The Biogeography and Natural History of Snakes: Featuring the Family Boidae
Snakes
are closely equivalent to birds and mammals. Biologically they are highly
specialized and very diverse. More than 2,700 species of snakes are currently
recognized, placed in about 420 genera and 18 families. Snakes inhabit all
major ecosystems outside of the Polar Regions and are among the most common
predators on other vertebrates. Snake belong to the kingdom: Animalia, phylum:
Chordata, class: reptilia, and order: Squamata. This research focuses the most
known family, Boidae. The research presented will give an overview of the
biology, distribution, evolution, history, and the natural history of the
snakes and the most famous family Boidae.
Physical
Characteristics
Snakes
are elongate animals either with no girdles or limbs, or occasionally with
vestigial pelvic girdles and hind limbs. They lack a sternum, external ear
opening, tympanic membrane, middle ear, and Eustachian tube. Except in some
burrowing forms, the immovable fused and transparent eyelids form a protective
window, the brille, beneath which the eye moves. The viscera are elongated, and
the left lung is smaller than the right or altogether absent. The tongue is
long, forked, and protractile. There is no urinary bladder. The skull is more
specialized than that of most lizards. The brain cavity is completely enclosed
anteriorly by dermal bones. Higher forms have the bones of the facial region
and jaws loosely joined to each other and to the cranium so that they can
spread apart. The two halves of the lower jaw are not fused, but are connected
by a ligament. Each half of both the upper and lower jaw can be moved
independently of the other half. This is important because it allows the snake
to engulf objects that are larger than its head. Basically snakes possess all
the essential internal organs that mammals have, but they are modified to fit a
narrow space. The body is covered with dry over-lapping scales. In most
non-burrowing and non-aquatic species the ventral scales are many times larger
and broader than those on the rest of the body. The belly often has a single
row of these broad scales that play an important part in the locomotion of
terrestrial and arboreal species. The color of snakes varies from species to
species. The color also varies from environment to environment. Snakes are
poikilothermic. They can control their body temperature by a process known as
thermoregulation. They do this by moving in or out of the sun or other warm
places, by flattening their bodies, and by using various appendages to gain
extra warmth from the sun’s rays. Snakes have evolved to adapt to almost every
habitat.
Evolution of Snakes
Evolution is a
process that works by “natural selection,” in which those individual animals
that possess superior survival traits tend to live longer than others and
reproduce, in turn passing those same traits on to their offspring. The fossil
history of snakes is very poorly known, since snake skeletons are very delicate
and do not fossilize easily. One
of the earliest snakes to appear in the fossil record has been given the
scientific name Lapparentophius defrenni. It was found in the Saharan Desert and has been dated to
the early Cretaceous period about 130 million years ago. The fossil consisted of
only a few backbones. It was missing all the ribs and the entire skull. The
fossil was still recognized as having the characteristics of snakes. Some fossils from earlier deposits have been
called snakes, but American paleontologist Alfred S. Romer has regarded them as
lizards. Snake bones are delicate and do not tend to fossilize well. Most
snakes are small, and fossil hunters tended to concentrate on larger, more
spectacular specimens. Today techniques have begun to turn up material, much of
very small size, including snake skull bones and vertebrae. Much of this
material is from Pliocene, Pleistocene, and older deposits. Snakes are a young
group of animals. They arose from a lizard ancestor probably very similar to
the modern varanids (monitor lizards). Some
fossils show certain characters
very similar to those found in the Bornean lizard Lanthanotus,
a platynotan lizard related to the Varanidae. Many theories have been proposed
as to the habitat preference of the snake ancestor, including aquatic, terrestrial
and burrowing. Support for the burrowing hypothesis has come from study of the
eye. Losses caused by ancestral adaptation to burrowing were not regained by
the descendant nonburrowing snakes, and other devices to help with vision
problems were developed and refined. The adaptive radiation that produced many different species
of colubrid snakes was a very recent event. It took place within the Tertiary
(from about 65,000,000 to 2,500,000 years ago), and represents step forward in
the evolution of limb loss, which is often a confusing when talking about
evolution. The distribution of snakes is wide spread because of plate
tectonics. The continents were all joined into one large super- continent near
the equator, known as Pangea (all earth). Back then places such as Antarctica
and northern Canada were warm, humid climates with tropical forests. Pangea
began to separate because of plate tectonics. Plates move slowly on top of the
earth’s mantle this process. This caused the various distributions of modern
snakes.
Snakes now occupy almost all available niches.
Family Boidae
After the dinosaurs disappeared, the boids were the
dominant snake family on earth, and became widespread and very diverse. The
boids are the most diverse and widespread of the henophidians. Henophidians are
a diverse group, linked more by primitive characters than by close
relationship. The boa fauna of South America is the single most famous snake
fauna in the world. They are predominantly tropical in distribution and possess
a variety of unusual specializations. The boids range in size from small
(200-300 mm) West Indian Ground Boas, Tropidophis, to the large (10-12 m) South American Anaconda, Eunectes murinus.
They occur throughout the tropics and subtropics, and a few, such as the
western United States Rubber Boa (Charina bottae), extend into temperate areas. The boids are divided into
five subfamilies: Boinae Pythoninae, Erycinae, Tropidophidae, and Bolyeridae.
Physical
Characteristics
Boas are
typically stout-bodied and short-tailed. The palatomaxillary arch is movably
attached to the rest of the skull. Teeth are present on the maxillary,
palatine, pterygoid, and dentary bones, and sometimes on the premaxillary. The
ventral scales form enlarged, transverse plates; the dorsal scales are small
and sometimes iridescent. In all but one subfamily, there are vestiges of the
pelvis and hind limbs. Some boids also have heat-sensing pits, which allow them
to track their prey or ambush them with pinpoint accuracy.
Habitat
Boids occupy a variety of habitats from deserts to rain
forests and even occur in the temperate coniferous forests of the Northwestern
United States (Lichanura and Charina). Many of the small forms are burrowers in
sandy soils. Some are arboreal, with short, prehensile tails. The huge
Anacondas of South America (Eunectes murinus) is largely aquatic and can remain submerged in water for
a long time. A number of the large boids, such as the Indian Python (Python
molurus), seem enjoy life in water and trees. Boas can be found thriving in
various biomes: tropical rainforest, tropical deciduous forest, tropical scrub
forest, freshwater lakes, freshwater rivers, temperate forest and rainforest,
tropical savanna and grasslands, and finally deserts.
Food Habits
Boids feed
on large lizards, opossums, bats, mongooses, rats, squirrels, deer, foxes,
jaguars, fish, other snakes, pigs, caimans, and sometimes small people
(children). Boids feed largely on birds and mammals, and usually kill their
prey by constriction. They do not crush the bones of their victims. Two or
three coils of the snake’s body are wrapped around the upper trunk of the prey.
These exert enough pressure to stop the prey from breathing, and the animal
suffocates. They usually swallow the prey whole, and because they can unhinge
their jaws, they can swallow the prey head firs, as this way the limbs tend to
fold in and smoothly move down the throat. The muscles contract in waves as
swallowing occurs, compressing the prey with each surge forward.
The Boa Constrictor
and the Giant Anaconda
The boa constrictor and the anaconda are the most famous
snakes from the Boidae family. The group is found generally in Central and
South America. Some species occur in North America, southeastern Europe,
Madagascar and New Guinea.
The boa
constrictor lives in the subtropical regions of central and South America. The
boa constrictor has the widest distribution range of the boines. It has a
triangular head with a large muscular body. Their color varies from reddish to
reddish-brown, grayish-brown, yellowish-brown or a light steel gray. The boas’
habitat is generally hot open woodlands and thick brushy areas near vegetated
waterways, where it feeds on birds, rodents, and small mammals. The boa is
arboreal and climbs trees in search of birds. It is a good ambush predator,
which lies camouflaged among the branches, striking out with lighting speed to
grab birds from the air. The giant relative of the boa is the anaconda.
The
anaconda is the most massive of the boids. They have the largest body mass.
They are the largest but not the longest. The body is stock, as compared to
other boas, and extremely muscular reaching lengths in excess of 30 feet. They
are native to South America. It is always near water where it hunts, and is
often found sunning itself on branches, rocks or warm sand. Lying submerged in
the water with only its snout extended, the giant predator will wait for
anything that passes by. Today the
anaconda’s natural history is being studied by Dr. Jesus Antonio Rivas, in the
Venezuelan llanos.
Modern Snakes,
Boids, and Man
Members of the Boidae family have
shared a long history. They live in a variety of habitats and different
adaptive zones. Snakes are used in zoos as educational tools, which gives the
public a better understanding of these animals. Most of these snakes will
continue to flourish as long as humans do not exploit and destroy the snake’s
food source. Current foresting, farming, and land development projects have
caused the habitat of snakes to become smaller. The jungle habitat is currently
being destroyed, which can lead to many problems. Destroying their habitat has
caused snakes to conflict with human habitats in search of food and shelter.
The pet trade is causing the distribution of boids to become more widespread.
The skin trade, however, is causing the number of large boids to decline.
Despite their wide appeal in zoos, movies, and the pet trade, some of the basic
aspects of the large boids biology are unknown. These snakes are unique and
play an important role in ecosystem. We need to preserve these wonderful
marvels of nature so that they can be further studied and so that we can learn
about ways, in which they may be beneficial to us.
The Biogeography and Natural History of Snakes: Featuring the Family Boidae
Jairus L. Johnson
April 26, 2002
BY 408 Spring 2002
Dr. John McCall
The University of West Alabama