Chilabothrus argentum

Scientific Name

Chilabothrus argentum 
The Silver Boa, C. argentum. Photo Graham Reynolds
Described and named by Dr. Graham R. Reynolds et al., Assistant Professor, UNC and  Asheville; Associate, Museum of Comparative Zoology, Harvard University.

Holotype

Museum of Comparative Zoology (MCZ): R-193527.  Paratype: MCZ R-193528.

Type Locality

Conception Island Bank, Bahamas.

Subspecies

None.

Synonyms

Common Name

Silver Boa, Conception Bank Boa, Conception Bank Silver Boa.

A long history of discovery

Remarkably, the research team describing Chilabothrus argentum was not the first group of scientists to take note of this enigmatic boa from Conception Island. The boa had previously been collected from Booby Cay, as a publication from 1978 indicates .  The authors believed this would be a new subspecies of Epicrates striatus (sensu lato) as they write:

REMARKS. A large specimen (5 feet) of this species was collected on Booby Cay, not far from Conception I. The coloration was such to eliminate known subspecies. It thus becomes apparent that Conception I. may harbor an undescribed subspecies of E. striatus that also occupied the San Salvador-Rum Cay-Conception group of cays. Despite the recent discovery of E. chrysogaster on the Crooked-Acklin's Bank, it seems unlikely that E. striatus occurs on Rum Cay or San Salvador I. now; these islands have been visited by the senior authors and others in the recent past and natives uniformly state that the only "snakes" on San Salvador are wood-worms (Leptotyphlops columbi) and that there are no snakes of any sort on Rum Cay. San Salvador is well settled and well known. Rum Cay much less so, but the possibility of large snakes as E. striatus on small islands such as Rum Cay and San Salvador still persisting there seems remote.

Unfortunately, they did not provide a catalog number, thus the whereabouts of the specimen they collected is unknown.  Schwartz and Henderson cite this publication ten years later in 1988 mentioning, again, the mysterious boa .  Thirteen years after the initial recognition, they provided in their book two maps showing the geographical distribution of the Epicrates striatus ssp.(sensu lato). Here they marked all known localities of defined subspecies on the maps; additionally they marked Conception Island as a locality, putting a “?” instead of a subspecies assignment.  They were still convinced that this boa represents a unique boa species or subspecies from this island .

The same happened another two years later when Tolson and Henderson provided a map of the Bahamas, showing known localities of  subspecies of Epicrates striatus.  Again marking Conception Island as a certain locality but using a question mark (?) as indicator for an undescribed new subspecies .  Unsurprisingly, this boa continued to be scientifically recognized and was mentioned another time by Franz et al. who were aware of the records from the literature.  However, they could not verify the existence of this boa from the Conception Bank in their preliminary survey of the herpetofauna of Conception Island.  They mention the boa as Epicrates striatus without indicating the unclear subspeciefic status .  It was also mentioned later by McDiarmid, even pointing out the unusual coloration Schwartz et al. had earlier described .

Another reference to this boa was made in 2011 in a book chapter about the conservation of the Bahamian Herpetofauna . After this, the boa was recognized from Conception Island in a later work by Buckner et al. then described as Epicrates striatus ssp. .

Finally, almost forty years and at least eight publications after the boa was first mentioned in the scientific literature, it became recognized as a full species through the works of Reynolds and co-workers .

Description and taxonomic notes

A largely arboreal species of Chilabothrus boid snake possessing the following characteristics: silver to tan dorsal background coloration with or without scattered dark brown scales, which appear either individually or in small clusters; a reduced to absent dorsal pattern consisting of faint light-gray elongate dorso-ventral blotching, occasionally no pattern is obvious; pure cream-white ventral scales; a single loreal scale; ventral scale count ranging from 275 to 282; subcaudal scale count ranging from 82 to 91; a low number (11–12) of supralabials; 9–10 circumorbitals. *

This boa has phylogenetic distinctiveness at mitochondrial locus CYTB consisting of 0.3% pairwise divergence from other Bahamian Chilabothrus species. The largest of six specimens collected by Reynolds and co-workers measured 1232 mm total length. They averaged 952.5 mm SVL (range 876–1029) and 196.6 mm tail length (range 184–209). Of particular note is the large specimen collected by Schwartz and colleagues that was significantly larger, with a total length of 5 feet (1524 mm) .

In the most thorough morphometric study for the species to date, the authors  found that females are moderately longer than males and have relatively longer tails, whereas male and female overall head shape is similar, suggesting a lack of allometric scaling in trophic sexual dimorphism .

 

C. argentum meristics. *
* Source

Distribution

Found on the Conception Bank.

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Habitats

This species is apparently largely arboreal and has been found in Cocothrinax argentata and Bursera simaruba. Other than the observations made by Reynolds and co workers, the habitat on Conception Island is largely unknown.  In his research about the vegetation on the different Bahamas Islands, Coker noted that the cactus Opuntia triacantha is present on Conception Island . Opuntia triacantha, found on Rum Cay, is said to occur on Watlings and Conception Islands as well, as reported by Gardner, Brace and Dolley .

Biology

Other than the threats to its conservation and some aspects of its biology, little else is known about the biology of the Conception Bank Silver Boa.

Conservation status, threats and population size in nature

CITES: Appendix II
Bahamas joined CITES on 20 July, 1979; entry into force on 18 September, 1979.

Red List: Critically endangered (CR):  (click here)

Catalogue of Life:  (click here)

The National Center for Biotechnology Information:  (click here)

CITES import/export data:  (click here)

The Silver Boa Trust:  (click here)

The 4 August, 2017 IUCN rating of CR is described as follows:

Chilabothrus argentum is a newly discovered species with a known distribution on a single island bank. Repeated surveys reveal that the species exists in a single isolated population, with an extent of occurrence of less than 7 km2 and an area of occupancy of 4 km2 (actually <1 km2 based on the area of the remaining habitat). It is likely that fewer than 1,000 individuals of all age classes exist, based on three mark-recapture sessions conducted on the population. Acute threats include small available habitat size, small population size, habitat damage owing to hurricanes and erosion, and the potential for human exploitation which are all causing a continuing decline. Chilabothrus argentum is therefore assessed as Critically Endangered based on criteria B1ab(i,ii,iii,v)+2ab(i,ii,ii,vi); making  the species one of the most threatened boid species globally.

The species is critically endangered. The population is in decline. Major threats are habitat loss,  introduced predators (mice, rats and feral cats have invaded the island) and climate change. One has to assume, in the case of critically endangered species, that safeguards are in place to prevent the illegal taking of specimens. It is unknown whether any rodent/cat eradication program is in the planning or has been conducted.  The population size is considered to consist of fewer than 1000 individuals of all age classes in total. . This population number had been stated in the initial paper, however, on the website of the Silver Boa Trust, established to save the boa, Reynolds and Geneva state that only 135 individuals live on the island (link).

A subsequent study from 2022 confirmed the suspicion about the population size of this boa. The authors conducted four research expeditions to Conception Island over a 19 month period where they were able to encounter 57 boas. They stimated a census population size of 111.3 boas for the entire species, and a density of 21.9 boas/ha in the core boa area. Making this boa species the most endangered boid species globally .

A 2023 study analyzed marine vessel traffic to Conception Island National Park using satellite image time series. The findings revealed that the park is visited much more frequently than previously thought. The peak visitation period in spring coincides with critical times for local wildlife: migrating songbirds, mating Silver Boas, mating White-tailed Tropicbirds, and egg-laying Green Turtles .

While we generally agree with the risk analysis they provided, we have so far seen no evidence that C. argentum is being poached for the pet trade. Surprisingly, to substantiate their claim, Reynolds et al. cite a more than 30 year old source who analyzed data for the years 1977-1983. While we can’t exclude poaching activities with certainty, we saw market trends for West Indian Boas of the genus Chilabothrus in decline for the last forty years. Boas of the genus Chilabothrus are not highly sought after and do not command high prices in the pet trade as claimed by Reynolds et al. We are aware that this can change and it is therefore important to safeguard the boas in situ. We are fully aware that poaching occurs in many species world-wide as has been demonstrated numerous times e.g. .

We consider human caused climate change to be a severe threat to island herpetofauna, especially for small islands. Hurricane intensification and increase in numbers of storms will change many West Indian Islands and affect all life on them. We therefore consider it a possibility that the species might be lost in a couple of years on the island because of environmental changes that this magnificent animal is maladapted to.

We share the opinion of Reynolds that ex situ conservation at one or several approved institutions would certainly be an additional useful measure to secure species survival. Yet, the main measures should be taken in situ. These include as suggested: habitat restoration and  removal of invasive vertebrates- namely cats, rats and mice. Protection of prey species is of crucial importance for the survival of this newly described boa.  A rat infestation, unnoticed or left unchecked, could decimate the newly described species.  Case in point, the exterpation of C. granti on Cayos Ratones by an undetected re-infestation of rats.

Conception Island Bank

The CIA World Factbook lists the following environmental threats for The Bahamas: coral reef decay; solid waste disposal

Population ex-situ

To our knowledge, no animals of this species exist outside their natural habitat.  It is not unreasonable to suggest this boa enter a program like the hugely successful C. granti  and C. monensis SSPs.  It is our recommendation a group of adult boas be secured for study in country or an equally successful Zoo in the US.  We know literally nothing about the ecology of this new boa. A captive population would therefore help to determine its ecology, morphology and reproductive biology.
Questions such as:

  1. When does reproduction take place?
  2. How long is gestation?
  3. How many neonates are in a litter?
  4. What do the neonates weigh, what is their SVL?
  5. What do the neonates look like, what color(s) are they?
  6. What do the neonates feed on for first foods?
  7. Do the offspring undergo an ontogenetic color change like many other species in the genus?  If so, how long does it take to complete?
  8. How long does it take to reach sexual maturity?  At what age are the males and females considered mature?
  9. How long lived are the adult boas?

Only by answering these questions will we understand the life cycle of this amazing new boa.  This knowledge, in turn, would help to determine conservation measures and a thriving ex-situ population can function as a safety backup population.  With few exceptions, much of what we know and understand about the genus has been through captive breeding programs in the public and private sectors.

Stunning Silvers

Very little photographic material exists of this enigmatic boa. To see pictures from R. Graham Reynolds websites click here, here, here or here.  Further photographic evidence from Joseph Burgess here and here.

Links & further reading

Silver Boa Trust

The Reynolds Lab

Continue to Chilabothrus chrysogaster

 

 

 

 

 

 

 

Citations