This poisonous rattlesnake species is a known threat throughout Whiteheart. The presence of the snake on the island impacts several of the novel’s key characters, even before the start of the novel, for it caused the death of Baba Rose years ago. Later, Hermine saves Wild Will when he is bitten by a Massasauga; the bite subsequently makes him impervious to further venom, or so asserts the local lore. Donkey grows to admire this aspect of Wild Will, imagining herself as immune when she tangles with the Massasauga whom she initially attempted to save from Hermine.

The Massasauga that Donkey wounds with a pencil becomes a significant symbol throughout the novel. In the immediate aftermath of this scuffle, the lead of the pencil remains in her arm as a reminder of the snake, and Donkey interprets this as proof that her blood and the snake’s have intermingled. As the story continues, the snake remains hidden in Rose Cottage, known only to Donkey. Its presence is an important secret and a reminder of the fact that she failed to seek treatment of Hermine soon enough to prevent the loss of her grandmother’s hand. Donkey envisions the snake as her twin sister, which implies that they are two separate halves that together make up a single, unified girl.