Epsilon Archive for Student Projects

Abstract

The wood lemming (Myopus schisticolor) is known for its skewed sex-ratio and unpredictable population outbreaks followed by mass mortality. Both mammalian and avian predators include the wood lemming in their diet, but the role of predation in the population dynamics and mass mortality of wood lemmings is unclear. To test the predation hypothesis, I dissected 158 wood lemmings found-dead during population outbreaks in 2014 and 2017 in the county of Värmland, Sweden, and looked for injuries and haemorrhages. For comparison, I also included 55 wood lemmings that were snap- trapped in 1995 - 2014 in Sweden. The most common injury in found-dead specimens was large holes and the most common location of injuries was the chest and the head. Predators killed 40 % of the wood lemmings examined and injured 33 % after their death. The male ratio before and during the outbreaks varied from 10 - 36 %. It must be stated that it is possible that an unknown number were predated and completely consumed and therefore not included in this study. Of the wood lemmings found and killed by predators in 2014 and 2017 combined, 87 % were females and 13 % males. Among the wood lemmings snap-trapped in 1995 - 2014 there were 73 % females and 27 % males. Further analyses suggest that predators did not select for larger wood lemmings and that predation varied temporally. Predation increased from July until September and most predation occurred from late July until early August, a trend that coincided with the increase in found-dead wood lemmings.

Since 60 % of wood lemmings did not die from predation, predation can be refuted as the main cause of mass mortality. Future studies should therefore focus on testing alternative hypotheses of the mass mortality of wood lemmings including the disease, stress and food shortage hypotheses. Even though predation was not the cause of mass mortality, it affected a high proportion of wood lemmings. A future study should therefore aim to identify which predators that kill wood lemmings.