Brachyorrhos
alternans Reuss, 1833 Museum Senckenbergianum, 1:155, pl. 9, Figure 3. Type
locality: "Java", Dr. Peitsch collector. Holotype: SMF 19465.
Homalopsis decussata
Schlegel, 1837 Essai sur la physionomie des serpentes, 2:344, pl. 13, Figures
14-16. Type locality: "Java", Collector:
Reinwardt. Holotype RMHL - 1157.
Miralia alternans
– Gray, 1842 Zoological Miscellany, p. 68.
Eurostus alternans
- Duméril, Bibron, and Duméril, 1854 Érpétologie générale...Reptiles, 7:957.
Hypsirhina alternans
- Jan, 1863 Elenco systematico degli ofidi, p. 78.
Enhydris alternans
- Haas, 1950 Treubia 20(3):575.
BMNH 63.12.4.22 bears the name Rhabdrion borneensis Bleeker and the specimen is listed as the type
of this species in the museum’s catalog. However, the name and a description were
apparently never published, but Günther (1872) reports “Rhabdion borneensis
Blkr.” as a synonym of this species. This species and Raclita indica may belong to a clade within the homalopsids.
Etymology: The name alternans
presumably refers to the alternating dark and light transverse bands on the
dorsum of this species, and comes from the Latin alternus, meaning “alternate.”
Distribution: Tthe Greater Sunda Islands: Indonesia
(Bangka, Belitung, Java, and Sumatra), and Malaysia (Sarawak, Island of
Borneo). The specimen ZRC 2.3523 [Sarawak, Sg. Stuum Muda, 21.1 km before Lundu
Ferry Point at Bg. Kayan] confirms the species presence on Borneo, which was
questioned by Steubing and Inger (1999). Miralia
alternans inhabits the Sunda Shelf and Philippines Bioregion of
Wikramanayake et al. (2002).
Diagnosis: Nineteen scale rows at mid-body; very wide
ventral scales; eight upper labials; two postocular scales; and a divided
internasal scale. The gestalt of this
species is that of a burrowing snake, with a short, narrow head. It also has
white transverse marking on the dorsum and the sides. Other homalopsids with 19
scale rows at mid-body include Hypsiscophis.
plumbea which has a single internasal scale, lacks white blotches, and has
a head that is distinct from the neck in large individuals; and R. indica which has seven upper labials,
one postocular scale, and 173 ventral scales or more.
Size: The largest
specimens measured by Bergman (1951) were a 490 mm SVL female, and a 474 mm SVL
male. The largest male measured for this study had a total length of 430 mm
with a 50 mm tail. Another male measured 434 mm but had an incomplete tail. A
female had a total length of 365 mm with a 45 mm tail. Five specimens, probably
neonates (SMF19465-68, 66189) had SVL from 123 - 126 mm (= 124.4mm), and tails that
were 14 - 19 mm (=
16.2mm). Bergman (1960) suggested size at birth is about 170 mm.
External Morphology: The head is short, barley distinct
from the neck, and very slightly depressed. The eyes are dorsolateral, and the
diameter is about half the eye-nostril distance.
On the
head the rostral scale is pentagonal, broader than tall, barely visible from
above. The nasals are semi-divided with the cleft touching the first labial or
the seam between the first and second labial. The internasal scales penetrate
the nasals; are divided; and are about half the size of the prefrontals, and are
in contact with the loreals. The prefrontals touch the loreal, supraocular, and
are symmetrical. The frontal is relatively short being equal to, or shorter
than the interorbital distance. The parietals are relatively long, about 1.4
times the length of the frontal. The supraocular is rectangular. The loreal is
in contact with the second and third labials. There is one preocular that is
taller than wide. There are two postocular scales; the bottom scale is smaller
than the upper scale. The temporal formula is 1 + 2 + 3. The upper labials
number eight; the largest can be the fourth or the sixth, the fourth labial
enters the orbit.
On the
chin the lower labials number 10 or 11. The sixth is the largest. The first
five contact the anterior pair of chin shields. The anterior chin shields are longer
than the posterior pair. The posterior pair is separated by a pair of smaller
scales. The chin shields are tuberculate. Gular scales number 6 - 8.
On the
body the anterior dorsal scales are smooth, the first row is ovate and the
scales become more lanceolate toward the midline. Dorsal scale rows on the neck and at mid-body
number 19 near the vent they are reduced to 17 or 18 rows. The posterior scales
are smooth except for those in the first to third rows in front of the vent
which are heavily keeled or tuberculate with one tubercle on each scale; in USM
56022 they are better developed than in FMNH 11092. Additionally the scales in
this region may have 4 - 5 very small tubercles on the posterior edge. The ventral
scales are wide, about 3.5 times the height of a nearby dorsal scale. Oddly the
ventral scales numbered 143 in three male specimens and 120 in one female; and this
species may have the widest ventral scales of any homalopsid. The anal plate is
divided and about twice as long as the preceding ventral.
On the
tail dorsal scales are similar to those on the body. They are ovate with 4 - 5
tubercles on the posterior edge. The subcaudal scales are divided and number
about 36 in two males (some of these are damaged count may be slightly off) and
27 in one female. The female’s tail is 13.6% of the SVL. Two males had tails
that were 14.8 and 16.8% of the SVL. At the base of the tail the width is 76%
of the height (average of two specimens).
Color and
Pattern. The crown of the head is a uniform black with a white transverse band
two scales wide posterior to the parietals. There are a few light spots on the
upper and lower labials and on the gulars. The black dorsum has 41 transverse bands
on scale rows 4-8, these are less than one scale wide. Some of these bands show
an incomplete alternating pattern while others completely transverse the
dorsum. There are also white transverse bands on scale rows 1 - 3; these are 1 -
1.5 scales wide and number 57. Some of these completely transverse the ventral
scales, others do not. Overall the venter has an appearance of alternating
black and white bands, some being incomplete. The dorsum and venter of the tail
are similar to the body in pattern and color.
Sexual
Dimorphism. Bergman (1960) noted no differences in the number of ventral scales
between males and females. However, he reported subcaudal scales in females to
number 23 - 27, and in males 32 - 34.
Examination
of the FMNH Borneo specimen Gyi (1970) reported to be alternans and found it to be Hypsiscophis.
plumbea. Gyi’s scale count data for this species appears to include scale
count data for H. plumbea, which he
considered to be a closely related species.
Natural History: The natural history of this rare snake
remains unknown. Iskandar (1987) reported two specimens were eaten by Cylindrophis ruffus, and he considered
the similarity in appearance of the two species aggressive mimicry; he also
reported this snake from the Kali Cakung, a river flowing through suburban
Jakarta, Indonesia. Overall the body form (small head, short tail) of this
snake suggests a burrowing species.
Reproduction: Bergman
(1960) collected snakes at "Djakarta" from 1933-1940. He had a sample
of 20 specimens. Seven of 12 females were gravid. Clutch size ranged from 6 - 16
(=
9). And his data suggested litter size increases with female body size. The
smallest gravid snake he measured was 403 mm SVL.