Tetrapod Zoology

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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.

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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].

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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.

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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].

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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.

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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.

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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…

And for more on snakes be sure to check out…

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.

Comments

#1 tai haku
September 3, 2010

I would’ve liked a little roadcruising section as they tried to find their python. A scene in which they pulled up on a big python at night (or even a mid-sized python) the way I and a lot of other herpers have in the glades over the past few years would’ve probably brought home how dense the population is to joe public a bit more than the scenes we actually saw. That said the ‘glades pythons have been subject to so much crap tv of late that such a well done programme was a real treat.

On the topic of florida’s non-native herpetofauna, I suspect that it’s only a matter of time before the next break-out exotic joins the iguanas, burms and nile monitors as a public enemy in the view of various news media.
#2 Casey Holliday
September 3, 2010

These invasive animals are going to science as well as documentaries. There is an active group of researchers studying these invasions from an ecological perspective (the monitors are colonizing barrier islands quite readily). However given they are invasive, there are no active mark & recapture studies-simply “mark” with a .22 or other means of euthanasia. Iguanas are often shipped to Veterinary schools for their token week of “exotics” anatomy. We’ve been lucky to be on the receiving end of the Varanus niloticus and Iguana specimens from Florida’s Gulf Coast (some of which made it into our recent symphysis paper). One of the Niles we received was about 7ft long (total length) with a head about 7in long (most are around 2-4ft long). Little did we know, then, they had used rancid, raw chicken as bait…until we were thawing it to CT the whole thing–and the belly gases found their way out–it cleared the entire floor of our building, whoops! no CT for you Can’t wait to see this episode!

Casey
#3 Cameron
September 3, 2010

For those in the US, PBS recently aired a documentary called Invasion of the Giant Pythons, dealing with Burmese Pythons in Florida. While not as flawless as ING, Nature’s documentaries are the best on American television and definitely worth checking out.

Florida is a notorious hotbed of weirdness and for some reason people feel compelled to release the strangest things: Griffon vultures, dozens of Parrot species, Cane Toads, Anaconda, Coyote, Elk, Capybara, Snakeheads, Armored suckermouth catfishes, Swamp Eels, and Knifefish, to name a few…
#4 Tamara Henson
September 3, 2010

When I was a kid I lived in Ohio for several years and, despite the hard winters there, we were told to look out for Nile monitors and report any sightings to the local library (the library was a part of the local museum so collected all sorts of local information on wildlife, including bird sightings from local birdwatchers). I never saw a wild monitor (saw a few in pet shops) but it seemed to be common knowledge among the authorities that the lizards inhabited the local swamps and woodlands. It appears that the so-called Loveland Frog was one of these escaped feral pets as well. Sigh, people seem to just dump their pets anywhere when they get tired of them.

We had pythons (both the Bermese and Ball/Royal species) as pets but never let them out. Unfortunately one of our neighbors wasn’t so smart. One morning (at about 8 AM) we responded to some frantic knocking on our front door only to find our neighbor who said “Help me, my snake is stuck in your fence!” Baffled we went outside and there was a 12 foot Indian Python trapped in our chain link fence. It had started to crawl through one link then changed its mind and crawled back through another so that it’s coils were trapped between both links. My dad got out some wire cutters and freed the snake and when we asked how it got outside the owner said that “he just let her out to play outside”. Duh, Lots of laughs though, especially as by then the entire neighborhood had come out to enjoy the show!
#5 Gordon Worley
September 3, 2010

Having been born and raised in Florida (and still living there, despite the heat, humidity, and human infestation) I am all too aware of how alien species are taking hold in the state. In general, though, none of it was intentional: people have just tried to bring anything from another tropical or subtropical part of the world and get it to live in Florida.

The best story I recall about our invader species relates to the Rhesus Monkeys. They were brought in to be part of a “jungle cruise” near Silver Springs, often referred to as the headwaters of the Everglades (which, of course, is not true; it’s just the first major extant spring along path that leads to the Everglades proper). They were put on an island because, according to the cruise’s operator, monkeys can’t swim. Naturally they escaped and until the 1980s they were regular pests to residents of surrounding areas, raiding trashcans and attacking dogs. That is, at least, according to accounts.
#6 Bill
September 3, 2010

As someone who spends a lot of time in Florida I can agree that it is one strange place. I was speaking to a USDA official who mentioned colleagues who wanted to hunt and trap as many pythons in one area as possible to see if they could actually get them all; this is hampered by simply not knowing how many there are (if any in particular areas) and not having a decent hunting trapping method. All I could suggest was night cruising likely roads (which we used to do in South Africa for snakes and I am sure works elsewhere) or maybe using a big female with a transmitter as a honey trap to bring in males – he said that they have tried that, but did not know if it had worked.
#7 AnJaCo
September 3, 2010

Anybody know if there are any legal issues with collecting and consuming any of these exotics in Florida? Seems like a nifty opportunity to try some exotic fare without feeling guilty.
#8 Cameron
September 3, 2010

AnJaCo,

Check out the FWC Page – most fish cannot be possessed alive, but they seem to be open game otherwise. Python catching also requires a permit. I’m sure for the most part there would be no objections to removing as many invaders as possible…
#9 Joy Reidenberg
September 4, 2010

regarding the statement: “[the photo above shows both snakes prior to dissection”
Actually, there were 3 snakes. Only the smaller two are shown here. The very large one was so soft it was deemed unsuitable for dissection and was not placed on the table. It would have really dwarfed the other two if placed next to them. The smaller male on the dissection table was the one that was also laid out initially on the asphalt next to the very large rotten one that Simon brought.

It’s not clear whether the freezing alone caused the very large one to get so soft. It died in the fire at the reptile distributer’s warehouse, and it’s body probably began to rot before it was recovered and frozen.
#10 tai haku
September 4, 2010

Anjaco – you may want to be wary of consuming florida python – I believe I read somewhere they are rather heavy on the mercury contamination front. Would make a nice belt though. I think the python-catching permit is only needed for inside the national park itself but definitely check with FWC. Also make sure you know what you’re going to do when you find one; big pythons are (and I realise this will be obvious to almost anyone who enjoys tetzoo but still….) dangerous.
#11 Mo Hassan
September 4, 2010

Re: other introduced oddities in Florida, whilst in the Everglades in 2007, I was more than shocked to see an adult male green iguana in a tree right next to the boardwalk, minutes after seeing alligators. Other than the iguana, the only squamates I saw in my 2 weeks in Florida were also non-native brown anoles (Anolis sagrei). Either it’s bad luck, poor herp-watching skills, or the pythons really are destroying the ecosystem.
#12 Jerzy
September 4, 2010

I wonder what is mechanism which causes rapid growth and shrinkage of snake internal organs (and same in eg. migratory birds)? If there is some liver growth factor or liver shrinkage factor, this would be of great medicinal value.
#13 Jerzy
September 4, 2010

BTW, on birdforum there was talk about even stranger naturalized animals in southern U.S. Jaguarundis and squirrel monkeys in Florida, and eg. zebras and several antelope species in other states.
#14 Vladimir Dinets
September 4, 2010

Last May there were rumors that all pythons in the Everglades had been killed by the “big freeze” of last January. I don’t know if it’s true, but I haven’t seen any on 2 all-night drives in August.
I’ve also seen leaflets about the new arrival: tegus from South America.
Jerzy: jaguarundis are certainly present in central Florida. There was one roadkill record, and many tracks. I’ve photographed jag tracks there once, too.
#15 David Marjanović
September 5, 2010

If there is some liver growth factor or liver shrinkage factor, this would be of great medicinal value.

There is a liver-and-only-liver growth factor, and we produce it. Liver cells that don’t get enough of it undergo apoptosis. This is why partly amputated livers regrow and then stop — liver transplants work much better than kidney transplants for this reason.
#16 John Scanlon, FCD
September 5, 2010

I look forward to the new series being shown in Australia, presumably on SBS as the first series was. Might take a while though.

From what’s visible of the CG ‘protosnake’ above, I’d agree that it’s ‘not that good’. I’ve never been able to see why it’s necessary for some people to invent hypothetical snake ancestors, when we actually have had, for over a century, a series of (good candidates for) stem-snake fossils illustrating the transition in vertebra number and limb reduction, and (in less detail, due to road-kill preservation) skull and jaw structure. The preference for fantasy over actual fossil intermediates seems to come down to ‘personal incredulity’ (thank you Prof. Dawkins for that term) that snakes descend from aquatic lizard ancestors.
#17 John Scanlon FCD
September 5, 2010

Oh, and thanks for the link to the Wang lab, Darren. I just watched the video of rats being digested, which is great fun (though lower resolution than we DigiMorph addicts have become used to).
#18 Cale
September 6, 2010

What an interesting article. Coincidentally, I stumbled on to it just after feeding my rubber boa.

Charina Bottae is a nest robber and tends to eat very huge meals at a time, while also occasionally going up to a year without. This of course is due to their specialized diets.

I never considered the physical changes she must go through to digest these feasts.
#19 David Marjanović
September 6, 2010

The preference for fantasy over actual fossil intermediates seems to come down to ‘personal incredulity’ (thank you Prof. Dawkins for that term) that snakes descend from aquatic lizard ancestors.

I do prefer to ascribe it to correlated characters in cladistic analyses that make the elongate squamates cluster together…
#20 John Scanlon, FCD
September 6, 2010

That might be fair, David, if the opposition to mosasauroid- and/or dolichosaur-snake relationships hadn’t started in the 19th century, long before anyone was consistently attempting to cluster by synapomorphy. Instead, there seem to have been campaigns against the main precladistic proponents of the aquatic origin scenario based on misrepresentation and ridicule (e.g. Marsh and Owen vs. Cope, everyone vs. Nopcsa and McDowell), followed by neglect in textbooks. And… all the cladistic analyses that actually include mosasauroids and dolichosaurs cluster them with snakes, and very far from other elongate/limbless groups.
#21 Darren Naish
September 6, 2010

What about Conrad (2008)? He found snakes (and amphisbaenians and dibamids) to be nested within Scincoidea, whereas dolichosaurs and mosasaurs are deep within Anguimorpha.
#22 John Scanlon, FCD
September 6, 2010

Oh yeah, you would bring up that cladistic analysis…

Get off my lawn!

(But I mean, snakes as scincoids? Would anyone think of that as a result, rather than a red flag? In the context of Conrad’s thesis, a small flag but definitely a red one.)
#23 David Marjanović
September 7, 2010

Conrad’s analysis is impressive in its size; it has lots of characters and shitloads of taxa.

Unfortunately, it has way too few characters for its number of taxa. Below a ratio of 3 I wouldn’t even bother publishing an original data matrix.

Far be it from me to question Conrad’s worthiness of a doctorate. His analysis is a big step in the right direction because it represents a mind-boggling amount of work. But we’re not there yet.

Having too few characters per taxon is a fairly common problem. The biggest published analysis of the phylogeny of limbed vertebrates is that by Ruta & Coates [2007]: 102 taxa and 339 characters. On the low side, but still fine, isn’t it? OK, 6 of the characters are parsimony-uninformative (“included for future expansions of this dataset”… what for). 102 taxa and 333 characters is still fine, isn’t it? A chapter of my thesis merges correlated characters in that matrix, so the number of (informative) characters has shrunk to 289, and I don’t think that’s the end of it. Lots of mostly postcranial characters — a short look at Pawley’s (2006) thesis will help — are not included in the data matrix.

all the cladistic analyses that actually include mosasauroids and dolichosaurs cluster them with snakes

What the fuck. None of the Rieppel/deBraga/whoever analyses includes any mosasauroids or dolichosaurs? That’s hard to believe. I haven’t paid attention, because I just wouldn’t have got the idea.
#24 DDeden
September 7, 2010

OT: I’m at Miami Beach, Florida, and was swimming with manatees on saturday at the sandy beach surf shallows. I noted that they like to spin axially, very smothly (like a bearing), I expect dolphins and sharks can’t due to their dorsal fin. The manatee algae-coated mottled skin looks pitted, but isn’t, it feels like oiled leather.
#25 Nathan Myers
September 7, 2010

Pythons and boas get released in Hawai’i with fair frequency, but not enough yet to seed breeding populations. The parthenogenetic Island Blind Snake has been established there since the ’30s, though most residents don’t recognize them. They were too fast for my daughter to catch.
#26 Monado
September 11, 2010

I don’t suppose there’s a hope in hell that they’ll ever get the pythons out of the Everglades, is there? And we’re one power failure away from having Asiatic catfish (the cane toad of fish) in the Great Lakes.
#27 Tommy Tyrberg
September 13, 2010

It’s the same with birds. I remember a Florida birder who suggestad that “Parrots of the World” should be renamed “Birds of Dade County”.
#28 james
March 17, 2011

whats thepr