Reproductive behaviour of female Siberian lemmings during the increase andpeak phase of the lemming cycle

The reproduction of female Siberian lemmings in the increase and peak phase s of the lemming cycle was investigated in connection with a ship-borne exp edition along the Siberian arctic tundra. The cycle phase of each studied l emming population was determined using demographic analyses, i.e. current d ensity indices (captured lemmings per 100 traps per 24 h), information on p revious density (frequency of old lemming faeces and runways), and informat ion from dendrochronological analyses revealing the most recent winters wit h a high intensity of willow-stem scarring caused by lemmings. The cycle ph ase determination was corroborated with data on the age profiles of the pop ulations. The reproductive behaviour of female lemmings differed markedly i n relation to cycle phase. In increase-phase populations, all captured fema les (including young and winter born) were reproducing (had embryos or were lactating), and females started to reproduce early in life, i.e. when <2 m onths old. By contrast, in peak-phase populations, only 6% of the young fem ales and 63% of the winter-born ones were reproducing, and females did not start to reproduce until they were 5-6 months old. The average number of em bryos per reproducing female was significantly higher in increase-phase pop ulations than in peak-phase ones. It is concluded that the rapid population growth in lemmings during the increase phase can largely be explained by t he early (young age) reproductive start and, consequently, the shorter gene ration time, the high proportion of females taking part in reproduction, an d the large litters produced. Similarly, a delay in the start of reproducti on, a lower proportion of reproducing females, and smaller litter sizes pro duced by peak-phase lemming populations can contribute substantially to the deceleration in the population increase and possibly lead to a decline.