Do female northern vipers (Vipera berus) really stop feeding during pregnancy?

A temporary reduction or even cessation of feeding has been documented in a wide range of vertebrate species and is usually attributed to conflicts between foraging and other activities. It is generally recognised that females of Old World vipers (Vipera spp.) reduce or even stop feeding during pregnancy, even though detailed quantitative information for most species is limited. We conducted a long-term (2000 – 2017) and intensive mark–recapture study in a large population of northern vipers (Vipera berus) and employed two indices of feeding frequency in adult females during their breeding years. The first index uses cross-sectional data and estimated that a strict minority (7 %) of captures of females in breeding years revealed detectable signs of recent food intake (swelling of the mid-body, voiding of solid faeces). A second index is based on the magnitude of increases in body mass over recapture intervals of individual females, which can be attributed to the intake of food. Depending on the criteria used to consider an increase in body mass as a consequence of prey consumption, about 12 % (range: 2 – 29 %) of recaptures revealed indications of prey consumption. Overall, we estimated that ca. 20 % of the reproductive females fed at least once before parturition. We thus confirm that most pregnant female V. berus fail to feed, but also point out that the number of females that feed occasionally during pregnancy is higher than has often been assumed. We suggest that reduced foraging by pregnantnorthern vipers is, presumably in part, a consequence of their behaviour of residing in habitats where feeding opportunities are rare because prey encounter rates are low.