Why are some adders black?

Melanism refers to the excessive production of melanin pigments which results in a dark or completely black phenotype. Melanism occurs either as polymorphism within populations or as a fixed character between closely related species, and it is rather common in both invertebrates and vertebrates. Melanism is also quite common in snakes, but it has mostly been studied in adders (Vipera berus) and in common garter snakes (Thamnophis sirtalis). The most common hypothesis for melanism in adders (and other ectotherms) suggests that dark individuals have better thermoregulation, which may for example result in faster growth, larger body size and thus greater mating success in males as well as higher fecundity in females. However, since not all adders are black, natural selection must also act against melanism. The most likely mechanism is through increased predation as melanistic adders (without their aposematic zigzag pattern) are more likely to fall prey to avian predators.