How does a rattlesnake get their venom?

Their poison is a fluid stored in venom glands in the back of their mouth that will automatically refill itself - so they basically make it themselves.

Some snakes, however, like the Tiger Snake, can sequester the toxins from the animals that they eat. Tiger Snakes sequester bufadienolide compounds from the toxic toads they eat. When they are found in an environment without these toads, however, they lack the toad's toxic bufadienolide compounds completely.

The mother snake can pass these toxin to young, in the egg, and confer a degree of protection to their young.

Additionally, the snakes that eat toads seem to know they have an additional degree of defense. They will stand and defend their territory. Snakes on a nearby island, lacking bufadienologenous toads, lack these compounds, lay eggs, bearing young, also lacking bufadienolide compounds completely. These snakes tend to flee when challenged.

Its unusual for animals to sequester defensive toxins. Poison Darts Frogs, and Mantella Frogs from Madagascar eat ants possessing an alkaloid the frog sequesters in its skin, that confers defensive abilities. Garter snakes are known to sequester tetrodotoxin from poisonous newts in it's diet, and birds in New Guinea sequester toxins from the insects they eat.

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