It’s about bioaccumulation. Bioaccumulation happens when chemicals, like pesticides or heavy metals, build up in an organism’s body. These toxins come from polluted environments, from waste products of human activities like manufacturing. They pollute water or soil and gradually accumulate in plants and animals.

If toxins are present in the environment, they may first be taken in by plants, and then by animals that eat the plants, and animals that eat those animals. Black mambas are quite high up the food chain, so a lot of the toxins would accumulate in their bodies. These poisonous substances can reach dangerous levels, causing health problems for whatever eats them.