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Introduction

The family Viperidae is a widespread group of snakes known for their advanced fang structures. The long-recurved fangs are held against the roof of the mouth when not in use and hinge forward when biting. The group is monophyletic and contains over 300 species across 38 genera. The family is split into three subfamilies with the two largest being Crotalinae (New World), containing rattlesnakes and pit-vipers, and Viperinae (Old World), containing adders and vipers

Viperidae are well represented in Africa. The Viperinae consist of the vipers/adders with numerous small head scales and being ovoviviparous (retaining eggs and giving live birth). They are medium-sized snakes, usually under 2 meters (7 feet) in length. However, they can be massive in girth and the larger species such as Bitis gabonica or Bitis arietans can be as thick as a man’s arm. The genus Causus (Night Adders) was proposed to be in a separate family – Causinae – as they possess plated head scales much like the elapids and are oviparous (egg-laying). Additionally, they have short fixed fangs much like the Elapidae. However, molecular work showed the Causus were well nested within Viperinae.

The Viperinae have radiated throughout Africa, producing four genera. There are several arboreal Bush Vipers in the genus Atheris found in the rainforests of central, east, and west Africa. The genus Echis is also species rich and adapted for the dry regions of north Africa. The monotypic genus Proatheris has been designated for a medium-sized viper, Proatheris superciliaris, found in East Africa down into Malawi and Mozambique. However, the most specious radiation is found in the terrestrial adders of the genus Bitis with the majority of these species occuring in the deserts and mountainous regions of western southern Africa.

Currently 18 Bitis species, and one extinct species, are recognised in Africa. The genus Bitis is split into large and small, or dwarf adders. The Large adders are generally distributed around the equator in the rainforest regions of Africa. The exception being the Puff Adder (Bitis arietans) which is widely distributed across much of sub-Saharan Africa with an isolated population in Morocco and into the Arabian Peninsula, usually avoiding the central rainforest regions. The large Bitis are generally considered closely related and we see several hybrids, usually from the trade, appear – B. arietans x B. gabonica and B. rhinoceros x B. nasicornis.

The large adders are comprised of six species. Five of the six species are associated with tropical forests around the equatorial region, with B. gabonica extending as far south as north-eastern South Africa. Two species, B. parviocula and B. harenna are Ethiopian endemics, found in the high montane forests of the Bale mountains. The common and widespread Puff Adder is the sixth large adder. It is separated from the other large adders as it is not restricted to equatorial forest (in fact, it is often absent from true forest) and can be found from sea level to high altitude grasslands and desert dunes in most of sub-Saharan Africa. In a phylogenetic study, the Puff Adder came up separate to the other large adders and is in separate subgenus. Surprisingly, recent genetic work on the Angolan Dwarf Adder (Bitis heraldica) has shown a close relationship to the large Bitis instead of the smaller dwarf adders. Dwarf Adders have disjunct populations and often show colour polymorphism and often locality dependent (geographically consistent). These adders can be common and widespread such as B. caudalis or localised and isolated (B. albanica and B.inornata). Dwarf Adders from the southern and eastern Cape regions of South Africa have been known to cause taxonomic confusion and are often referred to as the cornuta-inornata complex.

The genetic tree and relationship of Dwarf Adders is also unclear, and many phylogenetic trees have been proposed showing the relationship. It appears there are three major groups with the status of the Kenyan Horned Adder (Bitis worthingtoni) still questionable. In fact, the Kenyan Horned Adder has even been suggested to be in its own genus, Keniabitis.

The southern dwarf adder species appear to have originated around 13.5 million years ago from a basal species that become the Horned Adder (Bitis caudalis). This species then radiated as the African continent tilted and the landscape in the west became arid. This gave rise to Berg Adders (Bitis atropos) moving north-east along the great escarpment, reaching the eastern highlands of Zimbabwe before becoming split and disjunct as rivers eroded and separated mountain ranges. This is our first group and appears to be made up of at least four potential cryptic species, each found on a separate mountain range (Cape fold mountains, Drakensberg Mountains, Mpumalanga and Limpopo highlands and the Zimbabwe eastern Highlands). The Berg Adders are grassland or fynbos specialists, occupying the widest range of altitudes from sea level in the Cape provinces to over 2500 metres above sea level in the Drakensberg and Lesotho. The second group, formed by three species, (Bitis peringueyi, Bitis caudalis and Bitis schneideri) are arenicolous or sand dwelling species. The Horned Adder (Bitis caudalis) radiated throughout southern Africa along the west coast from Angola down to the South Africa and east through Botswana and the western edge of Zimbabwe. The other two species are more localised, hugging the cool, misty west coast of South Africa, Namibia and Angola. In Namibia there appears to be hybridisation between B. peringueyi and B. caudalis, producing a strange animal sometimes referred to as Bitis paucisquamata (see more under Bitis schneideri). Whether these hybrids are fertile is still unknown. These strange adders may also represent a separate, cryptic species.

The third major group is made up of rupicolous or rock dwelling species. They are however not entirely rock dwellers with Bitis cornuta in the Namib desert quite comfortable in the sand dunes. Bitis inornata is also comfortable in grasslands at high altitudes and Bitis armata lives among dense vegetation in coastal planes. However, for easier grouping and based on their shared morphological traits and genetic relationship, we therefore refer to them as the rock dwellers or the cornuta-inornata group. These species radiated along the Cape Fold Mountains and the Great Eastern Escarpment (much like Bitis atropos and often sympatric), but only extended as far east as Makhanda (Grahamstown), with Bitis albanica being the eastern most species in this group. The genetic status of Bitis albanica is unfortunately still unclear and proposals to synonymise it into Bitis rubida have been made, suggesting it is a very new split/radiation and does not warrant species status.

Subgenera

The genus Bitis has been split into four subgenera based on genetics. These subgenera show how species are related and are a way of grouping the Bitis. Genetics on Bitis is still relatively new and many species have few sequences as the animals are cryptic and samples are hard to obtain. Bitis heraldica was only recently (2019) sampled and the relationship within the Bitis was surprising. Macrocerastes – this group is made up of the large Bitis – B. gabonica, B. rhinoceros, B. nasicornis, B. parviocula and B. herenna and quite surprisingly, Bitis heraldica. These Bitis generally have a tropical distribution around the equator, with Bitis gabonica going as far south as the north coast of South Africa. Bitis contains only B. arietans. It is a widespread species across much of Africa and onto the Arabian Peninsula. It is generally absent from tropical forest. Calechidna contains most of the dwarf adders and is distributed mainly in the dry south-western region of Africa. – B. armata, B. albanica, B. atropos, B. inornata, B. schnideri, B. rubida, B. xeropaga, B. caudalis, B. cornuta, B. peringueyi, Keniabitis contains only the endemic B.worthingtoni.

Some previous names for the Bitis genus Cobra (Laurenti 1768) – ignored for more than a century – medical confusion to elapids Vipera – used by Merrem in 1820 Echidna – used by Merrem in 1820 Cerestes – Wagler 1830 Clotho (Gray 1842) Calechidna – Tschudi (1845)