Living together , sometimes feeding in a similar way : the case of the syntopic hylid frogs Hypsiboas raniceps and Scinax acuminatus ( Anura : Hylidae ) in the Pantanal of Miranda , Mato Grosso do Sul State ,

We studied the feeding ecology of two Hylinae anurans (Hypsiboas raniceps and Scinax acuminatus), living sympatrically and syntopically in the Pantanal of Miranda, Mato Grosso do Sul State, Brazil. The two hylid species had similar relative mouth width but differed in body size. The diet of the two frog species were composed of arthropodan prey. Both species consumed 11 different prey types, of which seven were common among them. Hypsiboas raniceps had a larger niche breadth (B A = 0.64) than S. acuminatus (B A = 0.48). Trophic niche overlap among frog species was 60.7 %. Our data are suggestive that although for many anurans the diet simply tend to reflect prey availability in the microhabitat, these two frog species, despite sharing similar microhabitat and period of activity (thus potentially exposed to a similar array of preys), tends to differ somewhat in diet (about 40%) which may result from some intrinsic ecological aspects to each of them (e.g. ecophysiology) and/or differences in body size.

For each individual frog we measured the snout-vent length (SVL), and mouth width (MW), to the nearest 0.1 mm, using a Vernier Caliper.The frogs were dissected and their stomach contents analysed qualitatively and quantitatively under a stereomicroscope.Food items were identified, measured (to the nearest 0.1 mm) and categorised to the taxonomic level of order.Unidentified arthropod remains were grouped in a separate category "unidentified parts of arthropods".Diet composition was estimated based on number, volume and frequency of occurrence (percentage of stomachs with a particular prey category) of each prey type in the stomachs.The length and width of each prey item were measured and its volume (V) (in mm 3 ) estimated using the ellipsoid formula (Dunham, 1983).An index of relative importance (IRI) of each prey category in the diet was estimated second Pianka et al. (1971).Food niche breadth (based on the number of food items) was estimated using the formula proposed by Levins (1968).The values of niche breadth were standardized (B A ) to a range of 0 to 1 using the appropriate formula.We used the Morisita-Horn index (C H ) (Horn, 1966) to estimate the trophic niche overlap.This index value varies from 0 to 1 where 0 indicates absence of overlap and 1 indicates total overlap.
Differences among frog species in body size (SVL) were tested using a T-test and differences in relative mouth size were tested using an Analysis of Covariance (ANCOVA), in which frog SVL as a covariate (Zar, 1984).All measurements are presented as mean ± 1 standard deviation and range.Prior to using each statistical test, we tested for the normality of the data distribution.When the data did not meet normality assumptions we used non-parametric statistics.We used the value of 0.05 as the cut-off for significance in all statistical tests.
Both H. raniceps and S. acuminatus consumed 11 different prey types in their diet.The prey items shared by the two frog species were spiders, ants, dipterans, coleopterans, blatarians, orthopterans and hemipterans (Table 1) with some other prey types being consumed exclusively by one or other frog species.
At the Pantanal subregion of Miranda, in Corumbá municipality, Mato Grosso do Sul State these two species are found living sympatrically and syntopically.Considering the large set of similarities in ecological (microhabitat, main activity period, reproductive mode and period) and in morphological aspects (e.g.body size), we could expect that a considerable similarity in trophic niche should arise as a result of living in syntopy and sharing such ecological aspects.
In this study we analyse the trophic niche of H. raniceps and S. acuminatus living in syntopy at the Pantanal of Miranda, in central-western Brazil.Specifically we addressed the following questions: i) What is the diet composition of each frog species?; ii) Which are the main prey types consumed by each frog species?; iii) Which is the niche breadth of each species and the niche overlap among them; iv) Do the two species differ significantly in body size?

Study area
The study was carried out at the subregion of the Pantanal called Miranda in Corumbá municipality, Mato Grosso do Sul State (MS) in the surroundings of the "Base de Estudos do Pantanal" of the Universidade Federal do Mato Grosso do Sul (UFMS) (57 o 00' W, 19 o 34' S).In the area, some formations of the Savannah arboreal vegetation predominate, with portions of semi-decidual forests, gallery forests and pastures (floodplain) being common elements of the landscape (Prado et al., 2002) fed by one of the largest rivers of the Pantanal, the Miranda river (Alho, 2008).Mean annual rainfall in the region is about 1,215 mm and annual temperature averages 25.1 °C (Prado et al., 2005).For a comprehensive checklist of frog species of the region see Prado et al. (2005).

Collecting methods and analysis
Collections were made at night during September 2008 between 8:00 PM and 00:00 AM.Frogs were collected by hand, posteriorly anesthetised with xylocain and fixed in 10% formalin as soon as possible.The two species occurred simultaneously in the habitat and were not calling on collection nights.

Discussion
Our data are suggestive that even living in syntopy, being relatively close taxonomically (same Subfamily), and potentially exposed to a similar prey spectrum, the hylids Scinax acuminatus and Hypsiboas raniceps differ in some aspects of their diet, with a niche dissimilarity of about 40%.This number could result, in part from the identification level of prey as emphasised Menin et al. (2005).Although both species fed on similar number of prey types (both species = 11 prey types), differences in terms of the quality of consumed prey potentially contributed to observed differences in the diets.For example, dipteran and lepidopteran larvae together with Dermaptera were preyed on exclusively by S. acuminatus, whereas coleopteran larvae, Odonata, Isopoda and Acari were consumed only by H. raniceps.Although for many anurans the diet simply tends to reflect prey availability in the microhabitat (Duellman and Trueb, 1994), our data on the diet of these two syntopic species (which are supposedly under similar prey spectrum) are suggestive that some ecological aspects intrinsic to each of them (e.g.ecophysiology) and/or size (the species differed in body size) tend to result in differences in types of prey consumed and in their frequencies.
The consumption of the prey items ingested by the two syntopic frogs are not unexpected since a diet composed by those arthropod preys are known also from the diet of other frog in the same area.Trachycephalus venulosus diet was exclusively composed by Coleoptera, Diptera, Hemiptera, Hymenoptera, Orthoptera, Pseudoescorpionida (34.3 %) (Table 1).The most frequent prey item in the diet of H. raniceps were coleopterans (22.9 %) and blattarians (20.0 %) (Table 1).On the other hand, numerically, the most common items in the diet of S. acuminatus were spiders (32.0 %) and orthopterans (20.0 %) (Table 1).Volumetrically blattarians dominated (71.6 %) followed by lepidopteran larvae (7.1 %).In terms of frequency, spiders (40.0 %) and orthopterans (25.0 %) were the most representative items.Coleopteran larvae were consumed only by H. raniceps whereas lepidopteran larvae were consumed exclusively by S. acuminatus (Table 1).Considering the estimated relative importance index (IRI), blattarians and coleopterans had greater importance in the H. raniceps diet whereas for S. acuminatus spiders and blattarians attained a higher IRI value (Table 1).Plant remains were found in eight stomachs (22.9%) of H. raniceps and in two (10.0%) of S. acuminatus.The mean number of preys in the stomach did not differ significantly among species (S. acuminatus = 1.47 ± 1.00 preys; H. raniceps = 2.05 ± 1.27 preys) (Mann-Whitney test: U = 206.50;p = 0.1513).Mean prey length independent of frog body size did not differ significantly among species (ANCOVA, F 1,33 = 0.366 p = 0.549).Similarly, the studied species also did not differ in terms of mean volume of prey consumed (S. acuminatus = 49.8 ± 115.3 mm 3 ; H. raniceps = 44.2± 144.9 mm 3 ) (Mann-Whitney test: U = 422.00;p = 0.8312).Trophic niche breadth for H. raniceps was 7.41 (B A = 0.64) whereas that for S. acuminatus was 5.84 (B A = 0.48).Trophic niche overlap among frog species was 60.7%.(Muri, 2005).For H. raniceps at another area, Rossa-Feres (1997) studying an anuran assemblage, recorded Coleoptera e Odonata for two adult H. raniceps resulting in a trophic niche breadth of 1.60.The few pieces of plant material we found in the stomachs of individual frogs corresponded to only fragments of dead leaves which suggested to us accidental ingestion during the prey capture.An accidental ingestion of plant material when capturing prey has been reported for some treefrog species (e.g.Oplinger, 1967;Rossa-Feres, 1997;Sluys and Rocha, 1998;Teixeira et al., 2002;Santos et al., 2004;Mahan and Johnson, 2007).
Our data indicated that the proportion of individuals having empty stomachs differed consistently among species (H.raniceps = 10.8% and S. acuminatus = 35.5%)which in turn may be suggestive that the two species may differ in their energy balance.The observed proportion of individuals of a species having empty stomachs has been used as an index of instantaneous energy balance for some organisms such as lizards (Huey et al., 2001), and fishes (Arrington et al., 2002).The idea is that individuals having empty stomachs are assumed to be in negative energy balance, depending on reserve stores (at least at that moment) provided by the incoming of previous feeding as source for metabolic processes and maintenance of energy needs (Boivin and Power, 1990;Huey et al., 2001;Arrington et al., 2002).Conversely, if individuals have food in their stomachs they will be gaining energy and thus, will be in positive energy balance (Huey et al., 2001).Although presently we still need to know to which extent these energy trends work among frogs, our data is suggestive that, comparatively, S. acuminatus is in lower energy balance compared with H. raniceps.Another plausible hypothesis regarding differences on proportions of empty stomachs may be that it arises from the differences in timing of feeding by the two treefrogs (Menin et al., 2005).In this study S. acuminatus apparently fed somewhat later than H. raniceps.However, the two hypotheses need to be tested.
We conclude that the diet of the two syntopic frog species studied are composed of arthropodan prey, and that despite sharing similar microhabitat and activity (thus potentially exposed to a similar array of prey), they tend to differ somewhat in diet (about 40%) which may result from some intrinsic ecological aspects to each of them (e.g.ecophysiology), differences in timing of feeding and/ or differences in body size.

Table 1 .
Diet composition of two anurans (Hypsiboas raniceps and Scinax acuminatus) living syntopically at the Pantanal of Miranda, MS -Brazil, with respective proportional values of prey consumption by number (%N), by volume (%V), by frequency (%F), and the estimated index of relative importance (IRI).