Current Herpetology 19 (1): 27-34., June 2000 (C)2000 by The Herpetological Society of Japan Ant Specialization in Diet of the Narrow-mouthed Toad, Microhyla ornata, from Amamioshima Island of the Ryukyu Archipelago TOSHIAKI HIRAI AND MASAFUMI MATSUI* Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501 JAPAN Abstract: Microhyla ornata consumed numerous ants, representing 77.1% in number and 44.6% in volume of the diet. The toad took ants in higher proportion than were present in the surrounding environment, and therefore, could be viewed as an ant specialized predator. Ants were the most numerously consumed prey in both spring and summer, while beetles and woodlice were less frequently taken in summer. Females have a larger body and wider mouth than males, and consumed significantly larger prey in maxi- mum size than did males. However, mean prey size, and frequencies of oc- currence for all prey taxa did not differ significantly between the sexes. These results suggest that the sexes do not differ in their use of food resources despite their morphological differences. Key words: Microhyla ornata; Ant-specialists; Prey availability; Ryukyu Ar- chipelago; Gape-limited predator INTRODUCTION go (e. g., Matsui, 1994; Ota, 1998), but eco- logical studies are much more limited, and The Anuran assemblage in the Ryukyu mostly confined to reproduction (e. g., Ut- Archipelago (excepting Osumi Islands) is sunomiya, 1980, 1989). A preliminary unique and more highly diversified than that report by Okochi and Katsuren (1989) is the found in the adjacent mainland of Japan. only information available about diet of While 21 anuran species/subspecies occur in anurans on Okinawajima Island. mainland Japan, 20 other species are dis- Microhyla ornata ranges throughout the tributed in the Ryukyu Archipelago, and 14 Ryukyu Archipelago from Amamioshima of them are endemic to the archipelago southward through China to Southeast Asia (Maeda and Matsui, 1999). and India (Frost, 1985), and inhabits vari- Quite a few comprehensive taxonomic ous habitats from lowlands to montane and biogeographical studies have been regions (Maeda and Matsui, 1999). The made on anurans in the Ryukyu Archipela- toad is presumed to be specialized for eating ants or termites because of its small body * Corresponding author and narrow mouth (Maeda and Matsui, . Tel: +81-75-753- 6846; Fax: +81-75-753-2891. 1999). However, detailed quantitative stu- E-mail address: [email protected] dies on the food habits of this species have (M.Matsui) not been done so far. 28 Current Herpetol. 19 (1) 2000 Berry (1965) reported that ants predomi- time as the collection of toads, we collected nated in the diet of two congeneric species, four 0.5×0.5m quadrate samples of leaf M. butleri and M. heymonsii from Singa- litter and brought them to the laboratory. pore. Many dendrobatid and bufonid spe- After large or inactive animals were re- cies are also well-known for eating moved from these samples, the remaining numerous ants, and are called ant specialists ones were extracted by Tullugren's funnel because they take ants in higher proportion method (Aoki, 1973). These potential prey than are found in the environment (Toft, invertebrates were stored in ethylene glycol 1980, 1981). for later analyses. Toft (1985), in reviewing resource par- Stomach contents were removed by dis- titioning studies in amphibians and reptiles, section of toads, and preserved in 10% considered that food partitioning plays an buffered formalin for later analyses. For important role in the organization of adult each toad, snout-vent length (SVL) and anuran communities. Therefore, mouth width (MW) were measured with a knowledge of food habits of each com- caliper to the nearest 0.1mm. munity member might provide pivotal in- We identified stomach contents and sights into the factors that are responsible potential prey to the level of class or order for frog community structures in the Ryu- except for Hymenoptera, which was clas- kyu Archipelago. sified into Formicidae and non-Formicidae. In order to obtain information on prey For holometabolous insects, larvae and selection by the narrow-mouthed toad, M. adults were separately treated. The occur- ornata, we conducted field work on rence of plant materials or minerals was Amamioshima which is near the northern recorded for each stomach. Details about limits of its range. measurement of stomach contents are given in Hirai and Matsui (1999). To detect seasonal variation in the diet, MATERIALS AND METHODS we compared the presence or absence of For the diet study, we collected toads in each prey taxon between spring and summer evergreen forests of the outskirts of Naze by Fisher's exact probability test. Next, we city on Amamioshima Is. (28°22'N, examined the relationships between prey 129°31'E).We made collections at night availability and diet composition by cal- (2100-2300h) on 6 April (spring) and 12 culating Kendall's rank correlation July (summer) of 1998. We captured all in- coefficients (τ). In this analysis, we used dividuals encountered on the forest floor, only taxa that were commonly found in and immediately fixed them in 10% buffered both potential prey samples and the formalin to preserve stomach contents with stomach contents because prey taxa found minimum digestion. only in one of these might have large sam- In order to estimate prey availability in pling errors. We presumed that the exclu- the habitat of the toads, we sampled leaf sion of these uncommon prey taxa would litter invertebrates by two different not affect the result of analyses because the methods. In spring, we set pit-fall traps common prey taxa accounted for more than near the temporal pool where toads were 75% of both diet and prey samples. abundant. Fifteen plastic cups (90mm in Moreover, we tested the sexual difference in diameter and 130mm in depth) were set in the diet by comparing the presence or ab- the ground at about 2m intervals for two sence of each prey taxon by Fisher's exact nights from 7 to 9 April. A small quantity probability test. In additition, we quan- of ethylene glycol was put into each cup to tified dietary overlap between sexes by cal- preserve samples. In summer, at the same culating simple similarity indices (Schoener, HIRAI & MATSUI-ANT SPECIALIZATION BY A MICROHYLID 29 1968): TABLE 1. Diet composition (in%) of Micro- Cxy=1-0.5Σ｜Pix-Piy｜ hyla ornata (1234 prey from 55 individuals, total based on proportion of prey taxa (i) in diet volume 2448.3 mm3). Abbreviations: F=fre- of two different sexes (x and y). We calcu- quency of occurrence; N=numeric proportion; lated both numeric and volumetric overlap V=volumetric proportion in this analysis. RESULTS Diet composition We identified 1234 prey items extracted from 55 stomachs (41 males, 14 females) of 56 individuals captured; the remaining stomach from an immature female was empty. Prey items included five arthropod classes (Arachnida, Crustacea, Diplopoda, Chilopoda, and Insecta), of which Insecta contained eight orders (Table 1). Ants (Formicidae) were consumed by all individ- ual toads with stomach contents, and predominated in the diet, representing 77.1% by number, and 44.6% by volume. The next most frequently consumed prey taxon was beetles (Coleoptera; 67.3%), but they made up only 6.4% by number, and 16.9% by volume. The other prey taxa constituted minute fractions, making up less than 5% by number, and 15% by volume. Plant (vegetable scraps) and mineral materials (pebbles and dirt) oc- curred in 47.3% and 1.8%, respectively, of the stomachs. Seasonal variation in diet The toads appeared to take more prey in for ants and wasps (non-formicids) were spring (Mean±SE=26.1±3.9) than in sum- found more frequently in spring than in mer (16.5±2.7), but the difference was not summer, but only two prey taxa, beetles and significant (U-test, p>0.05). However, the woodlice (Isopoda), differed significantly in volume of stomach contents differed fequency of occurrence (Fisher's exact seasonally, and almost three times more probability test, p<0.01 for Coleoptera; food was ingested in spring (59.2±8.1mm3) p<0.05 for Isopoda). Volumetric contri- than in summer (20.8±4.0mm3) (U-test, bution by these two prey taxa was great in p<0.01). spring, but it showed striking decrease in Ants constituted the bulk of the diet nu- summer. By contrast, termites (Isoptera) merically and volumetrically, and their made up a larger numerical proportion in proportions varied little seasonally (Table summer because of two individuals (9.5% 2). Among 10 prey taxa commonly con- in frequency) that contained unusually sumed in both seasons, all prey taxa except many termites (Table 2). Frequency of oc- 30 Current Herpetol. 19 (1) 2000 TABLE 2. Dietary comparison of Microhyla ornata between spring (887 prey from 34 individuals, total volume 2012.4mm3) and summer (347 prey from 21 individuals, total volume 435.9mm3). See Table 1 for abbreviations. currence of plant materials markedly in- sampled from the surrounding habitat. creased from spring (26.5%) to summer Specifically in spring, 11 of 14 potential (90.5%), and differed significantly between prey invertebrates sampled from the en- the seasons (Fisher's exact probability test, vironment were found in diet composition. p<0.01). Ants seemed to be the most readily available prey for toads, and were consumed in much Prey selection higher proportion than those found in the A total of 20 potential prey invertebrates environment. However, the next most were collected when samples in spring and abundant possible prey, collembolans, were summer were combined (Table 3). Among consumed much less in proportion by toads.