Venom Composition and Production

Snake venoms are highly complex chemical mixtures that “may contain many proteins, mainly particular enzymes, and strongly basic polypeptides” (Jiminez-Porras, 1968).  Though snakes venoms are often characterized as either neurotoxic (those that produce paralysis and death by respiratory shock; traditionally associated with elapids) or hemotoxic (those that have hemorrhagic effects; traditionally associated with vipers), many venoms, such as those from some rattlesnakes, show evidence of both types of effects (Jiminez-Porras, 1968; Greene, 1997).  Neurotoxins, which are rich in basic amino acids (Lee, 1972), act at the molecular level by disrupting neuromuscular junctions and hence limit muscle activity (Jiminez-Porras, 1968; Greene, 1997).  Greene (1997) notes that while the term hemotoxic implies effects only to the circulatory system, hemotoxic venoms often cause tissue destruction in other body systems.  Most of these tissue destructive properties are attributed to proteins and digestive enzymes such as phospholipase A2 which are commonly present in venom (Greene, 1997). In general, neurotoxins have lower molecular weights than hemotoxins which means they diffuse into the prey more easily and often act faster than hemotoxins (Tu, 1973; Greene, 1997).

The chemical compositions of venoms are known to vary intraspecifically for a number of reasons (Daltry et al., 1996; Andrade and Abe, 1999; Mackessy et al., 2003; Alape-Giron et al., 2008).  In some species, venom compositions exhibit considerable geographic variation indicating that venoms may need to be pooled in order to create antivenins that are effective across the whole of a snake’s geographic range (Daltry et al., 1996; Alape-Giron et al., 2008).  Daltry et al. (1996) observed that the geographic variation in venom was closely related to diet and thus reasoned that natural selection had directed venom composition to make the venoms most effective against the specific prey types that snakes in seperate areas would encounter.  Andrade and Abe (1999) observed ontogenetic changes in venom toxicity in Bothrops jararaca, with juvenile venoms being able to more effectively subdue frogs while adult venom was more toxic to mice (Table 1).  As a general rule, venoms seem to be either highly toxic (to bring about rapid prey death) or highly proteolytic (presumably to help with chemical digestion) (Mackessy et al., 2003).  In the case of Bothrops jararaca, it is hypothesized that juvenile venom is suited to quickly subdue prey that may easily escape a small snake while the adult venom has more proteolytic activity to aid in digestion of larger food items (Andrade and Abe, 1999).   

Table 1. Toxcitity (LD50) of the venoms of adult and juvenile Bothrops jararaca on mice and frogs (mg/kg). 95% confidence intervals are in parentheses. Adapted from Andrade and Abe, 1999. LD50 represnets a lethal dose for 50% of a population.  Note that juvenile venom is approximately two times more toxic to frogs than adult venom, while adult venom is roughly three times more toxic to mice than juvenile venom.

An innovative study by McCue (2006) measured the cost of venom production and showed that resting metabolic rate rose 11% in snakes while venom was being replenished.  The study indicates that venoms are more energetically costly to replace than other body tissues (McCue, 2006), thus providing support to the notion that snakes may meter their venom (discussed in the Metering page on this website).