Overly optimistic adder, Vipera berus (Linnaeus, 1758), killing and intending to swallow an oversized young hare, Lepus europaeus Pallas, 1778

In snakes, predatory attacks on oversized prey are not uncommon. Here, we describe an adult female Vipera berus that likely bit and intended to swallow an oversized young hare, Lepus europaeus . The estimated mass of the hare was about 10 times greater than that of the viper. The event took place in Denmark, on 10 August 2022. We suggest that the adder under these circumstances might have benefited from a large prey to gain maximum energy after likely having given birth. Greatly oversized prey items killed by snakes are poorly known in literature. Based on published and unpublished reports, we add to the knowledge of snakes’ killings and subsequently abandoning oversized prey items. It may be an underdocumented phenomenon.