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The synchrony of Siberian lemming (Lemmus sibiricus L.) population dynamics was investigated during a ship-borne expedition along the Palaearctic tundra coast in the summer of 1994. On 12 sites along the coast from the Kola Peninsula to Wrangel Island, relative densities of lemmings were recorded using a standardised snap-trapping programme. The phase position of the lemming cycle in each of the studied populations was determined based on current density estimates, signs of previous density and the age profile of each population (ageing based on eye lens mass). In addition, dendrochronological methods were used to determine when the last peak in the density of microtine populations occurred at each site. The examined lemming populations were in different phases of the lemming cycle. Some populations were in the peak phase, as indicated by high current densities, an age profile in which older individuals were well represented, and signs of high previous density (abundant old lemming faeces). Other populations were in the decline phase, as reflected in a moderate current density, a predominance of older individuals and signs of high previous density. Populations in the low phase had an extremely low current density and showed signs of high previous density, while populations in the increase phase had a moderate current density, a predominance of younger individuals and showed signs of low previous density. The results of phase determinations based on dendrochronological methods support the findings based on lemming demography. Recent Russian studies carried out on some of the sites also agreed with our phase determination results. Thus, on a regional scale (across the whole Palaearctic tundra), the population dynamics of Siberian lemmings can be considered asynchronous. However, sites situated adjacent to each other were often phase synchronous, suggesting a more fine-grained pattern of dynamics with synchrony over distances as long as 1000 km or so, e.g. the Yamal and Taymyr Peninsulas.

The reproduction of female Siberian lemmings in the increase and peak phases of the lemming cycle was investigated in connection with a ship-borne expedition along the Siberian arctic tundra. The cycle phase of each studied lemming population was determined using demographic analyses, i.e. current density indices (captured lemmings per 100 traps per 24 h), information on previous density (frequency of old lemming faeces and runways), and information from dendrochronological analyses revealing the most recent winters with a high intensity of willow-stem scarring caused by lemmings. The cycle phase determination was corroborated with data on the age profiles of the populations. The reproductive behaviour of female lemmings differed markedly in relation to cycle phase. In increase-phase populations, all captured females (including young and winter born) were reproducing (had embryos or were lactating), and females started to reproduce early in life, i.e. when