Wednesday, January 16, 2013

Galápagos Racers


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 Galápagan 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

ACKNOWLEDGMENTS


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.

REFERENCES

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.

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

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>

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