This is a slender and terrestrial species that grows to a maximum total length of 160 cm, although the average total length is much less. The head scalation includes 5-12 intersupraoculars that are weakly keeled, 7-9 supralabials of which the second is fused with the prelacunal to form a lacunolabial, and 9-13 sublabials. Midbody, there are 20-27 rows of dorsal scales. The ventrals number 170-216 and the 51-71 subcaudals are mostly paired. The color pattern is extremely variable, consisting of a dorsal ground color that may be tan, brown, gray, yellow, olive, or almost maroon. Midbody, this color is usually somewhat lighter than the head, anterior and posterior. The dorsal ground color is overlaid with a series of pale-edged, dark brown subtriangular or trapezoidal markings on either side of the body, the apices of which reach the vertebral line. These marking may be situated opposite each other, or partially or completely juxtaposed; most specimens have a pattern with all three variations. In juveniles, the tip of the tail is white. The head has a prominent dark brown stripe that runs from behind the eye, on either side of the head, back to the angle of the mouth, usually touching the last three supralabials. Dorsally, this stripe is bordered by a distinct pale area. The tongue is black, and the iris is gold to greenish gold with slightly darker reticulations.
Common names
The English common name is jararaca. In Argentina, it is called yarará and yararaca perezosa. In Brazil, it is referred to as caissaca, jaraca, jaracá, jararaca, jararaca-do-rabo-branco, jararaca-do-campo, jararaca-do-cerrado, jararaca-dormideira, jararaca-dominhoca and malha-de-sapo. In Paraguay and Uruguay, it is also called yarará.
Geographic range
This snake is found in southern Brazil, northeastern Paraguay and northern Argentina. The type locality is listed as "Lagoa d'Arara am Mucurí" by Wied-Neuwied in 1825. Occurs from near sea level to over 1,000 m altitude.
Habitat
The species prefers deciduous tropical forests and savanna country, as well as semitropical upland forests. It is said to favor open areas, such as farmland, with nearby vegetation cover.
Feeding
They prey on birds and small mammals.
Reproduction
Females are ovoviviparous, and are believed to produce about 20 young at a time.
Venom
Potency
This species is often abundant within its range of southeastern Brazil, where it was responsible for 52% of snakebites between 1902 and 1945, with a 0.7% fatality rate. The average venom yield is with a maximum of of dried venom. The venom is quite toxic. In mice, the median lethal dose is 1.2-1.3 mg/kg intravenous, 1.4 mg/kg intraperitoneal and 3.0 mg/kg subcutaneous. For humans, the LD50 is estimated to be subcutaneous.
Effects
Typical envenomation symptoms include local swelling, petechiae, bruising and blistering of the affected limb, spontaneous systemic bleeding of the gums and into the skin, subconjunctival hemorrhage and incoagulable blood. The systemic symptoms can potentially be fatal and may involve hemostatic disorders, intracranial hemorrhage, shock, and kidney failure.
