this is a misconception. The ammount and lethality of a snakes venom is relative to the size/age of the snake, and it's use for the venom.
Please note the following is not true in every case, but is the general tendency of venomous serpents.
ammount of venom injected - determined mostly by the size of the snake, larger snakes tend to utilize larger ammounts of venom. The situation can also be a factor in this aspect. A snake that is defending itself will tend to inject much more venom than it would if it was hunting.
Lethality of venom - this is where the age and purpose factors come in. Very young snakes tend to have more potent venom than their seniors. Young snakes need more potent venom for two reasons. first, they are smaller and cannot produce the volume of venom and older and larger version could, so the venom needs to be more potent to be effective. Second young snakes are more vulnerable to predators, and more likely to use their venom defensively, which leads me to the purpose factor. If a snake uses it's venom primarily to hunt, the venom tends to be less potent than that of a snake that uses venom primarily as a defensive tool.
the type of venom also varies from species to species, and can depend alot on use. IIRC, defensive use seems to prefer neurotoxins, as they tend to act very quickly, and will allow the snake to escape. While hunters tend to use hemotoxins. (my memory on venom types is not they best, and I may well be remembering incorrectly) Venom type also seemed to be affected by frequency of use, in as much as it relates to how costly it is for the snake to make the venom. Defensive uses are less frequent than hunting uses. It would make sense that a hunting snake will need to produce more venom over the course of it's life than a snake that uses venom primarily for defence. A snake that uses venom less often can afford to invest more resources into it's creation, while the snake that uses venom more often, needs a toxin that is less costly to make.
remember, I am an amatuer herpatologist, not an expert, so take what I'm saying with a grain of salt.
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