Etymology

From the Latin Boa for ‘large snake,’ after an animal mentioned in the Natural History of Pliny the Elder (AD 23-79).  He was an author, naturalist and natural philosopher, a naval and army commander of the early Roman Empire.

Geographic Range

The Genus Boa is restricted to the New World tropics, from northern Mexico to Argentina, and the West Indies.

Historical perspective

In 1758 Linnaeus created the genus Boa; he named  Boa constrictor and Boa orophias and placed them in the newly created genus.  Over the next two and a half centuries scientists placed newly described boas in the genus as specimens came in from the field.  Here we provide a timeline of those species and the currently recognized taxonomy of the genus.  We will look a closer look at the three species, Boa constrictor, Boa nebulosa and Boa orophias in excruciating detail in each account.  For now, we just present the date each species or subspecies was added to the genus Boa.

1758  Boa [Genus Boa erected]
1758  Boa constrictor
1758  Boa orophias
1768  Constrictor  [107]  (Genus Constrictor erected)
1768  Constrictor diviniloquus
1803  Boa imperator  [151]  [152]
1873  Boa occidentalis
1877  Boa ortonii
1906  Epicrates sabogae
1932  Constrictor constrictor amarali  [28]  [29]
1943  Constrictor constrictor sigma  [412]
1951  Boa constrictor imperator
1951  Boa constrictor sabogae
1951  Boa constrictor amarali
1951  Boa constrictor constrictor
1951  Boa constrictor occidentalis
1951  Boa constrictor sigma
1951  Boa orophias
1964  Boa constrictor orophias
1964  Boa constrictor nebulosa  [Separated from Boa constrictor orophias]
1969  Boa constrictor ortonii
1983  Boa constrictor melanogaster  [Nomen dubium]
1991  Boa constrictor longicauda
2007  Boa nebulosa  [Elevated to full species from 1964]
2007  Boa orophias  [Elevated to full species from 1964]
2009  Boa imperator  [Elevated to full species from 1951]
2016  Boa sigma [Elevated to full species from 1943]
2018  Boa blanchardensis  [Known only from fossil records] *
2019  Boa constrictor constrictor

* Blanchardensis refers to the holotype material found in Blanchard Cave on Marie-Galante Island; (ensis) is a Latin adjectival suffix meaning “pertaining to” or “originating in".

Taxonomy and phylogeny of the Genus Boa

The taxonomy and phylogeny of the boas was a long standing question and several views have been proposed since its description. After a couple of other categorizations the current taxonomic view is that the Genus Boa forms, together with the genera Chilabothrus, Corallus, Epicrates and Eunectes, the Subfamily Boinae.  The family Boidae was created by Gray almost 200 years ago .

According to Pyron and coworkers, this group can be distinguished from all other taxa by the following combination of characters: internarial septum with large fenestra, anterior margin of the ventral lamina of the nasal indented in lateral view, anterolateral margin of horizontal lamina of nasal noticeably indented viewed dorsally, horizontal lamina of the nasal does not overlap dorsal surface of frontal, most of palatine process of maxilla occurs posteriorly within the orbit, anterior end of ectopterygoid consists of indistinct lateral and medial heads, supratemporal inclined slightly in lateral view, posterior end of  supratemporal rounded but not dilated,  arasphenoid wing large and without pedicellate ventral surface, dorsal margin of prearticular noticeably curved upward near attachment of abductor posterior muscle, cornua of hyobranchium discontinuous anteriorly, and shallow labial pits .

The Genus Boa can be distinguished from the other members of the subfamily by the following dichotomous key

• Q1: greatly enlarged anterior teeth?
• yes – Corallus
• no – continue to Q2
  • Q2: eye, or circumorbitals in contact with supralabials?
  • yes – either Epicrates or Chilabothrus
  • no – continue to Q3
    • Q3: Loreal large and head scalation consisting of large scales?
    • yes – Eunectes
    • no – Boa

The molecular phylogeny within the genus has been studied, using mitochondrial genes as well as nuclear genes . The studies align in their results, concluding that two major clades exist within the genus, a southern (Boa constrictor) and a northern clade (Boa imperator). The study by Card and co-workers found Boa sigma is a valid taxon. Unfortunately, they did not include samples from the West Indies in their analysis. Therefore, the work from de Lima (2017) is particularly valuable, because she used many samples with known localities and included also Boa nebulosa,  Boa orophias as well as Boa constrictor from Trinidad in her analysis. De Lima estimated that the origin of Boa occurred around 59.7 Mya, in the Paleocene; followed by the separation of samples from the northern and southern portions of the continent that occurred around 34.24 Mya between the Eocene and Miocene. This is in contrast to the finidings from Card et al. who suggested that the clades split about 14.5 Mya. The study by de Lima suggests further that Boa nebulosa and Boa orophias are closest related and diversified from the next clade about 0.6 Mya. Interestingly, the specimen from Trinidad appears to be different from many other Boa constrictor and forms a clade with samples from Piripiri, Belem, VitoriadoXingu, Melgaco. This clade diverged from the rest about 0.79 Mya.

Boa species on the West Indian islands

Three Boa species occur in the West Indies; subfossil records of Boa constrictor have been found on Antigua but no living specimens have been observed.  Thus a population of Boa constrictor on Antigua might have existed but is considered extinct.  It is unclear whether this population was of natural origin or brought in by man from other Antillean islands .

In the light of recent taxonomic changes – Boa nebulosa and B. orophias were re-elevated to species level – it would be interesting to re-examine these subfossil remains to conlude which one of the three living Boa species is the closest living relative.  Boa orophias as well as Boa nebulosa occur on volcanic islands, whereas the majority of Chilabothrus species occurs either on coral islands or the large Antilles (tectonic shift results).  This is interesting in the light of behavioral observations. Vandeventer as well as Wilkins (pers. comm.) noted that Boa nebulosa could be found near hot steam vents.  J. Murray noted that his Boa nebulosa were more heat tolerant than many other snake species and he suspected that they do need higher temperatures than other boids.

The questions of the origin of Boa on the West Indian islands remains not entirely solved. Wallace reported “a large Boa constrictor was floated to the island of St. Vincent, twisted around the trunk of a cedar tree and was so little injured by its voyage that it captured some sheep before it was killed.” .  Wallace suggested that the snake came from the Latin American Mainland.  Even considering the possibility that the snake could have come from the closer Dominica, St. Lucia or Tobago, it is still a very impressive observation and a reminder that animals can travel large distances as blind passengers on flotsam.

Boa constrictor can survive also further north in the West Indies, as introduced populations of Boa constrictor as well as Boa imperator on several islands of the West Indies have shown . These introduced predators pose a severe threat to the indigenous fauna . Interestingly, Meerwarth reported a Boa constrictor from St Thomas (Virgin Islands) – “58 scales around midbody, 248 ventralia, 65 subcaudalia”. The specimen was collected by Becker 1897 . It remains unclear if the genus Boa once existed on St. Thomas, or if the animals collected were a dispersed animal or if the collection locality was simply falsely assigned.

Furthermore, Boa species were reported from other West Indian Islands. Historical evidence from naturalists described a large snake inhabiting Martinique (the island between Saint Lucia and Dominica) .  Breuil suggests that this species must have been a member of the genus Boa, however without providing further evidence other than the description of the naturalist Labat from 1724. Breuil claims that it became extinct from this island in the more distant past, since naturalists after Labat, describing the snakes from the island did not report or collect any specimens of this boa .

A recent study described Boa blanchardensis, a boa species that is now extinct and inhabited Marie-Galante and possibly other Guadeloupe islands. The Boa species was described based on fossil remains. The researchers could identify differences in the osteology between Boa nebulosa and Boa blanchardensis that justified the erection of this new species. The species was most likely a dwarf species measuring between 73 mm to 139 mm based on fossil remains. The species occurred in caves which most likely reflects predation on bats. It’s extinction history is interesting in that it shows how changing climates can have large effects on species. The Boa became extinct at the beginning of the Holocene.  Notably the Pleistocene-Holocene transition changed the West Indies dramatically. The island became smaller and wetter during the Holocene as a consequence of the climatic modifications occurring during this period.  The authors speculate that bat biodiversity changed and bird biodiversity was also affected and thus, a general rarefaction of prey may have had a strong impact on B. blanchardensis leading to its extinction on Marie-Galante .

Evolution

Fossil remains of Boa are not particularly rare and have been found from various sites e.g. . The earliest fossils stem from the Early Eocene (55 to 51 Mya) on the South American mainland. However these were only tentatively assigned to the genus Boa, because of the deficient preservation status .

The study of several exquisitely preserved skeletons of the booid snake Eoconstrictor fischeri from the Eocene Konservat-Lagerstätte of Messel, Germany provides considerable new insight into the biology of early boas (.  Currently E. fisheri is the only species in the genus; all known specimens come from the Middle Messel Formation (ibid).

Species

• Boa constrictor – This species has a wide distribution in Latin america. In the West Indies region, it occurs only on Trinidad and Tobago, adjacent to the Venezuelan coast of mainland Latin America. The conservation status of Boa constrictor on Trinidad and Tobago is not clearly assessed. Boa constrictor has a long and successful history in terraria worldwide.
  
• Boa imperator – This species does not occur naturally on the west indies but has been introduced by humans on Aruba, where it has been naturalized. Emperor Boas are commonly kept in public and private viviaria.
• Boa nebulosa – This species occurs naturally only on the Island Dominica. While it has been kept in private and zoo collections in the past, currently only a few  individuals work with the species.
• Boa orophias – This species occurs naturally only on the Island Saint Lucia. While it has been kept in private and zoo collections in the past, currently very few individuals exist outside their natural habitat.
• Boa sigma – has been previously described as a sub-species by Hobart Smith , but had been synonymized with B. imperator. The Western Mexico lineage has recently been elevated to full species status by Card and co-workers , thus making Boa sigma a valid species.

Boa species in vivaria

All Boa species have been kept in terraria with good success. The two insular forms B. nebulosa and B. orophias are unfortunately not as popular among hobbyists and in addition proved to be more difficult to keep. Therefore the Common Boa and the Emperor Boa are bred in large numbers in captivity, whereas the Clouded Boa and the St. Lucia Boa have almost entirely vanished from terraria.

Citations