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 188.8.131.52 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.