Snakes Families
Constrictor Family (Boidae)
Appeared 65 Million Years Ago
These nonvenomous snakes are the oldest and largest members of the snake family still existing on the planet. They were the dominant family of snakes from 65-36 million years ago before the more evolved Colubrid family appeared and began to compete with them. Because they existed before many of the landmasses separated they have a worldwide distribution. Boa constrictors exist predominantly in the tropical Americas and pythons, their close relatives, evolved in the regions of Africa, Asia, and Australia. Their Primitive features include a lack of venom, a rigid jaw, heavy skull, pelvic bone structures, and the remains of vestigial limbs in the form of spurs or claws inherited from their lizard ancestors.
Evolutionary Adaptations
Some species of constrictors show an evolved adaptation in the form of heat sensitive scales around their mouths. Although similar in function to heat pits on pit vipers, this physiological characteristic evolved separately and is not nearly as effective as the pit viper’s heat detection capability.
Hunting
Their hunting technique is characterized by a “lie and wait” ambush style as is true with most snakes that consume small mammals that are faster with more long range endurance than them. Their color pattern is ideal camouflage especially during nocturnal hunting. To catch prey a constrictor retracts its head and strikes the prey hard wrapping its body around for constriction. They do not crush their prey but rather suffocate it by gradually tightening their grip each time their prey exhales. Death occurs from lack of oxygen or because the heart cannot pump blood through the vascular system. Some arboreal constrictors have special teeth that allow them to strike and hold moving prey such as birds or bats. Without fail the constrictors always consume their prey head first. These snakes prefer a diet of mammals and birds because constriction is more effective on warm blooded animals that need to breathe more frequently. One of the primary differences between Boas and Pythons is their manner of reproduction. Boas bear live young (viviparous) while Pythons lay large quantities of eggs (oviparous) and protect them during incubation. The python is one of the few snakes that demonstrates parental care.
Colubrid Family (Colubridae)
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Appeared 36 Million Years Ago
This diverse family appeared about 36 million years ago and comprises two thirds of all species of known snakes. It is currently the dominant family of snakes on all the world’s continents except Australia. In Costa Rica this family represents 104 of the 137 species. They are found in trees, on the ground, underground and in the water. The most commonly recognized species are the garter, grass, rat, and whip snakes.
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Evolutionary Adaptations
Rear Fangs and the First Venom
Although two thirds of these snakes are completely nonvenomous the other one third evolved the first rudimentary venom produced by an organ in the head known as Duvernoy’s Gland. As a component of this adaptation came a primitive venom delivery system in the form of solid rear fangs and later, grooved rear fangs. Although effective on their prey the Colubrid venom is weak and their rear fangs very inefficient from a defensive standpoint. Because of this the Colubrids represent little danger to humans. However, their venom can cause localized symptoms in humans including pain, swelling and continuous bleeding. Almost all of the Colubrids harbor bacteria in their mouths that can cause infections if a bite wound is not properly cleaned. BEWARE-Venomous Colubrids are often listed as nonvenomous in many of the snake guides- the best advice is not to attempt to handle or capture any snake in the wild.
Hunting
Many colubrid species use their amazing speed and agility to actively search out and attack their prey. The venomous species hold on to their prey and allow the venom to take effect. The nonvenomous species may use constriction like the boas, or immobilize their prey by thrashing, or by the sheer force of their strike. Camouflage and stealth also play an important part in capturing prey.
Antipredatory Traits and Behaviors
The Colubrids have evolved a few sophisticated antipredatory characteristics that aid in their survival. Some species voluntarily break their tails, which keep moving, to distract an oncoming predator while escaping. A few others feign death by rolling over and exposing their stomachs. One of the most effective antipredatory trait is mimicking the color patterns and markings of the venomous Elapid and Viper species like this Milksnake from the Colubrid Family.
Elapid Family (Elapidae)
Appeared 22 Million Years Ago
All members of the Elapid Family are highly venomous. Although the family accounts for only 10% of the world’s snake species their members are the most deadly and include the Cobras, Mambas, Coralsnakes and Seasnakes. They are the dominant family of snakes in Australia and therefore are thought to have evolved about 22 million years ago just prior to the continent of Australia breaking off from the great landmass of Pangaea.
Evolutionary Adaptations
Front Fangs
These were the first species to evolve front fangs and powerful neurotoxic venom. In the more primitive species of elapids venom is conducted through grooves in the front fangs and in the more evolved species the fangs have an internal canal to conduct venom. The advantage of front fangs is the ability to inject venom with a quick strike and then release the prey to allow the venom to work without risking injury to themselves while the prey struggles.
Neurotoxic Venom
The venom travels from the bite wound to the bloodstream and within two to six hours begins blocking neurological transmissions leading to progressive paralysis of certain muscle tissues. Symptoms include loss of muscular function, heavy salivary secretion, headache and in severe cases cardiorespiratory collapse. The victim needs to be treated with a specific antivenom designed for a Coralsnake or Seasnake bite. Some Elapids also have hematoxins (like the Vipers) in their venom that attack and kill red blood cells and muscle tissue and begin to externally digest the prey. In Costa Rica there are only six species of Elapids with five species represented from the Coralsnake genus and one from the subfamily of Seasnakes. Although highly venomous these snakes represent 5% or less of the total number of annual snakebites in Costa Rica. Approximately 50% of Coralsnake bites are “Dry” bites where no venom is injected. The Coralsnakes and Seasnakes have bold color markings to advertise to predators, especially birds, their lethal nature. For this reason there are eleven Colubrid species which mimic the color pattern and share the protection of the Coralsnake markings.
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Viper Family (Viperidae)
Appeared 20 Million Years Ago
The members of the Viper family are the most advanced of all snakes from an evolutionary standpoint. They can survive in extreme climates ranging from the Arctic Circle to the hottest deserts and at the highest altitudes. They are not found in Australia and so are thought to have evolved after the continent separated from the major landmass Pangaea around 20 million years ago. Most vipers have short stocky bodies and cannot chase down prey. They employ an ambush strategy of lying motionless in the night awaiting small mammals to cross their path. This behavior consumes less energy so they can feed less often and reduces the chance of being exposed to a predator while traveling.
Evolutionary Adaptations
Hinged Front Fangs
The viper’s fangs are improved over that of the Elapids because they are longer and therefore can inject venom deeper into the tissue of prey. They are able to manage these long fangs because they are hinged and can be folded up into the roof of the mouth when not in use but deployed forward quickly for a strike. They are hollow and connected directly to the venom glands so they can inject large quantities of venom rapidly.
Improved Venom
Viper venom is a hematoxin comprised of over 20 digestive enzymes combined with various other toxic components that when combined yield a destructive effect on red blood cells and muscle tissues. The venom is the first phase in the digestive process and softens or breaks down the proteins and lipids of the prey to ease in the swallowing process. This destructive venom aids the snake’s digestive process especially in cold climates. The more venomous snakes such as the Bushmaster, Terciopelo and Neotropical Rattlesnake also have neurotoxins in their venom similar to the Elapids.
Heat Detection Ability of the Pitviper
The infrared imaging capability of the Pitviper is unique in the animal kingdom and makes it king of the night. These snakes have heat pits located on each side of their face that contain ultrasensitive thermoreceptor cells able to detect changes in temperature of less than .002 degrees Farenheit. They use the two heat pits like binocular vision to triangulate range and distance to their prey in complete darkness. Studies show they rely almost exclusively on this sense and not on eyesight to hunt for prey. |
Snakes evolved approximately 120 million years ago from their closest descendant, the Monitor lizard.
