The term pit viper comes from the presence of paired, highly sensitive, thermoreceptor organs (pits) present on the forward portion of these snakes' heads. These structures, also known as foveal organs, serve the snake in locating prey, aiming strikes, and adjusting venom dose. The foveal organs can detect temperature changes of as little as 0.003° C (0.0054° F). A neurologic feedback loop between the foveal organs and the venom delivery apparatus may allow the snake to adjust the volume of venom it injects into a potential meal or a perceived threat.
[...] Geographic differences occur in the venoms of other snakes as well. Canebrake rattlesnakes (C. horridus) from Florida, Georgia, and South Carolina possess more neurotoxic and myotoxic "canebrake toxin" than do specimens from Alabama, Mississippi, Tennessee, and North Carolina. Differences in concentration of this toxin correlate with variable clinical effects seen after bites by this species from different geographic regions. Neurotoxicity has been clinically associated with severe myotoxicity in many cases. Severe rhabdomyolysis and myoglobinuric renal failure have been reported after Mojave rattlesnake envenomation and are thought to be related to Mojave toxin. [...]
[...] Coral snakes are identified primarily by color pattern. U.S. coral snakes are banded in a red-yellow-black-yellow-red pattern, and the bands completely encircle the snake's body. The contiguity of the red and yellow bands distinguishes U.S. coral snakes from a number of harmless mimics (e.g., several king snakes and milk snakes, genus Lampropeltis), which generally have red and yellow bands separated by black bands. This can best be remembered by recalling the phrase "red on yellow, kill a fellow; red on black, venom lack" or by considering that the red and yellow lights on a traffic signal are the warning lights. [...]
[...] In one comparison the northern Pacific rattlesnake (Crotalus viridis oreganus) expended almost 4 times more venom when biting a hand model (defensive) than a mouse (predatory). The most important factor influencing potential venom delivery is the size of the snake. A direct relationship has been demonstrated between snake length and mass of venom expended in both predatory and defensive bites. A popular belief is that juvenile rattlesnakes are more dangerous than adult snakes because their venom is more toxic and they are unable to control the volume they release. [...]
[...] In these cases the elevations appear to more closely parallel local effects. Mojave toxin is thought to inhibit acetylcholine release at the presynaptic terminal of the neuromuscular junction. Myokymia, or muscle fasciculation, is often considered a manifestation of neurotoxicity. This phenomenon, however, occurs through a different mechanism than Mojave toxin-induced neurotoxicity. Muscle fasciculations are believed to be caused by the interaction of certain venom components with calcium or calcium binding sites on the nerve membrane. Fasciculations may occur after envenomation by various species of rattlesnakes, including northern and southern Pacific rattlesnakes (C. [...]
[...] However, in the second series of exposures, "experienced" snakes injected significantly more venom into larger prey. The clinical relevance of this is uncertain. In many species, venom composition appears to change as the snake ages. Phospholipase A2 activity decreases with age, probably accounting for some decrease in toxicity. Proteolytic activity, however, increases with age, possibly to aid digestion of larger prey eaten by older, larger snakes. Coagulopathic effects can be different between juvenile and adult western diamondback rattlesnakes (Crotalus atrox), partly due to greater amounts of thrombinlike enzymes in younger snakes. [...]
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