Evolution of the Snake Venom Delivery System

Abstract

There are over 3,000 species of snakes known to man. These limbless predators have been divided into two groups, the basal snakes (Henophidia) and the advanced snakes (Caenophidia). Venom evolved prior to the advanced snake radiation and, consequently, many use venom to subdue their prey. To do so, venom is injected via the use of a venom delivery system. The venom delivery system includes a postorbital venom gland on each side of the upper jaw that is associated with specialized venom-conducting fangs or teeth. Both the venom gland and fangs are considered to have originated from a common ancestor and are thought to be developmentally linked to one another. Even though the venom gland has a common ancestral origin, it can exhibit considerable morphological variation among the main snake families. Similarly, the fangs can occupy various positions on the upper jaw but are always found on the maxilla. Caenophidians are often referred to by the position of their fangs as either rear- or front-fanged snakes. The vast majority of snakes that are medically important to humans are front-fanged, and this character has evolved independently on at least three occasions. In addition, some front-fanged snakes have evolved a secondary gland associated with the venom system, known as the accessory gland. The venom glands, accessory glands, and fangs of different caenophidian snake families exhibit substantial morphological differences reflecting their evolutionary history. However, further studies are required to fully elucidate the ecological significance of differences in fang position, the function of the accessory gland, and the driving forces underpinning the convergent evolution observed in the snake venom delivery system.