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125
Diet of Hoplobatrachus occipitalis
All articles available online at http://www.salamandra-journal.com
© 2011 Deutsche Gesellscha für Herpetologie und Terrarienkunde e.V. (DGHT), Rheinbach, Germany
SALAMANDRA 47(3) 125–132 20 August 2011 ISSN 0036–3375
e diet of the African Tiger Frog, Hoplobatrachus occipitalis,
in northern Benin
M H M-O R
Museum für Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity at the Humboldt University Berlin,
Invalidenstr. 43, 10115 Berlin, Germany
Corresponding author: M-O R, e-mail: mo.roedel@mfn-berlin.de
Manuscript received: 27 May 2011
Abstract. e worldwide decline of amphibian populations calls for studies concerning their ecological role within eco-
systems and only knowledge about amphibian species’ diets may facilitate the identication of their respective position in
trophic cascades. Frog consumption by humans has recently increased to a considerable extent in some parts of West Af-
rica. We analyse herein the diet of the most commonly consumed frog species, Hoplobatrachus occipitalis (Dicroglossidae),
in Malanville, northern Benin. In order to determine its prey spectrum we investigated stomachs of frogs obtained from
frog hunters, and stomach-ushed frogs caught by ourselves. Overall, we investigated the gut contents of individuals (
ushed, dissected), of which had empty stomachs. We identied Coleoptera, Lepidoptera and Formicidae as the
most important prey categories in ushed frogs and Pisces, Coleoptera and Araneae in collected frog stomachs. Accord-
ing to these data, H. occipitalis is an opportunistic forager, able to predate on terrestrial as well as on aquatic taxa. e prey
spectrum revealed by the two dierent sampling methods diered only slightly. In contrast, the frequency of particular prey
categories (e.g., sh) diered strongly. ese dierences were most probably method-based, rather than reecting dierent
prey availability among capture sites.
Key words. Amphibia, Anura, Dicroglossidae, diet, savanna, stomach ushing, West Africa.
Introduction
In addition to climate change (B et al. ),
habitat degradation and conversion (C ,
E et al. ), and diseases (D et al. ),
the overharvesting by humans may play a major role in
the decline of amphibian populations (S et al.
, W et al. ). Since amphibians are im-
portant components in trophic cascades, vanishing popu-
lations may have crucial eects on ecosystem function-
ing (H , M R ). How-
ever, only detailed ecological and biological data, such as
diet, can facilitate estimating these eects (D ,
W et al. ).
Anuran larvae are mostly grazers or suspension feeders
(R et al. , A et al. ), whereas adults
predominantly prey on various invertebrates and, in re-
lation to their sizes, sometimes vertebrates. Studies con-
cerning the diet of adult amphibians have been conducted
for various species in almost all regions of the World; e.g.,
the Neotropics (W D , L et
al. ), Asia (H M b, Y et al. ),
Australia (L S ), temperate Amer-
ica (M J , H et al. ), and
Europe (B S , Kovács et al. ).
Although studies on African anurans are comparatively
rarer, the diets of several species have been studied (I-
M , H , B M
, K et al. ).
e large aquatic African Tiger Frog, Hoplobatrachus
occipitalis (G, ) inhabits savannas in sub-
Saharan Africa. It is known to be consumed by humans
in several countries (e.g., Benin, Burkina Faso, Ghana,
Guinea, Ivory Coast, Nigeria). e intensity of exploita-
tion of this species as human food varies among regions as
well as between dierent ethnic groups (M et al.
). e focus of our study was the town of Malanville
in northern Benin, where the collection of H. occipitalis
for local and regional food markets is especially intense,
i.e., a group of Nigerian frog hunters collected approxi-
mately frogs within two months (M et al.
). is intense exploitation seems to be unsustainable
and may lead to the local decline of H. occipitalis. is
could have ecological consequences. An Asian Hoploba-
trachus species, H. tigerinus is known to ingest as much
as of its own weight in insects per day (A
). In India, the collection of frogs’ legs resulted in the
survival of tonnage of insects, including disease-carrying
mosquitoes and agricultural pests (O ). Larval H.
occipitalis are opportunistic carnivores and mainly feed
on other tadpoles and mosquito larvae in temporary sa-
vanna ponds (S L , R ).
Detailed knowledge about the amount and composition
of the adults’ diet is so far limited to two studies from Sen-
egal (L ) and Ivory Coast (T ). e
aim of our study was to a) identify the diet of H. occipitalis
in northern Benin, as well as to b) evaluate the eciency
and accuracy of stomach ushing versus dissection.
126
M H M-O R
Material and methods
Study site and period
e study took place in northern Benin, close to the bor-
ders of Burkina Faso, Niger and Nigeria. All specimens
were collected in close proximity to the river Niger, near
Karimama (N °.’; E °.’) and Malanville
(N °.’; E °.’). is region is characterized
by a dry Sudan savanna with an annual precipitation of ap-
proximately mm (Faoclim – worldwide agroclimatic
data base, http://freegeographytools.com//fao-world-
climate-data). First rainfalls normally occur in late June
and last until mid-September. Our data were collected at
the end of the dry season (May and June) in .
Stomach contents
ere are various approaches to investigating the diet of
amphibians. Individuals can be directly observed (mostly
very dicult) or faeces can be collected (many prey items
may be completely digested or unrecognisable). However,
the two most common methods are dissection and stom-
ach ushing. ere are only a few studies comparing these
both methods directly (L C , W et
al. ). We applied both methods. No frog was killed for
this study.
Stomach ushing. Adult Hoplobatrachus occipitalis were
captured during the night ( to h) in ooded rice
paddies, at riversides and temporary ponds, and immedi-
ately (within one hour aer capturing) stomach ushed on
site, as described in S et al. (). We recorded weight
(spring scale: to g, accuracy: ± . g), snout–vent
length (digital dial calliper: to mm, accuracy: ± .
mm), and sex of each frog. For ushing, we used two types
of exible PVC tubes with diameters of mm for larger and
mm for smaller frogs. e tube was attached to a syringe,
which was lled with pond or river water (according to the
respective capture site). Regurgitated items were stored in
formalin () and later transferred to ethanol (). At the
beginning of our study, we kept ushed frogs in plastic
containers for three days to check for potential complica-
tions due to the stomach ushing (e.g., injuries, death). All
these frogs survived unharmed. All other individuals were
immediately released aer ushing.
Stomach sampling. Stomachs of H. occipitalis were
collected in the rice paddies of Malanville on three dierent
days. e respective frogs had been killed by Nigerian frog
hunters during the preceding nights (sampling time:
to h) by beating the frogs to death with long wooden
sticks (M et al. ). e hunters gutted the
frogs in the early morning and allowed us to separate the
stomachs from the remaining innards. Each of these frogs
was weighed with a spring scale (details see above) before
they were disembowelled by the hunters. e stomachs
were xed in formalin ().
Diet analyses
e stomach content of each frog was examined individ-
ually. e collected stomachs were sliced lengthwise to
extract the entire stomach content. All items were trans-
ferred into ethanol () for subsequent identication.
Prey items were counted, identied to a particular taxo-
nomic category (mostly order level), and length, width and
depth were measured (to the nearest . mm) with a dig-
ital calliper under a dissecting microscope. e volume of
completely preserved food items was calculated using the
formula of a prolate ellipsoid V = /π (½ × length) × (½
× width)². e original volumes of partly digested Formi-
cidae were calculated with the regression method as de-
scribed by H & M (a).
In order to characterize the diet of H. occipitalis in gen-
eral, several indices, adopted from diet studies in sh and
amphibians, were used (e.g. P et al. , G et
al. , D et al. ). Each calculation was done for
stomach ushing and sampling, respectively. e number
of stomachs containing a particular prey category was eval-
uated as the frequency of occurrence (FOi) and the propor-
tion of FOi [FOi = (FOi/nstomachs with diet) × ]. For each
prey category, we calculated the total volume as the sum
of all prey items of category i (Vi) as well the proportion of
Vi relative to the total volume of all measured food items
[Vi = (Vi/ΣVi...n) × ]. e importance of each prey cat-
egory related to the entire range of food items in all sam-
ples was identied via the index of relative importance
IRIi = (Ni + Vi) × FOi (P et al. ). We used
the Mann-Whitney U-test to search for potential prey dif-
ferences in relation to the frogs’ sizes, weights and sex and
to compare the two methods. e χ²-test was chosen to
compare frequencies. For correlative analyses, we applied
the Spearman-Rank correlation. All statistical analyses
were conducted with R .. (http://www.r-project.org).
Results
Investigated animals
frogs were ushed and the stomachs of individuals
were collected. e weight of the frogs ranged between
and g (mean ± SD: . ± . g, n = ), with ushed
frogs being slightly lighter (stomach sampling: range –
g, . ± . g, n = ; stomach ushing: range –
g, . ± . g, n = ). However, this dierence was
not signicant (Mann-Whitney U-test: W = , p= .,
nsampling = , nushing = ). Snout–vent length was only
measured in ushed individuals (range .–.mm;
mean ± SD: . ± . mm, n = ). Since size and
weight were tightly correlated (Spearman-Rank correla-
tion: rs = ., p < ., n = ), we used only weight for
subsequent analysis. Due to the killing method of the frog
hunters (see M et al. ), heads were damaged
and vocal sacs of male frogs were oen not recognizable.
Sex was thus only identied in ushed frogs (nmales = ,
nfemales = , two frogs were too young for sexing). In ushed
frogs, the sexes diered neither in size (Mann-Whitney
U-test: W = , p= ., nmales = , nfemales = ) nor in
weight (W = ., p = ., nmales = , nfemales = ).
Stomach lling
In total, . (sampling: .; ushing: .) of the
frogs had empty stomachs. e proportion of stomachs
127
Diet of Hoplobatrachus occipitalis
containing prey items did not dier between the two meth-
ods (χ²-test: χ² = ., df = , p = ., nstomach sampling = ,
nstomach ushing = ). Whether a stomach contained prey items
or not was independent from weight (all samples: Mann-
Whitney U-test: W = ., p = ., n = , stomach
sampling: W = , p = ., n = ; stomach ushing:
W= , p = ., n = ), size (stomach ushing only:
W = ., p = ., n = ), as well as from sex (stomach
ushing only: χ²-test: χ² = ., df = , p = ., nmales = ,
nfemales = ).
Overall, we recorded prey items, were gathered
by stomach sampling and by stomach ushing. e
number of items ranged between one and per stomach
(. ± ., n = , sampling range: –, . ± ., n=;
ushing range: –, . ± ., n = ). ere was no dif-
ference in the number of items found per stomach re-
lated to sex (stomach ushing only: W = ., p = .,
n=), nor to sampling method (Mann-Whitney U-test:
W=., p = ., n = ). We detected no correlation
between the number of food items and the weight of the
frogs (rs = -., p = ., n = ).
Volume of food items
Stomach contents at a more advanced stage of digestion
became increasingly dicult to identify both taxonomi-
cally as well as in volume. In order to avoid wrong catego-
rizations and misestimates in body sizes of the prey items
we included solely feebly digested or nearly intact items
for the content analysis. erefore, no volume calculations
were carried out in . of the prey items in the ushed
and in . in the collected stomachs. Flushed, single prey
items were signicantly smaller in size than those collect-
ed through dissection (sampling range: .–. mm³,
. ± .; ushing range: .–. mm³, .
± .; Mann-Whitney U-test: W = , p < .,
nsampling = , nushing = ; Figure ). When testing all frogs
or those that had been collected by the frog hunters only,
the mean size of prey was not correlated to the frogs’ weight
(Spearman-Rank correlation: all frogs: rs = ., p = .,
n = ; frog hunters samples: rs = ., p = ., n = ).
Prey size was weakly negatively correlated to the weights of
the ushed frogs (rs = -., p < ., n = ).
e volume of stomach contents (sum of all measured
preys items in one stomach) ranged between . and
. mm³ (. ± . mm³, n = ). Stomachs of
frogs that had been collected by the frog hunters contained
signicantly more prey volume than ushed frogs (stomach
ushing: range .–. mm³, . ± .mm³,
n= ; stomach sampling: range .–.mm³, .
± . mm³, n = ; Mann-Whitney U-test: W= ,
p < ., n = ). e volume of stomach contents was
positively correlated to the H. occipitalis weight, irrespec-
tive of the method (Spearman Rank correlation: rs= .,
p < ., n = ).
Prey composition
Most prey items were arthropods such as spiders or insects,
but vertebrates, i.e., sh and amphibians, were also com-
mon. e most common prey of ushed frogs were ter-
mites, whereas beetles, due to their frequency of occur-
rence and greater individual size, were the most important
prey animals. Ants and adult moths also had a high index
of relative importance in the ushed frogs. In the collected
stomachs, sh were the most common and most important
Figure 1. Volume of Hoplobatrachus occipitalis prey. Given are the calculated volumes of all measured food items in mm³ of collected
and ushed frog stomachs. e scale on the Y-axis is logarithmic; nsampling = 465, nushing = 430.
Volume of prey items [mm³]
.
. stomach ushing stomach sampling
128
M H M-O R
Table 1. Stomach contents of Hoplobatrachus occipitalis. N = number of prey items, Nm = number of measured items used for volume
calculations, N% = percentage of N, FO = frequency of occurrence, FO% = percentage of FO, V = sum of volume of prey items in
mm³, V% = percentage of V, IRI = index of relative importance, prey categories comprising several stages are divided in adults (a),
larvae (l) and egg (e); the three most important prey categories are highlighted; number of stomachs containing prey items: stomach
ushing = 65, stomach sampling = 166.
stomach ushing Prey animals N NmN% FO FO% V V% IRI
ANNELIDA
Oligochaeta 1 1 0.19 1 1.54 111.17 0.44 0.96
ARTHROPODA
Arachnida
Araneae 28 25 5.18 12 18.46 283.14 1.12 116.25
Crustacea
Decapoda 2 0 0.37 2 3.08 na na na
Amphipoda 1 1 0.19 1 1.54 20.70 0.08 0.41
Insecta
Blattodea 1 1 0.19 1 1.54 51.69 0.21 0.60
Coleoptera (a) 75 55 13.86 29 44.61 5819.27 23.04 1646.61
Coleoptera (l) 2 2 0.37 2 3.08 61.02 0.24 1.88
Dermaptera 7 6 1.29 6 9.23 679.16 2.69 36.77
Diptera (a) 9 9 1.66 4 6.15 30.34 0.12 10.98
Diptera (l) 3 3 0.56 1 1.54 69.40 0.28 1.28
Hemiptera 5 5 0.92 5 7.69 102.33 0.41 10.23
Hymenoptera
Formicidae 38 38 7.02 12 18.46 304.55 1.21 151.94
others 5 3 0.92 3 4.62 145.64 0.58 6.93
Isoptera 91 87 16.821 5 7.69 325.48 1.29 139.30
Lepidoptera (a) 62 29 11.46 19 29.23 3853.58 15.26 781.05
Lepidoptera (l) 6 6 1.11 6 9.23 234.01 0.93 18.79
Odonata (a) 2 1 0.37 2 3.08 298.01 1.18 4.77
Odonata (l) 1 1 0.19 1 1.54 117.67 0.47 1.00
Orthoptera 9 7 1.66 6 9.23 982.44 3.89 51.27
Phasmatodea 6 5 1.11 5 7.69 164.67 0.65 13.55
unidentied (a) 29 0 5.36 24 36.92 na na na
unidentied (e) 55 55 10.17 1 1.54 8.40 0.03 15.69
Myriapoda
Diplopoda 2 2 0.37 2 3.08 998.98 3.96 13.31
MOLLUSCA
Bivalvia 1 1 0.19 1 1.54 88.31 0.35 0.82
Gastropoda 1 1 0.19 1 1.54 58.50 0.23 0.64
VERTEBRATA
Amphibia
Anura (a) 2 2 0.37 2 3.08 7081.62 28.04 87.42
Anura (l) 82 82 15.16 1 1.54 1320.87 5.23 31.37
Mammalia 1 0 0.19 1 1.54 na na na
Pisces 11 2 2.03 9 13.85 2042.34 8.09 140.13
unidentied 3 0 0.56 3 4.66 na na na
TOTAL 541 430 100 168 258.46 25253.31 100 3283.94
129
Diet of Hoplobatrachus occipitalis
prey animals. ey were present in . of stomachs and
the index of relative importance (IRI = .) was almost
seven times higher than the second most important prey
category beetles (IRI = .). Furthermore, spiders were
frequently found in the frogs collected by the frog hunters.
Table summarizes the spectrum of recorded prey items.
Although most prey types were found in both collected
and ushed stomachs, the relative occurrence of each cat-
stomach sampling Prey animals N NmN% FO FO% V V% IRI
ARTHROPODA
Arachnida
Araneae 54 45 7.52 35 21.08 2179.03 1.580 191.89
Collembola 1 1 0.14 1 21.08 9.29 0.007 0.088
Crustacea
Decapoda 2 1 0.28 2 1.21 0.24 0 0.34
Insecta
Blattodea 2 2 0.28 2 1.21 277.34 0.201 0.58
Coleoptera (a) 72 45 10.03 45 27.11 5564.73 4.035 381.22
Coleoptera (l) 3 3 0.42 3 1.81 9.11 0.007 0.77
Dermaptera 1 1 0.14 1 0.60 0.35 0 0.08
Diptera (a) 27 19 3.76 19 11.45 254.42 0.184 45.15
Diptera (l) 18 16 2.51 10 6.02 337.02 0.244 16.57
Hemiptera 32 23 4.46 26 15.66 3615.40 2.622 110.87
Hymenoptera
Formicidae 49 49 6.83 19 11.45 293.47 0.213 80.55
others 8 6 1.11 8 4.82 133.43 0.097 5.84
Isoptera 58 41 8.08 8 4.82 459.18 0.333 40.54
Lepidoptera (a) 19 17 2.65 18 10.84 1140.28 0.827 37.66
Lepidoptera (l) 26 25 3.62 19 11.45 2133.17 1.547 59.15
Mecoptera 1 1 0.14 1 0.60 15.18 0.011 0.09
Mantodea 3 1 0.42 3 1.81 530.74 0.385 1.45
Odonata (a) 8 4 1.11 8 4.82 1554.47 1.127 10.80
Odonata (l) 4 2 0.56 4 2.41 330.88 0.240 1.92
Orthoptera 36 20 5.01 29 17.47 4553.92 3.302 145.28
Phasmatodea 14 8 1.95 13 7.83 205.07 0.149 16.43
unidentied (a) 74 0 10.31 52 31.33 na na na
unidentied (l) 2 0 0.28 2 1.21 na na na
unidentied (e) 16 16 2.23 7 4.22 46.52 0.034 9.54
Myriapoda
Chilopoda 1 1 0.14 1 0.60 102.87 0.075 0.13
MOLLUSCA
Gastropoda 9 9 1.25 8 4.82 1978.51 1.435 12.96
VERTEBRATA
Amphibia
Anura (a) 9 7 1.25 8 4.82 26379.25 19.128 98.22
Mammalia 2 1 0.28 2 1.21 6010.64 4.358 5.59
Pisces 150 101 20.89 56 33.74 79794.02 57.860 2656.68
Reptilia
Squamata 1 0 0.14 1 0.60 na na na
unidentied 16 0 2.23 16 9.64 na na na
TOTAL 771 509 100 461 277.71 140078.24 100 4122.18
Table 1 continued
130
M H M-O R
egory diered signicantly with the method used (χ²-test:
χ² = ., df = , p < ., n = ). Gravel, vegetation
and rice grains were also frequent in the investigated stom-
achs, with the latter being the most common non-food item
(–grains per stomach, . ± ., n = ). Vegetation was
gathered from stomachs and contained grains of grav-
el.
Discussion
We analysed the dietary composition of Hoplobatrachus oc-
cipitalis in rice paddies and small rivers close to the river
Niger around the end of the dry season of . We iden-
tied sh, beetles, moths and ants as the most important
prey. Our data on overall prey composition thus partly dif-
fered from other studies on H. occipitalis. In a Senegalese
population, L () found predominantly beetles,
ants and spiders by dissection. In southern Ivory Coast, Hy-
menoptera (including ants) were the most important prey
category of dissected H. occipitalis, followed by beetles and
amphibians (T ). It is known that many anurans
change their feeding habits according to the seasons; e.g.,
T () found dietary dierences according to the re-
spective supply in Peru, and I & M () report-
ed a seasonal dietary change in various Central African an-
uran species. In contrast, three of four analysed Hyperolius
species did not show any change between the dry and wet
seasons in Nigeria (L et al. ). Dierences in diet
composition may be based upon prey availability, varying
between seasons and/or habitats (K et al. ). Both
factors may explain dierences between our study, con-
ducted in a dry savanna around the end of the dry season,
and other populations (L , T ). Grav-
el, rice grains, and plant matter within the stomachs have
most likely been accidentally ingested (A et al. ,
K et al. ). On the other hand, some frog species
are known to feed on plants (D , S B-
-P ) and gravel could aid the digestion of the
diet (E L ). To clarify if these “prey items”
are deliberately ingested, direct observations are necessary.
e type of frogs’ prey items is oen associated with a
specic foraging mode. Active feeders may predate upon
large numbers of smaller prey that oen occurs in aggrega-
tions. T () called these frogs “ant-specialists”. Sol-
itary prey like beetles or spiders are preferred by “sit and
wait” feeders, which have been dened as “non-ant special-
ists” (T , L et al. ). Ants were consumed by
H. occipitalis in dierent quantities. L () found
them in . of the analysed stomachs (. of all ingest-
ed prey items). T () did not distinguish between
members of Hymenoptera. However, of the investigat-
ed stomachs contained individuals of this insect order. is
group, probably mostly ants, made up of all ingested
prey items in his analysis. In our study, ants occurred in
. of the collected and . of the ushed stomachs and
amounted to . and . of all identied categories, re-
spectively. us, ants were not avoided in northern Benin,
but played a less dominant role in comparison to other diet
components. Termites, which represent another aggrega-
tion taxa, are generally of less importance (L ,
this study) in, or even absent (T ) from, the diet of
H. occipitalis. e dietary composition of our H. occipitalis
populations speaks in favour of these frogs behaving as op-
portunistic “sit and wait” predators (G et al. ).
Terrestrial invertebrates usually dominate the diet of
anurans, even in aquatic or semiaquatic species (H
M b, M J ). Likewise, most
of the prey ingested by H. occipitalis was made up by ter-
restrial species. However, aquatic animals like tadpoles,
water bugs, and especially sh occurred as well (L
, T ; this study). A very high proportion of col-
lected stomachs (N = .) contained sh. is suggests
that H. occipitalis is capable of capturing prey above as well
as under the water surface as has been reported for only a
few other frog species such as Aubria subsigillata (K-
), Lithobates catesbeianus (H et al. ),
or Xenopus laevis (I M ).
e high proportion of sh was surprising, particularly
since in the study of T (), sh accounted for only
. of the prey, even though he studied a H. occipitalis
population at a sh farm, whereas our frogs predominantly
originated from rice paddies. e sh species in our sam-
ples (cichlids, cyprinids and catsh) cannot persist in tem-
porary waters and thus likely migrated into the rice paddies
when these were ooded.
While qualitative prey composition was similar between
both methods applied, quantitative results were dierent.
We identied beetles, moths and ants as the most impor-
tant prey categories in ushed frogs, and sh, beetles and
spiders in collected frog stomachs. Prey size was signi-
cantly smaller in the ushed frogs, potentially indicating
that not all large food items may have been ushed out.
Since sh were by far the largest prey items, this would ex-
plain why they were seemingly so rare in the ushed ani-
mals. Generally, altered abundance of particular prey items
might be related to site, prey availability varying between
habitats (H M ), as well as between natural
and human-dominated landscapes (S et al. ). For
instance, a large aquatic African rainforest frog, Aubria sub-
sigillata, was reported to mainly predate upon sh in Ga-
bon ( of all individuals investigated had ingested sh,
K ), whereas frogs from Ghana did not con-
tain any sh (H ). We captured our frogs within
rice paddies (as did the frog hunters), in shallow branches
of rivers, as well as in temporary ponds and near a well of a
village. However, we found sh in nine ushed individuals:
three captured in the paddy elds, six from the riverside,
and one from a temporary pond. It therefore seems unlikely
that our observed dierences in sh abundance were habi-
tat-specic. Most probably, the dierences can be ascribed
to the sampling approach. Stomach ushing might be less
eective in terms of larger food items like sh.
e reliability of stomach ushing data was evaluated in
various taxa. In penguins e.g., the eectiveness is relatively
high, but decreases with ingestion time (G ). In a
study on the diet of shes, the ecacy of ushing also var-
ied with the type of prey in combination with time aer
ingestion (P O’H ). F ()
concluded in his methodical comparison of diet analyses
in crocodiles, that problems will always occur with regard
to prey item size. In contrast, L & C ()
dissected some ushed frogs and found that less than of
the total volume had remained in their stomachs. W et al.
131
Diet of Hoplobatrachus occipitalis
() likewise found that nearly of the total volume
and prey item number could be recovered through ush-
ing. Unfortunately, we cannot provide such data since we
were not permitted to ush the animals before they were
killed by the Nigerian frog collectors and abstained from
killing the frogs we caught. Hence, it remains unclear if
some of the larger and/or a certain part of ingested items
remained in the stomachs aer ushing.
Many authors have reported that size of prey is relat-
ed to the snout–vent length and mouth width of anurans
(H , T , D et al. ) and/or that
diet preferences change with age (L M ,
H , B M ). We could not
nd any correlation between prey size and frog weight in
our study (however, only subadult to adult frogs were ex-
amined), although the prey covered a broad spectrum con-
cerning weight and size. Most of the examined frogs had
well-lled stomachs. e volume of all ingested items in
one frog adds up to a maximum of , mm³ and was
found in a medium-sized individual. e potentially enor-
mous quantity of daily consumption underlines the poten-
tially important position of H. occipitalis within its ecosys-
tems, especially where the species is very abundant such as
on river banks in the dry season, in swamps or rice pad-
dies. Since the consumption of H. occipitalis by West Af-
ricans is locally dramatically increasing (M et al.
), notable eects on the ecosystem of the species’ po-
tential decline are not unlikely. Regardless of the methodo-
logical approach, our data, based on nearly individu-
als, very probably provide a representative picture of the
species’ diet, at least in this particular region (rice pad-
dies and Sudanese savanna near a large river) and season
(dry season). It is known that H. occipitalis migrates from
river sites (occupied in the dry season) to newly formed
savanna ponds in the wet season (S L
). is switching of habitats may be also reected in a
change of prey items. Furthermore, H. occipitalis tadpoles
are known to be very ecient predators of, e.g., other tad-
poles and mosquito larvae (R , ). A decline
of adult frogs will consequently result in smaller numbers
of tadpoles. In the wake of unsustainable harvesting of
frogs, a dramatic increase of pest insect populations was
reported from India (A , O ). It would
therefore be interesting to investigate the diet of H. occipi-
talis during the wet season in their breeding habitats, far
from the rice paddies and riversides, as well as the exact
composition and quantities of the tadpoles’ prey.
Acknowledgements
We thank B S, as well as the respective authorities in
Benin for their support and the necessary research permits. We
are very thankful to M M, A T and
N A for their help during planning and conduct-
ing the eldwork. is study is part of the BIOLOG program of
the German Ministry of Education and Science (BMB+F; Project
BIOTA-West III, amphibian projects, LCJ).
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