Night Adder Snakes

The Causinae are a monotypic subfamily of venomous vipers found only in subsaharan Africa. It was created for the genus Causus; a group considered to be among the most primitive members of the family Viperidae based on head scalation, oviparity, venom apparatus and the fact that they have round pupils. However, this is contradicted by recent molecular studies. Six species are currently recognized. They are commonly known as night adders.

These snakes are fairly stout, never growing to more than 1 m in length.

As opposed to most vipers where the head is distinct from the neck and covered with small scales, in Causus the head is only slightly distinct from the neck and covered with 9 large symmetrical head shields. Also, the eyes have pupils that are round instead of elliptical like other vipers. The rostral scale is broad, sometimes pointed or upturned. The nostril is located between 2 nasals and an internasal. The frontal and supraocular scales are long. A loreal scale is present, separating the nasal and preoculars. The suboculars are separated from the supralabials. The mandible has splenial and angular elements.

The fangs are different too. Unlike other vipers there is no hinge action where the prefrontal bone engages the frontal. However, since the maxillary bones rotate almost as far, the fangs can still be erected. The fangs themselves are relatively short. A fine line, or suture, is also present along the length of the fang, representing the vestigial edge where the groove lips meet (from incomplete fang canal closure).

The body is cylindrical or slightly depressed and moderately slender. The dorsal scales are smooth or weakly keeled with apical pits. The ventral scales are rounded and the anal plate single. The tail is short and the subcaudals can be either single or paired.

Among the viperids, another unique characteristic of this genus is that several species have venom glands that are not confined to the temporal area as with most vipers, but are exceptionally long and extend well down the neck. These venom glands, located on either side of the spine, may be up to 10 cm in length, with long ducts connecting them to the fangs.

There are also other internal differences that set the Causinae apart: they have unusually long kidneys, a well-developed tracheal lung with two tracheal arteries, and the liver overlaps the tip of the heart.