Causinae

Common names: night adders.^[2][3]

The Causinae are a monotypic subfamily of venomous vipers found only in subsaharan Africa.^[1] 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.^[2][3] Six species are currently recognized.^[4]

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Description

These snakes are fairly stout, never growing to more than 1 m in length.^[3]

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.^[2]

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).^[2]

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.^[2]

Among the viperids, another unique characteristic of this genus is that several species have venom glands that are not confined the temporal area as with most vipers, but are exceptionally long and extend well down the neck.^[3] 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.^[5]

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.^[2]

Geographic range

Subsaharan Africa.^[1]

Behavior

Despite their common name, this genus is active during the day as well as at night. When disturbed, they will engage in a ferocious hissing and puffing threat display. It may lift the first part of its body off the ground in a coil and make a powerful swiping strike -- juveniles have even been known to come off the ground! Others specimens may raise the first part of their body off the ground, flattening the neck and moving forward, tongue extended, like a small cobra.^[3] The rather frantic strikes are often combined with attempts to quickly glide away.^[6]

Feeding

Feed almost exclusively on toads and frogs.^[3] There are reports of gluttony: when prey is abundant, they may eat until they are literally unable to swallow any more food.^[6]

Reproduction

All Causus species lay eggs (oviparious), which among vipers is considered to be a more primitive trait, though not unique.^[2] The average clutch consists of some two dozen eggs that require an incubation period of about 4 months. Hatchlings are 4-5 inches (10-12.5 cm) in length.^[6]

Captivity

Best kept in dry and well-ventilated cages that include places to hide, fresh water and a basking spot slightly warmer than the rest of the cage (26 to 27 °C). They will accept pre-killed mice as food, but as they are voracious feeders, care must be taken to prevent obesity by overfeeding.^[6]

Venom

In spite of their enormously developed venom glands, night adders don't always use them to subdue their prey. The venom would act fast enough, but often they simply seize and swallow their prey instead.^[5]

Causus venom is weak and tends only to dribble from the fangs, so that relatively little is ever injected. Envenomation normally causes only local pain and swelling.^[2] Antivenin treatment should not be necessary. Nevertheless, South African polyvalent serum is known to be effective against the venom of at least two species.^[7]

No recent deaths have been reported due to this species. Earlier reports of fatalities were based on anecdotal evidence; it is likely the species involved were not properly identified or that the cases were grossly mismanaged.^[7]

Species

^T) Type species.

Taxonomy

There has been a long-standing tendency among herpetologists to make assumptions regarding the phylogenetic position of this group. According to Dr. Roy McDiarmid, lead author of the highly influential 1999 checklist, Snake Species of the World: A Taxonomic and Geographic Reference, vol. 1, the consensus among the experts at the time this book was published was that Causus is basal to all viperids and therefore deserves a separate subfamily. On the other hand, Dr. Wolfgang Wüster, who was a dissenting voice in that same consensus, argues that, despite its very different appearance, Causus may instead be rooted within the Viperinae, in which case it should not be in a separate subfamily. More evidence will be needed before this dispute is finally put to rest.^[1][8]

See also

• Causinae by common name
• Causinae by taxonomic synonyms
• Snakebite

References