The well-known venomous pit-vipers range throughout the temperate and tropical Americas, and include the North American rattlesnakes. Left and below right: Two harmless garden snakes. Photos by Maya Walters.

We tend to associate poisonous snakes with the tropical forest, but many snakes are also found in temperate regions, and many do not have a poisonous bite. They are remarkably successful predators, having adapted to capture and consume prey without the aid of limbs or even any chewing action. Many snakes are even arboreal, managing to climb trees, move from branch to branch, even glide through the forest canopy.

As a group, snakes in the tropical rainforest have a more varied diet than any other group of vertebrate predators.

Lizards are another common forest reptile, with species ranging from tiny geckos to the prehistoric-looking iguanas. A large iguana can exceed 6 feet in length, and they spend much of their time in the forest canopy, often in trees overhanging streams. They are good swimmers, and will often jump down to the water if threatened.

Some snakes feed heavily on lizards. In tropical American forests, blunt-headed tree snakes have unique methods for capturing sleeping anole lizards. These lizards sleep out on the very tips of leaves and vines. Any approaching predator will make this perch vibrate, and with this early warning the lizard usually has plenty of time to escape. The blunt-headed tree snake, however, can sneak up on the anole lizards. These snakes have a special skeletal system that allows them to extend half of their body length into the air without support. The snake can therefore move along a plant next to the lizard, reach across through the air, and grab the lizard from its perch.

Unlike snakes, lizards are not strictly carnivores and eat both fruits and leaves, though many species also feed heavily on insects. Lizards aren't venomous and usually aren't aggressive.

It is the snakes' jaws that have allowed them to occupy their ecological niche. They can open their mouths a full 150 degrees, allowing them to swallow many types of prey that would otherwise be far too large. For example, the African egg-eating snake feeds on birds eggs that are several times wider than its head. Once swallowed, the eggs are broken by the snake's special vertebrae, which project into its gullet and crack the eggshells.

Some snakes use venom to kill other prey, while others have a less powerful bite and capture prey by constriction, a process in which the snake coils itself tightly and suffocates the prey. Some pit-vipers can consume prey that are heavier than themselves, but most snakes can only handle animals that weigh considerably less than themselves. The actual weight of the prey is not the limiting factor so much as its diameter: since snakes swallow their meals whole, some animals are simply too wide to fit between the snake's specially adapted jaws.

One of the most poisonous groups of snakes are the pit-vipers. The pit-vipers have a special sense organ: small hollows (or "pits") located below their nostrils. These pits are sensitive to heat, and snakes use them to help locate warm-blooded prey. Not limited to the tropics, pit-vipers include the rattlesnakes of North American dry coniferous forests. It can take over a month for a rattlesnake to completely replace its supply of venom. Above right: A rattlesnake. Photo credit Corel Photo Clipart CD.

Although pit-vipers are highly venomous, some animals have evolved a resistance to their venom. The common Virginia opossum actually feeds on rattlesnakes, and exhibits only temporary symptoms after receiving sixty times the amount of venom that it takes to kill comparably sized mammals. An animal with a similar adaptation is the common domestic house cat, which is relatively resistant to cobra venom!

Snakes' venom is designed to be used only when hunting, to help subdue prey. Each snake has limited quantities of the poisonous substance, and does not eagerly waste it on non-food animals. When threatened, snakes will usually either attempt to escape, or bite in defense without injecting much venom. The venom is produced by special glands in the snake's head, and snake venom is chemically much more complex than the compounds produced by other types of poisonous animals. Snakes can inject the venom through their tubular fangs when biting a victim. Since venom is an adaptation to make feeding easier, it is not produced in large quantities all year long: most temperate-zone snakes do not feed heavily during the fall and winter, and venom production is lower during these seasons.

A lizard well camouflaged. Photo by Maya Walters.

Snakes do have their own natural predators, including hawks, owls, and various mammals. Since the often poisonous bite of a snake is intended for prey and not predators, they have evolved a variety of other defenses, the most common which is camouflage or mimicry. Lizards, too, have need of defensive adaptations. Many lizards, when attacked, have the ability to shed their tails. After separating from the lizard, the tail continues to twitch, distracting the predator while the now tailless lizard makes its escape!