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.




            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