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There are only a few recent studies that have demonstrated senescence in ungulates and nothing is known regarding how patterns of senescence may vary as a function of density. Senescence in general is linked to the cost of reproduction, which probably increases with density in ungulates and may differ between the sexes. Further, senescence in ungulates is also regarded to be a function of tooth wear rates. As density dependence and sexual differences in food choice have been well documented, this may lead to different tooth wear rates and, thus, possibly density–dependent and sex–specific patterns of senescence. We therefore investigated the effects of age, sex, density and their possible interactions on the variability of body weight in 29 047 red deer harvested during 1965–1998 from Norway, out of which 380 males and 1452 females were eight years or older.There was clear evidence that spatio–temporal variation in density correlated negatively with body weights. In addition, there was evidence of senescence in both male and female red deer. Age at onset of senescence in females was after 20 years of age and independent of population density. In males, the onset and rate of senescence increased with increasing population density. The onset of senescence for males was at ca.12 years of age at low density, but decreased to approximately ten years of age at high density. The pattern of density–dependent senescence in males, but not that in females, can be explained if (i) the cost of reproduction increases with density more strongly in male than in female red deer, and/or (ii) tooth wear rates are density dependent in males, but not in females. We discuss the ability of these two different, not mutually exclusive hypotheses in explaining the observed pattern of senescence.

In order to evaluate the biogeographical hypothesis that the Norwegian lemming (Lemmus lemmus) survived the last glacial period in some Scandinavian refugia, we examined variation in the nucleotide sequence of the mitochondrial control region (402 base pairs (bp)) and the cytochrome b(cyt b) region (633 bp) in Norwegian and Siberian (Lemmus sibiricus) lemmings. The phylogenetic distinction and cyt bdivergence estimate of 1.8% between the Norwegian and Siberian lemmings suggest that their separation pre–dated the last glaciation and imply that the Norwegian lemming is probably a relic of the Pleistocene populations from Western Europe. The star–like control region phylogeny and low mitochondrial DNA diversity in the Norwegian lemming indicate a reduction in its historical effective size followed by population expansion. The average estimate of post–bottleneck time (19–21kyr) is close to the last glacial maximum (18–22 kyr BP). Taking these findings and the fossil records into consideration, it seems likely that, after colonization of Scandinavia in the Late Pleistocene, the Norwegian lemming suffered a reduction in its population effective size and survived the last glacial maximum in some local Scandinavian refugia, as suggested by early biogeographical work.

Skagerrak populations of Atlantic cod (Gadus morhuaL.) have been surveyed at several fixed stations since 1919. These coastal populations consist of local stocks with a low age of maturity and a short life span. We investigated 60 time–series of 0–group juveniles (i.e. young of the year) sampled annually from 1945 to 1994. An age–structured model was developed which incorporates asymmetrical interactions between the juvenile cohorts (0–group and 1–group; i.e. one–year–old juveniles) and stochastic reproduction. The model was expressed in delay coordinates in order to estimate model parameters directly from the time–series and thereby test the model predictions. The autocovariance structure of the time–series was consistent with the delay coordinates model superimposed upon a long–term trend. The model illustrates how both regulatory (density–dependent) and disruptive (stochastic) forces are crucial in shaping the dynamics of the coastal cod populations. The age–structured life cycle acts to resonance the stochasticity inherent in the recruitment process.

We analysed two 26–year long (1970 to 1995) time–series on annual population growth rates of Norwegian lemmings (Lemmus lemmus) from Finse, south Norway, using a threshold autoregressive (TAR) approach. We demonstrate that the population dynamics is both phase– and density–dependent. The phase–dependence accounts for the observed nonlinearity. We used the deduced stochastic model structure as a basis for evaluating the dynamic properties of this system. The dynamics is characterized either by limit cycles or chaos (the latter with a strong semi–periodic component). Stochasticity is seen to play an important role in the determination of the periodicity. The ecological implications of these statistical and mathematical results are discussed.