Feeding Habits of Two Sympatric Rocket Frogs (Genus Ptychadena) in a Forest Remnant of Southern-Central Ivory Coast, West Africa

The decline of tropical amphibian species calls for studies on their biology and ecology, in order to understand their role in the respective ecosystems. We studied the diet of Ptychadena oxyrhynchus and P. longirostris, two rocket frogs co-occurring along dirt roads with puddles in the Azagny National Park, southern-central Ivory Coast. Stomach contents of 62 individuals of P. oxyrhynchus and 75 specimens of P. longirostris were obtained using the stomach-flushing method. The diet of both species was essentially characterized by various terrestrial invertebrates (predominantly insects) and both species can be considered opportunistic feeders. The presence of tadpoles in the stomach of a female P. longirostris indicates that this species may occasionally prey on aquatic food items as well. The large overlap in prey between both study species can be taken as a hint that competition for food does not play a major role between them. An unexpected observation was the proportion of plant fragments in the diet of our frogs. However, it remains unknown if they devoured plant deliberately or not. Future research should investigate this question.

Ptychadena oxyrhynchus and P. longirostris are two species easily distinguishable from other congeners and both are common in West Africa [2]. Whereas P. oxyrhynchus is predominately a savannah species, P. longirostris is a forest dweller [2,3]. They are widespread in Ivory Coast and meet in the savannah-forest ecotone and in degraded parts of rainforests [2][3]5,[9][10][11]. Both species have been reported to live in sympatry in some forest remnants of south-eastern Ivory Coast [6,12], south-western Ghana [13] and south-western Nigeria [14]. However, despite the numerous publications recording both species [2,[13][14][15][16][17][18] we know only little about their biology. Apart from the reproductive biology, the diet of frogs may help to better understand their biology [19].
Ptychadena oxyrhynchus has been the object of a few ecological studies dealing with reproduction and population dynamics [20], as well as with diet [21]. In contrast, neither data are available for P. longirostris. An area where Ptychadena oxyrhynchus and P. longirostris co-occur is the Azagny National Park (ANP), southern-central Ivory Coast, where the species are found along dirt roads with puddles. We herein compare the prey composition of both species in this secondary rainforest.

Study site
The Azagny National Park (ANP; 5.15°-5.28° N; 4.78°-4.95° W, 1-100 m a.s.l.) comprises 21,850 ha of coastal rain forest, located in the department of Grand-Lahou, southern-central Ivory Coast (Figure 1). The mean annual temperature is 26°C; the mean annual precipitation 1,664 mm [22]. The equatorial climate includes a long rainy season from April to mid-July, followed by a short dry season from mid-July to mid-September. A shorter rainy season extends from mid-September to November and the long dry season lasts from December to April. Swamp-forests cover two-thirds of the park. Lauginie [23] provided an overview concerning the vegetation of the ANP.

Target species
Ptychadena oxyrhynchus is medium-sized and has extremely robust and long hind legs [2]. The basic dorsal color is beige with an olive tinge; the top of the snout is usually lighter colored. The dorsal ridges are well-defined ( Figure 2a). Ptychadena longirostris (Figure 2b), likewise have very pointed heads, and long hind limbs. Their bright brown to sometimes yellowish dorsal coloration may be uniform or showing an ill-defined darker pattern. In contrast to P. oxyrhynchus the dorsal ridges are less well-defined [2,3].
In ANP, P. oxyrhynchus and Ptychadena longirostris co-occur along dirt roads with numerous temporary water bodies, i.e., puddles of different size. dip-net along the dirt road puddles, between 22:30-00:30 h GMT. Detailed information concerning anuran survey methods in ANP are provided by Kouamé et al. [25] and Konan et al. [26]. After capture, frogs were sexed and their Snout-Urostyle-Length (SUL) measured with a dial caliper (accuracy ± 0.5 mm). Stomach contents of adult individuals were recorded by stomach flushing into a fine-meshed net [27,28], from which prey items were recovered with forceps, preserved in 70% ethanol and thereafter analyzed in the laboratory with the aid of a dissecting microscope. Frogs were released at their sites of capture. Invertebrates were determined to order level in insects, as insect larvae, spiders (Arachnida), woodlice (Isopoda), earthworms (Annelida) and unidentified preys (other prey) [29][30][31]. Further prey items were tadpoles and plants fragments. We determined and counted the prey items for each individual. For a quantitative analysis of the frogs' diets, we counted the number of individuals that contained one type of prey and calculated the frequency of occurrence (Fi%) of the different prey categories as the percentage of individuals containing a particular type of prey using the following formula [32,33]: Fi=100ni/n, where Fi=frequency of occurrence of the i food item in the sample, ni=number of individuals in which the i item was found, and n=total number of individuals with food in the sample.
The index of shared prey Cλ according to Morisita [34] modified from Horn [35] was used to calculate the prey overlap between P. oxyrhynchus and P. longirostris: Where: S=the total number of prey items; xi=the proportion of prey i consumed by the species x; yi=the proportion of prey i consumed by the species y.
Cλ would be 0 if the prey composition in both species was entirely different; Cλ would be 1 if the prey of both species was identical. A Cλ value smaller than 0.6 represents a significantly different trophic niche [36]. Other tests were performed with Statistica Version 7.1.
We identified nine prey categories consumed by P. oxyrhynchus and 13 in P. longirostris (Table 1). Male and female P. oxyrhynchus consumed prey items of the same categories. In contrast we found worms and a tadpole only in female P. longirostris, while only males consumed arachnids.
In Table 2 we summarize the percentage of the different prey categories (Fi%) consumed by both species. Insects represented the bulk of both species' diet. Insect categories were represented by Blattodea, Coleoptera, Hymenoptera, and Insect larvae, Isoptera, Lepidoptera and Orthoptera. Five prey categories (coleopterans, hymenopterans, lepidopterans, orthopterans and plant fragments) dominated the diet of all frogs (Tables 1 and 2). Female P. oxyrhynchus mainly fed on small cockroaches (growing up to 2 cm in length), orthopterans

Ptychadena oxyrhynchus
Ptychadena longirostris (grasshoppers), insect larvae and hymenopterans (predominantly ants) while males of this species mainly preyed on insect larvae, grasshoppers, beetles and plant fragments. Female P. longirostris mainly fed on butterflies, grasshoppers, beetles, and plant fragments; the dominant prey of males consisted of spiders, grasshoppers, ants and cockroaches.
The prey composition did not significantly overlap between P. oxyrhynchus and P. longirostris (Cλ=0.78). However, the index of shared prey between males and females was significantly different in P. oxyrhynchus (Cλ=0.52) and in P. longirostris was (Cλ=0.091).

Discussion
Not surprisingly the diet of P. oxyrhynchus and P. longirostris in ANP mainly consisted of arthropods (predominantly diverse insect orders). This is consistent with the diet of other congeners in Ivory Coast, e.g. P. mascareniensis and P. pumilio [37]. In contrast to our data, the diet of P. oxyrhynchus from a forest-savannah ecotone in central Ivory Coast was less diverse and mainly consisted of spiders and orthopterans [21]. These differences might be due to differing prey availability and/or other environmental conditions [37,38].
The large prey overlap between both Ptychadena species in ANP may be taken as a hint that competition for food does not play a major role between these two anurans. A similar observation for two sympatric forest frogs from the genus Phrynobatrachus has been reported in swampy parts of an Ivorian eastern forest remnant [39]. However, the values of Cλ beneath the threshold of 0.6 indicate that some intraspecific competitions for food could occur between males and females of respective species.
From a qualitative point of view, males and females of P. oxyrhynchus consumed similar prey, whereas in P. longirostris some differences in prey consumptions were found between sexes. In Malagasy P. mascareniensis the diet between males and females was similar [40], but it differed between sexes in Ivory Coast [37]. The later observation was attributed to a significant sexual dimorphism, with females being much larger than males. Regardless of sex the generalistic [41,42] diet of P. oxyrhynchus and P. longirostris was essentially characterized by terrestrial invertebrates (Tables 1 and 2), being in accordance with the terrestrial life of both species. However, the presence of a strictly aquatic prey, i.e., tadpoles, in the prey of a female P. longirostris indicates that this species also feeds in water. Many tadpoles are usually found in the road puddles, where they surface from time to time to breathe atmospheric oxygen. As only one P. longirostris specimen has fed on them, this seems to be a rather infrequent prey. The prey spectrum of P. longirostris indicates that this species is an opportunistic feeder. The fact that females did not consume isopterans (and spiders) and preyed on much less hymenopterans than conspecific males could be due to either their low number (N=15), which was four times inferior than captured males (N=60), or the significantly different trophic niche of both sexes (Cλ=0.091).
An unexpected observation was the proportion of plant fragments in the diet of our studied frogs. Although the ingestion of plants is commonly reported in many anuran species [37,39,[42][43][44]