A three year study covering one cycle in numbers of the brown and varying lemmings has been carried out at Baker Lake, Keewatin, N.W.T. An attempt was made to describe the events of the cycle in detail by snap trapping and live trapping techniques and by detailed autopsies on about 3U00 animals and from this to determine what explanations would fit the observations.
Increase began from very low numbers in the summer of 1959 with tremendous population growth occurring over the winter of 1959-60. Little further increase occurred in the peak summer of 1960. A great decline occurred over the winter of 1960-61, and this decline continued through the summer of 1961 on the Main Study Area.
Two changes in reproduction were associated with this cycle--changes in the length of the breeding season and in the weight at sexual maturity. Winter breeding occurred only in the period of increase, and a shortened summer breeding season occurred in the peak and to some degree in the decline. Young male Lemmus matured in the summer of increase but not in the peak or in the decline; young females matured in the increase and decline summers but not in the peak.
The major change in mortality was a very high juvenile mortality in the summer of the decline.
Changes in the quality of the animals were manifested not only by these reproductive and mortality changes but also by a 20-30% increase in mean body weights of the adults in the peak summer compared to the increase or decline summers.
The role of the extrinsic factors is considered. There was no widespread destruction of the habitat, or any evidence of quantitative or qualitative food shortage in the animals of the decline. Neither predators, disease, nor parasites seemed to be the cause of the observed changes in mortality.
The role of the intrinsic factors is also considered. Summer adrenal and spleen weights showed no clear relationship to the cycle. The amount of wounding on skins showed strong seasonal and yearly changes and was not a simple function of density.
It was concluded from these observations that the lemming cycle could not be adequately explained by the conventional extrinsic factors such as food supply, but rather that it is essentially a self-regulatory phenomenon. The stress hypothesis proposed by Christian was also rejected as an adequate explanation of these events. The idea of Chitty that populations change in quality during changes in density was fully supported by these results. Behavioral changes in the population may constitute the crux of the lemming cycle, and Chitty's suggestion that these fluctuations may represent a genetic polymorphism involving aggressive behavior is not refuted by these data. / Science, Faculty of / Zoology, Department of / Graduate
| Identifer | oai:union.ndltd.org:UBC/oai:circle.library.ubc.ca:2429/39018 |
| Date | January 1962 |
| Creators | Krebs, Charles J. |
| Publisher | University of British Columbia |
| Source Sets | University of British Columbia |
| Language | English |
| Detected Language | English |
| Type | Text, Thesis/Dissertation |
| Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
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