''Boa sigma'', known commonly as the Mexican west coast boa constrictor, is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

of snake in the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Boidae The Boidae, commonly known as boas or boids, are a family of nonvenomous snakes primarily found in the Americas, as well as Africa, Europe, Asia, and some Pacific Islands. Boas include some of the world's largest snakes, with the green anaconda ...

. The species is

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...

to western

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

. ''Boa sigma'' has previously been regarded as conspecific with ''

Boa constrictor The boa constrictor (scientific name also ''Boa constrictor''), also called the red-tailed boa, is a species of large, non-venomous, heavy-bodied snake that is frequently kept and bred in captivity. The boa constrictor is a member of the family B ...

'', and later with ''

Boa imperator ''Boa imperator'' or ''Boa constrictor imperator'' (in common usage) is a large, heavy-bodied, nonvenomous species of snake, of the boa genus, that is commonly kept in captivity. ''Boa imperator'' is part of the family Boidae and is found in Mex ...

''; however, in 2016, it was suggested on genetic grounds that ''Boa sigma'' should be regarded as a separate species.

Taxonomy

''Boa sigma'' was originally described as ''Constrictor constrictor sigma'' by

Hobart Muir Smith Hobart Muir Smith, born Frederick William Stouffer (September 26, 1912 – March 4, 2013), was an American herpetologist. He is credited with describing more than 100 new species of American reptiles and amphibians. In addition, he has been ...

in 1943. The subspecies only included snakes from the

Islas Marías The Islas Marías ("Mary Islands") are an archipelago of four islands that belong to Mexico. They are located in the Pacific Ocean, some off the coast of the mexican state, state of Nayarit and about southeast of the tip of Baja California. The ...

, and it didn't receive wide recognition; instead, it was synonymized with the mainland subspecies ''Boa constrictor imperator'' by many authors. In the 21st century, genetic studies revealed that ''Boa constrictor'' is a

species complex In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...

. First, ''Boa constrictor imperator'' was elevated to species level, but it was soon recognized that there was another species nested within the new ''Boa imperator''. According to Card et al. (2016), the name ''Boa sigma'' is available for this third species. However, further research is needed because the genetic study of Card et al. didn't include samples from the Islas Marías, from where the

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

and the

paratype In zoology and botany, a paratype is a specimen of an organism that helps define what the scientific name of a species and other taxon actually represents, but it is not the holotype (and in botany is also neither an isotype nor a syntype). Of ...

s were collected. Despite this uncertainty, ''Boa sigma'' has been widely accepted as a valid species.

Phylogeny

Suárez-Atilano et al. (2014) identified two lineages of ''Boa imperator'' with a divergence date of about 5.2 Ma. However, Card et al. (2016) found the divergence date of these lineages, which they recognized as ''Boa imperator'' and ''Boa sigma'', to be around 14 Ma. Both divergence dates could be explained with the rise of biogeographical barriers.

Distribution and habitat

''Boa sigma'' is distributed along the

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

coast of

west of the

Isthmus of Tehuantepec The Isthmus of Tehuantepec () is an isthmus in Mexico. It represents the shortest distance between the Gulf of Mexico and the Pacific Ocean. Before the opening of the Panama Canal, it was a major overland transport route known simply as the Te ...

. The northernmost records are from

Sonora Sonora (), officially Estado Libre y Soberano de Sonora ( en, Free and Sovereign State of Sonora), is one of the 31 states which, along with Mexico City, comprise the Administrative divisions of Mexico, Federal Entities of Mexico. The state is d ...

, where the species can reach altitudes of . Within the distribution range of ''Boa sigma'', three islands of the Islas Marías are included: María Madre, María Magdalena and María Cleofas. Based on

species distribution modelling Species distribution modelling (SDM), also known as environmental (or ecological) niche modelling (ENM), habitat modelling, predictive habitat distribution modelling, and range mapping uses computer algorithms to predict the distribution of a spec ...

, ''Boa sigma'' lives mainly within the

Tropical dry forest The tropical and subtropical dry broadleaf forest is a habitat type defined by the World Wide Fund for Nature and is located at tropical and subtropical latitudes. Though these forests occur in climates that are warm year-round, and may receive ...

ecoregion An ecoregion (ecological region) or ecozone (ecological zone) is an ecologically and geographically defined area that is smaller than a bioregion, which in turn is smaller than a biogeographic realm. Ecoregions cover relatively large areas of l ...

, whereas ''Boa imperator'' seems to prefer areas with more

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

. In

Oaxaca Oaxaca ( , also , , from nci, Huāxyacac ), officially the Free and Sovereign State of Oaxaca ( es, Estado Libre y Soberano de Oaxaca), is one of the 32 states that compose the political divisions of Mexico, Federative Entities of Mexico. It is ...

, there is a contact zone between these two species, but it's unclear whether or not hybridization occurs there.

Folklore

In eastern and southern Sonora, it's widely believed that ''Boa sigma'', locally known as ''corúa'', ''corúga'', ''corúva'', ''culebrón'' or ''limacoa'', is the guardian of water (''guardiana del agua''). Unlike other snakes, ''Boa sigma'' is protected by rural Sonorans, because it's thought that killing the boa would cause the spring (''aguaje'') to dry up.

References

{{Taxonbar, from=Q111369611

sigma Sigma (; uppercase Σ, lowercase σ, lowercase in word-final position ς; grc-gre, σίγμα) is the eighteenth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 200. In general mathematics, uppercase Σ is used as ...

Reptiles of Mexico Reptiles described in 1943 Taxa named by Hobart Muir Smith