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Tuesday, January 29, 2013

Africa's Giant Gaboon Vipers

For as long as I can remember, I've been impressed by Gaboon Vipers (Bitis gabonica). These western African behemoths can reach 5 3/4 feet in length and over 14.5 inches in girth, and weigh up to 25 pounds with an empty stomach. They are the heaviest vipers and possess the longest fangs, up to one and a half inches in length! Furthermore, their geometric dorsal pattern, as intricate as it is beautiful, is ideally suited to camouflaging them against the leafy forest floor, where they lie in wait for their endothermic prey: birds, rodents, rabbits, monkeys, small antelope, porcupines.

A Gaboon Viper, beautifully camouflaged
Like many vipers, Gaboon Vipers are ambush predators, a lifestyle to which they are supremely adapted. Long folding fangs and deadly venom allow them to kill their prey while keeping a safe distance from it. A very low metabolism permits them to wait in one spot for weeks, until the perfect opportunity presents itself. They spend between three-quarters and 95% of their time just sitting quietly, sometimes for up to three months at a time. Every so often, a viper, particularly a male during the breeding season of March through May, will embark on a long-distance movement of one quarter to two thirds of a mile, sometimes in a single day.1 The preferred habitat of Gaboon Vipers is a mosaic of forest, thicket, and grassland, although they will sometimes enter sugarcane fields and rural gardens. As with most snakes, life as a Gaboon Viper is probably pretty dull.

The impressive fangs of a Gaboon Viper
In spite of its impressive size, or perhaps because of it, Gaboon Vipers are, like many of their kin, docile and retiring. "On two occasions, I accidentally stepped directly on B. gabonica during the course of radiotracking, only becoming aware of this after feeling squirming movement beneath my foot. At no point during either encounter did the snake hiss or show aggression in any manner", writes Jonathan Warner in his dissertation, which also contains evidence that hippos, elephants, and leopards may walk right by Gaboon Vipers without noticing them. Being stepped on and squashed by these large herbivores might be the primary cause of mortality for adult vipers, which are not vulnerable to many natural predators.

You can see why
Although Gaboon Vipers produce prodigious amounts of venom (nearly 10 mL), the toxicity is rather low compared to other venomous snakes, and there are only a few detailed clinical reports of bites. They are undoubtedly dangerous snakes, but envenomations are few compared to such infamous species as the Russell's Viper. Like most snakes, particularly slow-moving ones with good camouflage, Gaboon Vipers usually sit still and remain unnoticed whenever a human comes nearby (so in other words, pretty much the exact same thing they were already doing).

Gaboon Viper plate from Duméril, Bibron, & Duméril's Erpétologie Générale;
unfortunately, this is one of the only plates not in color
Like most vipers, female Gaboon Vipers give birth to a litter of live young once every two to three years, usually between 20 and 40. Females do not eat while pregnant. Little is known about their reproductive behavior, but males combat one another over females, which must be an impressive sight. Much recent research on Gaboon Vipers has taken place in South Africa, where they are known as Gaboon Adders. In the southernmost populations, which are disjunct from the main range of the species, the climate is subtropical and seasonal differences in activity are observed, but radiotelemetry studies conducted in tropical areas of Cameroon and Nigeria show no seasonal changes in behavior.2

East African Gaboon Viper (B. g. gabonica)
What's the thing on their nose for? It is much larger in the West African subspecies than in the East African one. Hypotheses range from enhancing crypsis to doing nothing at all. Darren Naish at Tetrapod Zoology has addressed this question, but it seems he met with about the same amount of success as I did in finding a compelling, well-supported reason why these snakes have horns. I couldn't find any studies that examined whether the horns had a sensory function, although it certainly seems possible.

West African Gaboon Viper (B. g. rhinoceros)
I learned something new about these vipers recently. It seems that, among other heavy-bodied snakes, they have evolved the ability to retain their feces for incredibly long periods of time - months to years, after which time 5-20% of the body weight of a single snake may be feces. While this would kill a human, retained fecal material may be functioning as metabolically inert ballast in these species, which require a stationary inertial base for striking. Available data suggest that enhanced uptake of water and nutrients can also be achieved in snakes retaining feces - the poisonous urates (read: pee) are excreted more frequently. Amazing.

1 One exception is that these snakes always move following shedding, which occurs about twice a year, perhaps to distance themselves from potential predators attracted by the sloughed material or to avoid external parasites in the old skin that could reattach to the snake.

2 Snake biologists in Africa face challenges unfamiliar to we North Americans: "In several instances, I had to abort tracking efforts due to B. gabonica locations in close proximity to potentially dangerous game; namely [Water Buffalo, Rhinoceros, Elephant, Hippopotamus, and Crocodile]", writes Jonathan Warner in his dissertation.


Thanks to Tim Vickers, Wolfgang Wuster, Ivica, Markus Oulehla, and Jonathan Warner for their photos.


Lillywhite HB, de Delva P, Noonan BP (2002) Patterns of gut passage time and chronic retention of fecal mass in viperid snakes. In: Schuett GW, Höggren M, Douglas ME, Greene HW (eds) Biology of the Vipers. Eagle Mountain Publishers, Eagle Mountain, UT, pp 497-506

Linn I, Perrin M, Bodbijl T (2006) Movements and home range of the gaboon adder, Bitis gabonica gabonica, in Zululand, South Africa. Afr Zool 41:252-265

Luiselli L (2006) Site occupancy and density of sympatric Gaboon viper (Bitis gabonica) and nose-horned viper (Bitis nasicornis). J Trop Ecol 22:555-564

Marsh NA, Whaler BC (1984) The Gaboon Viper (Bitis gabonica): Its biology, venom components and toxinology. Toxicon 22:669-694

Warner JK (2009) Conservation Biology of the Gaboon Adder (Bitis gabonica) in South Africa. PhD dissertation, School of Animal, Plant, and Environmental Sciences, University of the Witwatersrand, Johannesburg, South Africa.

This Gaboon Viper quilt was made for me by
my mother on my 21st birthday.
The geometric pattern lends itself perfectly
to quilting.

Creative Commons License

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

Wednesday, January 16, 2013

Galápagos Racers

For answers to your questions about the Galápagos Racer/Marine Iguana chase scene from Planet Earth II, click here!

Sadly, snakes are inactive this time of year in northern Utah. With lower-than-average temperatures dipping below 0° F every night, I've had a lot of time indoors to read and daydream about snakier times and places. You can imagine my envy of my colleagues Susannah French, Nick Kiriazis, and Lori Neuman-Lee, who are currently in the Galápagos Islands studying Marine Iguanas. Nick, a student teacher at South Cache Middle School in Hyrum, Utah, is blogging about their research experiences at his blog, The Learning Scientist, which you should check out. Before they left, I instructed them that they were not to pass up an opportunity to observe the endemic racers of the Galápagos, the only snakes to inhabit the famous archipelago (other than one sea snake species, found offshore).

Galapagos Snake from Bartholome Island
Before I began the research for this article, I was under the impression that there was only a single species of Galápagos Racer. But speciation is quick to act in the birthplace of research on evolutionary principles, and in fact there are five species, each inhabiting different groups of islands. This isn't too surprising. As with most organisms inhabiting the Galápagos (and archipelagos in general), new species evolve on different islands over a relatively short period of time. Archipelagos are natural classrooms for evolutionary biologists, each island differing slightly in a number of qualities: size, rainfall, elevation, isolation. These differing characteristics, combined with long periods of reproductive isolation from populations on other islands, result in the evolution of endemic species to each island or group of closely spaced islands. Many groups of organisms have colonized the volcanic Galápagos from South America since the islands rose from beneath the sea some 8 million years ago. In that time, many of the plants and animals have diversified and speciated, leaving behind a pattern of relationships so telling that the idea of evolution by natural selection was formulated based on observations Charles Darwin made of the islands' fauna (for a recap, follow the link above to see a video to which my friend Rosemary Mosco contributed artwork).

Galapagos Snake eating a lava lizard (Microlophus sp.)
Although one might think that Darwin probably discovered most of the species native to the Galápagos Islands, others had been there before him. The first mention of these snakes in the scientific literature came 80 years before Darwin was born, by explorer William Dampier, a natural historian whose scientific accomplishments have been largely overshadowed by his reputation as a buccaneer. In his 1729 memoir, New Voyage Round the World, he wrote "There are some Green Snakes on [the Galápagos]; but no other land-animal that I did ever see." (Although they are hardly green.) In 1839 Darwin wrote "There is one snake which is numerous; it is identical, as I am informed by [French herpetologist Gabriel] Bibron, with the Psammophis Temminckii from Chile." This turned out not to be true, and the snakes were officially described as a new species in 1860 by Albert Günther, who named it Herpetodryas biserialis, a snake "light brown with a dark brown dorsal band" and having "maxillary teeth...of moderate size,...nearly equal length,...and entirely smooth". In 1912 John Van Denburgh, Curator of the Department of Herpetology at the California Academy of Sciences, wrote a monograph on the snakes of the Galápagos as part of a report on an expedition taken by the Academy in 1905-06. In it, he gave what is still the most complete account of the snakes' (by then moved to the genus Dromicus) natural history and ecology, and described four additional species (one of which was later synonomized with an earlier description by Austrian biologist Franz Steindachner). In accordance with the prevailing geologic thinking of the day, Van Denburg suggested that the snakes, along with the other Galápagos fauna, had reached the Galápagos via a land bridge, rather than by oceanic dispersal.

Plate from Steindachner 1876
Galápagos racers mating
Today these five species are known as Pseudalsophis biserialis (which occurs in two phases, dark and light, and is found on most islands), P. dorsalis (which is striped and found on Santiago, Rábida, Baltra, Santa Cruz, and Santa Fé), P. hoodensis (which is striped and found only on Gardner and Española, formerly known as Hood Island), P. slevini (which is banded and found on Fernandina, Isabela, and Pinzón), and P. steindachneri (which is striped and found on Santiago, Rábida, Baltra, and Santa Cruz). One interpretation is that these five species comprise a clade closely related to the mainland species P. elegans, found in Ecuador and Peru and supporting Bibron's suggestion via Darwin that Galápagos racers are related to and descended from mainland snakes. Variation in pattern and scalation is present but relatively minor. Another is that there were two independent colonizations of the Galápagos by South American snakes: one by Philodryas chamissonis from Chile (this is Bibron's Psammophis temminckii) that gave rise to P. hoodensis, and the other, by P. elegans, responsible for the other species. Others have suggested that P. slevini and P. steindachneri are descended from one ancestor (possibly a species of Antillophis from the Caribbean), P. hoodensis from another (P. chamissonis), and P. biserialis and P. dorsalis from yet a third (P. elegans, which might have invaded more than once). These hypothesized dispersal pathways are similar to those suggested for Tachysphex wasps from Chile, Microlophus lizards from Peru, and shrubs of the family Nolanaceae from Peru and Chile.

Galápagos Snake among some Marine Iguanas
To me, the first explanation seems the most parsimonious; that is, it is simplest to assume that all snakes on the Galápagos archipelago are descended from a single ancestor (P. elegans), and this is what the most up-to-date taxonomy reflects. Similarities in ecology might account for the morphological similarities that led earlier herpetologists to suggest Chilean and Caribbean ancestry. Although on some islands multiple species are found, this could be explained by multiple movements among islands of the archipelago following clonization and subsequent speciation. A 2008 review by University of Texas evolutionary biologist Christine Parent and her colleagues found that most of the Galápagos terrestrial fauna with known phylogenies (family trees), such as tortoises and finches, have diversified in parallel with the geological formation of the islands, supporting the idea that they colonized the islands only once. Because no detailed molecular phylogeny is available for Galápagos snakes, the final answer to the question of their origin and relationships has not yet been revealed.

Galápagos snake eating a small iguana
I was surprised to find how poorly known these snakes were given the infamy of the islands they inhabit. Because they are probably important predators on Galápagos finches, mockingbirds, and small lizards, as well as on non-native rodents, they deserve more study (although as Nick has pointed out, permission to study animals in the Galápagos can be difficult to obtain). Along with many of the Galápagos' other reptiles and birds, the snakes were probably almost driven to extinction by introduced cats and rats, against which they had not evolved defensive behaviors. Their natural predators probably include Galápagos mockingbirds. Today, tourism and development, although limited in the Galápagos, probably threatens these snakes as much as invasive species, which are beginning to be brought under control. Nick, Lori, and Susannah are busy studying the effects of tourism on the Marine Iguanas - who is studying the Galápagos snakes?

Even in the Galápagos, snakes are not immune to vehicular manslaughter


Special thanks to Nick Kiriazis for inspiring me to write this article with his blog, and thanks to Lori Neuman-Lee, Manuel Mejia, Dave Irving, Jim Moulton, Rosalind Gomes, and Phillip Marsh for their pictures.


Grehan J (2001) Biogeography and evolution of the Galápagos: integration of the biological and geological evidence. Biol J Linn Soc 74:267-287 <link>

Günther A (1860) On a new snake from the Galápagos islands. The Annals and Magazine of Natural History 3:78-79

Parent CE, Caccone A, Petren K, 2008. Colonization and diversification of Galápagos terrestrial fauna: a phylogenetic and biogeographical synthesis. Philosophical Transactions of the Royal Society B: Biological Sciences 363:3347-3361 <link>

Steindachner F (1876) Die schlangen und eidechsen der Galapagos-inseln. Zoologisch-botanischen Gesellschaft, Wien, Germany. <link>

Thomas R, 1997. Galápagos terrestrial snakes: biogeography and systematics. Herpetol Nat Hist 5:19-40 <link>

Van Denburgh J (1912) Expedition of the California Academy of Sciences to the Galapagos Islands, 1905-1906. IV. The snakes of the Galapagos Islands. Proceedings of the California Academy of Sciences (Series 4) 1:323-374 <link>

Zaher H, Grazziotin FG, Cadle JE, Murphy RW, Moura-Leite JC, Bonatto SL, 2009. Molecular phylogeny of advanced snakes (Serpentes, Caenophidia) with an emphasis on South American Xenodontines: A revised classification and descriptions of new taxa. Pap Avulsos Zool (Sao Paulo) 49:115-153 <link>

Creative Commons License

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.