The observation took place on 10 August 2022 between 13:53 and 13:57 local time, just ten metres from a sandy beach on Læsø, an island of 118 square kilometres in the Kattegat where adders are commonly encountered. The setting was a dirt road crossing open grassland with heather on sandy substrate, dotted with scattered human habitations. The observer, KB, first noticed the adder examining the hare and biting its left hind leg.

The leveret lay on the ground, moving its four legs jerkily and its head with difficulty, clearly unable to stand. When the snake was disturbed by the observer's presence, it slipped into the tall grass, only to return within seconds and resume its inspection, focusing especially on the hare's front legs and head. Over the following minutes the adder repeated this pattern several times.

Each time the observer moved around, the snake retreated into the vegetation, and each time it came back within about twenty seconds to continue its careful and energetic examination. It then bit and pulled a front leg, and subsequently also bit the hare's head. By this point the hare was still breathing but moving considerably less than before.

What makes this record particularly valuable is the light it sheds on how adders evaluate prey. Experiments by Forsman and Lindell in 1993 showed that when an adder is confronted with a large prey item, it assesses its size by moving its snout over the body and then pushing and pulling it. The behaviour recorded on Læsø corresponds remarkably well with this experimental description.

After such an examination, an adder may either attempt to swallow the oversized prey or refrain from doing so. The authors of the new report consider it likely that the Læsø female would ultimately have abandoned her excessively large meal after careful examination, though whether she would actually have attempted to swallow the leveret had the observer not intervened remains an open question. The size estimates underlying the story are grounded in published morphometric data.

The adder's total length and body mass were roughly estimated at 60 centimetres and 110 grams respectively, based on comparisons with the surrounding vegetation visible in the videos and on the knowledge that female Common European Adders generally attain total lengths of 60 to 65 centimetres, seldom more than 70, and typically weigh 100 to 110 grams after giving birth. The hare's dimensions were estimated at 30 centimetres and 1,000 grams, drawing on extensive morphometric data for the European hare published by Zörner in 1981. Adult hares range from 2,500 to 6,500 grams, with a maximum of 8,000 grams, and reach full size after four to six months, while newborn leverets weigh approximately 100 to 150 grams.

The observed leveret's age was estimated at about three months. Notably, the female adder appeared to be in poor nutritional condition, probably having recently given birth. Attacks on oversized prey are not unknown among snakes, but cases in which the prey is killed and then abandoned because it simply cannot be swallowed remain poorly documented in the scientific literature.

A comprehensive review by Kornilev and colleagues in 2023 compiled 101 publications describing at least 143 cases of snake mortality associated with the ingestion or attempted consumption of injurious prey. Oversized prey accounted for 40.6 percent of these cases, with reptiles and fish making up 33 and 26 percent respectively, while mammals represented only 16 percent. Other examples reinforce the pattern.

On the island of Shedao off northeastern China, small Gloydius shedaoensis pit vipers frequently kill birds larger than they themselves can swallow, and larger individuals of the same species later scavenge the carcasses, a remarkable system in which oversized kills serve a communal function. Large vipers also kill raptorial birds that pose little or no threat to them, behaviour that reduces predation risk for smaller individuals. On Guam, the introduced brown treesnake, Boiga irregularis, has had a devastating impact on native Micronesian Starlings, with nearly half of ingestion attempts on fledgling birds failing because the prey was too large, and every one of those oversized fledglings was killed regardless.

Perhaps the most extraordinary parallel comes from southern Africa, where two species of spitting cobras, Naja mossambica and Naja nigricincta nigricincta, frequently enter houses at night and bite sleeping humans, normally on the hands or feet. These bites are interpreted as genuine predatory attacks, probably triggered by the smell of mammals rather than by snakes merely seeking warmth or being accidentally rolled onto. The human victims weigh between ten and thirty times the mass of the cobras, making them, in effect, vastly oversized prey.

Closer to the Læsø case, snakes have been recorded fatally overestimating their abilities with leporid prey: a gopher snake, Pituophis catenifer, and a Ladder Snake, Zamenis scalaris, both attacked young rabbits and died in the attempt to swallow them. In one documented case, a dead Common European Adder with a total length of just over 40 centimetres was found having just swallowed a large Field Vole whose mass, at 26 grams, amounted to 92.9 percent of the snake's own body mass after the prey was removed.

Subject of Research: Predatory attack by a Common European Adder on an oversized young hare in Denmark

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

Article References: Bringsøe, H., Jablonski, D., & Birch, K. (2025). Overly optimistic adder, Vipera berus (Linnaeus, 1758), killing and intending to swallow an oversized young hare, Lepus europaeus Pallas, 1778. Herpetozoa, 38, 155-159. https://doi.org/10.3897/herpetozoa.38.e143850

Image Credits: AI Generated

DOI: 10.3897/herpetozoa.38.e143850

Keywords: Vipera berus, Common European Adder, Lepus europaeus, European hare, oversized prey, snake predation, Denmark, Læsø, Herpetozoa, envenomation, snake feeding ecology, reptile behaviour