Do newborn adders suffer mass mortality or do they venture into a collective hide-and-seek game?

In long-lived snakes estimates of survival rates in the immature age classes are notoriously difficult to obtain because the small, secretive juveniles are rarely caught in field studies. Hence, it is assumed that in many species juveniles suffer high mortality. An alternative view holds that the youngest life stages are so elusive that they 'disappear' temporarily from the population. We conducted a long-term (2000-2016) mark-recapture study in a large population of European adders (Vipera berus) and obtained demographic data for a large sample of individuals, including respectable numbers of immature and newborn snakes. Estimates obtained by the Cormack-Jolly-Seber method revealed dramatic age-related differences in yearly capture probabilities: they were much lower in the immature classes than in the adults. Concurrently, we found no evidence for age-dependent differences in survival rates. Hence, our inability to capture large numbers of immature snakes should be attributed to their low detection probabilities, not because they suffer high postnatal mortality. At least three traits contribute to the poor detectability of the immature snakes: (1) their small body size, (2) their lower thermal requirements and (3) their non-permanent emigration to the 'summer' or foraging habitats, which possess a greater food supply than the 'winter' habitats where searches for adders are habitually conducted. We tested two hypotheses addressing the proximate causes of time-dependent variation in survival during the adders' first year of life. Results indicated that newborn adders suffered higher mortality during harsh winters, but we found no support for the hypothesis that the feeding status of the newborn snakes affect their survival probabilities.