Episode 2 of series 2 of Inside Nature’s Giants was devoted to pythons (for an article reviewing ep 1, go here). Specifically, to Burmese pythons Python molurus. And, quite right too. Snakes are among the weirdest and most phenomenally modified of tetrapods: in contrast to we boring tetrapodal tetrapods with our big limb girdles, long limbs and less than 100 vertebrae, we’re talking about tubular reptiles with a few hundred vertebrae, stretched organs, distensible jaws and a total or virtual absence of limbs and limb girdles [montage above shows Simon Watt with captive Burmese python (© Windfall Films), a CG proto-snake (© Windfall Films/Channel 4) and an alligator and a python in combat (photo Lori Oberhofer)].
WARNING: major spoiler ahead – last warning!
As is now pretty well known, some of the warmer parts of the USA (specifically, regions of southern Florida) are suffering from a python invasion. Introduced animals are now breeding, reaching large size and taking out native wildlife. While there are thousands of adult pythons in captivity in the USA, some people contend that American pythons most likely represent imported hatchling pythons – not deliberately released captive-raised adults. There has been some suggestion that these snakes might – aided by global warming – end up colonising as much as a third of the United States (Rodda et al. 2009). Pyron et al. (2008) found these claims to be suspect, in part because the same climatic changes will cause any available habitat outside of Florida to become unsuitable.
Part of the episode concentrated on the ING team’s efforts to find a suitable snake for dissection. They did get hold of a very large (more than 4 m) individual (a former pet that had died in a fire), but unfortunately it had been kept frozen for about three years and its insides had turned to mush, making it a poor subject for dissection. They ended up using two more modest-sized (c. 3 m) individuals [see comment 9 below]. Mark Evans and Joy Reidenberg were assisted in the dissection by herpetologist Jeanette Wyneken [the photo above shows both snakes prior to dissection. At right (l to r), Evans, Wyneken and Reidenberg. Image © Windfall Films].
After looking at locomotion and musculature, they examined the relictual hindlimbs that are present in pythons. Members of various snake lineages retain pelvic girdles and hindlimb remnants*; boas and pythons possess ‘spurs’ that are used in male-male combat and in courtship [Burmese python spurs shown here, visible on either side of the gaping cloaca. Photo by Joy Reidenberg, used with permission]. Embryology shows that pythons start out with hindlimb buds but that limb development is aborted as the apical ectodermal ridge – a thick epithelium rim that normally forms at the distal end of developing limb buds – fails to develop (Cohn & Tickle 1999). The spur present in an adult python is apparently a relictual femur with a cartilaginous tip that is sheathed with a claw.
* Typhlopids, leptotyphlopids, anomalepidids, Anilius, Loxocemus, Cylindrophis and the many taxa traditionally grouped together in Boidae have pelvic girdles, but only leptotyphlopids, Anilius, Cylindrophis and taxa traditionally grouped together as boids have femora (Lee & Scanlon 2002). There are the various Cretaceous limbed snakes too, of course.
The presence of relictual hindlimbs in snakes (relictual organs are present all over the place in nature, no matter what creationists say) is an obvious link to the limbed, quadrupedal ancestry of these reptiles. This led to a segment where Richard Dawkins looked briefly at hypotheses of snake evolution. A CG lizard skeleton (actually not that good… it looked superficially like a crocodilian) was used as a sort of snake ancestor that then evolved a super-long, near-limbless body via the duplication of dorsal vertebrae and the reduction of the limbs and limb girdles [CG proto-snake shown at very top of article]. Dawkins didn’t touch on the debate over snake ancestry (viz, whether snakes are anguimorphs that descend from marine relatives of mosasaurs, or are relatives of amphisbaenians, dibamids and/or skinks that started out as burrowers), but I can see that there wasn’t time for this.
In the dissection of the head, Reidenberg and the others examined the anatomy related to the sensory abilities: the bifid tongue, the vomeronasal or Jacobson’s organ, and the heat-sensitive facial pits. Facial pits are present in various boas, pythons and vipers: their distribution within these groups is interesting and some ‘logical’ predictions you might make (for example, that their presence correlates with predation on endotherms) are not borne out (though they mostly are). Little known is that at least one colubrid possibly has facial pits: more on this soon. The team finished on the head by looking at cranial kineticism and jaw distension. Snakes do not ‘dislocate’ their jaws as popular lore would have it. Instead, mobile intramandibular joints, a flexible dentary symphysis and mobile maxillae and toothed pterygoids and palatines allow snakes to distend their jaws massively, and to ‘walk’ prey into the throat (during ‘pterygoid walking’, the maxillary and pterygoid teeth on the left side are used to drag the prey toward the throat, the snake then disengages these teeth and now does the same with the maxillary and pterygoid teeth on the right side, and so on. This left-right-left-right ‘walking’ technique is called unilateral feeding). Snake jaw and palate function has been covered a few times on Tet Zoo already: see Scolecophidians: seriously strange serpents, Side-stabbing stiletto snakes and The ‘python bites fence’ photo [photo above showing some of palate © Channel 4].
Moving then to digestion, the episode used Tobias Wang’s fascinating research to illustrate how python intestines, hearts and kindneys undergo remarkable increases in size during digestion. Wang’s research got a lot of media attention back in July because his MRI and CT-scanning work showed exactly what happens to the ingested bodies of rodents as they’re digested inside the snake (for some of the images concerned, and lots of links, go here. One of his images is shown above).
Since 2006 it’s been known that Burmese pythons are breeding in the Everglades. And the big female contained a huge number (c. 40) of developing egg folicles, with scars on its ovaries indicating the production of previous clutches. A pretty impressive quantity of fat – arranged in a strip that extended throughout most of the snake’s abdominal cavity – was also discovered. Rather than providing insulation, this serves as an energy store for egg production. The presence within snakes of highly elongate organs, and of such things as strongly asymmetrical lungs (the right lung is always largest, and the left lung is either small, vestigial or absent), was also looked at.
The dissection ended with an examination of the snake’s stomach contents. Mark Evans’s identification of a single hoof in the stomach [shown here; image © Channel 4] showed that the snake had eaten a White-tailed deer Odocoileus virginianus, while sticks present in the stomach most likely represented the stomach contents of the ingested herbivore. Here was a good opportunity to discuss the fact that the Burmese pythons living and breeding in the USA are hardly neutral or beneficial components of the ecosystem: they could well be having a serious impact on beleaguered native fauna. In 2007 it was discovered that a Burmese python had eaten two highly endangered Key Largo woodrats Neotoma floridana smalli, though it’s worth noting that pythons are not putting the woodrat in direct danger: its numbers have already crashed catastrophically due to cat predation and habitat destruction.
The famous 2005 photo of a 4 m python that seems to have ‘burst’ after ingesting a large alligator was shown [and... here it is again]. The message that many people seem to take from this photo is that pythons are regularly killing alligators, but this is contested by those who argue that predation is more normally the other way round. Some people say that there are also a few suspicious things about the photo that make it look unlike a fatal attempt at predation, and hoaxing has even been suggested (alligator skin was discovered elsewhere in the snake’s digestive tract, however, so this is unlikely to be correct). The fact that the python was missing its head have led others to conclude that the snake was killed by an even larger alligator, and the position of the split means that yet others have suggested that collision with a boat – not ‘bursting’ – led to the snake’s rupturing. We’ll never know.
Of incidental interest is that, in addition to Burmese pythons, Florida has a pretty diverse alien fauna; there are also introduced anoles and chameleons, Green iguanas Iguana iguana, Black spinytail iguanas Ctenosaura similis, Nile monitors Varanus niloticus, Spectacled caimans Caiman crocodilus and several monkey colonies!
Anyway… once again, an excellent TV show, featuring a huge quantity of information. There was never, ever the implication or suggestion that snakes are nasty, unpleasant or icky creatures. Instead, they were rightly portrayed as fascinating, important and enormously successful.
For other Tet Zoo articles on ING, see…
- Inside Nature’s Giants: a major television event worthy of praise and accolade. Part I!
- Inside Nature’s Giants part II: whale guts and hindlimbs ahoy
- Enough mammals for the time being: crocodiles on Inside Nature’s Giants (part III)
- Inside Nature’s Giants part IV: the incredible anatomy of the giraffe
- Inside Nature’s Giants, series 2: does Carcharodon bite?
And for more on snakes be sure to check out…
- Stupidly large snakes, the story so far
- Scolecophidians: seriously strange serpents
- Side-stabbing stiletto snakes
- Terrestrial elapids, take 2
- Why do some snakes have horns?
- Close encounters with the Father of Death
- Not two, not three, but FOUR anacondas
- The tiniest snakes
- “What was that cute little Mexican snake?”, and other musings…
- Snake 195 mm long eats centipede 140 mm long. Centipede too big. Snake dies.
- Micropechis ikaheka, the Small-eyed snake
- Help identify the snake. Please.
Many thanks to Zach Buchan for invaluable assistance, to Joy Reidenberg, and to Tom Mustill at Windfall Films. More on ING series 2 coming soon.
Refs – -
Cohn MJ, & Tickle C (1999). Developmental basis of limblessness and axial patterning in snakes. Nature, 399 (6735), 474-9 PMID: 10365960
Lee, M. S. Y. & Scanlon, J. D. 2002. Snake phylogeny based on osteology, soft anatomy and ecology. Biological Reviews 77, 333-401.
Pyron, R. A., Burbrink, F. T. & Guiher, T. J. 2008. Claims of potential expansion throughout the U.S. by invasive python species are contradicted by ecological niche models. PLoS ONE 2008; 3 (8): e2931 DOI: 10.1371/journal.pone.0002931
Rodda, G. H., Jarnevich, C. S. & Reed, R. N. 2009. What parts of the US mainland are climatically suitable for invasive alien pythons spreading from Everglades National Park? Biological Invasions 11, 241-252.