The term adder (アダー,adā?) refers to any of a number of viper species found around the world, of which the Pit vipers can be found in North America.
A number of adders native to Raccoon Forest and living in and around the grounds of the Arklay Laboratory were infected with the E strain after eating mutant animals. Most did not exhibit major mutations apart from a more potent venom, though the "Yawn" exhibited rapid production of growth hormones. The normal sized adders continued to live and hunt in the trees as they had done before, dropping on warm blooded animals.