|Galápagos Racers (Pseudalsophis occidentalis)|
on Fernandina Island, from the BBC's Planet Earth II footage
What kind of snakes are they?
Throughout the clip, Attenborough calls them "racer snakes"1, but herpetologists would normally call the snakes on the screen Galápagos Racers. Although these snakes are called "racers", they're not closely related to North American racers (genus Coluber); it's been about 45 million years since these two snakes last shared a common ancestor.
Galápagos Racers belong to the genus Pseudalsophis. Depending on which sources you consult, there are between 4 and 7 species of Pseudalsophis in the Galápagos, as well as one in mainland South America.
|Pseudalsophis slevini eating a gecko on Pinzón Island|
None of the sources reporting which species is shown in the film are authoritative, but without exception when the species is given it is given as Pseudalsophis biserialis. This is not correct under any modern taxonomy, although there is also a good explanation for why it is mistakenly being used—P. occidentalis was briefly a subspecies of P. biserialis, but has mostly been and is now treated either as a subspecies of P. dorsalis or as its own species. See below for much more (probably too much) detail.
|Galápagos Racer (P. dorsalis) among adult Marine Iguanas|
on Santa Cruz, which are much too large for it to eat
|Two P. occidentalis trying to eat the same iguana|
Jaw-walking is a fixed action pattern in snakes and they
may eat things that only vaguely resemble their food
once they start jaw-walking them.
From Planet Earth II Behind the Scenes
|Fates of rock iguana hatchlings, over half of which were|
eaten by Cubophis and Epicrates snake predators in their
first month of life. From Knapp et al. 2010
Many species of reptiles nest in areas where they otherwise do not spend much time, especially aquatic species (reptile eggs need to "breathe" air and cannot be laid underwater). Female Marine Iguanas may all use the same nesting sites because those are the only sites available, or they may choose to nest near one another because, just like with sea turtles, synchronous hatching of the young increases their probability of survival.
In a study of Bahamian rock iguanas (Cyclura cychlura), snake predation was the most likely cause of mortality for newborn iguanas dispersing away from their nests. They estimated that about 20-30% of hatchling iguanas survived their first month, and those that moved quickly and linearly away from their nests were the most likely to survive, perhaps because predators had learned to hang around the nesting area. Another study of Galápagos land iguanas showed that predation attempts by Galápagos hawks were more than three times as likely to be successful when the body temperature of the iguana hatchlings was below 90°F. And, baby Galápagos marine iguanas that hung around their hatching area had about a 10% lower survival rate than those that moved to the coast, which the researchers attribute mostly to higher risk of predation at the nesting area.
|Map showing the estimated age of each of the|
Galápagos Islands. From Ali & Aitchison 2014
One of the few phylogenies to include Galápagos Racers
From Pyron et al. 2013
In 1973, herpetologist Charles Myers wrote: "The classification of colubrid snakes in general, and of South American colubrids in particular, is in a notoriously unsatisfactory state." Unfortunately, we are not that much better off today when it comes to Galápagos Racers. It seems pretty clear that the nearest relative of P. biserialis, P. dorsalis, and P. occidentalis is Pseudalsophis elegans, the only species in the genus found on the mainland (in Ecuador, Peru, and extreme northern Chile). Beyond that, there isn't a lot of clarity about their next-closest relatives. They are possibly most closely related to obscure South American "groundsnakes" in the genus Psomophis, or to the even more obscure genus Saphenophis, which was described by Myers as "quite lacking in peculiar or unique features" and so named "in allusion to one incontrovertible fact about these snakes...from the Greek saphenes (evident truth, clear) + ophis (a serpent), meaning 'clearly a snake'". We don't really have a great hypothesis about how the different lineages of Galápagos Racers are related to one another, or even if they are all descended from a single common ancestor, because we only have DNA from one of them so far.
|Hypothesized scenario for the evolution of Pseudalsophis snakes|
So far, we have no DNA evidence that would support or refute this model
From Ali & Aitchison 2014
Maglio (1970) noted that the tooth counts and arrangement and the and shape of the premaxilla bone was most similar among the three Galápagos species that he examined (P. biserialis, P. dorsalis, and P. slevini), and different from the West Indian species that Taylor later suggested are P. slevini's closest relatives. More recently, a study led by Grazziotin claimed that they "unequivocally support...Zaher's (1999) hypothesis based on morphology that continental Pseudalsophis elegans is closely related to the Galápagos Island species of Xenodontinae (herein represented by Pseudalsophis dorsalis), rather than to West Indian Alsophis and Antillophis, and mainland Philodryas (Thomas, 1997)." However, they obviously didn't read Thomas's paper very carefully, because he also hypothesizes that P. dorsalis is closely related to P. elegans, and the Grazziotin paper didn't sequence any DNA from P. slevini, P. steindachneri, or P. hoodensis, and therefore didn't test any hypotheses about them.
As for whether or not the snakes in Planet Earth II should be called P. occidentalis or P. dorsalis occidentalis, that's really a lumper/splitter question. But, both the IUCN and the 2014 edition of Snakes of the World recognize P. occidentalis as a full species; it was originally described as such by Van Denburgh in 1912, sunk to a subspecies of P. dorsalis by Mertens in 1960, and re-elevated to a full species in a 1999 paper by Zaher that was not primarily concerned with taxonomy and appears to have subsequently been neglected. The Reptile Database is currently a holdout for the subspecies designation, which has not been disputed but which is also not explicitly supported by unambiguous data. Perhaps wisely, the official webpage of Galápagos National Park chooses not to use scientific names and refers to the Fernandina racers as the "western subspecies". The truth is that, until more research is done, we won't be able to settle on an accurate taxonomy for these snakes.
1 This sounds a bit redundant to a snake biologist, but it isn't incorrect. The one thing that I wish BBC programs would do is identify the species in them more precisely. I'm advocating for a "biologist mode" that can be activated which would show the location and identity of species in all clips, similar to the old MTV show Pop-up Video.↩
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