Short-term movements of Boiga nigriceps (Günther, 1863), with notes on its diet (Squamata: Colubridae) - Biotaxa

Short-term movements of Boiga nigriceps (Günther, 1863), with notes on its diet (Squamata: Colubridae) - Biotaxa
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
  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,
  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
  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
  Research, Development and Innovation Division, Sarawak
                                                                  north of the pond was a paved mountain trail stretching
   Forest Department, 93250, Kuching, Sarawak, Malaysia
  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:
                                                                  are commonly observed (Kanto Nishikawa, unpubl.
© 2020 by Herpetology Notes. Open Access by CC BY-NC-ND 4.0.      data): Abavorana luctuosa, Chalcorana raniceps,
Short-term movements of Boiga nigriceps (Günther, 1863), with notes on its diet (Squamata: Colubridae) - Biotaxa
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
( Movement          the ground, using an Extech 39240 waterproof stem
data are deposited in Movebank (https://www.                thermometer with 0.1°C accuracy., 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
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,
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