Exploring the main threats to the threatened African spurred tortoise Centrochelys sulcata in the West African Sahel - Exploring the main threats ...
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Exploring the main threats to the threatened African
spurred tortoise Centrochelys sulcata in the West
African Sahel
FABIO PETROZZI, EDEM A. ENIANG, GODFREY C. AKANI, NIOKING AMADI
EMMANUEL M. HEMA, TOMAS DIAGNE, GABRIEL HOINSOUDÉ SEGNIAGBETO
L A U R E N T C H I R I O , G I O V A N N I A M O R I and L U C A L U I S E L L I
Abstract The African spurred tortoise Centrochelys sulcata Keywords CITES, conservation, pet trade, Sahel, spurred
is the second largest terrestrial turtle, with a scattered distri- tortoise, tortoise–livestock competition, transhumant graz-
bution across the West African Sahel. This species is threa- ing system, wildfires
tened and declining consistently throughout its range, but
The supplementary material for this article can be found on-
little is known about the causes of its decline. It has been hy-
line at https://doi.org/./S
pothesized that the decline is attributable to () competition
with domestic cattle, () wildfire, and () the international
pet trade. We conducted a series of analyses to investigate
these three causes. Hypotheses and were analysed Introduction
using a spatially explicit approach, using a database of the
Food and Agriculture Organization of the United Nations
and logistic regression modelling; hypothesis was tested
by analysing the CITES trade database for –. We
T he African spurred tortoise Centrochelys sulcata
(Plate ) is a native species of the Sahara Desert and
the Sahel, where its distribution is fragmented (Trape
found a significant negative correlation between intensity et al., ; Petrozzi et al., , ). All populations of
of grazing (expressed as density of cattle, km−) and the this large tortoise (up to kg weight) are reported to be de-
presence of spurred tortoises, and this negative effect in- clining, mainly as a result of two independent threats: com-
creased when coupled with high fire intensity, whereas wild- petition with domestic livestock for food and space (Branch,
fires alone did not have a significant influence on the ), and collection for the pet trade (CITES, ) and for
species’ distribution at the global scale. There was a decrease subsistence (Branch, ). The species is categorized as
in the annual export of wild individuals for the pet trade Vulnerable on the IUCN Red List (Tortoise & Freshwater
after the introduction of export quotas by country and by Turtle Specialist Group, ).
year, but trade data must be considered with caution. The hypothesis that cattle are negatively affecting the
tortoise’s status and distribution had not been tested previ-
ously but is theoretically valid because of the characteristics
of the Sahel ecosystem where the species occurs. The Sahel is
a transitional ecoregion of semi-arid grasslands, savannahs
FABIO PETROZZI*, GODFREY C. AKANI, NIOKING AMADI and LUCA LUISELLI† and thorn shrublands across Burkina Faso, the Central
(Corresponding author) Department of Applied and Environmental Biology, African Republic, Chad, Eritrea, Ethiopia, Mali,
Rivers State University of Science and Technology, Port Harcourt, Nigeria
E-mail l.luiselli@ideccngo.org Mauritania, Niger, Nigeria, Senegal and Sudan (Trape
EDEM A. ENIANG Department of Forestry and Natural Environmental
et al., ; Mallon et al., ). In the Sahel, which includes
Management, Faculty of Agriculture, University of Uyo, Akwa Ibom State, several of the world’s economically poorest regions, most
Nigeria people rely on cattle grazing and derivate activities to sur-
EMMANUEL M. HEMA Université Ouaga 1 Professeur Joseph Ki ZERBO/CUP-D, vive (Touré et al., ; Barry et al., ; Kagoné, ).
laboratoire de Biologie et Ecologie Animales, Ouagadougou, Burkina Faso
Two main types of grazing systems tend to coexist in coun-
TOMAS DIAGNE African Chelonian Institute, Ngaparou-Mbour, Senegal tries of Sahelian Africa: traditional extensive systems and
GABRIEL HOINSOUDÉ SEGNIAGBETO Department of Zoology, Faculty of Sciences, modern, semi-intensive to intensive ones (Kagoné, ).
University of Lomé, Togo
Traditional systems are low-input systems, with no supple-
LAURENT CHIRIO Grasse, France mented food or chemicals being used except during emer-
GIOVANNI AMORI National Research Council (CNR) Institute of Ecosystem gency periods when forage availability is scarce. They
Studies, Rome, Italy
include the transhumant Fulani system (c. % of total cattle
*Also at: Ecologia Applicata Italia srl, Rome, Italy stock), sedentary village stock-raising, and stock rearing in
†Also at: IDECC–Institute for Development, Ecology, Conservation and
Cooperation, via G. Tomasi di Lampedusa 33, I-00144 Rome, Italy developed pastoral areas. In the Fulani system, herds may be
Received June . Revision requested July . single-species (Sudanian Fulani zebu) or mixed, with small
Accepted September . First published online January . ruminants and cattle, and cattle are fed by opportunistic
Oryx, 2018, 52(3), 544–551 © 2017 Fauna & Flora International doi:10.1017/S0030605316001125
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. https://doi.org/10.1017/S0030605316001125Threats to the Sahel tortoise 545
were poor and were not considered in the analysis, although
spurred tortoises do occur in these countries (Largen &
Spawls, ).
From these data we deleted records of specimens that
were not observed directly in the wild, as indicated in
Petrozzi et al. (, ), or from areas where the species
is now extinct (e.g. Cameroon; Chirio & LeBreton, ).
For each presence point we used the longitude and latitude
graticule used by Trape et al. (), in which each grid cell
was , km ( × km), in QGIS .. (QGIS
Development Team, ). We deleted grid cells with . %
of the area covered by sea, leaving cells (n = ).
Using QGIS we drew a minimum convex polygon inclu-
PLATE 1 An adult Centrochelys sulcata in the ‘W’ National Park, sive of all presence points (n = ). Within this we created
Niger, West Africa. Photograph by B. Cornelis.
random absence points, using QGIS functions (Fig. b).
When randomly selected absence grid cells coincided with
exploitation of forage resources according to a yearly se- presence cells the cells were replaced with new, random ab-
quence of grazing involving five seasons and the various sence cells. Square grid cells (Trape et al., ) were used for
types of pasture available (Barry et al., ; Kagoné, both the presence and absence points.
). Both this system and modern intensive grazing are Livestock density distribution data were extrapolated
invasive for the natural vegetation of the Sahel and, as a con- from Robinson et al. (). We clipped a raster of cattle
sequence, large areas have been altered (Kagoné, ) and density (Robinson et al., ) with a shape file covering
many species are threatened (Mallon et al., ). the whole of the species’ range, in which both presence
Sahelian pastoralists burn wide areas to facilitate cattle and absence grid cells were included (Fig. c). We then re-
grazing, and fires also spread naturally throughout the classified the resulting raster file using the FAO format, with
Sahelian region (Behnke, ; Smet & Ward, ; eight classes of density for livestock, yielding eight raster
Solomon et al., ). It is predicted that the spread of files. These were transformed into shape files, which were
fires may have a negative effect on the distribution of then merged into a single shape file. This was clipped for
spurred tortoises throughout the region. each presence and absence grid cell.
The hypothesis that the international pet trade may be a All the shape files (classified shape files, merged shape
factor in the depletion of spurred tortoise populations had file, and presence and absence grid cells with cattle density)
not previously been analysed in depth, as it is difficult to were processed using SpatiaLite v. .. (Furieri, ) to
test with field data. Nonetheless, CITES data are available correct potential invalid geometries, thus obtaining an out-
and may provide some insight into potential conservation put file (.sql) for each grid cell. The .sql files were imported
issues. into QGIS and saved as a new shape file to obtain the surface
Our aims were () to explore the tortoise–livestock com- of all the classes, both for each grid cell and in total. From
petition hypothesis by evaluating whether the distribution the surface of each livestock class in each grid cell we calcu-
of spurred tortoises is negatively correlated with the density lated the percentage of cell territory covered by the various
of domestic livestock, () to analyse whether there is an ap- classes of livestock density. Eight classes of livestock density
parent effect of the spatial distribution of fires on the distri- were used, as also used by FAO. Classes of predicted live-
bution of tortoises, and () to analyse the CITES database to stock density (expressed as number of animals per km),
identify general patterns in the international trade of as obtained from the FAO dataset (FAO, ) and used
Centrochelys sulcata. in our analyses, were: –; –; –; –; –; –;
–; . .
Methods Fire analysis
Data sources and geographical information system (GIS) The distribution of fire across the Sahelian region was calcu-
procedures lated using historical MODIS (Moderate-resolution Imaging
Spectroradiometer) data (NASA, ) for –; fires
We extracted distribution data for spurred tortoises in West detected covered at least , m. A point shape file was
Africa from the literature (Trape et al., ; Petrozzi et al., clipped, as for cattle distribution, with each presence cell
, ; Fig. a) and from original records collected by and each absence cell. In each cell, we determined the
ourselves. Available data for Sudan, Ethiopia and Eritrea mean number of fires per day.
Oryx, 2018, 52(3), 544–551 © 2017 Fauna & Flora International doi:10.1017/S0030605316001125
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. https://doi.org/10.1017/S0030605316001125546 F. Petrozzi et al.
FIG. 1 Location of (a) presence and absence grid cells in a survey of the African spurred tortoise Centrochelys sulcata across the West
African Sahel, with the reference numbers used in Table S; (b) the potential distribution of cattle density, and the six sites with the
highest recorded densities of spurred tortoises (white circles); and (c) the spatial distribution of fire intensity. Black grid cells in (a)
were excluded from analyses.
Oryx, 2018, 52(3), 544–551 © 2017 Fauna & Flora International doi:10.1017/S0030605316001125
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. https://doi.org/10.1017/S0030605316001125Threats to the Sahel tortoise 547
Trade analysis student t-test, with α set at %. One thousand bootstrap
samples were calculated, and contrasted with real data. As
We extracted all raw data available for spurred tortoises for the inequality of variance was found in the dataset (Levene’s
period – from the CITES trade database (CITES, test, F = ., P , .), equality of variance was not as-
). Each record of import/export included the country of sumed in the analysis.
origin, whether the tortoise had been bred in captivity or For the trade analysis, the correlation between (log) num-
taken in the wild, the purpose of the trade (e.g. commercial, ber of traded individuals and year was examined using a poly-
scientific), and whether specimens were exported dead or nomial model. The Akaike information criterion (AIC) for
alive, and whole or as parts (e.g. skins, shells, meat). It would this relationship was calculated.
be naïve to assume that all species trade in the study area was
legal, based on national quotas recorded in the CITES datasets.
Many specimens are collected illegally in the countries of the Results
Sahel and exported to Togo for the international pet trade
(Auliya et al., ), and the species is also sought for the Tortoises and cattle
local pet trade (Petrozzi et al., ).
The geographical distribution of presence sites in relation to
the density of cattle across West Africa is shown in Fig. a,b.
Statistical analyses In general the density of cattle per grid cell tended to be
higher in the southern part of the tortoise’s distribution,
All statistical analyses were conducted using PASW .
in particular in the cells located in Nigeria and southern
(IBM, Armonk, USA). For each presence and absence grid
Burkina Faso (where classes , and were predominant),
cell the proportionally dominant cattle density category
as well as in southern Mali (where classes and were predom-
(Supplementary Table S) was used as a proxy for the overall
inant, with some small extensions of class ). The grid cells in
effect of cattle on tortoises in the target grid cell; for ex-
southern Senegal also showed a relatively broad extension of
ample, if in a given grid cell the proportionally highest cattle
classes and . In Chad only one of the presence grid cells
density category was the fourth (i.e. . x $ per km),
showed a broad extension of classes and . The northern
the value was entered for that grid cell in the covariate
part of the tortoise’s distribution was characterized by a con-
variable of the logistic regression analysis. Presence/absence
comitant low cattle density (classes , and, in small part, ).
grid cells were then entered as dependent binary variables
Mean cattle density, expressed in terms of the dominant
( = absence of tortoises in a given grid cell; = presence
cattle density category in a given grid cell, was significantly
of tortoises in a given grid cell) and their cattle densities
higher (t-test: P , .) in absence grid cells (x = . ± SD
as covariates in a logistic regression model. Although the
., n = ) than in presence cells (x = . ± ., n = ). At
predominant cattle density is a good indicator for a first ana-
the six sites where our surveys found higher densities of
lysis, with eight categories this dominant category may actu-
spurred tortoises (authors, unpubl. data) the corresponding
ally represent a tiny surface compared to the surface
cattle density values were always – individuals per km
occupied by the other seven categories even if all of the
(Fig. b).
seven occupy surfaces that are smaller than that of the pre-
In our logistic regression .% of cases were classified cor-
dominant category. This introduces a bias. To prevent this
rectly (i.e. assigned correctly as random or used basking sites)
we also calculated an indicator score by dividing the per-
at step (i.e. no variables in the model, and before adding the
centage of occurrence of the predominant category by the
variable cattle density, which improved the model) and .%
raw proportion of the surface it covers in the grid cell.
of cases at step (i.e. after the final model was assessed by add-
This indicator score was then inserted into a logistic regres-
ing the variable cattle density, which significantly increased the
sion model in which the presence/absence of tortoises was
model fitting). Overall, the model was statistically significant
the dependent variable. The higher the score, the higher
(B = –., SE = ., Exp(B) = ., Wald = ., df = ,
the impact of grazing was, both in intensity and surface
P = .).
(per grid cell). For each presence and absence grid cell the
fire intensity value was entered in a logistic regression
model, as covariate, having verified that cattle density and Tortoises and fires
fire intensity were not collinear in each cell (P . .).
We used the forward stepwise model, which adds the sig- The spatial distribution of fire intensity is presented in
nificant variables one-by-one to the model, starting with no Fig. c. Our logistic regression model showed that % of
variables (step ) and testing the addition of the variables of cases were classified correctly (i.e. assigned correctly as ran-
interest (cattle density and fire intensity) to improve the dom or used basking sites) at step (i.e. no variables in the
model. Differences in mean cattle density between presence model, and before adding the variable cattle density, which
and absence grid cells were analysed using a two-tailed improved the model) and .% of cases at step (i.e. after
Oryx, 2018, 52(3), 544–551 © 2017 Fauna & Flora International doi:10.1017/S0030605316001125
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. https://doi.org/10.1017/S0030605316001125548 F. Petrozzi et al.
without indication of the country of origin (the country of
export is reported). Ghana, Togo, Benin, Mali and Sudan
were the main countries of export reported for the species.
The number of wild individuals traded decreased signifi-
cantly from (when export quotas were introduced),
from a mean of . ± SD . per year during –
to a mean of . ± SD . per year during –
(Mann–Whitney U-test, z = –., P , .). A het-
erogeneity of slopes test showed that the slopes of the regres-
sion lines of number of exported individuals vs year were
negative for wild individuals of known and unknown origin,
and were significantly different between wild individuals of
known origin and wild individuals of unknown origin (one-
way ANCOVA: F = ., df = ,, P , .).
Discussion
FIG. 2 Relationship between log number of traded individuals of Spurred tortoises, cattle density and fire intensity
Centrochelys sulcata and year (–). Data are from the
CITES trade database (CITES, ). The cell size in our grid of tortoise distribution ( × km)
is larger than ideal. For this type of study an ideal grid cell
the final model was assessed by adding the fire intensity size would have been × km, as is typically used in
variable and then the cattle density × fire intensity inter- modern atlases of vertebrate distribution. However, we
action variable, which significantly increased the model fit- were limited to the larger grid cell size because this was
ting). Overall, fire intensity alone did not influence the the size used for tortoise presence records in Trape et al.
model significantly (B = ., SE = ., Exp(B) = ., (). Our results should therefore be interpreted with
Wald = ., df = , P = .), whereas the cattle density × some caution.
fire intensity interaction variable was statistically significant Our study provided correlational evidence that livestock
(Wald = ., P = .). When the indicator score of live- density affects the presence of spurred tortoises throughout
stock density was entered in the logistic regression model it their range in the West African Sahel. This was shown by
was found that higher values of the score had a significant two kinds of analyses (a statistical test of the differences in
influence on the absence of the species, thus confirming pat- mean livestock density between areas where tortoises are
terns explained above (B = –., SE = ., Exp(B) = ., present and absent, and a logistic regression model in
Wald = ., df = , P , .). which the addition of the cattle density variable significantly
enhanced the fit of a model of tortoise distribution, with a
Tortoises and international trade high density of cattle significantly depressing the probability
of presence of the spurred tortoise). As the analysis was per-
According to the CITES database –, spurred tortoises formed at the global scale of the tortoise’s range, and surveys
were traded annually during –. In total, , wild in- were probably more intensive in the more accessible areas of
dividuals were traded during this period (yearly x = . ± SD this logistically problematic and socio-politically unsafe re-
.). There was a negative polynomial correlation between gion (i.e. in areas where livestock density may have been
year and (log) number of traded individuals (r = ., n = , higher), the negative correlation between cattle density
P , .; y = –.x – .x + .x + .; AIC = .; and spurred tortoise presence is likely to be correct.
Fig. ). However, the relationship was clearly heteroscedastic, Circumstantial evidence has shown that the few sites
with less variation in more recent years. where spurred tortoises are still abundant are characterized
A considerable portion of specimens (.%) came from by a total absence of domestic cattle (e.g. in the ‘W’ National
countries where the species does not occur (e.g. Togo, n = ; Park and the Termit Massif Total Reserve in Niger, where
Ghana, n = ,) or where it is extinct (Cameroon, n = ; there is no livestock grazing pressure, LC encountered
Chirio & LeBreton, ). Thus, we may conclude that for c. tortoises per day).
many of the individuals traded as wild, the true country of ori- The low density of spurred tortoises in areas where live-
gin is unknown. It appears that specimens traded from Togo stock are present may be a direct result of interspecific com-
originate in Burkina Faso, Niger and Mali (Petrozzi et al., ). petition for food and habitat but may also be a result of
According to the CITES () database , individuals hunting by livestock rearers. The tortoises are eaten by peo-
(.% of the total traded) were reported to be wild but ple throughout most of the Sahel, including in Burkina Faso
Oryx, 2018, 52(3), 544–551 © 2017 Fauna & Flora International doi:10.1017/S0030605316001125
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. https://doi.org/10.1017/S0030605316001125Threats to the Sahel tortoise 549
(Petrozzi et al., ), the Central African Republic, Mali Mali appears to have experienced the highest losses to the
and Niger (LC, pers. obs.). Contemporary grazing patterns pet trade, with specimens exported by Ghana, Niger, Togo
necessitate continual bush burning by livestock rearers and even Hungary and the USA declared to be of wild origin
(Behnke, ; Smet & Ward, ; Solomon et al., ), in Mali (a total of , tortoises in and in ;
and wildfire is known to be a threat to free-ranging popula- CITES, ). The decline of the species in Mali was there-
tions of tortoises elsewhere (e.g. Hailey, ; Esque et al., fore predictable (Lambert, ), and new population sur-
; Bertolero et al., ; Santos & Cheylan, ) because veys are needed urgently.
of its indirect effects on desert habitats, which can result in As the species has a scattered distribution (e.g. Diagne,
changes in dietary composition and loss of vegetation cover, ; Arvy et al., ), and most specimens originate from
an increase in predation and loss of protection from tem- single collection localities (G.H. Segniagbeto & L. Luiselli, un-
perature extremes (Esque et al., ). Our logistic regres- publ. data), it is essential to collect data on population trends
sion modelling showed that fire intensity decreased the at exploitation sites in order to minimize the risk of further
probability of finding a tortoise population within a given local extinctions (Chirio & LeBreton, ). The majority
grid cell even more than the density of cattle alone, although of so-called wild specimens of Centrochelys sulcata traded
fire intensity alone did not determine the presence/absence during – were of unknown origin. This raises ques-
of tortoises within a given grid cell. Future studies should tions about the reliability of the CITES database and should
verify experimentally the competitive mechanism between be taken into consideration in future analyses of trade in
spurred tortoises and domestic livestock, to guide more pre- spurred tortoises and other chelonians.
cisely the conservation management actions for this threa- The putative impact of illegal trading in this tortoise spe-
tened species while minimizing negative impacts on cies is considerable, as the trade of reptiles from north-west
impoverished human populations throughout the Sahel re- Africa is mostly illegal (and hence not recorded by the
gion. Where cattle are present, people are too, and they may CITES database), facilitated by hyperporous geographical
eat tortoise meat (Petrozzi et al., ). Thus, grazing may boundaries between countries and social instability. In
not be the only cause of the tortoise’s decline, and other fac- some regions spurred tortoises are eaten regularly (Petrozzi
tors are also relevant (e.g. habitat loss, wild and human- et al., ), which could heighten the risk of extinction.
made fires, hunting, and competition with livestock).
Conclusions
Spurred tortoises and the international pet trade
We analysed three factors that could be threatening wild po-
Data on the potential threat to the spurred tortoise from the pulations of C. sulcata throughout its range: competition for
international pet trade are incomplete. In particular, the sig- food and space with domestic livestock (Branch, ),
nificant variation in the number of tortoises traded legally wildfire intensity, and the international pet trade (CITES,
each year suggests that the CITES database failed to record ). We found () a clear negative correlation between in-
all the tortoises exported from the study region. tensity of grazing (expressed as density of cattle per km)
It appears that the number of wild individuals traded has and presence of spurred tortoises, () an increased detri-
been decreasing over the years and that the number of wild mental effect of cattle density when coupled with high fire
individuals for which the country of origin is unknown is intensity, and () a consistent decrease in annual exports
now approaching zero. The number of wild individuals of wild individuals for the pet trade following the introduc-
traded during – is remarkable for a large tion of export quotas by country and by year, possibly be-
K-selected species with delayed sexual maturity (Branch, cause the trade moved partially from legality to illegality
). The reduction in yearly exports is probably attribut- (i.e. given the increased difficulty in exporting wild indivi-
able to multiple causes, including the establishment of export duals, such exports may now be hidden). Comparatively,
quotas by country and by year, and possibly the reduced domestic livestock appear to be a greater threat to the tor-
number of individuals remaining in the wild. Experienced toise than fire or international trade, as there is no evidence
reptile dealers in Lomé, Togo, who usually trade the tortoises that management of grazing has been better in recent years
with Europe or the USA, reported to us that it is increasingly compared to the previous decades across the Sahel region.
difficult to obtain the species from local hunters because it is On the other hand, the number of wild individuals exported
considerably rarer than in the recent past in the traditional for the pet trade appears to be relatively low at present, al-
places of capture in Burkina Faso, Mali and Niger (G.H. though monitoring should continue. We recommend: ()
Segniagbeto & L. Luiselli, unpubl. data). The spurred tortoise precautionary listing of the spurred tortoise on Appendix
is now apparently rare in nearly all sites where it is known to I of CITES, to prevent further exploitation of wild indivi-
occur (e.g. Petrozzi et al., , ), apart from a few remote duals for the international pet trade, as already suggested
semi-desert areas of Niger and Mali (L. Chirio, unpubl. data). by the CITES authorities (CITES, ); () that more
Oryx, 2018, 52(3), 544–551 © 2017 Fauna & Flora International doi:10.1017/S0030605316001125
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. https://doi.org/10.1017/S0030605316001125550 F. Petrozzi et al.
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Some of the data were recorded during field surveys funded by University of Arizona Press, Tucson, Arizona.
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rangeland management practices and perceptions of in the Sahelian region. ED E M EN I A N G , GO D F R E Y AK A N I and
pastoralists towards rangeland degradation in the Borana zone NI O K I N G AM A D I work on the ecology and conservation of Nigerian
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D I S S E T , R. () Etude générale de l’élevage au Burkina Faso. FAO, conservation and biology of mammals and reptiles in Africa and
Ouagadougou, Burkina Faso. elsewhere. LU C A LU I S E L L I works on the community ecology
T R A P E , J.F., C H I R I O , L. & T R A P E , S. () Lézards, crocodiles et tortues and conservation biology of West African vertebrates, particularly
d’Afrique occidentale et du Sahara. IRD Editions, Paris, France. reptiles.
Oryx, 2018, 52(3), 544–551 © 2017 Fauna & Flora International doi:10.1017/S0030605316001125
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