Dwarf Boas - Reptilenesia

Ambergris Cay Dwarf Boa (Tropidophis g. greenwayi)

Now that the USA and Cuba are finally warming up to one another after a chilly fifty years, we might be poised to learn a lot more about a really interesting group of snakes that reach their highest diversity on Cuba. These are the tropidophiids, or "dwarf boas". Their name is a little misleading—like the splitjaw snakes, they were once thought to be related to the true boas, and the name sticks even now that we now know better. At least the dwarf part is accurate: most tropidophiids are only 1–2 feet long. But this unassuming group of drab, nocturnal, live-bearing snakes holds more surprises and lessons about snake evolution that one would expect at first glance, with no shortage of interesting natural history to boot.

Top: Tropidophis melanurus constricts an anole From Torres et al. 2014 Bottom: Madagascar Ground Boa (Acrantophis madagascarensis) constricts an oplurid lizard

Tropidophiids eat mostly frogs and lizards, and they constrict their prey in the same way as true boas: by winding the anterior part of their body neatly around their prey like a rope around a windlass, usually with an initial twist in the first loop, so that the snake's belly faces its head. This behavior, along with their relatively large gape size, seemed to suggest that they were related to the true boas (family Boidae, sensu stricto), including well-known tree boas, boa constrictors, and anacondas. All true boas are neotropical and there are quite a few in the West Indies, so unlike many of the other boid "hangers-on" (such as the Malagasy Sanzinia & Acrantophis, African Calabaria, North American rosy and rubber boas, Pacific Candoia, and Old World sand boas), a close relationship between tropidophiids and boids was easy to accept in terms of the biogeography of the living species. A comparative analysis of constriction behavior in extant alethinophidian snakes done by Harry Greene and Gordon Burghardt showed that this pattern of constriction is shared by essentially all "henophidian" snakes, including booids, pythonoids, and some uropeltoids, notwithstanding a few fossorial species that have apparently secondarily lost constriction behavior alltogether, because it doesn't work in tight spaces.¹

Top: Panamanian Dwarf Boa (Ungaliophis panamensis), a member of the group to which tropidophiids were once thought to be most closely related. Bottom: Red Pipesnakes (Anilius scytale) don't resemble tropidophiids very closely, but we now think that they are each others' closest living relatives.

In particular, what we now call tropidophiids were thought to be particularly closely related to two other small genera of neotropical boids, Exiliboa and Ungaliophis, which they superficially resemble both morphologically² and ecologially. These still share their common name of "dwarf boa", but about 15 years ago a new picture began to emerge. While DNA from Exiliboa and Ungaliophis suggested that they were indeed related to true boas, evidence from both mitochondrial and nuclear DNA and immunological proteins of Tropidophis and Trachyboa, along with details of their muscular, circulatory, and reproductive anatomy, suggested that they were most closely related to the monotypic family Aniliidae, which contains a single South American species known as the Red Pipesnake (Anilius scytale). As far as we know, Anilius doesn't normally constrict its prey³, because it mostly forages underground on elongate vertebrates such as eels, caecilians, amphisbaenians, and other snakes, similar to various Asian pipesnakes to which it was once thought to be closely related. But, we are now fairly certain that these Asian pipesnakes are convergent with Anilius, that tropidophiids and aniliids are each others' closest relatives, and that the similarity between the gape size and constriction behavior of tropidophiids and that of boas and pythons probably still represents the shared retention of a paired morphology/action pattern used by their common ancestor, it's just a common ancestor that is much older than we originally thought. Estimates suggest that tropidophiids and aniliids diverged from one another 60-110 mya in South America⁴, after their common ancestors were isolated from those of all other modern alethinophidian snakes, which radiated in Africa following the mid-Cretaceous split-up of west Gondwana 70-120 mya. This was the split that formed South America and Africa, and we are now getting used to diving the alethinophidians into two major lineages, Amerophidia (tropidophiids and aniliids) and Afrophidia (everybody else), instead of into a monophyletic Macrostomata containing boas, pythons, and caenophidians, and a basal group of non-macrostomatan pipesnakes more similar in ecology to scolecophidians. My snake taxonomy article from 2013 is actually out-of-date with respect to this major shift in snake taxonomy, because at the time it was still unclear to me (and there are still some strong arguments from paleontologists that the molecular data may be misleading).

The Greater Antilles, Bahamas, and Turks & Caicos

Tropidophis xanthogaster bleeding from the mouth, with blood behind the spectacle making the eyes appear red. From Torres et al. 2013

Tropidophis melanurus, the largest species of Tropidophis and the first described, from Cocteau & Bibron's 1843 volume on reptiles in de la Sagra's Histoire physique, politique, et naturelle de l’Ile de Cuba

Just when you thought things couldn't get any more interesting, brace yourself, because most Tropidophis can change color! They are light silver-white at night, when they are active, and dark grayish-brown during the day, when they are not. It takes a Tropidophis 1-2 hours to go from completely light to completely dark, which they accomplish via mobilization of melanosomes (organelles containing the light-absorbing pigment melanin) from the core of a melanophore cell deep within their skin into finger-like extensions of the melanophore that are closer to the surface of the skin, partially blocking stationary xanthophores and iridiophores, which contain yellow, blue, or green pigments. Both adults and juveniles undergo diel color change, and it does not seem to be affected by age, sex, pregnancy, or feeding, although prior to shedding snakes remain dark and inactive for several days. The change is probably predominantly triggered by photoperiod, but exposure to cool temperatures (<63°F) can elicit a partial change from dark to light even in the middle of the day. When captive snakes were transported from Cuba to Czechoslovakia, they became jet-lagged—it took them several days to synchronize their rhythm to the new photoperiod, and keeping them in complete darkness for several days desynchronized their rhythm from that of the sun. The proposed function of this color change is to help nocturnally-active snakes retain their body heat, as light-colored objects lose heat more slowly than dark-colored ones. This is probably similar to the reason that Round Island Splitjaw Snakes, Pacific Keel-scaled Boas (Candoia carinata), and the Hogg Island race of Boa constrictor also become lighter-colored at night.

Tropidophis pardalis on a Cuban stamp

There's much more to learn about tropidophiids, the Cuban radiation of Tropidophis in particular. To date, little ecological information has been collected on most species, owing in part to their rarity and in part to the difficulty of working in the region. How do five or six sympatric species partition resources and coexist in various parts of Cuba? What was the order of speciation and colonization of the islands, and when did it happen? Hopefully tropidophiids will be around long enough for us to find out. They are faced with numerous threats. As in many places, local people not especially fond of them, despite the fact that no Greater Antillean snakes are dangerous to people. Collection for the pet trade may also be a concern, particularly since one former government official in the Turks & Caicos Islands apparently granted a permit to reptile dealers to remove thousands of Tropidophis greenwayi from North Caicos for the pet trade, allegedly implying that it would be preferred if they removed all of the snakes! Throughout the West Indies, most native ecosystems have been absent for centuries, and increasingly rapid development, especially due to tourism, threatens what little remains. And introduction of non-native predators, particularly the Small Indian Mongoose (Herpestes javanicus), may be their biggest threat. As early as 1919, herpetologist Thomas Barbour wrote "In Jamaica [Tropidophis maculatus] is almost extinct owing to the appetite of the introduced mongoose". Ironically, Operation Mongoose was the codename for the Kennedy administration's attempt to create Cuban diplomatic, political, and economic isolation in hopes of weakening Castro’s regime. Cats, dogs, rats, goats, pigs, cane toads, and even other introduced snakes also threaten not just tropidophiids, but all 120+ snake species endemic to the West Indies as well as the rest of the native fauna. Improved PR and conservation programs have benefited several lizard species, and could help snakes too.

Tropidophis haetianus

I'm going to go ahead and wager that we'll discover a few new species of Tropidophis in the not-too-distant future, and that possibly the mainland species will get moved into a new genus. I also think that we need a more creative common name for them than "dwarf boa", preferably one that doesn't include the word "boa" at all. One existing option is "wood snakes", which is mediocre at best. They are also called "rock pythons" in the Caicos Islands, an equally misleading name as "dwarf boa", "culebras bobas" (dumb snakes) in Cuba, and "shame snakes" on Andros Island in the Bahamas, both of which may refer to their head-hiding defensive behavior. However, my favorite is the name they are known by in many parts of the West Indies: "thunder-snakes", because they are more frequently seen after severe rainstorms. Caribbean Thunder-snakes has a nice ring to it, and it could help improve their image.

1 1: Constriction behavior has become a lot more variable within the Colubroidea, where it has also been lost in several venomous lineages. Venom and constriction can be thought of as two different solutions to the same problem—how to kill large prey without exposing yourself to undue risk. Also, the contention that constriction and large gape size were lost in fossorial henophidians (aka "regressed" macrostomatans, including uropeltids, anomochilids, and aniliids) is seemingly contradicted by the complex multipinnate morphology of their jaw adductor muscles, which is sufficiently similar to that of their lizard ancestors that it is unlikely to have re-evolved in the exact same way multiple times. This problem might also be an issue for scolecophidians, given that they have similar jaw muscle morphology to pipesnakes but appear to be more closely related to other living snakes than they are to some basal fossil macrostomate snakes with limbs (symoliophiids). Stay tuned for more on the unresolved relationships at the base of the snake family tree, including a look at what fossil snakes can tell us.^↩

2 2: All four genera (Exiliboa, Ungaliophis, Tropidophis, and Trachyboa) either completely lack a left lung or have a greatly reduced one, a characteristic they share with anomochilids and some caenophidians, but not with most other henophidians, which have a somewhat reduced but functional left lung. In addition, all four genera also have a "lung" on the dorsal wall of the trachea: the tracheal cartilages do not form closed rings but remain open on the top, where a greatly expanded ligament forms the tracheal lung. It has alveoli just like a regular lung, which are especially deep near the head, and is contiguous with the true lung in the vicinity of the heart. But, although this might seem like very strong evidence that these four genera are closely related, tracheal lungs of diverse structure are widespread among snakes, being found in certain scolecophidians, xenophidiids, acrochordids, vipers, atractaspidids, sea snakes, and many colubroid snakes.^↩

3 3: A tantalizing bit of evidence emerged in 2008—biologists in Brazil videotaped the prey subjugation behavior of a captive Anilius scytale, which essentially constricted an amphisbaenian that they tried to feed it. In general its constriction behavior agreed with that of other henophidia, although it was more variable in the particulars, which could have been due to the difficulty of holding onto the elongate, "vigorous and constantly twisting prey". But, data from a single observation do not a generalization make, and more studies are needed.^↩

4 4: Fossils of ten extinct species in five genera from the Paleocene, Eocene, and Oligocene of Europe, Africa, & North and South America have been assigned to the Tropidophiidae, although all of them are probably actually either ungaliophiines or stem afrophidians. Two genera, Falseryx and Rottophis, both from the Oligocene of western Europe, have some similarities with living tropidophiids as well as with ungaliophiines, but for the most part their skulls are poorly preserved, leaving paleontologists to work on just their vertebrae. Paleogene erycines dominated the snake fauna of North America prior to the Miocene explosion of colubroids, but as far as we know all of these species were much more closely related to modern rosy and rubber boas than they were to tropidophiids. The only unequivocal tropidophiid fossils are from the Pleistocene of Florida and the Bahamas.^↩

Tropidophis paucisquamis from Brazil