between two different
regions of Peru.
Figure 8.0:
Mean mass (g) of prey categories in stomach contents of
C.crocodilus + M.niger
1.6
1.4
1.2
1
Mean mass (g) 0.8
0.6
0.4
0.2
0
C.crocodilus
n
No
er
Ot
h
In
se
ct
h
Fis
ro
st
Ga
Cr
u
st
ac
M.niger
Prey category
It can be seen that C.crocodilus is a consumer of all prey types.
Its most
common prey type was fish with a total of 12.6 g, which accounted for the
greatest biomass in the sample. While insects account for the lowest biomass in
the sample with only 0.3g. The most common prey type for M.niger was fish,
which comprised a total of 24.2g this accounts for the greatest biomass, while
gastropod accounts for the lowest biomass of the species at 1.3g (Figure 9)
(Street 2003).
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5.0 Discussion:
Five important points must be considered when interpreting data on Crocodilian
stomach contents:
1. Different prey types are digested at vastly different rates. Jackson et al
(1974) discuss this in relation to secondary ingestion, but a more important
effect is that the frequency of occurrence of a prey type in stomach
samples will be inversely proportional to its rate of digestion (Garnett,
1985).
2. Within prey types, larger items will take longer to digest, and larger
animals will digest equivalent sized prey faster than small individuals.
3. Some prey have indigestible parts that accumulate in the crocodilians
stomach, allowing estimation of the total number of prey eaten (e.g. the
carapace of crustaceans), whereas the least digestible parts of other prey
types do not allow such estimation (e.g. mammal hair and fish scales).
4. The retention of indigestible items may depend on the overall passage
rate through the stomach, i.e. the amount of other food eaten (Magnusson
et al 1987).
5. Different prey of equivalent mass or volume may have vastly different
nutritive value for a carnivore (Lance et al 1983).
The choice of prey categories can critically influence the interpretation of results.
I have used categories that I believe may reflect different foraging methods used
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by the caiman.
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The prey categories I have used have been used in previous
major studies of crocodilian diet (Da Silveira et al 1999, Magnusson et al 1987,
Santos et al 1996). The types of prey consumed by C.crocodilus and M.niger in
the Lago Preto Conservation Concession were similar, as they are in many other
areas of the Amazon basin (Magnusson et al 1987).
In regards to the proportional analysis of the caiman’s diet, the following
conclusions can be made:
Vegetation was very common in the diets of C.crocodilus and M.niger in this
study; in fact it made up the highest percentage of both species samples.
However, the ingestion of plants by caimans within Lago Preto was probably
accidental, since plant protein is not digested or assimilated by crocodilians in the
wild (Coulson et al, 1983).
Fish was highlighted as the second highest prey category for C.crocodilus,
making up 20.6% of the sample. In contrast fish made up 16% of the M.niger
sample, which highlights fish as an important prey item for both species. The
importance of fish in both species diet was to be expected, as most other major
diet studies have demonstrated this (Silveira et al 1999, Santos et al 1996, and
Magnusson et al 1987). Insects were also highlighted as important prey items for
both species (C.crocodilus -16.7% / M.niger – 23.6%); however the significance
appeared more apparent for M.niger. Further study is required to correlate size
of caiman and insects as a prey item, as past studies demonstrate that insect’s
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significance in the caiman’s diet will reduce with greater size (Silveira et al 1999,
Santos et al 1996, and Magnusson et al 1987).
A relationship made apparent within the analysis was that fish made up a higher
part of both species stomach samples within the lake habitat. This would make
sense as fish will be easier to catch within a lake habitat. This is due to the lack
of powerful currents within an oxbow lake compared to the rivers. Also the lakes
get cut off from the main rivers during the dry season, so there will be only limited
space to hide for prey.
This relationship is supported by previous studies
(Magnusson et al 1987).
In regards to the Schoener’s index to quantify dietary overlap between the two
species, overall this study highlighted a significant overlap between the diets of
M.niger and C.crocodilus. Both habitats highlighted dietary overlap; however the
dietary overlap seemed more significant within the river habitat. The fact that a
significant dietary overlap was highlighted was to be expected as other studies
have demonstrated the similarities in the two species diet (Magnusson et al
1987).
Further investigation will be required to establish the reason behind the
higher dietary overlap within the river habitat compared to the lake. A current
suggestion to this question is that a more specialist feeding strategy is adopted
by the different species within the lake habitats. This is believed to be due to the
fact that during the dry season the oxbow lakes are often cut off from the main
rivers, and the regular incoming supply of prey stops for several months. This
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compared to the river habitats where even in the dry season there is a constant
incoming supply of prey. This enables both caiman species to target the most
nutritious prey types which most likely are the cause of the dietary overlap. This
suggestion however needs further investigation (Magnusson et al 1987).
When compared to previous dietary studies from Lago Preto, the results from this
study are very similar.
In all the studies, fish, insect and crustacean are
highlighted as important prey types for both C.crocodilus and M.niger.
The
similarities continue with vegetation making up large proportions of most of the
Lago Preto study samples for both species. However, the highest proportions of
vegetation are highlighted within the M.niger samples (Swan 2005, Mountain
2005). As vegetation is not considered to be a deliberate part of a crocodilians
diet, the higher vegetation proportions within the M.niger samples may be a
reflection of its foraging methods.
When the Lago Preto studies are compared to the results collected from the
Pacaya Samiria National Reserve; certain dietary differences become apparent.
For both species, fish is again highlighted as an important prey type. However,
regarding C.crocodilus, insect is very rarely identified within the sample and the
crustacean proportion is much lower than Lago Preto.
The M.niger sample
highlights much lower proportions of insect and crustacean. High proportions of
‘other’ and ‘non’ prey categories are highlighted for both species, especially the
‘non’ prey category with M.niger. However, it is unclear what these categories
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contain, and as vegetation was not a separate category within this study; it may
come under the ‘non’ prey category.
This may explain the high ‘non’ prey
category proportion within the M.niger sample, if correlation between the two
regions was to be assumed. The diets of the two species do differ from those in
Lago Preto. However, in Pacaya Samiria the two species diets are still very
similar, indicating dietary competition.
It has to be remembered however, that the comparative dietary studies have
used a different method of analysis to calculate their prey proportions.
The
previous studies have used the biomass weights of the prey to calculate the prey
proportions.
This is compared to this particular study which has used the
numerical and frequency occurrences of the prey items.
The difference in
analysis does not allow a completely accurate comparison, as over and under
representation of particular prey categories may affect the other studies (see
section 5.2).
5.1 Stomach Flushing Techniques:
Stomach flushing techniques have often been used to obtain specimens stomach
contents (Fitzgerald 1989).
Such techniques are useful for diet analysis of
protected species such as the Black Caiman.
The ‘hose with Heimlich
manoeuvre’ was used by myself in the field. It is also important to stress that
although methods of stomach flushing cause minor irritation, (Fitzgerald 1989)
observed no other ill effects.
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5.2 Digestion Rates:
As Caiman are unable to digest chitin, an analysis of diet based solely on prey
category weights and biomass is inappropriate.
This would lead to over
representation of certain prey groups such as gastropods, crustaceans and
insects. However, on the other hand flesh is rapidly digested by caimans, so
prey categories such as fish may be under represented (Plough 1983).
Therefore this dietary analysis was done using the numerical frequencies (N%)
and the frequency occurrences (O%) of the prey categories.
Due to the slow rates of digestion in reptiles and particularly in crocodilians, the
amount of food recovered from all caimans was low.
This would therefore
suggest that the populations of each species are unlikely to demonstrate an
impact on the regional fish populations in the area (Silveira et al 1999).
During this study the difficulty in observing animals at night was realised. This
highlighted that caiman hunting behaviour and aspects of their ecology have had
little investigation, and will need to be studied further (Silveira et al 1999).
Diets of Caiman vary depending on age, size and habitat of the individuals along
with season and prey availability (Webb et al 1982). Gorzula (1978) carried out a
study on C.crocodilus in Venezuela and identified that they consume more food
during the wet season when food is more available. Thus conducting surveys
throughout the year would allow for the calibration of seasonal differences.
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This study was just to gain an overall picture of what the two species of Caiman
are preying upon within the Lago Preto Conservation Concession.
Further
studies are required within this area, preferably with a longer period in the field to
gain a larger sample from M.niger. This would enable a better picture to be
created of exactly which prey groups the species are competing over. Also diet
of the caiman and the size of those caiman need to be combined and analysed
within Lago Preto. But again a larger M.niger sample is required, along with
other capture methods for larger individuals for the analysis to be significant.
Further comparison between Caiman populations in Lago Preto and Pacaya
Samiria needs to be done. This would allow investigation into why M.niger’s
recovery has been more successful within Pacaya Samiria.
6.0 The Future:
One of the main points that this study has highlighted is that further research on
Neotropical crocodilians is essential for their future
conservation
and
management.
Important questions need to be answered regarding M.niger within the Lago
Preto area of Peru. These important questions include:

Where is M.niger breeding?

What conditions do they require when breeding, e.g. habitat preferences?

Which areas do the large M.niger individuals inhabit within Lago Preto?
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New avenues of research on M.niger within this area may include:

Live capture of adult individuals using baited traps

The capture of these individuals would allow radio telemetry research to
be conducted if the funding is available.
This research would answer
many important ecological questions.
This type of research has proved vitally significant in the conservation and
management of saltwater crocodiles in Australia
(http://www.flmnh.ufl.edu/natsci/herpetology/act-plan/plan1998b.htm).
Regarding C.crocodilus, this study has highlighted the fact that there is a very
healthy population within the Lago Preto area. This knowledge raises issues
such as sustainable culling of the population by local human communities to raise
an income.
This option can definitely be considered as it would potentially
reduce any illegal poaching activity, and would definitely lighten the current
ecological pressure on the population of M.niger in the area.
However, this management option can only be seriously considered if the correct
monitoring and enforcement measures are put in place. Other major studies,
have demonstrated that sustainable culling of particular species can have
beneficial effects on the local ecosystem when the natural balance has been
previously disrupted by human action (Silveira et al 1998).