Short-term movements of Boiga nigriceps (Günther, 1863), with notes on its diet (Squamata: Colubridae) - Biotaxa
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Herpetology Notes, volume 14: 83-89 (2021) (published online on 09 January 2021) Short-term movements of Boiga nigriceps (Günther, 1863), with notes on its diet (Squamata: Colubridae) Kanta Fujishima1,*, Tomonori Kodama2, Yosuke Kojima3, Mohamad Yazid Hossman4, and Kanto Nishikawa5 Abstract. The Dark-headed Cat Snake Boiga nigriceps is Malay Peninsula southwards to Sumatra and Java, and a poorly studied arboreal colubrid from Southeast Asia. eastward to Borneo, where it mostly occurs in lowland In Kubah National Park, Malaysia, we radio-tracked two forests. These snakes prey upon a variety of vertebrates, individual B. nigriceps for three and 13 days, respectively, to evaluate their movement, and we also obtained gut contents including reptiles, birds, and small mammals (Greene, to assess their diet. Snakes were arboreal in 91% of telemetry 1989; Cox et al., 2012; Stuebing et al., 2014; Ahmad fixes. They were located in the understorey (< 3 m above Sah and Mačát, 2016). Although widespread, available the ground) at 20% and 62% of fixes during daytime and data regarding habitat use, activity patterns and diet night-time, respectively. Mean daily displacement of the two of this species are limited. Effective conservation and snakes was 29.0 m/day and 23.6 m/day, respectively. We used the dynamic Brownian Bridge Movement Model (dBBMM) management measures rely upon essential information to quantify the occurrence distribution for one of the snakes, pertaining to a species’ ecological role (Campbell et and the 95 and 99% dBBMM isopleth contour confidence al., 2008; Costa et al., 2010; Hjarding et al., 2015). To regions of the snake were 0.13 ha and 0.20 ha, respectively. quantify movement and gain insights into the ecology of Two of the three snakes we encountered contained prey items, B. nigriceps, we conducted a short-term radiotelemetry which in each case were Harlequin Tree Frogs, Rhacophorus pardalis. We hypothesize that the snakes’ tendency to be in study and also obtained gut contents. the understorey at night is associated with the distribution of anuran prey. Materials and Methods Keywords. Reptile, Spatial ecology, Displacement, dBBMM, Study site. We carried out our study in Kubah National Foraging strategy Park in Sarawak, Malaysian Borneo from 17–23 August 2017 and 11–30 August 2018. The climate of the region Introduction is equatorial. Temperature and rainfall are relatively uniform throughout the year, with daily air temperature The Dark-headed Cat Snake Boiga nigriceps (Günther, ranging from 23–32°C and annual rainfall from 6000– 1863) is a slender, nocturnal and arboreal colubrid 7000 mm (Hazebroek and Morshidi, 2000). We sampled reaching 175 cm in maximum total length (Malkmus the area around an approximately 20 m long and 8 m et al., 2002). This tropical species occurs from the wide natural pond (1.6065°N, 110.1887°E; elevation 307 m; Fig. 1A). The pond was surrounded by a slightly disturbed primary mixed dipterocarp forest, dominated 1 Graduate School of Informatics, Kyoto University, Yoshida by species of the family Dipterocarpaceae, along with Honmachi, Sakyo-ku, Kyoto 606-8501, Japan. species of Myristicaceae, Lauraceae, Burseraceae, 2 Graduate School of Science, Kyoto University, Kitashirakawa Ebenaceae, and Sapotaceae. The canopy covered the Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan entire pond. Trees around the pond were up to ca. 20 m 3 Department of Biology, Toho University, Funabashi-shi, Chiba in height (calculated by a trigonometric tangent formula; 274-8510, Japan Korning and Thomsen, 1994). Approximately 30 m 4 Research, Development and Innovation Division, Sarawak north of the pond was a paved mountain trail stretching Forest Department, 93250, Kuching, Sarawak, Malaysia 5 Graduate School of Human and Environmental Studies, Kyoto from east to west, which created a gap in the canopy. University, Yoshida Nihonmatsu-cho, Sakyo-ku, Kyoto 606- A 2-m-wide raised timber boardwalk encompassing 8501, Japan; and Graduate School of Global Environmental the pond allowed visitors to walk through the area and Studies, Kyoto University, Yoshida-Honmachi, Sakyo-ku, observe wildlife. The pond serves as a spawning site for Kyoto 606-8501, Japan several species of anurans, and the following species * Corresponding author. E-mail: email@example.com are commonly observed (Kanto Nishikawa, unpubl. © 2020 by Herpetology Notes. Open Access by CC BY-NC-ND 4.0. data): Abavorana luctuosa, Chalcorana raniceps,
84 Kanta Fujishima et al. Figure 1. Radiotelemetry study of Boiga nigriceps in Sarawak, Borneo. (A) The pond at the study site in Kubah National Park. (B) Transmitter being attached to the tail of B. nigriceps with adhesive (left) and tape (right). (C) Snake 1 observed during telemetry. (D) Snake 2 observed on a rattan palm branch (shown in the white circle). Photo by authors Kanta Fujishima (A, B, D) and Tomonori Kodama (C). Limnonectes leporinus, Microhyla malang, Polypedates the snake) with a flexible measuring tape. Observers otilophus, P. leucomystax, P. macrotis, and Rhacophorus measured snout–vent length (SVL) and tail length of pardalis. captured snakes with a measuring tape to the closest 1 Survey protocol and animal handling. One to three mm, and body mass with an electronic scale to the closest observers haphazardly searched for B. nigriceps every 0.1 g, then determined sex by tail shape and hemipene night during our survey period by walking along the eversion. When a snake was suspected to contain prey, we boardwalk with flashlights. When observers found a gently palpated the ventral surface to elicit regurgitation snake, they recorded the location on a handheld Garmin (Luiselli and Amori, 2016). These procedures were eTrex 20 GPS unit (accuracy 5–10 m) and measured done in the field without anaesthesia, follow the Animal perch height (the vertical distance from the ground to Experiment Guidelines of Kyoto University, and were
Movements and Diet in Boiga nigriceps 85 approved by the institutional review committee of the from varying distances (< 30 m) while circling around Graduate School of Human and Environmental Studies that tree. Perch height was directly measured when of Kyoto University and the Research, Development & snakes were visible. Otherwise, we categorized their Innovation Division of the Sarawak Forest Department arboreal locations into three groups: understorey (< 3 (approval no. 30-A-7). Prey items were then identified to m), middle storey (3–10 m), and above storey (> 10 m). the lowest taxonomic level possible. As an indicator for the thermal quality of the habitat, air Supplementary materials. Supplementary materials temperature at each fix was measured within 3 m of the associated with this article are available on Zenodo snake’s horizontal location at approximately 1 m above (https://doi.org/10.5281/zenodo.4032443). Movement the ground, using an Extech 39240 waterproof stem data are deposited in Movebank (https://www. thermometer with 0.1°C accuracy. movebank.org, ID: 1260272117). Telemetry evaluation. To evaluate overall horizontal Radiotelemetry protocol. In total, four individual B. movement, we calculated mean daily displacement nigriceps were captured during our nocturnal surveys (MDD: m/day) for each individual as the sum of linear (Supplementary Materials, Table 1). All individuals distances between consecutive fixes, divided by the total were found on vegetation overhanging or adjacent to number of days over which that individual was tracked. the boardwalk, at perch heights of 0.5–2 m. Two males To quantify occurrence distributions, we used the (Snake 1, SVL = 882 mm, tail length = 287 mm, 38.5 dynamic Brownian Bridge Movement Model (dBBMM; g; Snake 2, SVL = 675 mm, tail length = 203 mm, 34.5 Kranstauber et al., 2012). Unlike conventional methods g) received dorsally attached temperature sensitive such as minimum convex polygons or kernel density radio-transmitters (BD-2X; 0.35 g, 11.5 × 5.3 × 2.8 estimators, the dBBMM derives the confidence mm; Holohil Systems, Carp, Ontario, Canada) with a region for space use during the study period from an standard battery life of 21 days. Transmitters were < 5% animal’s movement trajectory, accounting for temporal of snake body weight. We attached transmitters to the autocorrelation between relocations. The model shows base of the tail (to avoid interference with digestion) high performance in detecting frequently used passages using adhesive and tape (Madrid-Sotelo and García- and shelter sites and has been successfully applied to Aguayo, 2008; Fig. 1B). We kept each snake in a plastic several snake species with varying space use patterns cage for up to 6 h prior to release to confirm that the (Silva et al., 2018; Barnes et al., 2019). We derived 95 transmitter attachment did not hinder its movements. and 99% dBBMM isopleth contour confidence regions We released snakes at the same location and perch (or occurrence distributions) to represent the maximum height as where we captured them. area used by the animal (Walter et al., 2011). We set Individuals were located once or twice a day dBBMM parameters as follows: the moving window (including at night-time) with a wideband radio-receiver size to 7 (accounting for 7 days), location error to 3 m, (IC-R10; Icom Inc., Osaka, Japan) and a handheld and margin size to 3 (12 h) to detect potential diurnal directional VHF antenna (RA-24K; Telonics, Mesa, and nocturnal differences in movement pattern. We Arizona, USA). Fixes were made by circling around a calculated dBBMM in the R environment (R Core Team, small area where the strongest signals were obtained, 2020) using the move package (Kranstauber et al., 2020). and assuming that the telemetered snake was within Diet. We obtained recently consumed prey items it (White and Garrott, 1990). To avoid disturbing the from two of the four snakes we captured: Snake 2 animals, we maintained a distance of at least 3 m from (measurements previously mentioned) and another their presumed locations while searching. Some of the individual not included in the radiotelemetry survey displacements were smaller than the accuracy of our (Snake 3, male, SVL = 799 mm, tail length = 268 mm). GPS unit. Therefore, we recorded the snakes’ horizontal Snake 2 regurgitated its prey on its own after capture, location at each fix by measuring the distance and while the prey of Snake 3 was obtained by palpation. direction from certain points on the boardwalk, or from the location of a previous fix, which we then manually Results plotted onto a map using ImageJ (Abramoff et al., 2004). Radiotelemetry. During telemetry activities, six fixes When a snake was not on the ground, we assessed its from 20–23 August 2017 were made on Snake 1, and vertical location by gradually changing the direction of 17 fixes from 18–30 August 2018 on Snake 2 (Fig. 2; the antenna from ground to canopy, searching for the Supplementary Material, Table 2). Average (± SD) time strongest signal. This was repeated at least three times lags between subsequent fixes were 601 ± 390 min
86 Kanta Fujishima et al. Figure 2. Locations of Snake 1 (orange, filled symbols) and 2 (blue, open symbols). Circles, squares, and triangles correspond to fixes at < 3 m, 3–10 m, and > 10 m above the ground, respectively. ‘R’ shows where the snakes were released, and ‘E’, where the tracking ended. The X- and Y-axis are the Universal Transverse Mercator (UTM) coordinates (Easting, Northing) of zone 49N. and 1005 ± 481 min for Snakes 1 and 2, respectively base of a tree with a 50 cm diameter. MDD of Snake 1 (density plots are available in the Supplementary and 2 were 29.0 and 23.6 m/day, respectively. The 95 Materials, Fig. 4). Air temperature at each fix ranged and 99% dBBMM of Snake 2 were 0.13 and 0.20 ha, from 21.5–27.8°C. Snakes were not visible for 86% (20 respectively. Snake 1 was omitted from the dBBMM out of 23) of fixes, presumably due to their position in analysis due to an insufficient number of tracking days. dense foliage in the canopy, bushes near the ground, We directly observed the snakes on three occasions. or terrestrial shelters. They were located above the Two observations occurred at night, when Snake 1 (Fig. ground in 91% (21 out of 23) of fixes. Snakes were 1C) and 2 were slowly moving on thin branches at 0.7 located in the understorey (< 3 m above the ground) and 2.5 m above the ground, respectively, and one was at 20% (two out of 10) and 62% (eight out of 13) of made before dark at 17:47 h, when Snake 2 was coiled fixes during daytime (06:00–18:00 h) and night-time motionless at 0.9 m above the ground on a spinose (18:00–06:00 h), respectively. Both individuals used rattan palm (Calamus sp.) branch (Fig. 1D). shelter sites on the forest floor, each on one occasion: Diet. All prey items were Harlequin Treefrogs, R. Snake 1 was located in the cavity of a fallen tree pardalis (Fig. 3). Snake 2 had ingested two frogs and (approximately 6 m in length and 30 cm in diameter), Snake 3 had ingested only one. and Snake 2 in an approximately 1 m wide hole at the
Movements and Diet in Boiga nigriceps 87 speculate that B. nigriceps similarly adopts an active foraging strategy, and possibly also ambushing to some extent. However, further observations on foraging behaviour are necessary to confirm this. The dBBMM occurrence distribution of Snake 2 during the 12-day tracking period was very small. The 95% dBBMM of T. macrops and T. vogeli, recorded by Barnes et al. (2019), averaged 0.66 and 2.34 ha, respectively (but with different tracking frequencies and dBBMM parameters than our study). These ambush-foraging pitvipers are expected to have limited space-use compared to active foragers. The amount of space used by snakes is generally correlated with tracking duration (Madsen, 1984), and given our short tracking period and apparently active nature of this snake, it is likely Figure 3. Rhacophorus pardalis regurgitated by a Boiga that seasonal and annual space use of B. nigriceps is nigriceps (Snake 3). Photo by Kyosuke Hamanaka. much larger than the occurrence distribution which we recorded. Long-term radiotracking would yield broader Discussion insights into the home range, movement and habitat use of this species. Our spatial and dietary data provide a snapshot of The two snakes we tracked were largely arboreal, the life history of B. nigriceps. Although quantitative using a wide range of heights. However, both ecological information on this species is scarce to date, a individuals were moderate in size, possibly juveniles. large database is available for the congeneric Brown Tree These snakes may show more terrestrial behaviour as Snake B. irregularis, which has been a major invasive body size increases, a trend seen in B. irregularis (Tobin species in Guam (Rodda and Savidge, 2007). Boiga et al., 1999) and many other snake species (Lillywhite irregularis is similar to B. nigriceps in morphology and and Henderson, 1993; Shine et al., 2002). The two presumably in ecology as well; therefore, we will mainly individuals were found in the understorey (< 3 m) more discuss our findings in the light of current knowledge on frequently after dark (62%) than during the day (20%). B. nigriceps and B. irregularis. Our small sample size and short tracking periods We first compared MDD and dBBMM data from preclude us from making any generalizations on the diel our study with those from previous studies of B. movement patterns of this species. Habitat selection and irregularis and other tropical arboreal snakes to infer movement of arboreal snakes is possibly influenced by a the spatial ecology of B. nigriceps. Lardner et al. complex interplay of various factors including substrate (2014) studied the movement of 18 B. irregularis for structure, microclimate, prey availability, and predation 1–15 days and found that the daily displacements of all risk (Lillywhite and Henderson, 1993; Anderson et al., individuals averaged 38 m. MDD of arboreal, ambush- 2005). Prey availability is known to have a particularly hunting vipers Trimeresurus macrops and T. vogeli profound effect on the foraging site selection of snakes were < 1 m during a 78-day tracking period (Barnes (Shine and Li-Xin, 2002; Wasko and Sasa, 2012). et al., 2019). Temporal resolution affects MDD, and Given that B. nigriceps is primarily a nocturnal forager, although our tracking frequencies were not exactly it can be hypothesized that the snakes’ presence in the equal to those of Lardner et al. (2014) and Barnes et understorey at night is associated with the distribution al. (2019), it can be said that MDD of B. nigriceps of prey, particularly frogs in the case of our study site. were comparable to B. nigriceps rather than that of During the day, the middle and above storey may provide the arboreal vipers. In addition, movements of B. favourable conditions for thermoregulation (Anderson nigriceps were seemingly continuous throughout the et al., 2005), foraging (Lillywhite and Henderson, tracking periods. The relatively constant, long-distance 1993), and/or protection from terrestrial predators movements of B. irregularis are associated with active (Shine and Fitzgerald, 1996; Tobin et al., 1999). Snakes prey searching, although individuals are known to were located twice in shelters on the forest floor. Boiga flexibly switch between active and ambush foraging irregularis also occasionally utilizes terrestrial shelters (Rodda, 1992; Lardner et al., 2014). We can thus
88 Kanta Fujishima et al. such as dead logs during daytime (Tobin et al., 1999). dimensional home ranges have been developed (Cooper For arboreal snakes, these confined spaces may serve as et al., 2014; Tracey et al., 2014). While these methods important refugia against predators, drought, or harsh require a large dataset of three-dimensional locations, weathers involving strong winds or rain (Lillywhite and they hold great promise in better representing the space Henderson, 1993). use of arboreal animals (Chandler et al., 2020). In There are few records on the diet of B. nigriceps. conclusion, further ecological research encompassing Greene (1989) reported a bird which had been three-dimensional methods is needed to ascertain swallowed head-first. Stuebing et al. (2014) recorded a the basic life history of B. nigriceps. Additionally, snake (ca. 40 cm) and some bird feathers as gut contents. integrative studies of both predator and prey are Ahmad Sah and Mačát (2016) observed an individual essential for testing the hypothesis of snake-frog (ca. 1 m) constricting a bat in a field station in Brunei relationships raised by this study, and for understanding Darussalam. Captive specimens have been maintained the complex food web in the area. on a diet of House Geckos, genus Hemidactylus (Cox et al., 2012). These observations suggest an opportunistic Acknowledgements. The State Government of Sarawak generalist diet for this species. In the present study, gut kindly permitted us to conduct our surveys in Kubah National Park (research permit NPW.907.4.4.(Old.14)-129, Park permit contents of B. nigriceps revealed a single anuran species: 66/2017, research permit (185)JHS/NCCD/600-7/2/107, and Park R. pardalis. It is presumably the most abundant anuran permit WL92/2018), and Kubah National Park and the Research, species at the pond (Kanto Nishikawa, unpubl. data). Development, and Innovation Division of the Sarawak Forest These treefrogs are usually sparsely spread across the Department provided facilities for research. We are grateful to canopy to forage or to rest (Inger et al., 2017). At night, Runi Anak Sylester Pungga, Paulus Anak Meleng, and Takao male frogs form breeding groups at low vegetation (1–3 Itioka for their support in obtaining permission for field research m above the ground) around spawning sites, which are and specimen exportation, to Akihiro Noda, Takaki Kurita, Kyosuke Hamanaka, and Shimpei Ochi for their support in the typically ponds or small streams with reduced gradients field, to Shuichiro Tagane, Asano Iku, and Usun Shimizu-kaya (Malkmus et al., 2002). Apart from this, little is known for plant identification, and to Hideaki Nishizawa for technical about the movements and abundance of R. pardalis, assistance and valuable comments on the manuscript. This study which could potentially have an influence on their was partly supported by the Asahi Glass Foundation, JSPS predators such as B. nigriceps. Ecological studies on Core-to-Core Program Type B Asia-Africa Science Platforms the prey species of B. nigriceps could provide valuable (Coordinator: Masaharu Motokawa), JST/JICA, SATREPS, and insights into the life history of the snake. Grant-in-Aid for JSPS Fellows to KN and YK. All procedures in this study followed the Animal Experiment Guideline of Kyoto In understanding the spatial ecology of arboreal University and were approved by the Graduate School of Human species such as B. nigriceps, we emphasize the and Environmental Studies, Kyoto. importance of assessing vertical movement, along with three-dimensional structure, vegetation type, and References canopy/understorey connectivity of the habitat. By doing so we can better represent habitat use, predator- Abramoff, M.D., Magalhaes, P.J., Ram, S.J. (2004): Image processing with ImageJ. Biophotonics International 11: 36-42. prey interactions, competition and niche partitioning, Ahmad Sah, H.H., Mačát., Z. (2016): Natural history notes. Boiga and provide important implications for conservation nigriceps (Dark-headed Cat Snake). Diet. Herpetological and management (Rodda, 1992; Martins et al., 2008; Review 47: 141–142. Barnes et al., 2019). 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