GENTLEMEN, The reptiles have been comparatively neglected by recent zoologists, perhaps on account of the popular prejudices against this interesting and curious class of animals which Linneaus designates "Animalia pessima tetra nuda."  It is only necessary to overcome these prejudices, and to examine them even superficially, and we cannot but be struck with the beauty of their colours, the wonderful nature of their structure, and the peculiarities of their habits and manners.  Indeed I do not know any class of animals better calculated to excite the wonder and astonishment of a student of nature.

~John Edward Gray, Esq. FGS. &c.  (To the Editors of the Annals of Philosophy, British Museum, July 12, 1825)

The Boas – well known snakes?

The term “Boa” is well known in the general public. Prominent members of this snake lineage are Boa constrictor or the Anaconda – snakes that are associated with adjectives such as big or colossal.  Contrary to this, surprisingly little knowledge is present in the general public about the Superfamily as a whole and its diverse lineages and members. In their recent checklist, Reynolds and Henderson recognized a total of 66 species and 33 subspecies of boid snakes.  These are distributed among  6 Families, containing 14 Genera .  Of these 66 species, some are exceedingly small, such as the two species of Bromeliad Boas (Ungaliophis) or Sand Boas (Eryx/Gongylophis) that reach sizes well below one meter in length.  Most members of the lineage reach sizes of around 2 meters or less in total length, which is contrary to the public sentiment of boas as giant constrictors.

Unifying anatomical characters that differentiate Booids from other snake lineages

So what makes a snake a boa?  The unifying characteristics Superfamily Booidae stem from the shared evolutionary history and thus relatedness.  This common evolutionary history and relatedness is reflected on a molecular level in the DNA of the different species, as well as in a set of unifying morphological characters – size is not among them!  These are: hind limb vestiges present as a set of spurs on either side of the vent, a moderately to well developed left lung, absent tracheal lung and, in females, both oviducts are well developed.  Most species have cranial heat receptors .

Several of these anatomical characters are shared with pythons, which is why pythons and boas have been considered to be closely related.  Differences between Booidea and Pythonoidea are the lack of postfrontal bones and premaxillary teeth in the Boa lineage.  Almost all species of Boids (the enigmatic Calabaria reinhardtii being the exception) are live bearing, whereas all pythons are exclusively egg laying.  More indicative of the loose relationship are, however, the genetic differences.  Research showed that Booidea and Pythonoidea are not as closely related as previously assumed.

In comparison, pythons and boas are two globally distributed and distantly related snake lineages which underwent radiations, resulting in remarkable phenotypic and ecological diversity.  A recent study tested whether pythons, boas and their relatives have evolved convergent phenotypes when displaying similar ecology.  The study concluded that the two groups display strong and widespread convergence in cases where they occupy equivalent ecological niches and that the history of phenotypic evolution strongly matches the history of ecological diversification. Thus suggesting a coupling of both processes, resulting in replicated adaptive radiation in both groups.  Presumably strong selective pressures related to habitat-use have driven convergent evolution in phenotypes, resulting in a similar ecomorphology .

The  Booidea are still being researched and several surprising facts came to light only recently.  These include that the genus Calabaria belongs to the Booidea Superfamily, not to the Pythonoidea.  Tropidophis and it’s allies form a unique lineage not included in the Booidea.  The Genus Chilabothrus was resurrected, several species were elevated from the rank of subspecies….the list goes on.

Taxonomy and zoological phylogenies in the molecular age

Taxonomists used, until the early to mid 1980’s of the last century, exclusively meristic characters to differentiate species e.g. .  The first transition brought biochemical approaches in the game e.g. .  Even though these analyses were expensive and more difficult to perform as compared to counting scales, they paved the way for novel perspectives and intellectual approaches.  Previously unavailable data was  seen for the first time and could be taken into consideration.  These new data points would bring new aspects, in terms of relationships and evolution, to light.  The age of molecular biology saw its dawn and with it scientists started to reevaluate concepts in biology that once were considered robustly standing.

It was the readily available DNA sequencing, insights into molecular evolution and models based on these insights, as well as other technologies associated with it, that motivated researchers to start to review phylogenetic concepts.  These first molecular phylogenies had (like everything in science) some pitfalls and many were later refuted or refined. Science is a continuously ongoing process.  Some of the difficulties that are related to molecular phylogenies are: choice of genes on which the phylogeny is based upon, choice of model/technology to calculate the phylogeny, too small sample size (based on few animals’ DNA), too few datapoints, too many datapoints etc.  Even if, from today’s point of view, an old molecular phylogeny might look primitive these paved the way for today’s more sophisticated models.

Replicability, iterative improvement, inclusion of new data and challenging of hypotheses are some of the core principles of science and differentiate it from religious beliefs.  With today’s mature state and easy applicability of molecular biological assays, most researchers rely on molecular data to analyze the differences and similarities in specific genetic or genomic sequences, and thus define relationships within animal groups and draw conclusions about evolutionary history (e.g. ).  This technological revolution and its continuous improvement (e.g.) has greatly improved our knowledge of relationships among groups of living (and sometimes even extinct) fauna .  Molecular data, used in combination with the large set of well analyzed morphological characters, shapes the picture of our understanding of animal evolution e.g. .

Evolution of Boid snakes

Molecular phylogenetic analyses clarified that the superfamily Booidea is a monophyletic group .  Within the Superfamily the Boidae family (Genera: Boa, Chilabothrus, Corallus, Epicrates, Eunectes) accounts for the highest number of species.

The Boidae emerged in the mid Cretaceous period about 145 million years ago (Mya) . Several fossil records from snakes belonging to the booidea have been discovered in North America and, most recently, in Europe.  The German town of Messel hosts the biggest fossil snake assemblage discovered to date.  The fossils are from the early–middle Eocene, some ~ 48 Mya.  Recently researchers re-investigated Eoconstrictor fischeri and concluded that this snake was a medium- to large-sized snake (total body length ~200 cm), similar to the extant Chilabothrus inornatus.  The general shape of the skull of Eoconstrictor is remarkably similar to that of neotropical boas, especially Boa constrictor.  In regards to the behavior of the early boas, the researchers found that Eoconstrictor fischeri spent considerable time on the ground.  Two other small Messel booids Rieppelophis ermannorum and Rageryx schmidi are inferred to be ground-dwelling forms.  Messelophis variatus, a Tropidophiine snake in contrast was largely arboreal .  It is commonly accepted that the Booidea evolved in South America.  The large assemblage in Messel shows that this lineage was geographically wide spread from South America to North America and Europe and became extinct in large parts of its distributional range presumably due to climatic changes for which the boas were maladapted.

A study investigating the Karyotypes of different Boid species (seven species and three subspecies comprised the study), found three subspecies of Boa, two species of Eunectes and three species of Epicrates that exhibit a diplooid chomosome set of 36 chromosomes. For the first time, it was shown that Corallus hortulana held a totally different karyotype than the rest of the Boidae family, a diploid set of 40 chromosomes with a greater number of macrochromosomes.  Chromosomal mapping of telomeric sequences revealed the presence of interstitial telomeric sites (ITSs).  The researchers concluded that chromosomal rearrangements have contributed to the diversification of the boid snakes and that ITSs are common in basal and derived lineages within the suborder Serpentes .

A follow up study investigated Boid snake karyotype evolution and its mechanisms.  It showed that the family Boidae differs from other families by presenting the greatest chromosomal diversity among the Booidea families. The karyotypes in this group range from 2n =  36 to 2n = 44.  Most snake lineages have a set of 2n = 36 chromosomes (16 macro and 20 microchromosomes). This is considered a plesiomorphic feature for the suborder serpentes.  Notably, the karyotype of the Genus Corallus differs from its sister group, the clade Eunectes, Epicrates, and Chilabothrus (all species with 2n = 36) in that the Corallus lineage exhibits two karyotype configurations, the 2n = 40 (C. hortulana, C. grenadensis) and 2n = 44 chromosomes (C. caninus).  Corallus shared the most recent common ancestor with its sister lineages until the early Paleogene. The study proposes that the two karyotype configurations found in the Corallus lineage arose in the middle of the Paleogene/Eocene from centric fissions in all meta and submetacentric chromosomes .

The question of sex chromosome evolution has been intensively studied in different vertebrate classes.  Until recently it was assumed that all snakes possess ZW sex chromosomes .  A paradigm shift was the work of Gamble and Co-workers, who demonstrated that boas and pythons do not have ZW sex chromosomes, but XY sex chromosomes.  Interestingly, the XY sex chromosomes in boa and python evolved independently as comparative genomics revealed .  It is particularly remarkable that  the two lineages not only phenotypes evolved convergently, but also the genomic evolution followed similar trajectories, at least in the sex chromosomes.

Exceptional in the lineage in terms of karyotype are the Corallus species, as was demonstrated by the work of Viana et al.  They showed that XY chromosomes in Corallus caninus underwent fission events, which gave rise to four small acrocentric chromosomes. However, the researchers point out that it is uncertain whether a functional XY sex chromosome system exists in the C. caninus lineage or if a multiple XY system evolved through this fission event .  Further research is needed to fully understand the genome evolution trajectories and determine if the genome evolution in general occurs at a faster rate in Corallus compared to other snakes from the boid lineage.

Members of the Booidea

The Families and Genera in the Booidea according to :

• Calabariidae (Calabaria)
• Sanziniidae (Sanzinia and Acrantophis)
• Charinidae (subfamilies Charininae (Charina and Lichanura) and Ungaliophiinae (Exiliboa and Ungaliophis))
• Erycidae (Eryx)
• Candoiidae fam. nov. (Candoia)
• Boidae (Boa, Corallus, Eunectes, Epicrates, and Chilabothrus).

A short history of Boidae

Here we present the Class, Family and each genus, their creators and how the genus’ are assembled to form Boidae.   Added are additional terms we think important to the study of our selection of Boas.  See the Evolutionary Trees below for a species level view within the genus.

1755  Herpetology  Klein
1758  Boa  Linnaeus  [215]  [216]
1768  Constrictor  Laurenti  [107]
1768  Reptilia  Laurenti  [20]  [21]  [22]  [23]
1803  Corallus  Daudin  [257]  [258]
1824  Xiphosoma   Wagler & Spixx
1830  Serpentes  Wagler
1830  Epicrates  Wagler  [165]  [166]  [167]  [168]
1830  Eunectes  Wagler  [165]  [166]  [167]  [168]
1842  Boidae Gray  [42]  [43]  [44]  [45]  [46]
1844  Chilabothrus  Dumeril & Bibron  [563]
1844  Pelophilus  Dumeril & Bibron  [524]
1856 Homalochilus  Fischer  [101]

Herpetology according to Klein, 1755:

The Boidae family as depicted by Dumeril & Bibron, 1844:

The Boa tree expanded from the table above.

The species tree in 1988:

The Species Tree, based on fossil evidence, in 2013:

*  Currently, additional genetic testing is underway and the results will provide a better understanding of the phylogeny; these results will be available later in 2021 (Reynolds pers. comm).  The 2013 web version of the table above is here.

Occurrence & Biology

The geographic distribution of the superfamily is large.  Members of the superfamily have successfully radiated in Europe (Eryx), Africa (Eryx and Calabaria), Madagascar (Acranthophis and Sanzinia), Asia (Eryx), Asia-Pacific (Candoia), North America (Boa, Charina and Lichanura) Middle and South America (Exiliboa and Ungaliophis, Boa, Corallus, Eunectes, Epicrates, and Chilabothrus).  Geographically, the boas have their biggest radiaton success in Middle and South America (including the West Indies), where the majority of the species occurs.  They are, however, absent from Australia and North Asia as well as from the polar regions .  In comparison, the pythons are restricted to the old world (Asia, Africa, Australia), with the exception of Loxocemus bicolor.  The latter being an uncertainty in terms of its phylogenetic placement  in the Superfamily Pythonoidea.

The superfamily Booidea successfully conquered a vast diversity of habitats, from deserts (e.g. Lichanura) to rain (e.g. Corallus batesii and C. caninus) and cloudforests (e.g. Ungaliophis) to Mangroves (Corallus ruschenbergerii), from swampland (e.g. Eunectes) to savannahs (e.g. Eryx) and every habitat in between.  With regards to the ecology, the lineage is likewise diverse, with ambush predators like Corallus species or active hunters like Chilabothrus.  Harrington and colleagues analyzed the life styles of different snake families in regards to arboreality. Arboreality presents a major shift in lifestyle and is thus interesting to investigate across families. They found that 46% of boa species are arboreal (36% primarily arboreal and 64% semi arboreal). Therefore the Boidae are the second most arboreal snake family, after the Pareidae .  While many boas are generalists in terms of food, some specialists have evolved, feeding almost exclusively on one prey item (e.g. reptiles/lizards in the case of Chilabothrus gracilis).  Strategies for breeding and survival are likewise diverse and range  from litters of very few young (Chilabothrus angulifer, C. fordii, C. gracilis, Lichanura trivirgata, Ungaliophis, Sanzinia) to large litters of more than 50 young (Chilabothrus strigilatus, Eunectes murinus, Boa constrictor).  The neonate sizes differ even among closely related species (e.g. Chilabothrus angulifer (203.6 g) vs. Chilabothrus gracilis (2.0 g)).

Booidea on the West Indies

Present in the West Indies are only species of the Family Boidae from the Genera Boa, Corallus, Eunectes, Epicrates, and Chilabothrus.  The Genus Chilabothrus is endemic to the West Indies, whereas the other Genera are present also in South America and Middle America.  Contrasting to the situation of genera is the species specific situation.  Most of the boa species in the West Indies are true endemists and occur only on some of the islands.  In a general overview article Tolson and Henderson found that of 120 snake species (representing six families and 20 genera) which inhabit the West Indies, 115 (95.8%) are endemic to the region .  These are just the boas we know about; extinctions of species on the islands and cays have no doubt occurred without leaving any fossils or records .

An exception are the islands of Trinidad and Tobago, which harbor the species Boa constrictor, Corallus ruschenbergerii, Epicrates maurus and Eunectes murinus.  These species are not exclusively island species but are present on the South and Middle American mainland as well.  The close proximity of Trinidad to the South American continent suggests the possibility that the isolation barrier is permissive on occasion, facilitating genetic exchange between island and mainland populations.  Molecular biological analysis will determine the degree of relationship and distinctness of the Trinidad and Tobago and mainland populations.  Westindianboas.org continues to collaborate with scientists and the herpetological community to investigate the biology,  evolution and conservation of West Indian Boa species.

Overview map of West Indian Boas

Each flag represents a single species or subspecies and the location of the flag represents the type location as originally described. Click on a flag to see the species occurring there and then on the link to get to the species account.

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Citations