Dynamics of Freely Growing, Confined Populations of the Norwegian Lemming Lemmus lemmus

Abstract

Two populations of lemmings in 6 m2 indoor pens, provided with surplus food, were characterized by increase, peak and decline phases. The founding females in both populations produced 16-18% of all litters. The sex ratio at birth was approximately 1:1 but as the populations increased there became an excess of females among the adults. Growth rates of animals older than 35 d were similar during the various phases of population development. The mean individual growth rate was considerably greater than in our stock colony and may reflect a selection for fast growers. For both sexes there was a tendency for a single individual to be heaviest, and these animals also appeared to be socially dominant. The population increase phase was characterized by high survival rates, good reproduction success, relatively low post-natal mortality and little overt agonistic behavior. During the peak phase there was relatively more overt agonistic behavior and fighting. In the peak and decline phases reproductive success was poor, and preand post-natal mortality was high. Animals which died before weaning showed lower growth rates already during their first few days of post-natal life. Biting, often followed by infection, appeared to be one of the main mortality factors for both young and adults. Pathological conditions such as fatty infiltration of the liver, ascites and internal hemorrhage were observed. Several of the observed physiological and behavioral changes occurred more frequently in the decline than in the increase phase indicating that these changes are not direct functions of population density per se.