Lying in Ambush for Nocturnal Frogs: Field Observations on the Feeding Behavior of Three Colubrid Snakes, Elaphe Quadrivirgata, E

Home , Elaphe, Japanese striped snake

Lying in Ambush for Nocturnal Frogs: Field Observations on the Feeding Behavior of Three Colubrid Snakes, Elaphe quadrivirgata, E. climacophora, and Rhabdophis tigrinus

AKIRA MORI, MITSUHIKO TODA, SEISHI KADOWAKI AND HAJIME MORIGUCHI

Abstract: The nocturnal activity of Elaphe quadrivirgata, E. climacophora, and Rhabdophis tigrinus hitherto known as diurnal, heliothermic predators, was observed around a breeding pond of the Japanese treefrog, Rhacophorus arboreus. The peak of seasonal nocturnal activity of the snakes largely coincided with that of R. arboreus. Snakes were observed lying motionless on a tree branch, with the anterior part of the body extended and the head directed towards the trunk and/or downward. Often the chin and/or temporal region of the snakes made contact with the surface of the trunk. Predation on R. arboreus by the snakes was directly observed on 10 occasions. These facts suggest that, from the above position, E. quadrivirgata, E. climacophora, and Rhabdophis tigrinus "actively" ambushed Rhacophorus arboreus that used the trees as diel vertical pathways during its breeding season. Possible factors that affect the foraging tactics of the snakes are discussed. Key words: Elaphe quadrivirgata; Elaphe climacophora; Rhabdophis tigrinus; Noc- turnal ambush; Foraging tactics

Foraging ecology is one of the most important Jaeger and Barnard, 1981; O'Brien et al., 1989). aspects in understanding life history strategies in All snakes hitherto known are carnivorous carnivorous animals. To analyze the putative (Vitt, 1987) and various morphological and feeding adaptations of predators, it is essential physiological adaptations for foraging have been to know not only what they eat but also how demonstrated (Cundall, 1987 for review). they interact with their prey (e. g., where, when, Although these morphological and physiological and how they eat prey). Numerous studies sug- adaptations constrain the flexibility of their gest that predators search for prey by using one foraging tactics, intraspecific shifts in foraging of two basic modes, sit-and-wait and active mode, site, and period in response to en- foraging (McLaughlin, 1989 for review). These vironmental factors are reported in several search modes are generally species-specific partly snakes (Patterson and Davies, 1982; Hailey and because physiological and morphological traits Davies, 1986). However, more basic field obser- constrain the predator's ability to switch modes vations are still extensively necessary to draw any (e. g., Huey and Pianka, 1981; McLaughlin, conclusions about the all-inclusive foraging 1989). On the other hand, there are indications strategy of any snake. that some predators switch between sit-and-wait The Japanese striped snake, Elaphe and active search modes (e. g., Jaeger and Bar- quadrivirgata, (Colubridae) is one of the best nard, 1981; Pietruszka, 1986). Intraspecific studied snakes in Japan and its natural history changes of foraging tactics such as temporal has been well documented (Fukada, 1954, 1958, shifts in foraging habitat and diel activity pat- 1959, 1960, 1985; Ota, 1986; Hasegawa and terns are also reported (e. g., Shine and Moriguchi, 1989; Mori, 1989, 1991; Kadowaki, Lambeck, 1985; Sjoberg, 1989). Based on em- 1992). This snake is mainly a terrestrial and pirical field studies and foraging theories, these diurnal active forager which maintains a relative- intraspecific shifts have been interpreted as ly high activity temperature (mean body responses to changes in environmental factors temperature=28.7C: Fukada, 1985), achieved such as food resource availability in order to in- by basking in the sun (i. e., heliothermy; Pough crease foraging efficiency (e. g., Norberg, 1977; and Gans, 1982). Extensive investigations of stomach contents revealed that the species feeds Accepted 30 Apr. 1992 on various kinds of prey including frogs, lizards, 108 Jpn. J. Herpetol. 14 (3). 1992 small mammals, and reptile eggs (Mori and The vegetation around the pond is mainly com- Moriguchi, 1988 for review). At least two posed of Acer amoenum, Eurya japonica, different foraging tactics are known in E. Magnolia praecocissima, and Castanopsis quadrivirgata: catching diurnally active prey by sieboldii (3-15m high), of which some branches directly chasing it (Ota, 1986) and detecting and hang over the pond (Fig. 1). We visited the subduing nocturnal prey sleeping in its retreat pond two to 29 nights per month usually after (Mori, 1989). Although both of these tactics are midnight. used by a single individual (Mori, personal obser- With a small flashlight, we carefully searched vations), feeding opportunities may also affect for frogs and snakes around the pond. The which foraging mode is employed (Mori, 1989). number of frogs and snakes was counted, and Elaphe climacophora and Rhabdophis location, behavior, and posture of snakes were tigrinus are also common snakes in the Japan recorded. Because of dense vegetative cover in Main Islands (Fukada, 1958; Moriguchi and the higher portions of the trees, vertical censuses Naito, 1982; Kadowaki, 1992). Both species are were restricted to approximately an area below mainly diurnal and the mean body temperature 4m. Some snakes were captured by hand and is 27.7C in the former and 26.8C in the latter brought back to the laboratory where snout-vent (Fukada, 1989). Elaphe climacophora is a ter- (SVL) and tail lengths and body mass (BM) were restrial to semi-arboreal snake feeding on en- dothermic animals, whereas R. tigrinus is a terrestrial to semi-aquatic snake predominantly ex- ploiting anurans (Fukada, 1959, Sengoku, 1979, Moriguchi and Naito, 1982). However, detailed field observations on feeding behavior of these snakes are meager. Rhacophorus arboreus, one of the food resources for E. quadrivirgata and Rhabdophis tigrinus, is a well-known foam nesting treefrog distributed in Honshu of Japan (Maeda and Matsui, 1989). During the breeding season from April to July, the frogs congregate at still water and breed on trees, among the grass or on the ground above or adjacent to the water, construct- ing foam nests usually on branches and leaves. Breeding activities are usually observed from night to early morning (Toda, 1988). During an ongoing study on the reproductive biology of Rhacophorus arboreus, we observed another foraging tactic of E. quadrivirgata: ambushing, from a tree branch during the night, nocturnal frogs which were congregating at a small pond to breed. In this paper, we present field observations on the feeding behavior of E. quadrivirgata and discuss some proximate factors that may affect the foraging tactics of the snake. Similar feeding behavior of E. climacophora and Rhab- dophis tigrinus, observed at the same study site are also reported. MATERIALSAND METHODS Field work was conducted from May to July FIG. 1. Schematic map and cross section of a breeding pond of the Japanese treefrog, Rhacophorus 1987 and from May to October 1988, 1989 and arboreus. Dashed lines indicate overhead canopy of 1990 at a small pond (17m long) located in the trees. Aa: Acer amoenum, Ac: Aralia cordata, Aj: Botanic Garden (5ha) of Kanazawa University Aucuba japonica, Cs: Castanopsis sieboldii, Ej: Eurya (36°34'N, 136°40'E), Ishikawa prefecture, japonica, Hc: Houttuynia cordata, Io: Idesia polycar- Japan. The Botanic Garden is dominated by pa, Is: Iris pseudacorus, Mp: Magnolia praecocissima, Castanopsis sieboldii and Machilus thunbergii. Rt: Rhus trichocarpa. MORI ET AL. -SNAKE FEEDING BEHAVIOR 109 measured. These snakes were marked by ven- and 12 individuals of E. quadrivirgata, E. tral scale clipping as a permanent marking climacophora, and R. tigrinus were marked, (Brown and Parker, 1976) and painted on the respectively. Marked individuals comprised head with quick-drying paint for a temporary sixty-two % (32/52) of the sightings of E. marking, after which, they were released at the quadrivirgata, 61% (11/18) of E. climacophora, site of capture as soon as possible. In 1989 and and 44% (14/32) of R. tigrinus. It is probable, 1990 snakes were palpated at the time of capture however, that these values were underestimated to recover prey items from the stomach. since ventral or painted markings could not Regurgitated prey was identified to the species always be identified. Both males and females level if possible and returned immediately to the were captured in all species. No juvenile snake snake's stomach. Cloacal body temperature of was observed or captured except for R. tigrinus. some snakes was measured with a thermistor. Most of the snakes were observed from late We believe that these handling procedures did May to early August, when Rhacophorus ar- not affect the normal feeding behavior of the boreus congregated at the breeding pond and snakes. Ambient temperature at about 0.6m was abundant on the surrounding ground and above the ground was measured at a fixed point trees during the night (Fig. 2). In spring of beside the pond during each observation. 1988-1990, it seemed that snakes did not appear at Behavior of several snakes was continuously the pond until the air temperature measured dur- observed through several hours in June 1989 and ing the census exceeded approximately 15 C. July 1990. Measurements of SVL and BM of Few snakes were found at night from mid- frogs being swallowed by snakes were obtained August to September even though the air from data, by referring to individual marking temperature was higher than 15 C. numbers, previously recorded during mark- All E. quadrivirgata were found lying mo- recapture study of the frogs. Brief censuses in tionless on branches 0.7-3.6 meters high (x= 1.8, the Botanic Garden both in the daytime and at N = 52). All E. climacophora but one, which night were also made sporadically to investigate was coiled in the water, were observed lying on food resources used by snakes in this area. branches 0.8-3.0 meters high (x= 1.9, N= 17). Among 32 observed Rhabdophis tigrinus, 82% RESULTS were found on branches 0.2-3.0 meters high Snakes were observed on a total of 70 nights (x=1.6, N=27) and the remaining were seen on out of 280 night-time censuses over the 4 years the ground (N=2) or in the water (N=3). The (1987-1990). One snake found on 8 June 1988 tree most frequently used as a perch was M. could not be identified as to its species so this praecocissima (31 times), followed by Castanop- record was disregarded in the subsequent sis sieboldii and Eurya japonica (24 and 20 analyses. Elaphe quadrivirgata was observed times, respectively). more frequently each year (13, 17, and 11 nights Most individuals of E. quadrivirgata found on in 1987, 1988, and 1989, respectively) than the trees showed a typical posture (Fig. 3). The other two species, except in 1990 when no in- snakes were lying motionless with the anterior dividual of E. quadrivirgata was found (Table part of the body extended and the head directed 1). The total numbers of E. quadrivirgata, E. towards the axis of the tree (i. e., trunk). The climacophora, and R. tigrinus observed at night head was usually positioned around a knot and were 52, 18, and 32, respectively. Ten, five, directed downwards, with the lower jaw or tem-

TABLE 1. Number of nights when snakes were observed around the breeding pond of Rhacophorus arboreus. Eq: Elaphe quadrivirgata, Ec: E. climacophora, Rt: Rhabdophis tigrinus 110 Jpn. J. Herpetol. 14 (3). 1992

FIG. 2. Average numbers of snakes and Rhacophorus arboreus observed per night at a breeding pond of R. arboreus during a nighttime census. The average air temperature was also measured at the time. Averages were calculated for every five or six nights. Stars indicate no censuses were made for those five or six consecutive days. Dashed lines indicate air temperatures (AT) of 15 C. In 1987, field study was conducted from May to July only. poral part of the head making contact with the extended on branches and twigs. Elaphe surface of the trunk. The posterior part of the climacophora and R. tigrinus also positioned body was either irregularly coiled or loosely themselves in a similar manner, but the latter

FIG. 3. Typical ambush posture of Elaphe quadrivirgata on a tree. Note the physical contact of the snake's lower jaws or temporal region with the surface of the tree and the direction of the head, which is oriented towards the axis of the tree (trunk) or downwards. MORI ET AL. -SNAKE FEEDING BEHAVIOR 111

TABLE2. Data of direct observations of nocturnal predation by snakes at the breeding pond of Rhacophorus arboreus. All prey items were R. arboreus. Snout-vent length (SVL) and attack distance in cm, body mass (BM) in g, and duration of swallow in min. Eq: Elaphe quadrivirgata, Ec: E. climacophora, Rt: Rhabdophis tigrinus.

species seemed to be less stereotyped in its temperature (AT) were measured two times after posture than the two species of Elaphe. ingestion of frogs: 19.3 C (BT) and 19.3 C (AT) Predation on Rhacophorus arboreus by (0335h), and 20.4 C (BT) and 20.5 C (AT) snakes was directly observed 10 times. Seven (0700h). The body temperature of nine other snakes attacked a frog from the posture mention- snakes (two E. quadrivirgata, two E. ed above and three were found swallowing a frog climacophora, and five R. tigrinus), holding the in a tree or on the ground (Table 2). A typical specific posture above, ranged from 20.0- predatory sequence was as follows. A snake, 25.0 C. The corresponding range of AT was which was lying in the predominant posture on a 18.7-25.0 C. In these nine cases AT was not branch, turned its head toward a frog as the frog measured at exactly the same time as BT was climbed the trunk several cm or jumped up to a measured. Nonetheless, there was little branch below the snake's head. The distance difference between BT and AT in all cases. between the snake and the frog when the former All stomach contents were identified as initially oriented towards the latter was approx- Rhacophorus arboreus. The proportion of in- imately 30-60cm. Until the frog came within dividuals of E. quadrivirgata, E. climacophora, striking range, the snake did not lunge at the and Rhabdophis tigrinus containing prey items frog but only turned its head or extended its neck was 15.8% (3/19), 14.3% (1/7), and 20.8% a few cm toward it. This behavior was more (5/24), respectively. noticeable in Elaphe than in R. tigrinus, of Snakes continuously observed at night rarely which some individuals approached the frogs changed their positions except when prey en- several cm before attacking in response to the counter occurred. Occasional daytime censuses movements of the frog. Distance between the of the pond revealed that few snakes retained snake and the frog when the former attacked the their nocturnal positions on the tree throughout latter ranged from 5-40cm. The snake always the daytime. In most censuses no snakes were attacked the frog just after the frog initiated observed around the pond. Three individuals of some movement (climbing or jumping). In E. E. quadrivirgata, which were continuously quadrivirgata, no apparent tongue flicks were observed on the previous night, began to move a observed between the initial response of the few hours after sunrise, climbing up the tree and snake and a subsequent attack by the snake. It going out of sight of the observers. All in- could not be confirmed whether or not tongue dividuals of R. tigrinus observed in the daytime flicks occurred in other two species. In all cases, were crawling on the ground or swimming in the the snake swallowed the frog alive and no con- water. Similar nocturnal behavior and an ac- stricting behavior was observed. tual feeding sequence of the snakes were observ- The body temperature (BT) of one individual ed in another pond where Rhacophorus arboreus of E. quadrivirgata and the ambient air congregated for breeding. 112 Jpn. J. Herpetol. 14 (3). 1992

1984; Diller, 1990). These rattlesnakes coil adja- DISCUSSION cent to fallen logs used as rodent runways with The current observations suggest that in- the head positioned perpendicular to the log's dividuals of E. quadrivirgata, E. climacophora long axis. Reinert et al. (1984) and Diller (1990) and Rhabdophis tigrinus, lying on a tree branch assumed that logs were actively selected by the around the pond at night, were not merely snakes with the aid of chemosensory percep- resting but "actively" ambushing Rhacophorus tion. In the present observations it seems that arboreus because 1) snakes were found only dur- there were some "preferred" ambush trees and ing the season when the frogs congregate at the branches that were predominantly used by some breeding pond, 2) most of the snakes showed a snakes although no quantitative evaluations typical posture which seems to be adapted to am- were made. Further observations are required bush (see below), and 3) they actually attacked to clarify whether the snakes actively select trees and swallowed the frogs at night. or specific spots in trees for ambush, where the The specific position and posture of the snakes probability of prey encounter is greater than in seem to facilitate detection of R. arboreus other spots. breeding at the present study site. During the It has been suggested, both from empirical breeding season, male frogs congregate at the and theoretical aspects, that prey density or pond and exhibit a regular diel movement pat- availability influences foraging site, time, and tern (Toda, 1988): in general, they rest in the mode of a predator (e. g., Norberg, 1977; Arnold crown of trees during the daytime, jump down and Wassersug, 1978; Andersson, 1981; Jaeger to the pond in the evening, vocalize around the and Barnard, 1981; Shine and Lambeck, 1985; pond during the night, and climb back to the Sjoberg, 1989). At Kanazawa University there crown before or at dawn. They may climb up are a number of possible prey animals for trees by jumping up from branch to branch, but the snakes of the present study, including often they clamber up the trunks directly. Hyla japonica, Gekko japonicus, Eumeces Although female frogs may not rest up in the latiscutatus, Takydromus tachydromoides, trees, they usually climb up a tree to make a Achalinus spinalis, and Apodemus speciosus foam nest on a lower branch. Thus, the snakes' (Toda, 1992, unpubl. data). Most of these prey positioning on lower branches, adjacent to daily animals, however, do not seem to be abundant pathways of the frogs (i. e., trunk), with the head in the present study field, since the University is directed to the trunk and downwards, may located in the center of Kanazawa City and enhance the probability of prey encounter dur- isolated from nearby natural habitats. It is like- ing the diel vertical movements of the frogs. ly that a temporal high density of R. arboreus Another characteristic of the ambush posture, around the pond and the scarcity of other prey the physical contact of the lower jaw or temporal animals affect the foraging tactics of the snakes. region with the surface of the trunk, is possibly We postulate that exploitation of the aggregat- related to sensory stimulation and prey recogni- ing frogs directly affects foraging time and tion. This physical contact may allow the snake site: the snakes may become active at night to to perceive any mechanical disturbance along the take advantage of increased availability of noc- tree caused by the movements (climbing or jump- turnal frogs and climb trees to increase the ing) of the frog, since vibration is one of the im- probability of prey encounter. portant cues involved in prey detection by snakes On the other hand, the foraging time of (e. g., Proske, 1969; Wever, 1978; Reinert et al., several snakes is reported to be temperature- 1984). Direct observations indicate that visual dependent (Gibbons and Semlitsch, 1987 for and/or mechanical cues are important in prey review). Most of these snakes are nocturnal dur- detection in all of the three species, because they ing the summer and diurnal during the spring oriented or attacked the frog immediately after and autumn. The change in foraging time may prey movements. In E. quadrivirgata, no be attributed to thermoregulation requirements tongue flicks were observed during prey en- that permit nighttime activity on warm summer counters, suggesting that vomerolfaction nights but necessitate basking on cooler days (chemical sense supported by the vomeronasal (Sanders and Jacob, 1980). It does not seem, system; Cooper and Burghardt, 1991) is not however, that nocturnal foraging of E. important in prey detection and attack by E. quadrivirgata, E. climacophora, and Rhabdo- quadrivirgata, at least in this context. phis tigrinus is merely a response to temperature: Similar ambush methods are observed in few snakes were observed at night from August Crotalus horridus and C. viridis (Reinert et al., to September when the air temperatures were not MORI ET AL. -SNAKE FEEDING BEHAVIOR 113 lower than those of the previous months when snakes (Thamnophis): social behavior as a possible nocturnal activity was observed. In addition, defense. Ecology 59 (5): 1014-1022. no snakes were seen at night in any other sites of BROWN, W. S. ANDW. S. PARKER. 1976. A ventral the Botanic Garden except the breeding season scale clipping system for permanently marking of Rhacophorus arboreus. snakes (Reptilia, Serpentes). J. Herpetol. 10(3): 247-249. However, temperature may affect the foraging COOPER, W. E., JR. AND G. M. BURGHARDT.1991. mode of these snakes, heliothermic predators Vomerolfaction and vomodor. J. Chem. Ecol. which depend on high body temperature to pur- 16(1): 103-105. sue and lunge at prey (Fukada, 1985, 1989; see CUNDALL,D. 1987. Functional morphology. In: Greenwald, 1974). It is probable that active R. A. Seigel, J. T. Collins and S. S. Novak (eds.), foraging, which involves rapid pursuit of the Snakes. Ecology and Evolutionary Biology. p. prey, is an inefficient method of prey capture dur- 106-140. Macmillan Publ. Company, New York. ing cooler nights. In fact, the body temperature DILLER, L. V. 1990. A field observation on the of E. quadrivirgata, E. climacophora, and feeding behavior of Crotalus viridis lutosus. J. Rhabdophis tigrinus was almost identical to the Herpetol. 24 (1): 95-97. ambient air temperature. Consequently, the FUKADA,H. 1954. Biological studies on the snakes. I. Observations on hatching of the striped snake, only possible foraging mode of these snakes Elaphe quadrivirgata (Boie). Bull. Kyoto Gakugei at night may be ambushing because of this Univ. Ser. B 5: 29-33. physiological constraint. The behavior of R. FUKADA,H. 1958. Biological studies on the snakes. tigrinus, whose reported mean body temperature IV. Seasonal prevalence in the fields. Bull. Kyoto during daytime activity is lowest among the three Gakugei Univ. Ser. B 13: 22-35. species (Fukada, 1990), was somewhat different FUKADA,H. 1959. Biological studies on the snakes. from that of the Elaphe: some R. tigrinus were V. Food habits in the fields. Bull. Kyoto Gakugei found on the ground or in the water, the ambush Univ. Ser. B 14: 22-28. posture was less stereotyped, and they appro- FUKADA,H. 1960. Biological studies on the snakes. ached the frogs in response to their movements VII. Growth and maturity of Elaphe quadrivirgata and attacked them from a greater distance than (Boie). Bull. Kyoto Gakugei Univ. Ser. B 16: 6-21. E. quadrivirgata (Table 2). FUKADA,H. 1985. Body temperatures of snakes in the fields. 1. Elaphe quadrivirgata. Jpn. J. The present findings, together with previous Herpetol. 11(1): 25-32. field observations (Ota, 1986; Mori, 1989), in- FUKADA,H. 1989. Body temperatures of snakes in dicate the flexibility of the foraging tactics of E. the fields. 2. Rhabdophis tigrinus, Elaphe quadrivirgata even under thermal constraints. climacophora, Amphiesma vibakari, and Elaphe It would be advantageous for the snake to be conspicillata. Jpn. J. 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要旨野外におけるシマヘビ,アオダイショウ及び,ヤマカガシの捕食行動の観察:夜行性のカエルに対する待ち伏せ行動森哲・戸田光彦・門脇正史・森口一シマヘビ,アオダイショウ,および,ヤマカ 10例観察された.以上の事実から,これらのヘガシは直射日光により体温を上げて餌を探しまビは,繁殖期間中に樹の幹を日周活動の通り道わる昼行性の捕食者であることがこれまで報告に利用しているモリアオガエルを,この位置で "積極的に"待ち伏せしていることが示唆されされているが,今回,モリアオガエルの繁殖池で夜間に活動しているのが観察された.これらた.ヘビの捕食戦術に影響を与えている要因にのヘビの夜間の季節活動のピークは,モリアオついて考察した. ガエルの季節活動のピークとよく一致した.観 (606-01京都市左京区北白川追分町京都大察されたヘビのほとんどは樹の枝上で胴体前部学理学部動物学教室(森)・920金沢市丸の内をのばして頭部を幹または下方向に向け静止し 1-1金沢大学大学院自然科学研究科生命科学ていた.これらのヘビは下顎または側頭部を幹専攻環境生物学講座(戸田・門脇)・379-23群の表面にぴったりと接していることが多かった. 馬県新田郡薮塚本町日本蛇族学術研究所(森 3種のヘビによるモリアオガエルの捕食行動は口))