Daboia russelli siamensis

Colubridae, subfamily Viperinae (True vipers)

128 cm

Localized areas of southern and south-eastern Taiwan, up to 500 m altitude. Rare.

South China (Fujian, Guangdong, Guangxi, Hunan, Yunnan), Taiwan, Myanma, Thailand, Cambodia, Indonesia (E Java, Komodo, Flores, Lomblen, Endeh)

Medium-sized snake; total length up to 128 cm. There are 21-33 (27-29 at mid-body) rows of scales, which are strongly keeled and rough (except for the outermost row). Head is triangular and distinct from neck; body is stout, slightly flattened; tail is short. Eye is medium-sized; iris is dark gray black, dappled with dirty yellow, pupil is vertically elliptical, black with conspicuous gray to yellow border. Nostril is very large, conspicuous, located in concavity on side of nose. The supraocular is well developed; there are no pits between eye and nostril. Tongue is dark gray to black, outer fork tips lighter. Fangs are large, hinged from upper jaw. Upper head is chocolate to dark brown black, with three large prominent designs on head; one between eyes, and the other two posteriorly. The scales on upper head are small and equal in size, heavily keeled and rough. The supralabials are white to cream mottled with dark pigment. Upper body and tail are brown to gray brown or chocolate, with white- to light brown-edged blotches or designs of variable size, shape and distribution. The areas around mid-dorsal series of blotches appear as a chain; the lateral series of designs contain smaller blotches tending to alternate with the mid-dorsal ones. Ventral head and neck are white to very light gray, speckled with diffuse black pigment; chin plates and gulars possess areas of black in posterior field of each scale, presenting an overall polka-dot appearance. Ventral body is progressively darker posteriorly due to flecks of dark gray or black pigment, with one to five semicircular designs on the ventrals. Underside of tail is darker than body. Anal scale is entire, and subcaudals are paired.

D. russelli is mainly nocturnal, but can alter its behavior in cool weather and become more active during the day.

This species is ovoviviparous. Mating generally occurs early in the year, although gravid females may be found at any time. The gestation period is more than six months. Young are produced from May to November, but mostly in June and July. It is a prolific breeder. Litters of 20–40 are common, although there may be fewer offspring and as little as one. The reported maximum is 65 in a single litter. At birth, juveniles are 215–260 mm in length. The minimum length for a gravid female is about 100 cm. It seems that sexual maturity is achieved in 2–3 years. In one case, it took a specimen nearly 4.5 hours to produce 11 young.

Although this genus does not have the heat-sensitive pit organs common to the Crotalinae (pitvipers), it is one of a number of viperines that are apparently able to react to thermal cues, further supporting the notion that they too possess a heat-sensitive organ. The identity of this sensor is not certain, but the nerve endings in the supranasal sac of these snakes resemble those found in other heat-sensitive organs.

The LD50 in mice, which is used as a general indicator of snake venom toxicity, is as follows: 0.08–0.31 μg/g intravenous, 0.40 μg/kg intraperitoneal, 4.75 mg/kg subcutaneous. For most humans a lethal dose is 40–70 mg. In general, the toxicity depends on a combination of five different venom fractions, each of which is less toxic when tested separately. Venom toxicity also varies within populations and over time.

Envenomation symptoms begin with pain at the site of the bite, immediately followed by swelling of the affected extremity. Bleeding is a common symptom, especially from the gums, and sputum may show signs of blood within 20 minutes post-bite. There is a drop in blood pressure and the heart rate falls. Blistering occurs at the site of the bite, developing along the affected limb in severe cases. Necrosis is usually superficial and limited to the muscles near the bite, but may be severe in extreme cases. Vomiting and facial swelling occurs in about one-third of all cases.

Severe pain may last for 2–4 weeks. Locally, it may persist depending on the level of tissue damage. Often, local swelling peaks within 48–72 hours, involving both the affected limb and the trunk. If swelling up to the trunk occurs within 1–2 hours, massive envenomation is likely. Discoloration may occur throughout the swollen area as red blood cells and plasma leak into muscle tissue. Death from septicaemia, respiratory or cardiac failure may occur 1 to 14 days post-bite or even later.

Because this venom is so effective at inducing thrombosis, it has been incorporated into an in vitro diagnostic test for blood clotting that is widely used in hospital laboratories. This test is often referred to as Dilute Russell's viper venom time (dRVVT). The coagulant in the venom directly activates factor X, which turns prothrombin into thrombin in the presence of factor V and phospholipid. The venom is diluted to give a clotting time of 23 to 27 seconds and the phospholipid is reduced to make the test extremely sensitive to phospholipid. The dRVVT test is more sensitive than the aPTT test for the detection of lupus anticoagulant (an autoimmune disorder), because it is not influenced by deficiencies in clotting factors VIII, IX or XI.

The Chinese name 鎖蛇 (suo3she2) means "chain link (鎖) snake".

Some herpetologists believe that, because D. russelli is so successful as a species and has such a fearful reputation within its natural environment, another snake has even come to mimic its appearance. Superficially, the rough-scaled sand boa, Gongylophis conicus, has a color pattern that often looks a lot like that of D. russelli, even though it is completely harmless.