The Boa Clade

Taxonomic comment. The boa constrictor is arguably the most famous of snakes. It was first described by Linnaeus based upon two specimens (NHRM 10 and NHRM 20001) and a third, now lost specimen, in the Museum de Geer. The syntypes were said to be from the “Indiis.”  Between 1758 and 1991 Boa constrictor has been described 17 times and recent works often recognize as many as ten subspecies. Ivana Hynková and colleagues sequenced a fragment of the mtDNA gene cytochrome b from 115 specimens representing six different subspecies. They found 67 haplotypes and two distinct clades. The Central American clade and the South America clade are separated by a genetic divergence of 5 to 7%. Hynková and colleagues recommend the name Boa constrictor Linnaeus be applied to the South American clade and the name Boa imperator Daudin applied to the Central American clade. Within the South American clade they recognized the distinctness of the Argentina population (B. c. occidentalis) which is basal to the remaining South American populations and up to 4.3% divergent from it. It seems likely that occidentalis will eventually be recognized as a distinct species. They also report Central American clade haplotypes from the northwest coast of South America. A more robust study may alter this arrangement but for now I will follow their suggestions. Unfortunately, Hynková and colleagues did not include any genes from the Lesser Antilles populations of Boa. However, Henderson and Powell (2009) recognize both these island populations as distinct species. Given the facts that the island snakes are isolated, morphologically distinct, and on their own evolutionary trajectory I follow their arrangement.

Boa constrictor Linnaeus

Boa constrictor Linnaeus 1758:215
Constrictor formosissimus Laurenti 1768:107
Constrictor rex serpentum Laurenti 1768:107
Constrictor auspex Laurenti 1768: 108
Constrictor diviniloquus Laurenti 1768:108
Boa diviniloqua — Dumeril and Bibron 1844:515
Boa constrictrix Schneider 1801:247 (emendation)
Boa occidentalis Philippii 1873
Boa ortonii Cope 1878:35
Boa constrictor — Boulenger 1893:117
Boa occidentalis— Boulenger 1893: 118
Epicrates sabogae Barbour 1906:226
Constrictor constrictor occidentalis Ihering 1911: 321
Constrictor occidentalis— Serie 1921
Constrictor constrictor sabogae — Barbour and Loveridge 1929:139
Boa constrictor amarali Stull 1932
Constrictor constrictor amarali Stull 1932:27
Constrictor constrictor ortonii — Schmidt and Walker 1943:305
Boa constrictor constrictor — Focart 1951
Boa constrictor occidentalis—Focart 1951: 199
Boa constrictor sabogae — Focart 1951
Boa constrictor ortonii — Stimson 1969
Boa constrictor longicauda Price and Russo 1991

BoaconstrictorMap
 
Map Key: Green Triangle - Boa nebulosa; Yellow Square - Boa orophias; Blue Triangle - Boa c. constrictor; Red squares - B. c. occidentalis. This map is a rough guess!
Type Locality: "Indiis." In error. Syntypes: NHRM 10, NHRM 20001.

Distribution: Boa constrictor Clade ranges from northern South America southward to southern Bolivia, Paraguay, and Argentina. Boas are also present on the Lesser Antilles islands of Trinidad, Tobago, Dominica, and St. Lucia. The species has been introduced by humans into Florida, Aruba, and Cozumel. Presumably the genus Boa evolved in western Gondwana (South America) and it has apparently expanded its distribution into Central America with the closure of the Panama Isthmus about 3 to 3.5 MYA. This scenario is supported by the mtDNA study by Hynková and colleagues (2009). Two species I am including in this clade are Boa nebulosa from Dominica and Boa orophias fromSt. Lucia.

Habitat: In South America includes wet and dry forests, shrublands, secondary growth in lowland areas.  The southernmost subspecies (B. c. occidentalis) uses semiarid plains with xeric vegetation and is often associated with burrows of the Vizcacha (Lagostomus maximus). Chiaraviglio and Bertona (2007) examined reproduction and thermoregulation in the Argentine Boa (B. c. occidentalis) and found reproductive female preferred habitats that had less ground cover but more shrubs and trees.

Diet: Like most large constricting snakes its diet is very general. While many authors note its dependence on mammals, it will not hesitate to feed on birds, lizards, other snakes.

Size: Clifford Pope’s (1967) classic text, The Giant Snakes, reports what Pope considered to be the record size for the Boa Constrictor.  He wrote,

“The accepted record length for the boa constrictor is unusual in that it is based on a field measurement (by a scientist) that appreciably raises the formerly accepted maximum. This field measurement was made by Colin F. Pittendrigh, who encountered the big snake one morning in a swampy area of the Central Range of Trinidad. It was coiled up in the hollow end of a tree trunk, from which it had to be extracted by means of poles. After it was shot, Pittendrigh determined its length in the flesh as 18½ feet. This is the kind of field observation which cannot be lightly discredited.”

            An 18½ foot snake would be 5.64 m. James Oliver (1958) cites this same record as being the maximum sized Boa Constrictor. Sherman and Madge Minton (1973) also accepted this size record in their book Giant Reptiles as did John Mehrtens (1987) and Scott Weidesnsaul (1991) in their books on snakes. In the 1986 edition of Snakes of the World, Chris Mattison reported the Boa constrictor to reach 6 m or 20 feet, a length not supported by any specimens. But in his Encyclopedia of Snakes (1996) he reports a maximum length of 4 meters.

            However, Hans E. A. Boos (1992) curator of the Emperor Valley Zoo in Trinidad investigated further and contacted Pittendrigh. Boos received a letter from Pittendrigh dated March 12, 1980. Pittendrigh was then Director of the Hopkins Marine Station operated by Stanford University. Pittendrigh described the 1944 encounter with the huge snake in the Guico-Tamana area of Trinidad. The snake had been sunning on a fallen tree, the snake was killed and skinned. The skin was said to be 30 feet long but the animals was “about 18 feet long.” That night the skin was destroyed by stray dogs. Boos pressed Pittendrigh for more information and the confusion between the Boa Constrictor and the Anaconda. In a second letter Pittendrigh admitted that he really did not know the difference between the Anaconda and the Boa Constrictor. Boos later made contact with Yussuf  Khan, a local man who had worked with Pittendrigh and asked him about the big snake, Khan said it was a large, greenish, and was a “Huille,” the local name for the Anaconda.

            Therefore, the record sized 18.5 foot Boa constrictor was based upon a specimen of the Anaconda. The question of how large Boa Constrictors get falls to a record reported by Watkins-Colwell and Leenders (2003:61). William Duellman (2005) considered this record to be the maximum sized for the boa. However, the measurement is based upon a skin, albeit a dried skin that had been rehydrated. Skinning a snake always results in a stretched skin (see special topic) and it seems unlikely that rehydrating the skin causes it to shrink back to its original size.

Table 1. Reported sizes for large specimens of Boa constrictor.

Length (m)

Determination

Reference

4.45  (skin)
Measured (see text)
Watkins-Colwell and Leenders (2003:61)
4.88
estimated
Porras (1999)
4.2
?
Martins and Oliveira (1998:)
4.0
estimate
Boos (2001:50)
±3
estimated
Duellman (2005:355)
3.8
Measured (Kartabo, Guyana)
Beebe (1946:18)
5.45 (skin)
measured
Lancini (1986:61)
3.355
?
Mole (1914:143)
3.2
measured
Mole and Urich (1894:502)
3.51
measured
Mole and Urich (1894:502)
3.11
measured
Mole (1914:143)
3.35
estimate
Hagenbeck (1909:186)
 
 
Boa imperator Daudin
Boa imperator Daudin 1803: 150
Boa eques Eydoux and Souleyet 1842: 144
Boa diviniloquax var. mexicana Jan 1863: 83
Boa constrictor var. isthmica Garman 1883: 9
Boa mexicana — Boulenger 1893: 119
Constrictor constrictor imperator — Ihering 1911: 8
Constrictor constrictor sigma Smith 1943: 411
Type Locality: Central America, principally Mexico. Types: MNHN.

Distribution: Boa imperator ranges from northern Mexico through Central America into the northwest coast of South America. On the northern most edge of its range it occurs in Tamaulipas, southern Sonora, and extreme southwestern Chihuahua (Lemos-Espinal and Smith, 2007). The southern edge of its distribution is in Columbia. The map above is based upon literature records and other maps, it is at best a guess, particularly for the Columbian distribution which is based upon Pérez-Santos and Moreno (1988). Not all localities were plotted for reasons of resolution.

Habitat In Mexico it inhabits semi-desert and scrub. At Rancho Guirocoba in Sonora (about 435 m ASL) it occurs in an area where there is a mixed tropical and desert fauna. Central America includes wet and dry forests, shrublands, and secondary growth in lowland areas.  Boback (2005) found island populations used buttonwood and mangrove areas on the islands off Belize.

Diet: Its diet is very general. While many authors note its dependence on mammals, it will not hesitate to feed on birds, lizards, and other snakes. Boback (2005) found island population off Belize to feed on small passerine birds, such as the gray-breasted martins (Progne chalybea).

Reproduction: Few studies have been done on this snake in the field, but B. imperator is frequently captive bred. Copulation often occurs in the fall or winter. Courting in captivity has been reported to last 2–5 months. Females become sexually mature by the end of their third year, males may be sexually mature at 18 months. Females shed after ovulation and the young are born 100–105 days later (Stone, 2007). Some island populations are dwarfs, Culkers’ Cay off the coast of Belize reported has females reproducing at 1.14 m and giving birth to litters of just a few offspring.

 

Boa nebulosa Lazell

Type Locality: Woodford Hills, St. Andrews Parrish, Dominica. Holotype: MCZ R65493
Constrictor constrictor nebulosa Lazell, 1964
Boa constrictor nebulosa — Binder and Lamp, 2007
Boa nebulosa — Bonny, 2007

Boa constrictor nebulosa is restricted to the island of Dominica. Lazell (1964) suggested it is found throughout the island to elevations of at least 350 m. In dry portions of the island it is limited to wet ravines. Henderson and Powell (2009) have summarized the habitats used by this snake and report it from coastal xeric and littoral woodlands to montane rainforest. It also uses agro-ecosystems (banana plantations) and anthropogenetic habitats such as abandoned buildings and botanical gardens. Aggregations of 2 to 7 snakes have been reported in rock piles. This snake may be less arboreal than other species of Boa.

Size: The largest specimen reported by Lazell (1964) was a 1.847 m female with a damaged tail. Binder and Lamp (2007) found females are often 1.9–2.6 m, males are 1.2–1.8 m in SVL.  

Diet: Rats and agoutis (Dasyprocta noblei) are frequently eaten, but it will also eat chickens. It has also been seen probing the burrows of the endemic and endangered Iguana delicatissima.

Reproduction: Near full term embryos have been reported on 25 April. Litter sizes of 16 and 25 have been reported (Henderson and Powell, 2009).

 

Boa orophias Linnaeus

Type Locality: None give. Restricted to Praslin, Staint Lucia by Lazell (1964)

Boa Orophias Linnaeus 1758:215
Boa diviniloqua — Cope 1893: 429
Constrictor constrictor orophias — Amaral 1929:142
Boa constrictor  orophias  ― Stimson, 1969
Boa orophias ―Bonney, 2007

Common Name: Tet’chien

Distribution: Boa orophias is restricted to the windward and leeward coasts of St. Lucia and cuts across the island in the moderately dry zone, avoiding the very dry northern peninsula. It does exceed 350 m ASL (Lazell, 1964). It is often in banana (Musa) plantations and is arboreal in bushes and trees to heights of 12 m.

Diet: Includes small mammals (rats, mongooses, bats) and birds.

Size: Females are 2.4 to 3.0 m, males are 1.7 to 2.2 m in SVL.

Reproduction: A breeding aggregation of one female and five males was reported in February.

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JCM Natural History © John C. Murphy
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