Sunday, 30 May 2010

Squamates Part IV

The final part of the series on squamates consists of the snakes. There are many ways to categorise snakes: traditionally and colloquially this is between the venomous and non-venomous varieties. In reality, the distinction is not so clear cut. There's also constricting and non-constricting snakes, again fraught with the same difficulty.

Boas belong to the family Boidae, often considered a subfamily of the pythons of the Pythonidae. The majority of the snakes belong to the family Colubridae, usually known as rear-fanged snakes or colubrids - these are mostly non-venomous and non-constricting. Members of the Elapidae include the notoriously venomous cobras, taipans, coral snakes, and sea snakes. The Viperidae and Crotalidae (sometimes considered under the same family) include vipers and pit vipers, the latter group including the rattlesnakes. There are many other smaller families of snakes, none of which I have photos of (all photos below: order Squamata; class Sauropsida; phylum Chordata).

Dumeril's boa
Boa dumerili (Jan in Jan & Sordelli, 1860) - Boidae
London Zoo
November 2008

Formerly included in the genus Acrantophis, the Dumeril's boa is a ground-dwelling boid from Madagascar and the island of Reunion in the Indian Ocean. It is closely related to that other famous boa (I don't mean the feather variety), Boa constrictor, which has the same vernacular name as its scientific one. Boids are native to the Neotropics, that is Central and South America and the West Indies (with a few species in North America), as well as the Old World in parts of Europe, Africa, southern Asia and New Guinea.

Emerald tree boa
Corallus caninus (Linnaeus, 1758) - Boidae
London Zoo
December 2009

There are many species of tree boa of the genus Corallus distributed in the Neotropics. The emerald tree boa is probably the most well known - the adult is green with white bands, while the young are a shocking orange or scarlet in colour. They bear a remarkable resemblance to the unrelated green tree python (see below), an excellent example of convergent evolution.

Green tree python
Morelia viridis (Schlegel, 1872) - Pythonidae
Whipsnade Wild Animal Park
April 2009

The green tree python is an arboreal python from Australia and New Guinea, green in its adult stage and red or maroon as a juvenile. The heat pits present in boids and pythons can be seen in the above photograph - these enable the snake to 'see' a thermal image of its warm-blooded prey. This extra sense makes up for the lack of hearing common to all snakes.

Jaguar carpet python
Morelia spilota mcdowelli Wells & Wellington, 1984 - Pythonidae
Natural History Museum
May 2009

This is a beautifully-coloured morph of the coastal carpet python native to eastern Australia. Carpet pythons are closely related to green tree pythons, but are, as their name suggests, mostly terrestrial.

Reticulated python
Python reticulatus (Schneider, 1801) - Pythonidae
Paradise Wildlife Park, Broxbourne, Hertfordshire
May 2008

The reticulated python is a Southeast Asian snake which holds the record for the longest living snake, commonly exceeding 8.7 m (28 feet) in length. The green anaconda (Eunectes murinus) is often thought to hold that title, but although it is a bulkier and heavier animal, it rarely gets as long. Note the iridescence on the scales of this beautifully-patterned individual.

Indian python
Python molurus molurus (Linnaeus, 1758) - Pythonidae
Jardin des Plantes, Paris
January 2010

One of the more commonly kept large snakes belongs to the species Python molurus, but not to the nominate race (see below). The Indian python is less commonly seen in captivity - it is less boldly marked than the Burmese python.

Burmese python
Python molurus bivittatus Kuhl, 1820 - Pythonidae
Jardin des Plantes
January 2010

It is the Burmese python that is a more common pet, with a more easterly distribution in the wild than P. m. molurus. They have become established in parts of southern Florida in recent years as ignorant former pet-owners release their unwanted serpents into the wild.

Ball python
Python regius (Shaw, 1802) - Pythonidae
Linton Zoo, Cambridgeshire
July 2009

The ball, or royal, python is native to western and central Africa and is quite small for a python, making it the ideal pet for someone who wants a python but not a P. reticulatus or P. molurus! They habitually curl into tight balls in defence, giving them one of their common names.

Baron's green racer
Philodryas baroni Berg, 1895 - Colubridae
Jardin des Plantes
January 2010

A classic member of the Colubridae, a large group of rear-fanged snakes, very few of which are any danger to humans. It's not a commonly seen species in captivity, but is kept by a few people, some of whom report that its bite is venomous.

Dispholidus typus (Smith, 1829) - Colubridae
London Zoo
June 2004

The name 'boomslang' is Dutch for 'tree snake'. It is a tree-dwelling snake, native to Africa, and is notorious as being the most venomous of the back-fanged snakes. It is often confused with the even more venomous green mambas (Dendrophis spp.) of the Elapidae.

Pachyophis woodwardi Nopcsa, 1923 - Colubridae
Oxford Museum of Natural History
July 2008

One of the earliest colubrid snakes, Pachyophis woodwardi was described by Franz Nopcsa, the Hungarian palaeontologist who famously killed himself shortly after killing his lover. Pachyophis is believed to have been a marine snake, as it was found in Cenomanian (mid-Cretaceous) marine deposits in Bosnia & Herzegovina.

Black mamba
Dendroaspis polyaspis Gunther, 1864 - Elapidae
London Zoo
December 2009

The black mamba is a deeply-feared and infamous snake throughout much of Africa. It's not black, as you can see, more of a pewter-grey, but it is the inside of its mouth which is apparently jet black. I haven't seen that for myself, and one would think that that would be the last fact you ever learn as the mouth clasps around some extremity or another. It is a fast animal for one without any legs, moving at a top speed of 20 km/h (12 mph).

Gaboon viper
Bitis gabonica (Dumeril, Bibron & Dumeril, 1854) - Viperidae
London Zoo
December 2009

This is quite a shocking animal - it is a bulky yet short viper with a huge head, huge eyes, and an unbelievably complex pattern on its body. The Gaboon viper is native to west Africa where it lies on the forest floor, blending in perfectly. You can see by the size of its cheeks that it must have a potent amount of venom in its glands, indeed, although it is docile and only bites when absolutely provoked, the venom is indeed very toxic, but not necessarily fatal.

Western diamondback rattlesnake
Crotalus atrox Baird & Girard, 1853 - Crotalidae
London Zoo
June 2007

The rattlesnakes are a specialised group of pit vipers endemic to the New World. They are notorious for the sound they make, produced by hollow scales at the end of the tail which are rattled in warning. The scales are added with each successive moult, indeed hatchling rattlers can't rattle! Rattlesnakes are cryptically coloured in order to prevent their prey, usually small desert rodents, from detecting their presence as they wait for such animals to come into reach (they are detected using heat pits, much like those of boids). However, if the snake senses the presence of a more malicious and less delicious threat, it will shake its tail in warning. Only when this warning fails will it strike, delivering a potent bite of venom.

Aruba Island rattlesnake
Crotalus unicolor Lidth de Jeude, 1887 - Crotalidae
London Zoo
June 2007

Not all rattlers are found in deserts though. The neotropical rattlesnake (C. durissus) is distributed throughout much of South and Central America, including tropical forests, and there are species like the timber rattlesnake (C. horridus) and eastern diamondback rattlesnake (C. adamanteus) from temperate forests of eastern North America. The Aruba Island rattlesnake is often considered a subspecies of C. durissus, and is critically endangered, with less than 230 individuals estimated to survive in the wild on the island of Aruba off the coast of Venezuela.

So that's it for the squamates, onto the crocodylians next!!

1 comment:

Anonymous said...

Very informative, thanks. :)